ALL of CIE IGCSE Physics! | The ONLY revision video you need! | 2025 onwards | 0972 / 0625

Summary

Highlights

• Learn about precise measurement tools – the foundation of good experiments! 🧪
• Discover physics magic with forces and turns in everyday scenarios! 🌀
• Understand energy transfers and impress with real-world applications. 🌍
• Hear the waves, see the spectrum – master sound and light waves! 🎶🌈
• Dive deep into the wonders of the cosmos with engaging space physics! 🚀🌌
• Explore the essentials of magnetism and its practical uses! ⚡📎
• Momentum made simple – from vehicles to sports, it’s all around us! 🚴‍♂️💨
• Unravel the complexities of atomic and nuclear physics effortlessly. 💣🔬

Key Takeaways

• Master IGCSE Physics with Hazel’s all-in-one video – your ticket to success! 🎓
• Don’t just study hard, study smart with Hazel's revision guides. 📚
• Learn precise measurement techniques for the best results in exams! 📏
• Discover the wonders of forces and how they shape our world. 💪
• Eureka! Understand energy transfers and their real-world applications. ⚡
• Demystify waves and make sense of sound and light! 🌊🔦
• Get inspired by the mysteries of the universe and the physics that govern it! 🌌✨
• Embrace magnetism and discover its magic in everyday life! 🧲
• Ace momentum and movement with simple steps and clear explanations. 🚗💨
• Unlock the secrets of atomic structure and nuclear physics! 🧪🔬

Overview

Chapters

• 00:00 - 00:30: Introduction The introduction sets the stage for an all-in-one CIE IGCSE Physics video. The presenter expresses excitement about the video's release and aims to offer a comprehensive guide for scoring maximum marks in exams. She mentions that 'perfect answers' will appear throughout the video to aid students. Additionally, viewers are encouraged to purchase her revision guides, which include these perfect answers and additional resources.
• 00:30 - 03:00: Measuring Volume, Length and Time The chapter titled 'Measuring Volume, Length and Time' begins with useful details about school licenses and revision guides available on the website. It then explains how volume is measured using a measuring cylinder, which is described as a plastic tube with increments on the side. It is noted that while the measuring cylinder is not very precise, it serves the purpose for basic volume measurement by filling it with liquid and reading the measurement at the liquid's level. Further details might not be provided in this excerpt, but the chapter likely continues with instructions or explanations on measuring length and time.
• 03:30 - 09:00: Resultant Force and Parallelogram of Forces The chapter discusses the measurement of forces and the concept of resultant force. It introduces the parallelogram of forces as a method to determine the resultant force when two forces are applied at an angle. Different tools like meter rulers and stopwatches are mentioned for measuring lengths and intervals of time, which are essential in experiments involving forces. This chapter emphasizes the importance of accurate measurement in understanding and calculating forces.
• 09:00 - 21:00: Speed, Distance, Time and Graphs The chapter focuses on the concepts of speed, distance, and time, along with their representation using graphs. It begins with an explanation of two main types of time measurement: analog and digital. Analog involves measurements akin to a clock face, such as using a stopwatch to measure seconds. Digital timekeeping, however, is noted for its precision, allowing measurements to the nearest hundredth of a second.
• 21:00 - 32:00: Forces and Motion The chapter 'Forces and Motion' discusses the process for calculating the average value of a short distance, such as the distance a javelin is thrown. It explains the importance of measuring the distance multiple times, adding all the results together, and dividing by the number of trials (n) to find the average distance.
• 32:00 - 52:00: Pressure and Moments This chapter introduces the concept of calculating average values by repeatedly measuring and summing quantities, then dividing by the number of measurements. This methodology is applied specifically to a pendulum system, which is a weight hung at the end of a string that swings backward and forward. The introduction sets up the framework for understanding the mechanics of pendulums, likely leading into discussions of pressure and moments as they relate to pendulum motion.
• 52:00 - 60:00: Momentum The chapter titled 'Momentum' discusses the concept of momentum in the context of pendulums. It explains that the time period of a pendulum is the duration it takes to complete one full oscillation. For accuracy, it is suggested to measure the total time taken for a number of swings (n) and then divide by n to get the average time for a single oscillation. This method provides a more precise measurement of the pendulum's time period.
• 60:00 - 76:00: Energy The chapter 'Energy' begins with an explanation of the concept of resultant force, which is challenging to define due to multiple forces acting on an object. The resultant force is described as the single force that represents the combined effect of all individual forces acting on an object. The chapter intends to provide mathematical insights and examples to demonstrate how to determine the resultant force given various types of questions. The initial complexity in understanding usually arises from the questions themselves, rather than the concept.
• 76:00 - 83:00: Units and Conversions In this chapter, 'Units and Conversions,' the focus is on learning how to measure distances and angles using basic tools such as a protractor, ruler, and pencil. The emphasis is on using these tools to solve problems rather than relying on more complex mathematical methods like trigonometry. Examples are provided to demonstrate how to calculate the magnitude and direction of forces acting on an object.
• 83:00 - 93:00: Heat Transfer and Insulation The chapter 'Heat Transfer and Insulation' discusses the concept of forces and their interaction. It uses the example of a parallelogram of forces to explain the mechanics of pulling forces in a pulley system. The key takeaway is understanding how a force of 350 newtons pulling on a pulley results in a net upward force of 50 newtons, after accounting for the opposing force of 300 newtons exerted by the weight of the box. The chapter emphasizes the importance of analyzing the forces at play without always resorting to complex diagrams, focusing instead on the intuitive understanding of force interactions.
• 98:00 - 118:00: Pressure, Solids, Liquids, and Gases The chapter titled 'Pressure, Solids, Liquids, and Gases' explores the concepts of force and pressure. The transcript mentions examples of forces in action, such as calculating the resultant force when two men push a trolley uphill using force arrows. It explains how to add forces working in the same direction, demonstrating with an example of 300 newtons and 200 newtons combining to make a 500-newton force. The chapter also introduces the parallelogram of forces, used to determine resultant forces when forces act on a point.
• 118:00 - 167:00: Waves The chapter discusses the process of calculating the magnitude and direction of the resultant force from two given forces. It highlights the importance of choosing an appropriate scale for measurement, such as one centimeter representing one newton, and explains that the magnitude refers to the size of the force. The discussion involves practical steps, such as using a ruler to measure the forces accurately.
• 167:00 - 213:00: Magnetism and Electromagnetism This chapter covered the basics of magnetism and electromagnetism, particularly focusing on understanding angles and precise measurements which are crucial in experiments and drawing. It involves using tools like protractors to ensure accuracy, as demonstrated in drawing a 90-degree angle and a specific length of a line, in this case, a four-centimeter line.
• 213:00 - 268:00: Electricity and Circuits The chapter "Electricity and Circuits" starts with a demonstration of converting a diagram into a parallelogram. It involves labeling forces and understanding the nature of a parallelogram, which is a shape with two parallel sides. As part of the learning process, a line parallel to a three newton force is drawn, and a subsequent line runs parallel, extending four centimeters down.
• 268:00 - 339:00: Radioactivity and Particles The chapter titled 'Radioactivity and Particles' involves completing a parallelogram to find the resultant force, which simplifies to a rectangle in this specific context. The process to find the resultant force includes joining the corner of the parallelogram to the origin of the forces and drawing a straight line, preferably using a ruler. To find the magnitude of the resultant force, measure the length of this line, which in the provided case equals five.
• 339:00 - 359:00: Astronomy and the Universe This chapter, titled 'Astronomy and the Universe', begins with a physics problem involving the calculation of a resultant force of five Newtons and determining its direction. The direction is measured by identifying a specific angle, initially recorded incorrectly and later corrected to 55 degrees. The chapter uses this problem to explain fundamental physics principles including magnitude and direction calculation. Following this problem, the chapter transitions into discussing the speed-distance-time topic.
• 459:00 - 460:00: Conclusion In the concluding chapter, key equations are emphasized. The speaker expresses a fondness for formula triangles, introducing the primary example: distance (d) at the top, with speed (s) and time (t) at the bottom. To find distance, cover the 'distance' on the triangle, and the formula is speed multiplied by time. To find speed, cover 'speed,' and the formula becomes distance divided by time. The chapter serves to highlight these fundamental relationships in mathematics.

ALL of CIE IGCSE Physics! | The ONLY revision video you need! | 2025 onwards | 0972 / 0625 Transcription

• 00:00 - 00:30 [Music] [Applause] [Music] hi guys welcome to my all-in-one cie igcse physics video it's so exciting that this video is finally here i hope you find it really helpful as always my perfect answers pop up throughout the video this is the wording i think you should be using in your exams to score maximum marks if you want me to do the hard work for you you can check out my revision guides which are available to purchase on my website and they contain my perfect answers as well as some extra
• 00:30 - 01:00 bits and bobs which you may find helpful and there's also details about school licenses and revision guides which you'll also find on my website however we need to get started so jumping in with how is volume measured and we use a measuring cylinder for this so remember it's a plastic tube with increments on the side it's not a very precise piece of equipment and you fill it with the liquid say up to here read across
• 01:00 - 01:30 and therefore you have your volume looking at length now you'll use various pieces of apparatus here you could use a meter ruler if you're looking at quite a long item otherwise use a centimeter ruler so the rulers will measure millimeters to centimeters to meters looking at measuring the interval of time a clock stop clock or stop watch will be useful here
• 01:30 - 02:00 there are two main types you've got the one which looks like a clock face and that's an analog measurement of time and you often find if you're measuring seconds and you're using a stopwatch it will move around and then you press the stop button you can measure the time a more precise way is using a digital clock as this can measure to the nearest hundredth of a second
• 02:00 - 02:30 moving on a little bit and now we need to look at how we would calculate the average value of a short distance such as the distance a javelin is thrown so you want to crucially measure the distance multiple times and we're going to call this n then you want to add all the results together and then finally divide by n to get that average distance
• 02:30 - 03:00 similarly if you're calculating the average value of time you want to measure the time multiple times we'll call that n again add the times together and again divide by n now looking specifically a pendulum so that's a string which has a weight at the end which moves backwards and
• 03:00 - 03:30 forwards in an arc so the time period of the pendulum is the time taken to complete one oscillation and the best and most accurate way to do this is measure the time taken for n number of swings and then divide by n
• 03:30 - 04:00 now a resultant force is quite hard to define because there can be lots of forces acting on an object and resultant force is kind of the combined effect of all those forces and how's the definition you'll need to provide it is a resultant force is a single force that has the effect of all of the forces acting on an object in terms of maths and the video we're going to look at now i'm going to show you how you can work out the resultant force when you've been given several different types of question now the issue with the questions isn't actually
• 04:00 - 04:30 the maths involved because actually all you need is a protractor make sure you have one of those and a ruler and a sharp pencil make sure you use a pencil in case you screw up so you can rub it out because if you're using pen you're going to get into quite a mess and then it's a matter of counting distances counting the length along your ruler rather than actually using trigonometry to help you solve it so i promise it's not as hard as it looks so i'm going to show you some examples now one figures six and seven show examples where two forces act on object x in each case work out the magnitude and direction of the resultant force on x now don't go
• 04:30 - 05:00 straight into drawing a parallelogram of forces because you've learned that and therefore you're determined to use it sometimes you won't need to use it and it won't make sense to just look at the figure closely to work out what's going on so you can see that this guy is pulling on a pulley and he's pulling with a force of 350 newtons but the crucial thing is that pulley is attached to here which means his force is actually pulling this weight upwards now the weight of the box is pulling the box down by 300 newtons so what's the difference in those two numbers well it's 50 newtons and obviously that will
• 05:00 - 05:30 be up so your answer is 50 newtons upwards then we've got two men pushing a trolley that's quite a heavy weight up a hill and some one of them is pulling it but you can see from these force arrows that they're both pulling in the same direction so all you have to do here is add together the individual forces so that's 300 plus 200 and that's the 500 newton force up the hill and that's done now i'm going to show you how when to use the parallelogram of forces so a force of three newtons and a force of four newtons act on a point determine
• 05:30 - 06:00 the magnitude and direction of the resultant of these two forces if the angle between their line of action is and i probably should have mentioned it before but magnitude is literally just the size of the four so the calculation that we literally just did so in order to do this you will need your ruler and you'll need to choose a sensible scale so i'm going to choose one centimeter to represent one newton it doesn't matter which way around you do this but i'm going to do my three newtons along here so that's going to be a three centimeter long line i'm using a
• 06:00 - 06:30 pen by the way because obviously i'm not answering an exam paper so it doesn't actually matter if i screw up here and then you're using your protractor because you want a perfect 90 degree angle to mark where 90 degrees is which is here and now you've got your two points you want to link that together with a oh that's annoying it's not actually as long as i want it to be but i'm fairly sure i've drawn that nice and accurately so that's a four centimeter line going upwards
• 06:30 - 07:00 and now we're going to turn it into a parallelogram i'm just going to label my forces so what is a parallelogram well it's the shape that has two parallel sides so obviously parallel to three newtons will be this line and then parallel to this line we're going to draw four centimeters coming down
• 07:00 - 07:30 okay so we've completed our parallelogram which in this case is just a rectangle and then in order to find the resultant force all you have to do is join the corner of that parallelogram to where the forces originated from you're just going to draw a nice straight line you must use a ruler for this and then in terms of working out the magnitude just measure the length of that line yeah and that's five
• 07:30 - 08:00 so your resultant force is five newtons and then in terms of working out the direction you're just going to measure this angle here and then make sure you're reading the right way so obviously you'll be doing in this case this way i think i've done it slightly wrongly so i'm going to put that as 55 degrees and that's how i've worked out both the magnitude and direction now we're going to take a look at the speed distance time topic
• 08:00 - 08:30 so there are some very key equations that you need to be aware of so you know me i love formula triangles so i'm going to have my first one here which is d at the top then s then t now you cover whatever it is that you're after so i'm after distance so if i cover that over with my thumb i can see that it's there for speed times time if i'm after speed then we're going to do distance divided by time and then lastly time
• 08:30 - 09:00 is distance divided by speed so if they ask you for the equation sure use your formula triangles to actually work out what the equation is but you must write out the full equation to get the mark formula triangles do not count so as you can see on the left hand side i've drawn very poorly a distance time graph and i'm after this speed at point a so to work out the speed i know that i need to do distance divided by time i can read that straight off the graph so
• 09:00 - 09:30 i can see that the distance traveled is eight the time was four seconds so simple calculation the speed is therefore two meters per second meters per second due to the units we can see on the y-axis which was the distance was given in meters and time was given in seconds it's important to understand these graphs fully because they could ask you for example how long was the car stationary for so in terms of stationary you're looking at the flat parts of the graph so the car is stationary here why because it remains
• 09:30 - 10:00 at eight meters throughout that time so how long was the cast station before have a look along here and the answer here is four seconds this is a different type of graph so the velocity time graph or the speed time graph not to be confused with the distance time graph so we are being asked to find out the acceleration at point a on the graph so acceleration is given by the equation acceleration equals
• 10:00 - 10:30 final speed minus initial speed over time and a neater way to write that is a equals b minus u over t so looking at portion a of the graph what is the final speed well it's given here so read across and you'll see that it is five the initial speed is given here and we know that zero so five minus zero
• 10:30 - 11:00 the time taken was four seconds so the answer here is one point two five now be careful with your units you're looking for meters per second squared for acceleration another thing they like to ask you is to calculate the distance traveled now this is a longer task and remember distance traveled is given by the area under the graph so let's calculate the entire distance traveled by this particular vehicle so
• 11:00 - 11:30 the way i like to do this is by splitting it up you can work it out as a trapezium or you can work it out as two triangles and a rectangle which i personally find easier so i'm going to label them a b and c so area a is given by this formula well it's a triangle so i need to put in a half immediately times it by the height of the triangle which is 5 times it by its width do the calculation and that will therefore be 10 meters area b is more straightforward it's just
• 11:30 - 12:00 a rectangle so just make sure you're reading the correct length so it's 5 again this time times it by 3 you get a value which is 15 meters finally area c we're back to our triangle so we need a half times the height of the triangle which is five times the width of the triangle which is two so we get five meters if you add that up you get a total area of 30 meters and that therefore is the distance traveled
• 12:00 - 12:30 carrying on with this topic let's ask ourselves what is deceleration well as the name suggests it is the opposite of acceleration so we are slowing down so it's a decreasing changing speed which is the same as negative acceleration and remember the units will be meters
• 12:30 - 13:00 per second squared same as acceleration next up what is free fall so clearly it needs to be an object which is falling down towards the earth what is responsible for this well it is gravity and the acceleration of free fall is measured in g
• 13:00 - 13:30 and notice that this acceleration is constant the next thing we need to know is describe the motion of objects falling without air resistance and you need to remember this number which is that they have a downward acceleration of 9.8 meters per second squared and you'll find that the object continues to accelerate due to no air resistance
• 13:30 - 14:00 and that's obviously until it reaches the ground remember about terminal velocity we tend to use the example of a parachutist jumping out of an aeroplane and you'll often see lots of diagrams of areas of different sizes i'm going to try and talk to you about that now so let's start at the top we're starting in our airplane and our parachutist is looking out and he's about to jump the moment he jumps the only force acting on
• 14:00 - 14:30 him is weight so that will cause him to accelerate towards the earth's surface now the faster he accelerates the the bigger the air resistance because remember as he's dropping he's going to be hitting lots of air and that those air particles are going to be acting in the opposite direction to his motion trying to slow him down and the faster he travels the more particles will be hitting per second so the overall force of drag or air resistance will become very large so at a certain point you will find that the size of the
• 14:30 - 15:00 air resistance or the drag will match the size of the weight force acting downwards and we call that terminal velocity because all that means is that the two forces are balanced and the parachutist will no longer accelerate he'll just continue falling at a constant speed and that is what time velocity is basically neither accelerating or decelerating just traveling at a constant speed before too long the parachuters will choose to open his parachute because he won't obviously want to splatter on the ground when he
• 15:00 - 15:30 opens his parachute you see a massive increase in the surface area of the parachutist and therefore way more air particles will be will be trapped inside the parachute and therefore air resistance will increase hugely and he'll jerk upwards as a result of that and you'll see that he'll slow down however because his speed decreases that actually causes air resistance to decrease because if you think about it the slower he now travels the fewer air
• 15:30 - 16:00 particles will be hitting him and opposing his motion and therefore he'll slow down and eventually his weight and the size of the drag air resistance force will become the same and we call that terminal velocity again because he's now traveling at a constant speed now remember that terminal velocity will be at much lower speed than the initial one and that's due to the fact he's opened up his parachute a student investigates terminal velocity she uses a tall glass tube filled with oil she drops the metal ball into the
• 16:00 - 16:30 tube and the ball falls through the oil use ideas about forces to explain how a falling object can reach terminal velocity it doesn't matter what this question is if it's someone jumping out of a plane or a ball being dropped into a tube of oil if it mentions terminal velocity this is your perfect answer so first of all you want to state that the ball has a weight and that will be its downward force acting upwards will be the force of drag as the ball accelerates the drag force will increase and eventually the weight
• 16:30 - 17:00 will equal drag at this point the ball no longer accelerates it travels at a constant speed and there is no further acceleration and this means that terminal velocity has been reached so that's your perfect answer try and write it in sentences i've written it like that for ease of ipad use describe how the student could find out if the ball reaches terminal velocity as it falls through the oil in your answer you should include the measuring instruments that the student will need
• 17:00 - 17:30 the measurements she should take and how she could use her measurements to find out if the ball has reached terminal velocity you may include a label diagram in your answer so clearly we need to use a timer to time how long the ball falls for and we need to use a baller to make sure that the ball is falling over the same distance every time in terms of the measurements taken you need to take a measurement of time for the amount of time that it took for the ball to pass between two points you obviously want to work out that distance using your ruler
• 17:30 - 18:00 and then repeat and calculating average to increase the reliability to make it even more specific you could have used a light gate and then lastly once you've worked out the distance and the time you can obviously calculate speed using speed equals distance divided by time i've picked out some questions so question one a toy car rolls down a ramp and hits a cushion the graph shows how its velocity changes with time so this is effectively a speed time graph constant velocity on the graph is shown by what does the word constant mean well
• 18:00 - 18:30 it means same so where on this graph is it the same velocity now you can kind of imagine a y-axis and the numbers going up the y-axis so it could be something like 0 10 20 30 on the velocity front so where does that not change well it's obviously going to be the horizontal portion of the line because at that point the speed or the velocity is going to be the same everywhere so the answer here is b the horizontal part of the line and with these sorts of questions rather than reading the options through and getting confused i would look at the question first of all work out what you
• 18:30 - 19:00 think is the answer and then see if that is an option below the distance travelled is shown by well again i told you to work out distance that's the area under the graph line let's look at the options and that's a the average velocity of the toy car is given by so remember your equation velocity or speed equals distance divided by time therefore that is b a bus travels along a straight road the graph shows how the velocity of the bus changes during a
• 19:00 - 19:30 short journey okay so we've got a velocity time graph so again distance will be given by area under the curve acceleration will be given by the gradient anyway state the velocity of the bus after 25 seconds so you're looking up here the answer here is six meters per second how long is the bus stationary during its journey now i've seen loads of people get this wrong because they think that it's stationary here and here one of that's absolute rubbish because look at the velocity it's 12 so
• 19:30 - 20:00 stationary means that it was standing still so standing still means that the velocity must have been zero which is just this chunk here and therefore your answer here is 10 seconds state the equation linking acceleration change in velocity and time taken they're giving you the exact wording so you want to write it out as they've written it so you're going to write acceleration equals change in velocity over time taken or you could have written a equals v minus u over t calculate the acceleration of the bus during the first 10 seconds give the
• 20:00 - 20:30 unit that's worth three marks so we'll have a look back up here and we're looking at this portion of the graph the first 10 seconds we're looking at the gradient so that equation was acceleration equals v minus u over t so that is final speed which is 12 minus initial speed which was zero over time taken which was 10 so 12 take 0 over 10 is 1.2 the units of acceleration are meters per second
• 20:30 - 21:00 squared state the equation linking average speed distance moved from time taken average speed equals distance move divided by time taken the bus moves a total distance of 390 meters during the journey calculate the average speed of the bus so speed equals distance divided by time distance is 390 divided by look on the graph for whatever the time was and it was 60 seconds pop that into your calculator and your
• 21:00 - 21:30 answer will be 6.5 meters per second the bus travels further in the first 30 seconds of its journey than it does during the last 30 seconds of this journey explain how the graph shows this well remember that i told you distance is given by area under the graph line and you can see that the trapezium for the first 30 seconds is way bigger than the one for the seconds 30 seconds so therefore clearly the bus traveled further so for the first mark state the fact that distance is given by area under the graph and then for the
• 21:30 - 22:00 second one compare the two areas under the line and you're done question four the diagram shows some people waiting in a cutest supermarket the queue moves forward each time a person leaves the checkout person x spends seven minutes in the queue before reaching the checkout and the graph shows how distance changes with time for person x so it's the last person in the queue and notice that it is a distance time graph so what is the initial length of the queue and we're looking for that in meters so all you have to do is read
• 22:00 - 22:30 off here on the y-axis so read along and you'll see if my pen wasn't as fat that it is 6.1 meters explain how you could use the graph to work out the number of times person x is stationary so explain is going to be a very crucial word here and we also need to work out the number of times the person is stationary so stationary means that they don't move so when aren't they moving well it's all the flat portions of the grass so that's one two three
• 22:30 - 23:00 four five six seven so you're going to say seven times that they were stationary and we know this because the flat part of the graph indicates zero speed state the equation linking average speed distance moved and time taken so out of
• 23:00 - 23:30 the way i'm going to write my formula triangle dst so therefore average speed equals cover the s distance moved over time taken part two calculate the average speed of person x in the queue give the unit don't forget to give the unit so the distance moved we know is 6.1 meters the time is 7 minutes
• 23:30 - 24:00 don't get caught out here i know the graph looks confusing but it tells us up here that the person spent seven minutes in the queue i'm going to multiply it by 60 because the s i unit for time is seconds so obviously there are 60 seconds in a minute hence why i'm timesing it by 60. pop that into your calculator and you'll get a value which rounds to 0.015 to three significant figures and we know that the distance was in meters
• 24:00 - 24:30 i've already told you that i've converted to seconds which is why the unit here is meters per second we now need to discuss the difference between weight and mass so people constantly get this confused they talk about how much they weigh when actually they're talking about their mass and i'm going to explain what i mean by that so weight first of all is given by the unit n which stands for newtons mass is given by the unit kilograms
• 24:30 - 25:00 and so straight away we can see that they have different units now it's worth noticing that your mass is unchanged it doesn't matter where you stand with which planet you're standing on where you are in space your mass will remain the same so if your mass is 50 kilograms on earth it will be 50 kilograms on mars and that's because it's really a description of what you're made up of however with weight weight has to take into account gravity
• 25:00 - 25:30 and therefore when gravity changes your weight will change if we look at the equation linking the three you find that weight equals mass times gravitational field strength so if you have a massive 50 kilograms you're standing on the earth which has an approximate gravitational field strength of 10 your weight will therefore be 500 newtons so this would be the case on earth however
• 25:30 - 26:00 on the moon if we do the same calculation we find that although your mass is still 50 your gravitational field strength is hugely reduced on the moon to in fact only 1.6 meaning that your weight on the moon is only 80 newtons so this is a huge change your weight has drastically decreased on the moon whereas your mass has stayed the same so there in everyday life we talk about how much we weigh we save 50 kilograms remember that's inaccurate and then we're actually discussing our mass
• 26:00 - 26:30 and as long as you can use this equation weight equals mass times gravitational field strength you'll have no problems so we're looking now at the density and pressure topic so remember density is to do with how close together particles are heavy objects have high density and that's because particles are close together lighter objects are less dense and that's due to the further apart spacing of the particles now the equation you need to know about is
• 26:30 - 27:00 density equals mass over volume now i use a formula triangle as usual to remember this and one of my two t's taught me a good way to know this is drunk men vomit so you want d at the bottom of the triangle m at the top v at the bottom so to work out density it's mass over volume mass is density times volume and volume is mass divided by density they're quite interested in you being able to determine the density of various objects so let's start with symmetrical objects things that we can easily measure the volume of so things
• 27:00 - 27:30 like cubes cuboids brick for example now what you want to do here is obviously looking at the equation density cause mass over volume you want to measure the object's mass using a top pan balance and get that measurement in kilograms then to measure the volume we simply use a ruler we measure the length width and height multiply those values together to work out the volume substituting your mass and volume values into the equation and out pops the density
• 27:30 - 28:00 now it's going to be far more difficult measuring the density of an asymmetric object something like an ornament or a vase or a stone and that's because you can't easily measure the length width and height of the object so it's very difficult to determine its volume this time we're going to use the displacement method the simplest way to describe this is you fill a measuring cylinder with water you measure the volume then you add the object to that measuring cylinder and you measure the new volume and the
• 28:00 - 28:30 difference between those two values will clearly be the volume of the object so its volume is now sorted its mass is easy to determine using the top pan balance method substitute the values into the equation so make sure you state the equation in your answer that will be worth a mark and out pop's density let's look at a couple of density example calculations so question one 0.1 meters cubed of a liquid has a mass of 25 kilograms what is its density so we need to use this formula triangle
• 28:30 - 29:00 i use drunk men vomit to help me remember the order obviously that's disgusting but one of my treaties taught it to me and it's pretty helpful so we can see from it the density is given by mass over volume the mass has been given as 25 so volume is 0.1 so our final answer here is 250 and note the units the mass was given in kilograms the volume was given in meters cubed it's being divided which is why this is the unit
• 29:00 - 29:30 example two a solid has a mass four kilograms and a volume of one meters cubed what is its density so same formula density equals mass over volume our mass is 4 kilograms our volume is 1 meters cubed 4 divided by 1 is 4 and the units are the same as before next up how to use given density data to predict if an object will float so your first step in doing this is look at the relative density
• 29:30 - 30:00 and then you need to compare it to the number one and if that relative density is greater than one then clearly the object will sink if it is less than one then it will float we now need to look at the topic of force so just remember first of all the effect that a force has on an object and
• 30:00 - 30:30 that can be that the force changes the object's speed it can change the object's direction or it can indeed change the object's shape you need to be able to list the different types of force so i've written them out here and now we just need to have a quick chat about what all of those things mean so first of all notice that some of these forces are contact forces and these are forces which act between two objects that are actually physically touching each other non-contact forces is obviously as the name suggests when they don't touch
• 30:30 - 31:00 so let's actually have a look at this normal reaction first of all because that's quite a strange one this normal reaction force or reaction force or normal contact force it's all the same thing that's when an object is on the ground and it experiences a force which is perpendicular to the surface and what that means that's where the word normal comes from because remember in the light topic a normal line is a line for example drawn like this so here's your glass block and here are some normal
• 31:00 - 31:30 lines because they're 90 degrees to the surface of the glass block so in a similar way the reaction force is felt perpendicular to the surface and i'll show you an example of that later with a cast so don't worry too much now friction is a nice straightforward one that's when two objects slide past each other they experience a friction force so for example a toy car sliding down a slope will experience friction where its wheels touch the slope air resistance or drag that's to do with objects moving
• 31:30 - 32:00 through the air so it could be a card being driven along the road and air particles collide with the car creating a small force which acts to slow it down the faster the car is traveling the more particles that will hit per second hence air resistance increases and that's why things like cars have to be streamlined to reduce air resistance looking at non-contact forces so let's start with the magnetic force and that is experienced by any magnetic material inside a magnetic field do remember that opposite magnetic poles attract so north
• 32:00 - 32:30 and south poles will attract like poles such as two north poles or two south poles repel and electrostatic forces all to do with static charge so we're talking about the build-up of charge and it's experienced by any charged particle that's held within an electric field so a gravitational force is the name suggested to do with gravity and it's experienced by any mass which is found within a gravitational field and remember these masses may be attracted towards each other due to this
• 32:30 - 33:00 gravitational force the easiest example here is the sun and the earth they're both masses they're both very large objects and they're held in position due to the gravitational force between them weights i've already touched upon remember that is a force acting downwards from any object and it incorporates both the mass and gravitational field strength up to us this is when we're talking about water so for example a boat sitting on water although its weight is acting downwards there's the force of the water
• 33:00 - 33:30 pushing back up so it's the equivalent of the normal reaction force so we call this up thrust and if weight and upstress are equal then we know the boat will float the nuclear force is as the name suggests to do with the nucleus of atoms or the nuclei of atoms and it's the strong attractive force between the protons and the neutrons within the nucleus that helps hold the nucleus together remember that force is measured in newtons and that you often see a force diagram and that's basically an
• 33:30 - 34:00 object which is moving and it will have arrows and the size of the arrow represents how large the force is so obviously the larger the arrow the larger the force remember in your textbook i'll often talk about balanced forces that is when two forces acting on an object they'll be acting in opposite directions to each other and remember that they have to be the same size for them to be balanced forces and that basically means that an object which is standing still i.e stationary
• 34:00 - 34:30 it won't speed up it will stay exactly as it is and it also means that an object which is traveling at a certain speed when it has balanced forces acting on it will continue to travel at that speed it won't speed up it won't slow down so therefore if we're talking about unbalanced forces it makes sense that one of the forces opposing the other force is larger than the other so if the object is stationary it will start moving and if the object is traveling at certain speed it will either speed up or slow down so what are these forces that we're
• 34:30 - 35:00 talking about i'm going to use the example of a car driving along the street to help illustrate this so you have a car it starts on the tarmac and it's driving so the forward force is going to be the driving force from the engine and that will be causing the car to either accelerate or just carry on traveling at the same speed opposing that driving force will be several other forces first of all air resistance otherwise known as drag now what is that basically if you've got a car moving you've got air whistling past it and
• 35:00 - 35:30 these air particles they'll collide with the car and they generate a tiny tiny force which acts in the opposite direction to the direction the car's traveling and by doing that they oppose the motion of the cast they act to slow it down and you have lots of other forces opposing motions so there's friction remember friction is the force that occurs between surfaces so friction in this example will be between the car wheels and the road so remember it's that friction which is useful because it
• 35:30 - 36:00 allows the car to grip onto the road and in certain conditions like icy conditions wet conditions or if the tires have been too worn you'll decrease the friction between the tires and the road and that can lead to dangerous occurrences like the car skinning so a bit of friction is important you have other forces acting on the car you have weight remember that's the downward force due to gravity and don't forget this force it's called the normal reaction and that occurs between the tires and the road surface and it occurs upwards so perpendicular to the road and
• 36:00 - 36:30 basically all the normal reaction is is it's the force which stops objects kind of being forced into the earth so it acts against gravity that's quite a hard one to imagine but just remember that it occurs at 90 degrees to the surface another physics equation you need to be aware of is fam so f stands for force a stands for
• 36:30 - 37:00 acceleration and m stands for mass so let's work out the various variations so force equals we can see that it's acceleration times mass if for after acceleration cover that up you can see that it is force divided by mass and lastly mass equals force divided by acceleration
• 37:00 - 37:30 touching quickly on the units mass should always be in kilograms forces newtons so acceleration is meters per second squared so make sure you have that sorted and we'll look at a couple of examples now so in this question we're calculating the resultant force required to accelerate a 30 kilogram object at 1.5 meters per second squared so yeah we have mass we have acceleration we're looking for force so force equals
• 37:30 - 38:00 acceleration times mass it's a simple expedient of substituting in those values so 1.5 times 30 which gives you a value of 45 newtons question 2 the force on a moving object is 1000 newtons calculate the mass of an object if it is accelerating at 2.5 meters per second squared so we've been given the force and the acceleration mass is given by f over a our force is a thousand newtons divided
• 38:00 - 38:30 by the acceleration which is 2.5 giving us a value which is 400 don't forget your units it is in kilograms so if we had to describe what friction is in further detail we'd say that it's the force between two surfaces that they exert against each other and the point is that if one of those surfaces happens to be air we can't call it friction in this case we call it
• 38:30 - 39:00 something else which is air resistance sometimes also known as drag what effects does friction have well first of all you've probably felt if you've rubbed your hands together that creates a large force of friction and it generates heat and then the next more obvious effect of friction is that it will impede or restrict movement of the two surfaces
• 39:00 - 39:30 past each other so for example a toy car sliding down something like sandpaper because there's an awful lot of friction between the sandpaper and the toy car wheels you'll see lots of friction and it will indeed impede the movement of that toy car now we need to look at the forces which cause an object to move in a circle we call this sort of force where objects move in a circle centripetal force
• 39:30 - 40:00 and you need to be aware of newton's first law which remember states that an object that's stationary will remain stationary an object going along at constant speed will remain at constant speed so how does an object move in a circle well first of all you need an external force which causes a change in velocity
• 40:00 - 40:30 and that change in velocity by definition means that the object must be accelerating and if this force is applied perpendicularly so at right angles to the direction of travel you find that the object moves in a circle linking with that fact that i said it would be constantly changing its velocity so it'd be accelerating the acceleration actually occurs towards the
• 40:30 - 41:00 center of the circle and that is according to the equation f equals m a so force equals mass times acceleration so i want to quickly talk to you about hooke's law now hooke's law is a pretty boring experiment whereby a guy called hook amazingly originally hung various weights off a spring and measured the new length of the spring and then obviously as more
• 41:00 - 41:30 weights were added the spring got longer and then he plotted a graph which was force and it's okay to plot force because remember force and weight have the same unit which is newtons against the extension and you see on the graph that it should be a proportional relationship which means that as the length as the weight increases the length of the spring increases which makes sense then you'll find that the line distorts and that's because we're no longer increasing proportionally and that's because the elastic limit of the spring
• 41:30 - 42:00 has been reached it's deformed it spends out of shape and it won't return to its original length so that's what underpins hookshot now we're going to look at a past exam question on it so a student investigates whether spring obeys hooke's law she uses the operator shown in the photograph which additional measuring instrument does the student need for the investigation well i've told you it's all about measuring the length of the spring so we need a ruler explain how the student can investigate whether the spring obeys hooke's law so we're basically looking for experimental
• 42:00 - 42:30 details on what they're going to do so first of all you want to measure the original length of the spring using that meter ruler and then we're going to add a known weight to that hook and measure the new length of the spring and then we just repeat the process using a range of different weights so we could go up to 20 newtons 30 newtons 40 newtons or we could repeat at each weight because remember repeating calculating an average is really important and that's worth the mark for just pointing out that particular experimental detail
• 42:30 - 43:00 the diagram shows how a mechanic applies a force on a spanner to try and undo a bolt state the equation linking moment force and perpendicular distance from the pivot so that is moment equals force times perpendicular distance
• 43:00 - 43:30 calculate the force required to produce a moment of 4.8 newton meters so as always good practice to write that equation again we've been told the moment we're after the force now the crucial thing here is picking the correct distance is it 40 centimeters 30 or well remember the key word here is perpendicular that means that right angles so if the force is acting this
• 43:30 - 44:00 way which of those distances is at right angles well it's here because this is where that right angle would sit which is why we use this value you must convert it to meters because of this unit here so that's 0.4 to find f divide both sides by 0.4 to get a value of 12 newtons the mechanic is not able to undo the bolt because a moment of 9.6 newton meters is needed explain how the mechanic could produce a moment of 9.6
• 44:00 - 44:30 newton meters notice that this 9.6 newton meters is double the original moment how to increase the moment well either by increasing the force or the perpendicular distance so we're going to write increase either the force or perpendicular distance and for that second mark be very specific about that increase in this case double either of them
• 44:30 - 45:00 a toy train is placed on the middle of a bridge on a model railway the weight of the train acts through its center of gravity ignore the weight of the bridge which row of the table shows the correct values for forces x and y notice this is an example of a moment question which is forces on a beam so we don't have a single pivot we've got two supports here where the forces act up against now because we've been told that train is sitting in the middle of the bridge it means that the forces x and y must be equal so we simply need to divide 50 newtons
• 45:00 - 45:30 by two to find the forces at x and y which is why a is the answer 7.5 newtons at each describe how force x changes if the train moves from p to q so the train is moving in that direction which means that this force increases and this force decreases so force x will decrease and by the time the train reaches q x will equal zero because that 15
• 45:30 - 46:00 newtons will be acting entirely through y a man uses a wheelbarrow to carry some logs along a flat path as shown he pushes with a horizontal force of 140 newtons and the wheelbarrow moves 39 meters state relationship between work done force and distance moved so our moment question will appear slightly later just using my formula triangle here work done equals force times distance calculate the work done moving the wheelbarrow
• 46:00 - 46:30 so writing out that equation again the force is 140 the distance is 39. pop that into your calculator to get 5500 joules state how much energy is transferred to the wheelbarrow notice these units are the same it's only worth one mark so chances are it's the same value and indeed it is the man stops and holds the wheelbarrow horizontally as shown the man exerts the total upward force of
• 46:30 - 47:00 f newton's the weight of the loaded wheelbarrow is 470 newtons mark x on the diagram to indicate the center of gravity of the loaded wheelbarrow that needs to be in the center of the wheelbarrow so here state the equation linking moment force and perpendicular distance from the pivot so now we're getting into the nitty-gritty of four of moments again calculate the force f so let's state first of all the principle of moments which is that clockwise moment
• 47:00 - 47:30 equals anti-clockwise moments so let's work out our clockwise moment first of all so it's going in this direction so that's going to be the force times the perpendicular distance acting in the clockwise direction so that's 470 newtons multiplied by 0.6 meters and then the anti-clockwise moment acting in the opposite direction this direction so after f which is the force times its
• 47:30 - 48:00 perpendicular distance from the pivot the pivot is over here so we need the entire distance which is 0.6 plus 0.8 meters which is 1.4 meters let's do 470 times 0.6 on the left hand side equals 1.4 f divide both sides by 1.4 to get f by itself and our final value is 201 newtons
• 48:00 - 48:30 a yard arm used to measure weight consists of steel bar about one meters long with a basket at one end and a movable weight at the other end it is held up by a hook which is fixed to the bar close to the bracket the diagram shows a yard arm being used to find the weight of five bananas draw an x on the diagram to show the pivot point so the pivot is the turning point so where will that be well it will be just by that hook so here state the principle of moments well that
• 48:30 - 49:00 is the clockwise moments equals anti-clockwise moments the support for the basket is 14.1 centimeters from the pivot so that's here the movable weight is 84.6 centimeters from the pivot and weighs 1.25 newtons calculate the weight of the five bananas
• 49:00 - 49:30 so here we go so let's start with our principle of moments which is that the clockwise moment equals the anti-clockwise moment so the clockwise moment will be acting in this direction so we need to do that perpendicular distance from the pivot times that force i'm just going to convert 84.6 centimeters to meters so that becomes 0.846
• 49:30 - 50:00 meters times the weight of that movable weight which is 1.25 equals the anti-clockwise moment so that distance from pivot which is 14.1 centimeters so that's 0.141 meters times x which is the weight of the bananas because remember force and weight have the same unit so that becomes 1.0575
• 50:00 - 50:30 equals 0.141 x divide both sides by 0.141 your final answer here is 7.5 newtons calculate the mass in grams of one banana so we know that five bananas has a weight of 7.5 newtons so let's divide that by five to get the weight of one banana so 1.5 newtons is one banana then we need our equation which links weight and
• 50:30 - 51:00 mass so remember weight equals mass times gravitational field strength which on earth is 10. so we're going to put 1.5 as our weight equals mass which we're after times g which is 10 to get m by itself we'd need to divide both sides by 10 to get 0.15 kilos and then lastly convert that to grams so the final answer is 150 grams state two ways that the fruit seller could alter his yard arm so that he can measure larger weights
• 51:00 - 51:30 one thing he could do is use a heavier movable weight and the second thing he could do is have a smaller distance from the pivot to the basket
• 51:30 - 52:00 so the first thing to state is that momentum is a vector which means that it has both a direction and magnitude which means size the units of momentum are kilograms meters per second and you'll understand why if i show you the equation now the equation for momentum is mass times velocity so if we take the individual units that's kilograms and meters per second and then we just sit them closer to each other and that
• 52:00 - 52:30 is your unit now one thing you'll often see is the statement that momentum is conserved and that's going to be very important when we look at the calculations and all that really means is that momentum before equals momentum after and that's the statement that you'll find yourself making time and time again so now i want to show you some key examples so a truck of mass 500 kilograms is moving at four meters per second to the right and it collides with another track of mass 1500 kilograms moving at 1.5
• 52:30 - 53:00 meters per second to the right what is their common velocity after the collision if they stick together so our first statement is that momentum before equals momentum after so what is the first truck's momentum well it's its mass i'm just going to write the equation for momentum up here so you can refer to it equals mass times velocity so that first track has a mass of 500 kilograms and is moving a velocity of 4 meters per second the second truck has a massive 1500 kilograms and has a velocity of 1.5
• 53:00 - 53:30 meters per second now because they stick together it means that they have a common mass after the collision which is 1 500 plus 500 and then they have a common velocity which is what we're after so now you just need to sort out the equation what's 500 times 4 2 000 what's 1500 times 1.5 250 1500 plus 500 is 2 000
• 53:30 - 54:00 multiplied by x and then to work out x we divide both sides by 2 000 to get a value of 2.13 meters per second a second type of example you might get is a recoil velocity question and these seem difficult but i promise they're straightforward if you follow these steps so emerald standing still and fires a rifle the bullet has a mass of 0.045 kilograms and is traveling at 350 meters per second if emma has a mass of 60 kilos
• 54:00 - 54:30 with what velocity does she move backwards so going back to our favorite equation so momentum before equals momentum after emma is standing still and that therefore means by definition that her momentum before is zero so what is the momentum after well it's emma's momentum so it's her mass times her velocity which is what we're being asked to find plus the momentum of the bullets so that's its mass times its velocity so sort out your
• 54:30 - 55:00 numbers and then you need to take 15.75 away from both sides and then divide both sides by 60 to get x by itself to get minus 0.263 meters per second to three significant figures and it makes sense that it's minus because she's moving in the opposite direction to the bullet now the third type of equation you could be asked about is the equation stating force equals change in momentum over time
• 55:00 - 55:30 so here's an example a 1500 kilogram car accelerates from rest to a velocity of 30 meters per second this takes 20 seconds calculate the force acting on the car as always write out our equation so we're after force our change in momentum is going to be our final momentum minus our initial momentum over time so the final momentum will be the mass of the car 1500 kilograms times its velocity minus its mass
• 55:30 - 56:00 times its initial velocity which was zero because it was starting from rest over time which is 20 seconds so 45 000 divided by 20 equals 2250 newtons because remember newton is the unit of force another thing they do like to ask you about is safety procedures in cars so that includes things like crumple zones when the front bumper of the car collapses when it collides with an object it includes
• 56:00 - 56:30 seat belts airbags and they'll often ask you how do these features prevent serious injury and i promise the answer is always the same and you need to support your answer around the equation force equals change in momentum over time because what is effectively happening is that although the change in momentum obviously stays the same the time over which that change of momentum occurs increases because the
• 56:30 - 57:00 seat belt stretches the air bag inflates the crumple zone crumples so in your answer you're going to state the change in momentum stays the same but occurs over a longer time frame this reduces the force bout due to the equation force equals change in momentum over time the student is playing a game with some empty tens he throws a wet cloth of mass
• 57:00 - 57:30 0.15 kilograms at the tins the wet class moves a velocity of 6 meters per second state the equation linking momentum mass and velocity momentum equals mass times velocity calculate the momentum of the wet cloth and give the unit so as always write down your equation and then line up all your numbers and you'll avoid making silly errors
• 57:30 - 58:00 so the mass we know is 0.15 the velocity is 6. and therefore our final answer is 0.9 kilograms meters per second the wet cloth sticks to tin one massive tin one is 0.05 kilograms the cloth and tin one move away together calculate their velocity so you want to make the statement momentum before equals momentum after we know the
• 58:00 - 58:30 momentum before because it's the momentum of the wet cloth which was 0.9 and then the momentum after remember is mass times velocity because that's the equation for momentum the mass is going to be both the mass of tin 1 and the mass of the cloth which is 0.15 and then they have a combined velocity because they stuck to each other
• 58:30 - 59:00 so sort out your mass and then divide both sides by 0.2 to find x and your answer here is 4.5 meters per second carrying on with the momentum topic looking more closely at something called impulse so first of all what is impulse and it is a measure of the change in momentum and that has to be over time and notice that it has a unit of newtons
• 59:00 - 59:30 okay new for this specification is the types of energy they've kind of changed the naming system so some have stayed the same and some are new so we're going to go through each of them in turn talking about what they mean and some examples so starting with chemical energy this is to do with energy stores and it's associated with chemical bonds and really your examples here are things like food remember food is a storage of energy the same is true for batteries kinetic energy this is to do with moving
• 59:30 - 60:00 an object so anything that has movement has kinetic energy so a man running down the road a bus being driven along the street they both have kinetic energy gravitational energy as the name suggests is energy associated with an object gaining height so anything that has been lifted will gain gravitational energy for example a chair lift a ski resort an airplane these will have gravitational energy and apple being picked up for example next up elastic energy this is the energy stored when an object
• 60:00 - 60:30 is stretched squashed or twisted um rubber bands are the obvious place here catapults balloons that have been inflated anything that you distort and then pings back to its original shape will contain elastic energy nuclear energy will become very important when we look at nuclear fission because this is the energy associated with those reactions so remember the fuel uranium-235 which you do need to know that is a huge store of nuclear fuel and therefore nuclear energy thermal energies the name suggests is to
• 60:30 - 61:00 do with heat anything that has gone hotter has gained thermal energy so a hot cup of tea for example magnetic energy is new for you guys and this is the energy stored when light poles are pushed closer together or when unlike poles are pulled further apart so light poles remember is two south poles being pushed together unlike poles would be the north and south pole so just name any magnets here so just a simple bar magnet a fridge magnet that you put on the fridge door these would
• 61:00 - 61:30 all count as magnetic energy stores and lastly electrostatic that's the energy stored when light charges are moved closer together or when unlike charges are pulled further apart so very similar to the magnetic energy and your example here is is hard but it's things like clouds remember when there are lightning storms there's huge buildup of static charge van de graaff generators the same place wherever you build up static charge you will have electrostatic
• 61:30 - 62:00 energy how might an event or process change the energy an object has well first of all notice the conservation of energy states that energy cannot be created or destroyed it's simply transformed or transferred and let's list a few examples of a process or event that may transform energy for example burning fuel
• 62:00 - 62:30 now we know within fuel we have chemical potential energy and upon burning that will transform itself into heat energy there are a couple of very specific specification points which we need to go into now so firstly give an example of energy transfer by electrical working and a good example here is a battery which remember has a store of chemical potential energy
• 62:30 - 63:00 if you take a complete circuit so one containing a battery wires and then a resistor so don't forget that there is the battery here is the resistor well when that current flows from the battery to the resistor remember we get a transfer to heat energy so this is definitely an example of energy transfer by electrical working now we need to look at an example of
• 63:00 - 63:30 energy transfer by mechanical working as opposed to electrical working the easiest thing here is to consider forces and if you apply a force to a lever remember levers create moments which are turning effects and at the other end that will produce kinetic energy because it will actually cause something to move so taking an example such as a spanner or a wrench
• 63:30 - 64:00 terribly drawn but you apply a force here using your hand and that creates a moment which causes the wrench to turn which will turn that screw releasing kinetic energy next equation i'm interested in is kinetic energy which is half of mass times velocity squared i prefer not to put this into a triangle i think it's easier if you keep it exactly as it's written and i'll show you how to calculate the different parts so you can actually see that it can be
• 64:00 - 64:30 rearranged easily in this first question we're being asked to calculate the kinetic energy of a tennis ball traveling at 46 meters per second with a mass of 500 grams so as always write out our equation which is k is half mv squared be careful with your units because 500 grams isn't in kilograms so i'm going to put that into kilograms so remember 500 grams is 0.5 kilograms so start substituting in your values so it's half
• 64:30 - 65:00 times 0.5 times 46 squared so start to simplify so half of 0.5 is obviously 0.25 46 times 46 or 46 squared is 2116 times that by 0.25 and that gives us a value which is 529 it's an energy value so it's in joules calculate the velocity of a bus traveling through town with a mass of 5040 kilograms and kinetic energy of i'm
• 65:00 - 65:30 not going to read this number of 527 643 joules so writing out the equation ke equals half mv squared we know the ke this time so i'm just going to pop that in here i find it easier to answer these questions if i rearrange later so half of mass so that's half times 504 o and we're looking for velocity so we keep that as v squared
• 65:30 - 66:00 the easiest thing now is to work out half of the mass which is 2 520 times that by v squared now we need to get v squared by itself so the way in which we do that is divide both sides by 2520 so i'm going to do 527 643 divided by 2520 which gives me a value of 209.382 dot dot dot equals v squared and then we just need to square root to make
• 66:00 - 66:30 sure we just get a value for v so v equals 14.46 nine notice i haven't rounded too early i'm now going to round to three sig fig so it becomes 14.5 and it's a velocity which means the unit's meters per second let's look more closely at the term work done so what does work done actually
• 66:30 - 67:00 mean well it's a measure of a force moving something and crucially whenever work done energy is transferred an example could be potential energy might do work sliding a brick across the ground right i'm going to go through some miscellaneous calculations to show how you should be approaching questions if you're not quite sure which equation to
• 67:00 - 67:30 use so my first piece of advice is the moment they tell you to start during your exam turn over to a blank page and start scribbling out all your formula triangles so you're pretty much making yourself your own personal formula sheet even though that will probably take you about a minute it will be totally worthwhile because it means when you get to a particular calculation of which there are many in the physics exam you don't need to be scrambling around in your brain for the correct one you just refer to your formula sheet and it should literally just be sitting there
• 67:30 - 68:00 waiting for you so let's have a look at what we have here though so a student pushes a trolley of weight 150 newtons upper slope of length 20 meters the slope is 1.2 meters high calculate the gpe so that's the gravitational potential energy of the trolley and calculate the work done by the student so what do we have here let's identify look at the unit it's a newton which means it's both weight and force
• 68:00 - 68:30 we know the slope length is 20 meters and we know that it is 1.2 meters high so gravitational potential energy is given by mass times gravitational field strength times height we see an immediate issue which is that we don't have the mass of the object we only have its weight so we need to do a preliminary calculation using this formula triangle which is weight equals mass times gravity we're looking for mass so that's going
• 68:30 - 69:00 to be weight divided by gravity now the weight we've been told is 150 newtons gravity on earth is approximately 10 you could put 9.8 but i'm going to write 10 and therefore you're going to write the mass as being 15 kilograms now we're ready to use the original equation so mass is 15 gravity i've just said is 10. the height it says that the slope is 1.2 meters high hence 1.2 is substituted in here so
• 69:00 - 69:30 once i've substituted that in i get a value which is 180 it's an energy value so it needs to be in joules so i've rubbed the rest out so we've got more space so in b we're calculating the work done by the student if she pushed the trolley with a force of 11 newtons so we have work done we have force so the formula triangle will we need this time is work done equals force times distance so we're looking for work done let's write out the equation equals force times distance
• 69:30 - 70:00 the force we know is 11 newtons the distance look higher up in the question she pushed it up a slope of length 20 meters so that's why we substitute in 20 here pop it into the calculator and you know that it is 220 work done remember has the same units as energy which is why i'm putting 220 joules there's always something about renewable non-renewable energy so non-renewable
• 70:00 - 70:30 energy is things like fossil fuels so coal natural gas oil these are things which are running out they're very good at providing us with energy but if we carry on using them at the vape we're using them now we're going to run out and because they come from fossils they are not going to be replaceable so we're turning to renewable energy things like solar um wind hydroelectric geothermal all those sorts of things are renewable energy and you need to be able to state the advantages and disadvantages of both they all tend to have the same kind of
• 70:30 - 71:00 disadvantages which is that they're unreliable so for example if there's no wind there'll be no wind power no sun no solar um no waves no wave power the only type of reliable energies out there are things like hydroelectric because that's remember when water falls down the dam that tends to be quite reliable unless there's been no vein geothermal tends to be very reliable but remember there's only very certain places on earth where you can harness geothermal energy places like iceland where there's a gap between the two tectonic plates so places like
• 71:00 - 71:30 in england you won't be able to get geothermal energy they could ask you how energy transfers occur for example inside a hydroelectric power station they do like to ask you that because remember water gains gravitational potential energy because it's at the top of the dam as it falls over the dam it will gain kinetic energy transferred into electrical energy to therefore power houses this
• 71:30 - 72:00 might seem quite bitty but i'm just literally racking my brain to think of the questions i'll ask you they could ask you how fossil fuel power station works and actually any power station kind of works in the same way which is that either if they're burning biomass or the burning fossil fuels what you're doing is you're heating water that water turns into steam the steam turns a turbine a turbine turns a generator and generator generates electricity so those are all the steps involved in generating electricity nuclear power's quite an interesting one because it's not technically renewable
• 72:00 - 72:30 because you can't make more radioactive material however such tiny amounts are needed it means that it's very unlikely to run out so there is a huge potential for nuclear power however disadvantages include the fact that there's radioactive waste which needs to be stored very carefully because if that escapes into water supplies that could cause cancer there are high decommissioning costs and that means this is very expensive um taking the whole power station apart once it's been used for about 20 years or so
• 72:30 - 73:00 high startup costs obviously follow so it's going to be expensive to make those factories in the first place the diagram shows some electrical appliances which appliance is designed to transfer electrical energy to thermal energy so we're putting in electricity we're getting heat out of it it is clearly the kettle b which appliance is designed to transfer electrical energy to kinetic energy so again we're putting in electricity we're
• 73:00 - 73:30 getting movement energy out which would be the food mixer a in all appliances energy is conserved what is meant by the phrase energy is conserved and that is the energy cannot be created or destroyed it is simply converted from one form into another the lamp has an efficiency of 20 explain what this means and that means that 20 percent of the energy input has been transferred usefully so in the case of a food mixer 20 of the electrical energy in will have been converted to
• 73:30 - 74:00 kinetic energy or you could have argued that 80 of the energy input has been wasted draw a labeled sankey diagram for the lamp okay so sankey diagrams always look like this now you do find that a lot of it is wasted so that's why the wasted energy arrow down here will be much broader than the useful energy and now let's label all of them so the lamp coming into that will be electrical energy the useful energy out will obviously be
• 74:00 - 74:30 light energy and then there'll be wasted energy in the form of heat because do notice that lamps get pretty hot because we haven't been given any numbers you can't add any numbers so don't worry about that we now need to look at efficiency so using this example a filament bulb was supplied with 80 joules of energy 10 joules of energy was output as light the vest was wasted calculate the efficiency of the light bulb you need this equation so efficiency equals
• 74:30 - 75:00 use for energy out over divided by total energy in and i like to make it a percentage so times by 100 but that's not altogether necessary so useful energy out we know was the light so that's 10 total energy input was 80 times it by 100 so you can pop that into your calculator or just cancel it down like i've just done so the efficiency is 12.5 percent so what's the relationship between power
• 75:00 - 75:30 and work done well power is simply a measure of how quickly work is done and notice that they have different units work done has the same unit as energy so joules powers unit is what a hamster of mass 40 grams runs up a two meter curtain in five seconds calculates its power so we have mass
• 75:30 - 76:00 which i'm going to immediately convert into kilograms to make sure i don't make any errors so that becomes 0.04 kilograms we have distance which is 2 meters we have time and we're looking for power now there isn't a direct equation that links these components now this question is actually more complicated than it looks because you need quite a few equations as there's no
• 76:00 - 76:30 equation which directly links mass distance time and power first equation we need is weight equals mass times gravity the next one we need is a formula triangle which is work done equals force times distance and the last one contains power which after all is what we're after which is work done is power times time what is the common factor linking the two formula triangles well it is work done so we need to calculate work done
• 76:30 - 77:00 first of all which is force times distance unfortunately we don't have the force so we're going to have to work that out and this relies on you remembering that force and weight have the same unit so if we work out weight then we can work out the force so we need this equation first of all which is weight is mass times gravity i've already told you that the mass is 0.04 gravity on earth is approximately 10 so therefore the weight is 0.4 newtons
• 77:00 - 77:30 now we're going to calculate work done using the equation force times distance force and weight have the same unit which is why i can substitute in 0.4 as the force distance is 2 as given by the question so the work done is 0.8 joules and then finally we can work out power by doing work done over time look how i've laid out my answer with all my equals signs lined up with all my working out shown so the work done we know is 0.8
• 77:30 - 78:00 i'm only writing it here because i'm running out of space divided by time which is 5 so that becomes 0.16 don't forget your units for power which is what now pressure so pressure is acting all around us remember that pressure around atmospheric level is 100 000 pascals or 100 kpa and the pressure increases underwater and if anyone goes scuba diving you'll know this when you have to do your training you need to be aware of the pressure around you and the
• 78:00 - 78:30 reason the pressure increases is simply because there's more water above you the atmosphere above that and it's all bearing down on you hence you experience a high pressure at the top of mountains you'll have less pressure and that's simply because there's less air above you now in terms of when you go up in an airplane just to try and give you some contact has anyone ever noticed that that you get red rolls in those little plastic packets or a packet of crisps and it looks like a pillow in an airplane you get super puffy and that's
• 78:30 - 79:00 because when the packet crisp for example was sealed at ground level then you have the atmospheric pressure around so that gets sealed into the packet and so you've got those particles colliding creating pressure against the walls of those crisp packets however when you go into an airplane and you're at altitude then what you find is the surrounding air there's less pressure so you find there's more pressure within the crisp packet less pressure surrounding it which is why it puffs out
• 79:00 - 79:30 we know that pressure is given by the equation force divided by area so clearly the larger the area the smaller the pressure and that's because force will be being divided by a larger number so where is this used in everyday life something like a ski or a snow boot due to their shape
• 79:30 - 80:00 means that you have a large area the force so the weight of the person is spread over a larger area and therefore the pressure exerted has decreased and this helps to explain how snow boots work so when you walk in snow boots it stops you sinking into the snow and the same with skis your skis don't sink into the snow they simply slide over
• 80:00 - 80:30 let's take an opposing example so a knife for example so your force acts down here so you press down with your hand but the area over which this force is felt we know if you look at a knife is extremely small a knife's blade is really only about a millimeter in diameter so that force is spread over a very very small area meaning the pressure is huge and it's that increased pressure which actually creates that sharp cutting
• 80:30 - 81:00 surface which is the reason why knives work so effectively looking at an example which combines both here is a drawing pin used to pin paper to the wall or on a notice board so here you have a larger area to apply the force over so that larger area means that the pressure will have decreased which is a good thing because you don't want to stick it into your finger that would be very painful however if we look at the opposite side of a drawing pin so down here clearly we have a very small area
• 81:00 - 81:30 which means according to our pressure equals force over area our pressure will therefore be massive which explains how it can poke into walls and hold paper to notice boards just one point to notice about pressure we know that pressure increases the lower down you go due to the weight of air above and in terms of practical implications this means that deep sea submarines clearly have to be built to withstand very high pressure
• 81:30 - 82:00 okay this is obviously rubbish but submarines have very thick walls to withstand high pressure under the sea and weirdly wells where you collect water so a well is like a shaft where the water sits at the bottom and then the person can lower a bucket in in order to reach that water at the bottom now the pressure will be much
• 82:00 - 82:30 higher down here so the walls are built far more thickly here i'm looking at a question so which exerts more pressure on the ground a tank with a weight of 80 000 newtons or a cyclist with a weight of 1 000 newton the tank tracks have a contact area of 10 meters squared and each of the cyclist ties have a conduct area of 0.06 meters squared so what you need to do is begin by drawing the triangle to help you sort out your formula so it looks like this
• 82:30 - 83:00 force is at the top then it goes area and then it goes pressure so i'm going to change to black so i'm obviously looking at the pressure so i need the form of the equation which looks like this force over area i'm going to start by looking at the tank so its pressure will be its force and obviously that will be the 80 000 newton's y because weight and force have the same units so that is newtons and therefore we can use them interchangeably so that's 80
• 83:00 - 83:30 000 divided by the area which i've been told is 10 and here's the answer answer 8 000 newtons per meter squared now let's look at the cyclist okay so the pressure will equal the force again which is this time a thousand newtons divided by the area now don't make a mistake here the area is two lots of 0.006
• 83:30 - 84:00 the reason being because obviously the cyclist has two tires so i'm gonna do naught point naught six times two which equals one thousand please don't let me run out of space by naught point naught one two um going to use the calculator here and the answer here is 8 3 3 newtons meters squared so
• 84:00 - 84:30 as you can see it's obvious that the cyclist exerts more pressure because their pressure is 83 333 so make sure you write that the cyclist is the answer question four a diver works in the sea on a day when the atmospheric pressure is 101 kilopascals and the density of the seawater is 1028 kg per mm cubed the diver uses compressed air to breathe underwater 1 700 liters of air from the atmosphere is compressed into a 12 liter gas cylinder the compressor die quickly cools to its
• 84:30 - 85:00 original temperature calculate the pressure of the air in the cylinder the only reason these questions are hard is because you need to actually pull out the information in the question but once you've done that it's straightforward enough so what numbers have we been given well we've been given atmospheric pressure of 101 kpa density which is here but i don't think we need to use that quite yet we've been told that 1 700 liters of air is compressed into a 12 liter gas cylinder so we can see here straight away we've got two volumes we've got a pressure and we've got
• 85:00 - 85:30 another pressure so which equation contains two lots of volumes and two lots of pressures well it's this one correct me if i'm wrong but i think this is given on the front of your paper but it's quite straightforward to remember anyway just make sure your units are all the same and then start subbing in these numbers so pressure one is going to be 101 times volume one which is 1700 equals we're looking for p2 so i'll keep that there times 12 and then the way to
• 85:30 - 86:00 work this out is work out the left hand side first of all so 101 times 1700 and then divide both sides by 12 and then when you round that to three sig fig you're gonna get p2 equals one four three zero zero the units will be killer pascals
• 86:00 - 86:30 state the equation linking pressure difference depth density and g so you can write this out in words you can literally write pressure difference equals depth times density times g or you can write it out as symbols which would be p equals h times remember it's the greek letter rho which looks like a p so that's why it's a bit confusing times g so if in doubt i would write it out in words whatever you do don't write gravity for g they're very fussy over this you need to write acceleration due to gravity
• 86:30 - 87:00 calculate the increase in pressure when the diver descends from the surface to a depth of 11 meters so what you're going to do here i'm just going to write out the equation so pressure equals height times density rho times g so 11 times density which we were given right the beginning of the question which is 1028 times g which is 10 and we need to know that number you can use 9.8 also but 10 is easy
• 87:00 - 87:30 and then you work that out and you get an answer which is 110 kpa part three calculate the total pressure on the diver a depth of 11 meters assume that the atmospheric pressure remains 101 kpa well because it's worth only one mark they've only given you a small gap you shouldn't be doing any proper calculations here so no substituting into equations you simply need to add together your previous value to the atmospheric pressure so that's 101 plus 110 which equals
• 87:30 - 88:00 200 211 kilopascals see as the diver breathes out bubbles of gas are released and rise to the surface the bubbles increase in volume as they rise explain this increase in volume two marks made two separate points one of the most basic arguments you can make here is that it'll be warmer closer to the surface of the sea because you're closer to the sun that would be worth one mark and what that does because remember if it's warmer the particles vibrate more and therefore the pressure will increase inside the bubble which
• 88:00 - 88:30 leads to its volume increasing or you could give an argument to do with the fact that pressure decreases closer to the surface always provide an equation in this case so that would be pressure equals height times density times g and then you would state that the pv value is constant for a fixed mass of gas and if that sounds weird you can state the other pressure equation once again which is p1 times v1 equals p2 times v2 i'm going to show you some more density questions because i've touched on this density
• 88:30 - 89:00 triangle which i told you is drunk men vomit here but there's another equation you need to be aware of with density which i'm going to bring up now because i founded this particular example so i could talk about it so question six this question is about pressure in a liquid the teacher uses this apparatus to demonstrate pressure difference in water the apparatus is hollow and has three short tubes at different depths the teacher completely fills the apparatus with water and water comes out of all the tubes
• 89:00 - 89:30 so look path of the water from the top of the tube state the relationship between pressure difference height density and g so i wouldn't use a triangle here you do just need to learn this and it is pressure difference equals height times density times g that does not look like a g i don't know why i can't draw that times properly the diagram shows the path of water coming from the top tube complete the diagram by drawing the parts of water you would expect to see from the other tubes now
• 89:30 - 90:00 notice that this question exists i have a lot of people i teach that probably wouldn't even have spotted that that was a question and would have lost two marks immediately so do make sure you read every single question and that you indeed attempt every question now i already told you that the lower down you go in water so the further under the ocean the same is the case here the greater the pressure that is experienced that means that the path of water coming out the lowest pipe will have traveled the furthest due to that increased pressure so we're looking at something like this
• 90:00 - 90:30 and then for the middle tube you're looking for an intermediate path so like that explain the pattern of the paths of water from the tubes so i've accidentally already alluded to this you want to say that the water at the bottom of the tube has greater pressure and therefore the force acting on it is the greatest meaning that it travels furthest in another demonstrator the teacher uses this container the container is made of glass and each section has a different shape the teacher pulls water into the container until it reaches the level shown in the left hand section complete the diagram
• 90:30 - 91:00 by drawing the water levels in the other four sections so if we this is probably quite a hard question so just be aware that the air pressure is the same for all vases after all they are connected and that means that the water level will be the same for each regardless of the shape make sure you actually draw within the bars i'd have a habit of drawing outside of the bars so i'm going to use in the exam overall to show that it is constant for each vessel explain why the water fills the container in the way you
• 91:00 - 91:30 have shown so we need to describe why we've drawn it at that constant level well i've already said that the air pressure is the same for all so that's worth one mark we know that it's water we know we haven't changed liquids throughout therefore the density of the liquid will remain the same and then look at how that equation links so pressure difference equals height times density times g gravity hasn't changed density hasn't changed pressure hasn't changed therefore height won't change hence the same height for each and
• 91:30 - 92:00 lastly you could point out that the vessels are connected so when we look at solids liquids and gases be prepared to draw their particle diagrams notice that solids have particles which are in very fixed arrangements and that's because the particles vibrate around in fixed positions they have little kinetic energy and there are strong forces between them moving to liquids you see that the particles are slightly more widely spaced apart they're not touching quite as much so
• 92:00 - 92:30 they have intermediate forces between them and they vibrate more and they don't have fixed positions gases now so you need your particles to be further apart this is because they have large amounts of kinetic energy obviously they're not held in fixed position and there are weak forces between the particles and here's your summary now let's start naming the correct conversions between all these various states of matter so remember if you're going from a solid to a liquid that is melting like a solid
• 92:30 - 93:00 ice block turns into water melting if you go the other way and the water turns into ice clearly that will be freezing if you have a liquid and it turns into a gas that will be boiling or evaporating and then when you have a gas and it turns back into a liquid that is condensation so that's what happens when you have a shower and you see it getting all misted up on the windows condensation is occurring here let's now ask ourselves why if a gas is
• 93:00 - 93:30 heated in a container at a fixed volume why will the pressure increase and this is all to do with kinetic theory so we have a gas it's inside a container of fixed volume and you heat it well clearly the particles will move faster they'll collide more often with the container walls
• 93:30 - 94:00 thus generating a greater force therefore increasing the pressure within the container i keep mentioning kinetic theory but what actually is it first of all it's a theory used to explain the properties of solids liquids and gases now crucially these particles are always moving and they attract each other and if
• 94:00 - 94:30 they're closer together then you have a stronger attraction if the particles are further apart then you have a weaker attraction there's a couple of scientists you need to know a little bit about the first one is robert brown and his discovery led to the notion of brownian motion now all this states is that particles in suspension move randomly
• 94:30 - 95:00 and you can see this in everyday life so pollen grains on water smoke particles and what you can actually absorb if you look at pollen grains on water under a microscope is that they move randomly and that's due to small particles colliding with them ones which you can't see they create a small force
• 95:00 - 95:30 which actually acts to move those pollen grains so it's a very basic theory you need to know about looking at this in more detail so we need to first of all look at newton's first law which remember states that every object will remain at rest or stationary or continue at the same speed if it's already moving
• 95:30 - 96:00 unless an external force acts upon it so how does that link to brony in motion well i've just said that there are small particles which act on the pollen grains creating small forces which move them so how does that link to what we've just done well we know the pollen grains appear to be moving and that must therefore based on newton's first law mean that a small force is acting on them where does that force come from well it's the small fast-moving particles which we can't see
• 96:00 - 96:30 which bombard the larger particles so basically what i was just saying about brony in motion but just spout out a bit more in terms of kinetic theory the diagram shows some gas particles in a container the piston can be moved in or out to change the volume of the gas add arrows to the diagram to show the random motion of the gas particles just literally show them just moving around really randomly
• 96:30 - 97:00 just make sure you add enough arrows they're just a gas so it's quite straightforward explain how the motion of the gas particles produces the pressure inside the container this is an answer you can vote learn so you want to first of all talk about the fact that gas particles have random motion and that they collide with the walls of the container then this creates a force and then lastly states an equation which is pressure equals force over area and that
• 97:00 - 97:30 is the easiest way in which you'll get three marks i would really recommend you learn that answer off by heart state what would happen to the pressure if you push the piston into the container without changing the temperature well you've got a smaller volume so therefore the pressure would increase when the gas in the container is heated the piston moves outwards place ticks against the three correct statements the gas particles get bigger no that's not true the mass of gas particles stays the same yet that makes sense the gas
• 97:30 - 98:00 particles move faster well clearly if you heat them they're going to gain kinetic energy so they will move faster the average distance between the gas particles increases again true because if they're moving faster they'll be moving away from each other therefore increasing the distance the temperature of the gas decreases well no the container's been heated so that's actually absolute rubbish let's now look at the relationship between the temperature of an object and its internal energy so remember temperature is a measure of how hot an object is
• 98:00 - 98:30 and remember you need to know the units for temperature which is either degrees celsius or kelvin now clearly if you have a higher temperature you're going to have greater internal energy and that's because as the temperature increases remember those particles within the object vibrate more and therefore the vibration means that each molecule is carrying more energy and you need to be able to describe what
• 98:30 - 99:00 pressure is in terms of particles so taking particles within a crust packet or within a container or within a squirty cream container just remember that those air particles collide with the walls of the container no what are you doing laura no no claws no claws no claws no clothes
• 99:00 - 99:30 to get rid of this please be good be good she always meows when i talk to her she knows she's being naughty like her so those particles collide with the walls of the container creating a small force and that force given over a specific area is the pressure and we know this because of the equation pressure equals force over area so you can use the triangle fap to help you remember that do be prepared to provide answers to
• 99:30 - 100:00 what pressure is or how pressure is produced within a container as that could be worth four marks and do remember those particles within the container have random movement because they are air particles and they are a gas why does that pressure increase with increased temperature well clearly increased temperature provides those particles with greater kinetic energy which means they collide with the walls of the container both more frequently and with greater force hence you see an increase in the pressure so if we go the opposite way and we decrease the temperature clearly
• 100:00 - 100:30 decreasing the temperature will decrease the kinetic energy eventually the particles will have no kinetic energy whatsoever and at this point we say that absolute zero has been reached because particles have no kinetic energy there is no pressure it has a value which is minus 273 degrees celsius we've already talked about robert brown and his brownie in motion the next scientist we need to look at is boyle
• 100:30 - 101:00 his name was also robert so robert boyle we know that pressure is inversely proportional to volume which means if we decrease the volume we see an increase in pressure which makes sense let's say we have an original container which looks like this we have the volume look those particles have gotten far closer together so we've seen a increase
• 101:00 - 101:30 in pressure conversely if we increase the volume look at how spaced out those particles now so we've seen a decrease in pressure and these are proportional so if you halve your volume you get a doubling of pressure if you double the volume you get half the pressure
• 101:30 - 102:00 and remember that associated with boyle's law you need to know an equation which is p1v1 equals p2v2 so now let's look at an example if a gas has a volume of three centimeters cubed and a pressure of four atmospheres and the pressure is lowered to one atmosphere what will be the new volume so write out your equation p1v1 equals p2v2 let's do some identifying so we know our initial pressure is 4 so that's p1 the volume initially is three so that's
• 102:00 - 102:30 v1 we're lowering the pressure to one so that's p2 and we're after v2 and i always write it out like this so i know what's going on and now it's a matter of substituting in those values so 4 times d is 12 that equals v and that is our final answer so v equals 12 centimeters cubed let's go into greater detail to do with the topic on thermal properties and temperatures
• 102:30 - 103:00 so first of all why do solids and liquids expand when they're heated and this is a nice obvious answer that's because at higher temperatures gain kinetic energy so they vibrate more and obviously increased vibrations requires expansion in order to accommodate that vibration
• 103:00 - 103:30 and if we take a look at a simple diagram here when they gain kinetic energy they vibrate more they spread out and by definition they expand now we need to look at everyday examples where expansion affects us so using a thermometer remember that's used to measure the
• 103:30 - 104:00 temperature and in a mercury thermometer the liquid inside expands and contracts dependent on the surrounding temperature so the hotter it is the greater expansion you see the higher the reading of the thermometer when it's cooler the liquid contracts so you see a lower reading when we're looking at building you often
• 104:00 - 104:30 find that concrete is reinforced with steel girders the point to notice is that luckily they both expand by the same amount which is essential because otherwise you'd see your building degrade because imagine if the concrete expanded and the steel didn't also to do with construction power cables you notice these are often left slack so like this rather than this
• 104:30 - 105:00 and that's to enable them to contract on cold days and then lastly bridges terrible bridge these have small gaps in them which enable the road to expand on hot days we need to understand why solids expand
• 105:00 - 105:30 less compared with liquids or gases when heated and the most obvious thing to do here is look at the various particle arrangements now here in the solid you can see that particles have a tight arrangement they're held in fixed positions with strong forces between those particles and clearly this will limit expansion
• 105:30 - 106:00 and prevent expansion taking place however with liquids and gases there's far more space around the particles particularly gases look at all this space again with liquid far more space compared with the solids notice that they have weaker forces between the particles less fixed positions and then you can see greater expansion
• 106:00 - 106:30 can take place let's now look at the method we would use in order to actually calculate the specific heat capacity so i've already listed some steps so first of all you need an insulated beaker to prevent heat loss to the surroundings we would then submerge the heater in the water for a set amount of time let's say 230 seconds as an example and then we need to measure the
• 106:30 - 107:00 temperature change and the way in which we do that is using a thermometer we measure the initial temperature we measure the final temperature after 230 seconds and then that is everything you need now in order to substitute into the equation q plus m c delta t we need to know what q is so that's the change in energy which is power times time so we know that our heater had a power rating of 100 watts our time is 230 seconds
• 107:00 - 107:30 work that out to get this value here and then we can now substitute these values in to work out our specific heat capacity so q is 23 000 m is our mass which was 0.5 kilograms we're after the specific heat capacity which is c and then lastly the temperature change let's say that as an example that the change in temperature was 10 degrees so that's the difference between the initial and final temperature so that's 10 and now solve our equation
• 107:30 - 108:00 divide by 5 on both sides to get a value of c which is four thousand six hundred dollars per kilogram degrees c we need to look in greater detail at the process of boiling and melting condensing etc so starting with boiling what is actually going on well we know first of all that liquid is turning into a gas and how this actually works is that there are pockets within the liquid so
• 108:00 - 108:30 we know a liquid particle diagram looks a bit like this and those pockets within the liquid turn to a gas when they're heated this gas rises through the liquid and eventually releases a vapor at the surface of the liquid and then notice as you heat it further the boiling point stays the same despite
• 108:30 - 109:00 the fact that you're still heating it and that this energy separates the molecules to form a gas so we've already looked at boiling let's bring evaporation into the equation and to all intents and purposes they seem pretty similar because in both cases you're going from a liquid to a gas but we know that there are real differences between the two first of all
• 109:00 - 109:30 boiling is far more rapid than evaporation and that's because the vapor bubbles have a high amount of kinetic energy whereas evaporation is much slower the molecules have much less energy so you find evaporation is the reason why a puddle disappears on a hot day whereas boiling is why when a pan of
• 109:30 - 110:00 water is boiled for long enough all the water disappears inside the pan so we've looked at boiling how about melting and we know ice melts so it turns from a solid to a liquid and what you notice here is that as thermal energy is absorbed the temperature rises eventually the energy will be sufficient to overcome the molecular attraction so it means that these will all separate
• 110:00 - 110:30 and you'll have your liquid touching slightly more on evaporation so how does the evaporation of a puddle or any liquid happen really so what you find is that particles have differing kinetic energy now those particles with the most amount of energy will evaporate
• 110:30 - 111:00 first and they will leave the surface of the liquid and what will happen is it will mean that the remaining particles have lower average kinetic energy do notice that in a closed container condensation and evaporation will be occurring simultaneously which means at the same time so what factors affect how quickly evaporation occurs try and use your common sense here number one temperature clearly the higher the temp the more molecules
• 111:00 - 111:30 with higher kinetic energy therefore these molecules can escape so evaporation occurs more quickly the second factor affecting evaporation is surface area as you might expect the larger the surface area the greater the area and a higher proportion of molecules are near the surface of that puddle for example
• 111:30 - 112:00 therefore more evaporation occurs and then lastly windiness or let's call it moving air this means that molecules which have evaporated are then moved away from the liquid and they can't return and it's really a diffusion gradient point of view if there's less water in the air then more evaporation can occur so how does evaporation actually lead to
• 112:00 - 112:30 cooling and we know about this from the body because remember when sweat evaporates it cools the body which is why sweating is an efficient mechanism for helping us cool down in hot weather so how does it work well we know that kinetic theory states that high temperatures increases the energy of the particles
• 112:30 - 113:00 those highest energy particles will evaporate first meaning that the remaining particles have lower average kinetic energy remaining particles have lower average kinetic energy and that automatically means if they have less energy than they are to lower temperature and hence cooling has occurred
• 113:00 - 113:30 we need to now know how an object which is in contact with an evaporating liquid may also cool down so if we have an object here solid object it's touching evaporating liquid so how does that object itself get cooler and that's because thermal energy is transferred to the liquid molecules
• 113:30 - 114:00 these molecules retain that energy when they escape the liquid i.e when they're evaporated which leaves a cooler liquid to which more of the objects thermal energy can be transferred so the meaty topic of conduction
• 114:00 - 114:30 convection radiation this is all to do with thermal energy transfer so things heating up things cooling down starting with conduction now i'm sure you're aware that metals are good conductors the reason being is due to their delocalized electrons or you can say they're free electrons but delocalized sounds a bit fancier remember the structure of a metal you'll need to know this for both chemistry and the electricity topic of physics so to be honest it's worth
• 114:30 - 115:00 learning so remember that a metal is made up of positive ions surrounded by a sea of delocalized electrons so what happens in the process of conduction is that when heat is applied the positive metal ions start vibrating and they gain kinetic energy what you find then is that the kinetic energy is transferred from the hot parts of the metal to the cooler parts by the presence of these delocalized electrons so as the electrons move through the metal they carry the thermal energy with
• 115:00 - 115:30 them vibrating the positive ions more and before long you can see how an entire metal object becomes hot now the opposite of a good conductor is an insulator and these are clearly going to be poor conductors so to point out things which are poor conductors first of all plastic and wood why because they have no delocalized electrons and that is the key point here
• 115:30 - 116:00 air is also a poor conductor it is a good insulator and not just because it doesn't have any delocalized electrons but clearly it's air so the particles are going to be extremely far apart so the chance of them vibrating and colliding with other particles to pass on their kinetic energy is going to be very slim hence why air is a good insulator so touching on some exam style questions so polystyrene why is it a poor conductor and that's because it contains little air spaces and air is a
• 116:00 - 116:30 good insulator because it has no delocalized electrons and the particles are very far apart so you want to link every single object you're given whether it's a poor conductor or insulator next up for example a duvet how does it do they keep you warm it's full of feathers or synthetic material that looks a bit like cotton wool and the point is again that it traps air air is a good insulator it's a poor conductor so less heat transfer will take place meaning that you stay warmer so what
• 116:30 - 117:00 have we learned we have learned that metal is a good conductor air wood and plastic are poor conductors and that notice that solids will be better conductors than gases now we need to look at describing an experiment which allows us to investigate the properties of good thermal conductors the crucial thing here is they have to be good thermal conductors so really we're looking at metals
• 117:00 - 117:30 so let's take metal rods and we need to dip one end in wax so we could have a copper rod iron rod zinc rod and you want to heat up those metal rods so the way you could do that is by placing them in a hot water bath or using a bunsen burner and then obviously as they get hot that wax will start to melt and the wax will start to drip off the
• 117:30 - 118:00 end and you can collect the wax and measure the amount of it by maybe measuring its mass over a set amount of time let's say a minute and clearly the metal which produces the most wax will be the best thermal conductor now we need to describe an experiment to investigate properties of poor thermal conductors and in this case we're going
• 118:00 - 118:30 to be using water as our poor thermal conductor and it's a very specific experiment here so what we're going to do is we're going to get a test tube and we're going to place ice into it so ice is in the bottom we're going to use gauze to separate the ice from the water which is above then we're going to apply heat in the form of a bunsen burner and that's just
• 118:30 - 119:00 to the water at the top and what you find is that water boils without the ice below melting thus demonstrating that water is a poor conductor now convection is a very different type of heat energy transfer it only occurs in liquids and gases you cannot get convection in a solid it literally makes no sense whatsoever and the key example
• 119:00 - 119:30 we always use is a radiator in a house now my wonderful drawing comes out so here's our rubbish radiator and it's nice and hot because it's got hot water flowing through it and what happens is that heat causes particles of air above the radiator here to gain kinetic energy now because they've gained kinetic energy the air expands which means that the particles
• 119:30 - 120:00 occupy more space now because they're occupying more space it means that they become less dense so they rise up to here as they rise they become cooler by definition they become denser and therefore they sink and before you know a convection current has been set up whereby the process keeps repeating and that's how a radiator in the corner of a room can actually heat up the entire room so we've got coffee in here and we don't want it to be getting cold
• 120:00 - 120:30 too quickly so what can we do well i've already said you can put it in a polystyrene cup wipe to prevent conduction losses however because the coffee is hot can you imagine that bizarrely it will heat the air above the coffee and it will literally set up a mini convection current above the coffee that keeps repeating itself speeding up the process of cooling down that coffee so what you want to do here is add a lid and that's to prevent convection currents being set up the other great thing about adding
• 120:30 - 121:00 the lid is you add a layer of air in here and remember air is a good insulator so you're preventing heat losses by both convection and conduction by adding a lid from an experimental view how can we view convection where you might have done this in school you get a very specific glass apparatus which looks something like this it's a big glass tube and then you fill it with water and you put some potassium permanganate crystals in one corner
• 121:00 - 121:30 then you apply heat from a bunsen burner and obviously what happens here is the particles vibrate more they occupy more space and so they become less dense meaning that they rise so you'll see a purple vapor or purple color start to form on this side has nowhere else to go so continue along here as it gets cooler because there's no heat obviously in this top left corner it will begin to sink and then fluid over here replaces the
• 121:30 - 122:00 lost fluid on the right hand side and before long the whole of the tube will become purple demonstrating that convection has taken place last up radiation infrared radiation which we will have met in the electromagnetic spectrum it is a wave which means it is not dictated to by the state of mata what i'm trying to say here is that it can travel in solids liquids and acids and that kind of automatically makes it
• 122:00 - 122:30 the most difficult to try and prevent heat losses via however there are certain things you can do and that's all to do with the color of objects so do try to remember that white shiny surfaces reflect infrared radiation which is why in very hot countries like certain parts of spain you might see the houses are white same in same is true for greece and that's because the white color reflects infrared radiation um anyone with dark hair black hair you'll notice that your head gets really hot in the sun why because black matte
• 122:30 - 123:00 objects are very good absorbers of infrared radiation and notice that the hotter the object or the greater the difference between the object and the surrounding temperature the greater the rate at which heat is lost putting this more into context so looking at a thermos flask am i even going to try and draw this oh god hmm
• 123:00 - 123:30 okay so that's appallingly drawn but i can still point out a few things so first of all the most obvious thing here to point out is that it has a vacuum now remember that a vacuum is a space with no particles and we know that both conduction and convection require particles in order to occur so we say that there is no conduction or convection that can occur within this vacuum and as always i'm linking the property
• 123:30 - 124:00 with which type of heat transfer it prevents there are silvered surfaces which line the bottle or line the flask and the point of that is that it reflects infrared radiation back at the contents of the fast keeping it hot or keeping it cold the lid prevents convection currents which i've already touched upon and you can say that the whole thing is made out of plastic because plastic is a good insulator
• 124:00 - 124:30 and that's the way you get these answers nice and scientific is by pointing out the various parts of it and what it stops so looking at heat transfers within a home how can we stop all our heatscaping so first of all in the loft you can have loft insulation which contains lots of airspaces air is a good insulator preventing conduction heat losses you could talk about the cavity walls so cavity meaning a hole like a cavity in your teeth so there's holes within the
• 124:30 - 125:00 walls which contain air again because air is a good insulator do notice though that convection currents can still be set up in air which is why you often find there's also foam because that solid foam stops the convection currents being set up you could talk about the radiators now having a silvered surface and that's to reflect infrared radiation back at the room and lastly double glazing remember that's when you have two window panes which trap air between them helping to prevent conduction now we're hearing
• 125:00 - 125:30 more and more about global warming and the detrimental effect it's having on our planet and i'm just going to talk you through the science that surrounds this this all starts with the sun and obviously the sun gives off radiation and you can see with these arrows down here that there's the incoming solar radiation now the important thing to notice is that this incoming solar radiation is predominantly short wavelength now not all of that incoming solar radiation reaches the earth's surface in fact 25 of it is absorbed in the
• 125:30 - 126:00 atmosphere where the ozone layer absorbs much of the ultraviolet component the remaining 75 percent of the solar radiation reaches the earth's surface the next important step is that the surface of the earth re-emits this radiation but at much longer wavelengths so you can see it being emitted over here
• 126:00 - 126:30 and this much longer wavelength means that it is now predominantly infrared which remembers just heat now the fact that these waves have a longer wavelength means that they don't just pass out of the earth's atmosphere and escape into space instead they are trapped by a layer of gases and these are greenhouse gases and this is what's going on here only some of that radiation goes back out into space most of it gets absorbed by
• 126:30 - 127:00 the greenhouse gases which then reflect it back at the earth's surface and it's that reflection back at the earth's surface which is what causes this huge warming and is therefore known as the greenhouse effect so the wave topic let's try and cover as much of this as possible there are two
• 127:00 - 127:30 types of wave you need to know about transverse and longitudinal waves learn the perfect definition for both transverse waves vibrations occur perpendicular to the direction in which the wave is traveling key examples here are water waves light waves and any member of the em spectrum infrared radiation ultraviolet x-rays etc longitudinal waves vibrations this time occur parallel to the direction which the wave is traveling your key example here is the sound wave be prepared to draw a longitudinal wave and notice that
• 127:30 - 128:00 there are periods of refraction and compression so it's like a slinky where it comes close together that's compression where it moves further apart that's a rough action now the wave equation states that wave speed equals wave frequency times wavelength waves to be measured in meters per second frequency measured in hertz and wavelength measured in meters what is the frequency of the wave well it's the number of waves per second if you're labeling a wave be prepared to draw the amplitude which is from either the
• 128:00 - 128:30 middle line to the top of the wave or to the bottom of the wave it's not from top to bottom that's two times the amplitude the wavelength is the distance between two peaks or two troughs the time period of a wave is the time taken to produce one wave and it's given by the equation frequency equals one over time period so let's talk about reflection that's all to do with waves bouncing so remember the key thing here is to know the difference between the angle of incidence and the angle of reflection so the wave coming in and hitting the
• 128:30 - 129:00 substance is the incident ray it bounces off and what you find is the angle at which it bounces off so the angle of reflection will equal the incident angle don't forget to label your normal line which is a line it's like an imaginary line drawn at 90 degrees to the boundary surface refraction is all about a wave changing direction when it enters a new medium and that's due to it either slowing down or speeding up so it's due to a change in wave speed what you find here is that if you have a light wave entering a glass block it
• 129:00 - 129:30 will slow down and it will bend towards the normal on exiting the block it will speed up again and bend away from the normal so be prepared to draw that and also be aware of what happens when water waves move from a different depth so when they go from deep water you find that they're traveling really quickly and that the waves are very far apart from each other as they enter shallower water they slow down and the waves become closer together and you'll actually see the wavefront getting closer together if you're struggling to remember that just remember cars on a road they'll be further apart when
• 129:30 - 130:00 they're traveling nice and quickly and then when you're stuck in a traffic jam and they're traveling slowly they're basically on top of each other let's take a look at diffraction now so remember this is when waves spread out as they pass through a gap and we'll look at some examples diffraction can only really occur if the gap size is approximately the same as the wavelength of the wave and you do find that wider gaps cause less diffraction
• 130:00 - 130:30 so let's look at some examples of diffraction so in example one we've got quite a wide gap you've got the wave front approaching that from this direction and you notice here that little diffraction occurs you'll have something which looks pretty similar to this so you need to be prepared to draw these shapes now in the second example we've got a much narrower gap
• 130:30 - 131:00 you'll find that far more diffraction occurs and it will look something like this so you can really see that the waves have spread out a ripple tank is used to visualize waves and wave patterns and it's made using a shallow glass tank with an oscillating paddle or needle to create waves so ripple tank is used to visualize waves and wave patterns it's made by using a shallow glass tank
• 131:00 - 131:30 as you see here with an oscillating paddle or needle to create the waves a light is then shone down through it onto a white card below to more easily see the motion of the ripples created on the water surface so if we consider what sort of wave patterns we might be able to view well those could be ones associated with reflection refraction and diffraction now if a solid barrier is placed in a tank the waves will reflect off the
• 131:30 - 132:00 barrier casting a shadow like this so we can see the waves traveling hitting this barrier will reflect off creating these waves perpendicular to the direction of the original motion and in this picture you can see refraction taking place remember that refraction is a change in the waves direction as a result of the wave changing speed now that change in wave speed and something like a water wave will be due to a change in depth so if we consider this ripple tank over here we have deeper water and then a
• 132:00 - 132:30 plastic triangle has been placed in this section of the ripple tank which automatically makes the water there shallower so the waves are moving in this direction as they hit the shallower water first of all they slow down and that means that the waves get closer together so the wavelength of the wave has decreased and notice that as those waves slow down the waves bend towards the normal remember that the frequency of the waves does not change because the source of
• 132:30 - 133:00 the waves continues to vibrate at the same frequency so let's look more closely what this means from a mathematical point of view let's take the wave equation which states that wave speed equals frequency times wavelength well we've already said that this frequency remains unchanged we know that that wavelength has decreased and so by definition clearly our value
• 133:00 - 133:30 for v our wave speed will have decreased so that's something we can both calculate and observe finally ripple tanks can show the diffraction of waves by creating either a gap for the wave to pass through or creating an edge for the wave to go around remember that waves will bend around the side of an edge and longer wavelengths are more diffracted by an edge than shorter wavelengths
• 133:30 - 134:00 so here's a barrier our waves are hitting that barrier and then diffracting around the edge of it if we consider a situation like this in a harbor you can see that waves will diffract through a gap you can see that when waves pass through a gap they will spread out which is what diffraction is and the spreading is most significant when the gap size is about the same as the wavelength
• 134:00 - 134:30 so we can see greater diffraction taking place in this right hand diagram compared with the larger gap on the left hand side let's now think about images formed in a plane mirror so here's our plane mirror straight line with the cross hatching
• 134:30 - 135:00 behind shows that it's a plane mirror remember we have a normal line which is drawn at 90 degrees so let's pick an object such as a flower and think about the image that will be formed by that mirror so here is our object so it's the actual flower that we're looking at so how does that object actually become an image in our eye while light from an object strikes the plane mirror and is reflected from the mirror surface and then after that reflection the light
• 135:00 - 135:30 strikes the eye of the observer but what can we actually say about that image well first of all the image is obviously the same size as the object and it's worth making a note of everything i'm writing and these are quite common sense points so don't be surprised by them secondly obviously the image will be the same color as the object the image will be the same distance behind the mirror as the object is in
• 135:30 - 136:00 front but crucially the image will be laterally inverted which means the left will appear on the right and the right will appear on the left lastly this image will be virtual which means that it can't appear on a screen now one hard question that people do struggle is is working out how to draw the image when you've been given the object such as in this case so here's our plane mirror
• 136:00 - 136:30 now the easiest thing to do here is use your ruler to draw a straight line from the object which hits the plane mirror then draw a second line just beneath that and then you want to reflect that back off the mirror so you could do it really neatly by drawing normal lines here measuring the angle of instance but to be honest you can probably get away from drawing it by eye like i'm doing now and then
• 136:30 - 137:00 at the end of these lines you need to draw the eye which is actually viewing the object so something like this is perfectly adequate and then to finish i'm just going to change color you want to take an approximation probably use your ruler to measure this distance here so that you can draw the image behind from the same distance to make it make sense and then just use your ruler to draw a dotted line here and really this distance from the object
• 137:00 - 137:30 to the mirror should be the same as from the image to the mirror and if i rub out those kind of explanation lines really this is what your final answer should look like now the equation for the calculating refractive index is n equals sine i over sine r which you'll have to learn now sine i so remember i is the incident angle so you just need to pop that into your calculator with the sign in front of it and sign r that's that angle of
• 137:30 - 138:00 refraction so again pop it into your calculator but first of all pressing the sign button now the critical angle is something they're like asking you about and it's all to do with the angle of incidence so every type of medium every type of substance has its own critical angle now if the angle of incidence is less than the critical angle you get both refraction and reflection taking place if the angle of incidence is the same as the critical angle you get a refraction which occurs along the boundary if the angle of incidence is greater than the critical angle you get what's called
• 138:00 - 138:30 total internal reflection so you don't get any refraction all the light is totally internally reflected and they make use of this in optical fibers now to calculate the critical angle you simply need to do sine c equals 1 over n making sure you inverse sine on your calculator to get the correct value for c well a convex lens is otherwise known as a converging lens and if you know what the word converging means it is actually just a normal english word it means
• 138:30 - 139:00 coming together so if you've got a couple of rays and they pass through a convex or converging lens then they'll be brought together and they'll meet behind the lens an example of use of this is the magnifying glass now the focal length is the distance between the center of the lens and its point of focus the principal focus is the point where the parallel rays meet after they pass through the lens and then lastly the difference between a real image and a virtual image is that a real image can be formed on a screen and
• 139:00 - 139:30 a virtual image cannot now if i move and i show you this hopefully this will make it a bit clearer so with the convex lens we can see that when the parallel rays pass through it they then come together i.e they converge and the point where they converge behind the lens is called the focal point and that's what's going on here they're meeting and we look at someone that's long-sighted so this is what happens to older people they start needing glasses to read print like reading books or
• 139:30 - 140:00 whatever so this time you've got the parallel rays coming in they're hitting the lens but the lens has weakened over time so rather than meeting on the retina here they're meeting behind the eyeball behind the retina so you're going to get a very blurred image here so instead you need a convex or converging lens which is actually going to cause those rays to come in so that when they're brought onto the human lens they meet nicely on the back of the eyeball here on the retina so let's have a go at constructing various ray
• 140:00 - 140:30 diagrams this arrow here is supposed to be the object this is the lens and these are the focal points so based on my instructions first of all you want to draw a horizontal line from the top of the object use a ruler and a pencil for this and then you want to take it through the focal point like that oh gosh i really struggle on the ipad next step you want to draw a diagonal line which runs from the top of the object arrow through the middle of the lens and we'll see where it crosses
• 140:30 - 141:00 oh not very good not very good oh supposed to cross through the middle of the lens and then where the two lines cross that is your image and then you need to compare it to the original object now because it's underneath the line we know that the image is inverted if i join accurately the actual height of the arrow so from here to here compared with that believe it or not would have been shorter so therefore we know that the image formed is smaller
• 141:00 - 141:30 or we could write diminished if you're feeling fancy because that means smaller and then because it's formed after the lens then we know our image is real if they ask you for an example of a lens which works like this then you can talk about the eye or camera or something like that this one's slightly different we can see that the object arrows now move between the focal point and the lens so we're going to see a slightly different thing happening but it doesn't matter you can still use the same process horizontal
• 141:30 - 142:00 line to the lens then we're going to take the line down through that focal point and then from here on this is where it's a bit confusing because if i then draw the diagonal line here you can see those rays are never ever going to meet on the right hand side ever they're just going to keep going and going going so what you want to do is take a your ruler and carry on the lines and make them dotted make it nice and straight unlike what i'm doing do the same here
• 142:00 - 142:30 and then eventually they'll cross and that's actually where your image will be formed up here so i'm going to draw the arrow so now let's discuss what we can see well because it's the same not the same height but it's upright it's the same direction as the object arrow was facing we know that it's upright because the arrow is much larger we know that it's enlarged but because it's formed before the lens we know it must be a virtual image and an example of this would be the magnifying glass
• 142:30 - 143:00 and your arrows will go this way dispersion is all to do with light splitting up and spreading out so if you get white light remember it's made up of lots of different colors if you shine it into a prism it will split into those colors remember the order of the rainbow and that will help you with knowing what colors the light splits into so richard of york gave battle in vain red orange yellow green blue indigo violet now red is refracted the
• 143:00 - 143:30 least violet is refracted the most five astume uses a rectangular glass block to determine refractive index of glass the diagram shows a ray of red light in there as it enters the glass block at p the normal p is shown as a dotted line which as we would expect is perpendicular complete the diagram by drawing the ray that continues inside the block labeling the angle of incidence and the angle of refraction and drawing the way
• 143:30 - 144:00 that leaves the block so use your ruler here and a pencil so remember as it enters the block it enters a more dense medium so it bends towards the normal which is why i'm doing it here we need to draw a second normal line down here try and draw that more straight than i have and then remember as the ray leaves it is again parallel to that ray that came in which is why i'm doing it at this angle now we must label the angle of incidence and the angle of refraction so the angle of incidence is between the normal and the incident ray the angle of
• 144:00 - 144:30 refraction is going to be hard to show that that's in there and that's between the normal and the ray that has been refracted the student measures values for the angle of instance i and the angle of refraction are so i is given as 60 degrees r is given as 34 degrees so we need to complete the table by inserting values for sine i and sine r so that's quite straightforward just put into your calculator sine 60 and when you've done that that is 0.87
• 144:30 - 145:00 to 2 decimal places put into your calculator sine 34 and that is 0.56 to 2 decimal places state the equation linking refractive index angle of incidence and angle of refraction so remember refractive index is given by n so it's n equals sine i over sine r so in terms of calculating fractive index sine i is 0.87 sign r is 0.56 and when you put that into your
• 145:00 - 145:30 calculator you get a value which is 1.6 to two significant figures how should the student continue the investigation to obtain a more accurate value for the refractive index of glass so that's worth three marks to make three separate points so the most obvious point to make is to increase the reliability you want to repeat and calculate an average you want to vary the angle of incidence more so you could do 50 degrees 70 degrees 80 degrees and then next you could draw a graph plotting sinai against sinai remember
• 145:30 - 146:00 that the gradient will give you the refractive index electromagnetic spectrum now which is a little sub topic of the waves topic i do like this topic remember that it is a family of waves and they vary just in terms of their frequency and their wavelength so let's go through those waves first of all so starting with the longest wavelength wave so waves which look more like this so they have long wavelengths because remember the wavelength is the distance between two
• 146:00 - 146:30 peaks or two troughs as opposed to having short wavelengths whereby the wavelength is this big as opposed to this big so we're starting from the longest wavelength that is radio waves then we have microwaves infrared radiation visible light ultraviolet x-rays and gamma rays and i'll provide a link below to this video which i love one of my shooties told me about it a couple of years ago and it's i think it's sung by some south korean guides if you start playing it at 30 seconds
• 146:30 - 147:00 listen to the chorus through three times and you'll have the order nailed in your head otherwise i'm sure there are lots of mnemonics that people can remember and comment below if you know of a good one in terms of frequency clearly gamma rays will have the highest frequency because there's more waves per second with gamma rays because they're much closer together whereas microwaves and radio waves will have a much lower frequency because their waves are much further apart the key point to notice with this topic is the various uses of all of these so
• 147:00 - 147:30 we'll start with radio waves remember these are used in communication microwaves are also used in communication so satellite communication they're also obviously used in cooking food most of us have microwaves in our kitchen then we have infrared radiation and that is also used in communication and in this case we're talking about remote control so communicating between the remote control that you're pointing at the tv for example so infrared radiation is being given off there remember we met
• 147:30 - 148:00 infrared radiation when we were talking about the heat topics so conducting convection radiation because infrared radiation is simply heat being given off from objects and therefore infrared radiation is the type of radiation used in ovens because obviously they get hot to cook our food and they are giving off infrared radiation visible light as well as the usual uses so the fact that we can see stuff with them it's also used in optical fibers and photography now we're getting down to the other end of the spectrum so where the waves start
• 148:00 - 148:30 getting much closer together the next wave we're going to be looking at is ultraviolet so uv remember these are rays that come from the sun they have various uses such as tanning beds so sun beds that will have uv rays being emitted but also you can use it to check whether banknotes have been forged or not so you hold up to the uv light and you can tell if they're authentic then we're looking at x-rays obviously they're used in medicine to create images of the human body and lastly gamma rays gamma rays are
• 148:30 - 149:00 used to sterilize surgical equipment because they're such high energy they kill things effectively so that could be killing bacteria on surgical equipment it could be killing cancer cells so used appropriately they're very useful if they're used inappropriately they can actually cause cancer so that's worth noticing the way they'll ask the questions is it will say stuff like microwaves are used in communication name two other rays also used in communication you could have said infrared radiation you could said radio
• 149:00 - 149:30 waves or visible so be aware here what dangers associated with infrared radiation well because it's effectively heat if it gets too hot it can cause skin burns if you touch it dangers associated with ultraviolet that will be skin cancer because again if used inappropriately that can cause cancer as well as the rays from the sun if we're exposed to them for too long it can cause cancer how can we reduce danger from exposure to ultraviolet well wearing sun
• 149:30 - 150:00 cream wearing sunglasses to protect our eyes limiting our explosion time covering up wearing clothes to protect ourselves from x-rays we need to limit our exposure time hide behind lead screens wear protective clothing and the same is true of gamma rays you really don't want to be exposed to these rays for too long they could ask you what all these rays have in common and that is that they are all transverse waves so in all cases vibrations occur perpendicular to the direction which the waves traveling remember that's your perfect definition of a transverse wave
• 150:00 - 150:30 they clearly all transfer energy they may all be reflected refracted and diffracted so they can all bounce off objects they can all change directions they go through a different medium and lastly they travel at the same speed which is 300 million meters per second we're now going to look at some of the uses of electromagnetic waves in a little more detail focusing in on communication the first thing to note is that communication with artificial satellites is mainly done by the microwave part of
• 150:30 - 151:00 the electromagnetic spectrum and you'll find that some satellite phones use low orbit satellites which are satellites that are only between 200 and 1000 kilometers above the earth communications which use these low orbit satellites often use a network of satellites to pass on information using microwaves to locations far away from each other on earth we now need to consider geostationary satellites
• 151:00 - 151:30 which may be used by both satellite television and satellite phones now the important thing to be aware of with these is that these are satellites that orbit earth at the same speed as the earth is rotating now why is that so important well it's because these satellites therefore appear to always be in the same position in the sky or above the earth's surface
• 151:30 - 152:00 and this means that a satellite dish on earth can always be pointed towards the satellite if we consider different types of communication so things like mobile phones and wireless internet well these both use the microwave part of the electromagnetic spectrum why is that well the microwaves have a long wavelength and they can penetrate walls and practically speaking they only require a short aerial for transmission
• 152:00 - 152:30 and reception if we now consider bluetooth well this uses radio waves as opposed to microwaves why is that again because they can penetrate walls although the signal is weakened by doing so let's consider optical fibers now which are used for high speed broadband and cable tv these use the visible light
• 152:30 - 153:00 and shortwave infrared parts of the em spectrum to carry high rates of data down transparent glass cables we now need to consider digital and analog signals and it's easier standby if we look at a diagram like the one below now an analog signal varies
• 153:00 - 153:30 both in terms of frequency and amplitude as seen in this first diagram whereas the digital signal has two values only which is either on or off or one or zero so one being on zero being off and as you can see here here's the on value here's the off value
• 153:30 - 154:00 now sound can be transmitted as either an analog or digital signal and as you might imagine digital signals maintain their quality better than analog signals and we'll explain why in a moment so let's consider the explanation for why digital signals are better quality well first of all we need to realize that all signals become weaker as they travel longer distances
• 154:00 - 154:30 they may also pick up extra signals known as noise which you probably heard if you're listening to a radio station as crackles or hisses now this noise affects both analog and digital signals but we need to consider how the analog and digital signals deal with that noise so the noise adds extra
• 154:30 - 155:00 information to the analog signals and therefore every time the signal is amplified the noise is also amplified and gradually the signal becomes less and less like the original one sometimes it's impossible to make out the music against the background noise now if we look at how noise would affect
• 155:00 - 155:30 digital signal noise would also add extra information to the digital signal however this noise is usually lower in amplitude than the amplitude of the on states so it would feature lower and therefore digital signals can be cleaned up in a process known as regeneration because as we know with a digital signal there's only two values on a roth or zero or one which means that all other values can be removed and as such the quality of the signal is maintained over a larger range
• 155:30 - 156:00 so the removal of noise from a digital signal is known as regeneration because the original signal can be cleaned up and that means that the quality of a digital signal is maintained over longer range the other beneficial thing about digital signal versus analog signals is that they can carry more information per second leading to high quality sound so we'll label it here higher quality sound
• 156:00 - 156:30 more info carried per second what is an echo well it's a reflected sound wave remember they use echo sounding for working out to the depths of the ocean so they'll send down some sound waves they record the time it takes for sound wave to be transmitted and then be received again by the transmitter you use the equation speed equals distance over time so you know the speed of the sound wave you've recorded the time just make sure
• 156:30 - 157:00 you have the time because obviously if you use the full time then that's the time it took to for the wave to hit the bottom and come back up whereas the distance is only from the transmitter to the bottom which is why you have to halve the time what is ultrasound well ultrasound is a longitudinal sound wave which is above the range of human hearing so it's above 20 000 hertz try and be really detailed with your answers there infrasound is sound which is too low for the human ear to hear so it's below 20
• 157:00 - 157:30 hertz the last thing to do with sound waves is remember that pitch relates to the frequency of the sound wave so the number of waves per second the higher that number the higher the pitch so the squeakier the sound like high whereas amplitude so that's the height of the wave is all to do with the loudness of the sound so the higher the amplitude the louder the sound so if we take my rubbish finger here if i do that can you see it has a very low amplitude because it's very short so that means it's a quiet sound but because the frequency is very high it has a high
• 157:30 - 158:00 pitch so it's a very squeaky sound whereas if i were to do this we've got a much larger amplitude so a loud sound but because the waves are so infrequent it's a very low sound two a microphone is connected to an oscilloscope to display a sound wave the
• 158:00 - 158:30 diagram shows the trace on the oscilloscope screen the oscilloscope settings are in the y direction one square is one volts and in the x direction one square is 0.001 seconds so let's make a few annotations so that must mean that up to there is 2 volts and this is 0.01 seconds which means the entire wave cycle is 0.002 seconds so how many periods are shown on the trace i remember the period
• 158:30 - 159:00 is the time for one wave so that's a complete wave so there's the first wave and you can see that repeats three times which is why the answer here is three and what is the frequency of the sound wave so frequency is given by f which is 1 over the time period so we simply do 1 divided by 0.002 which gives us a value of 500 and they've already given us the units which is in hertz on the grid below sketch the trace of a
• 159:00 - 159:30 sound wave with a smaller amplitude and a higher frequency than the wave shown by the dotted line so a smaller amp amplitude means that it will be quieter so our wave needs to be less high so it needs to be much lower high frequency means that the waves need to get closer together so i'm going to draw it like this and try and keep it nice and even so i'm only going up one wave and down one way but because they're closer together i've definitely drawn a higher frequency the fact that they're lower means that it's
• 159:30 - 160:00 a smaller amplitude so magnetism well we're going to start by first of all stating which metals are magnetic that is iron steel cobalt and nickel and the two most common examples you'll come across are obviously iron and steel do notice that steel is an alloy of iron which means that it contains iron but it also contains the element carbon a few basics to point out which is if you have a bar magnet or any magnet remember that two north poles will repel
• 160:00 - 160:30 so if you try and push them together they'll slide past each other the same is true with south poles they'll repel as well and the opposites attract so a north pole will attract a south pole looking at the difference between a soft magnetic material and a hard magnetic material now the real difference is that a hard magnetic material maintains its magnetism so steel for example is a hard magnetic material it does not lose its magnetism easily whereas iron loses it very easily and to put this into context
• 160:30 - 161:00 if we use a scrap metal yard as our example here scrap metal yards are full of big old lumps of iron which need moving around now you actually use a magnet to move them around and what you do is you turn on a massive iron electromagnet and it becomes magnetized it clamps down and picks up that metal that it's trying to move and then when you've moved it across to the new position you can basically cut the power source and it will lose its magnetism you couldn't use steel in this case because you'd effectively find that all
• 161:00 - 161:30 that scrap metal would remain stuck to the magnet and you'd never be able to pull it off so what are the differences between a magnet and a magnetic material because they sound pretty similar well a magnet as we know a simple bar magnet will look something like this it has a magnetic field which runs from north to south so simply it looks something like this and if we list these points you can say it has a magnetic field as you look at the magnet you can see
• 161:30 - 162:00 that it has two opposite poles and we know that it will attract magnetic materials such as an iron nail will be attracted to this bar magnet in contrast a magnetic material doesn't have a magnetic field it can be attracted by a magnet which is again a common sense point although it doesn't have a magnetic field itself
• 162:00 - 162:30 a magnetic field can be induced around the magnetic material if we compare hard and soft magnetic materials we know hard ones include steel because they retain their magnetism soft magnetic materials put those in inverted commas such as iron lose their magnetism easily we've already mentioned the term but
• 162:30 - 163:00 let's look at it in further detail so what really is induced magnetism and that's when a non-magnetic material develops magnetism you find that atoms in the magnetic material have a small magnet force and that when these forces are pulled into line the material becomes a magnet looking at how material can become
• 163:00 - 163:30 magnetized well it can become weakly magnetized or strongly magnetized depending on how you magnetize it and weekly magnetizing occurs when you get the material and you hold it close to the magnet whereas if you want to strongly magnetize it you stroke the material with the pole of a magnet
• 163:30 - 164:00 so we've got our material here which is what we're trying to magnetize we've got our bar magnet over here and we're just going to stroke that north pole up and down the material until it becomes strongly magnetized remember by adding a solenoid that's a cylindrical coil of wire you can also increase the level of magnetism by the way if you like how i'm writing these notes it's very similar to the revision guide so if you want this level of details throughout your whole course it's very much worth having a look at my vision
• 164:00 - 164:30 guide which you can see a preview on on the website scienceofhazel.com anyway what will cause the loss of magnetism well we know that a substance becomes magnetized when the atoms all arranged in the same direction so clearly if you hammer the material you cause those atoms to disarrange themselves so they become disorganized if you also expose the material to heat then the atoms will also disorganize themselves
• 164:30 - 165:00 and lastly placing the magnet in a coil and passing an alternating current through it will cause loss of magnetism so what is a magnetic field line well it's the space around a magnet whereby magnetism can be detected and in terms of designing experiments whereby you can determine the shape of magnetic field lines you have two options here first of all iron filings and secondly using plotting compasses so what you do is you get a piece of paper you sprinkle iron filings onto it and
• 165:00 - 165:30 then you place a bar magnet underneath and you'll see that those iron filings align themselves with the magnetic field with a plotting compass instead you place your bar magnet on a piece of paper you place a plotting compass near to it and you make a note of the direction which the arrow on the plotting compass is pointing and then you pick it up put it in a new position keep repeating that process until you've drawn the magnetic field lines
• 165:30 - 166:00 now just remember that a solenoid is a cylindrical coil of wire touching now on the right hand grip rule so this is just a way of showing the direction of magnetic field lines so my thumb represents a wire so in this case it's going from north to south or from up to down and you might be asked to draw the
• 166:00 - 166:30 magnetic field line directions and just follow the direction which your fingers are pointing be aware of a uniform magnetic field and all you'll see here is that the lines the magnetic field lines are evenly spaced and that they're parallel and those are the two points you need to point out with a uniform magnetic field so how can an object's magnetism be induced so how can it become magnetic well first of all you need to start with a magnetic metal such as iron and steel and then you can place it inside the magnetic field of a permanent magnet and
• 166:30 - 167:00 its magnetism will be created or induced obviously the moment you remove that object the iron object from that magnetic field it will lose its magnetism steel i've already pointed out is a hard magnetic material so it will tend to retain its magnetism so looking at the differences between a permanent and electromagnet so starting with the electromagnet let's draw a really simple one here so we've got an iron rod it's within a solenoid so cylindrical coil of wire and we're connecting it to
• 167:00 - 167:30 a circuit so i'm just going to draw a simple cell here so that's appallingly drawn but hopefully you can see that we have a simple electromagnet so let's label what we have here we have a soft iron core we have a coil of wire and we know that when it's turned on so let's insert a switch so we can easily turn the circuit on and off
• 167:30 - 168:00 so when that switch is open the electromagnet is obviously off when we close that switch the electromagnet is switched on so crucially with electromagnets they're very easy to turn on and off and they have lots of uses including in relay switches circuit breakers
• 168:00 - 168:30 doorbells whereas we if we look at a permanent magnet so again because it's the easiest for me to draw i'm drawing a bar magnet here you can see that they are always magnetic they're made out of a hard magnetic material and they are used in compasses
• 168:30 - 169:00 speakers etc so remember in terms of electrical charges you're looking at two types which is positive and negative and as always opposites attract so these will attract
• 169:00 - 169:30 if you have two negative charges then they will repel and the same is true for two positive charges now how do things actually become charged so how does a plastic rod become charged and that's all to do with the transfer of electrons so charging occurs due to the transfer of electrons
• 169:30 - 170:00 so remember that electrons are negatively charged so if an object gains electrons it will become negatively charged if it loses electrons it will by definition become positively charged and don't forget the unit for charge is coulombs which we can give with a capital c just remember when you're thinking about
• 170:00 - 170:30 static electricity it's all to do with electrons being transferred from one place to another so for example a polythene rod which is rubbed with a cloth will become charged wide because electrons will transfer themselves from the polythene cloth to the rod so it'll become negatively charged that therefore means it attracts positively charged charged items because remember opposites attract the questions always go the same and they'll talk about why does it become charged and literally the two marks you need to specify are is the transfer of electrons
• 170:30 - 171:00 using friction and if you actually look at past exam questions you'll see what i'm talking about that is so often the answer i can't even explain why does a balloon stick to a wall well you've charged it using friction the balloon has become negatively charged due to the transfer of electrons and then it sticks to the wall because it repels those electrons away from the wall meaning that it can stick to the positive charges i'm trying to think of other questions so things like why do your hair stand on end when you touch a van de graaff generator that's because all your has become negatively charged they
• 171:00 - 171:30 therefore repel so they stand up on end why can't you become charged on a metal slide when you slide down it well that's because metal is a good conductor so often they'll ask you about metals and things not becoming charged you just need to say that it's a good conductor so the electrons can freely flow looking at objects which may or may not become charged so a conductor first of all is obviously something which allows charge to flow freely around it and your most common examples
• 171:30 - 172:00 here are obviously going to be metals so let's just list a couple such as copper and silver carbon in the form of graphite is a good electrical conductor from a semiconductor point of view you're looking at silicon and geranium and insulators so substances which do not allow an electric charge to flow we've already mentioned plastics so let's name a few
• 172:00 - 172:30 such as pvc polythene you've got glass rubber wood etc in terms of looking at the direction of electric field if we take a battery for example which looks something like this so it's made up of two cells remember that the taller one is the positive terminal the shorter fatter one is the negative terminal and you find
• 172:30 - 173:00 that the electric charge flows from the positive to the negative terminal so in the case of this example it would be in this direction what is an electric field well it's a region in which electric charge experiences a force if you're asked to describe electric field patterns you simply need to state the following and it's lines with arrows
• 173:00 - 173:30 which represent the fields the arrows show where the force on a positive charge would act and these field lines point away from the positive towards the negative so what is current after all we know we measure it using an ammeter
• 173:30 - 174:00 we know that it's measured in amps well it's the rate of flow of charge and in metals this means the flow of electrons the equation linking charge current and time so if i use my formula triangle over here q stands for charge i stands for current t stands for time so charge is current times time so taking an example so if a current of
• 174:00 - 174:30 three amps flows over a period of five seconds what is the charge it's always good practice to write out the equation you're using so it's q equals i times t q is charged which after i is current which is three time is in seconds so three times five is 15 and remember the
• 174:30 - 175:00 units of charge are coulombs and that is your final answer we will do touch briefly on an ammeter do notice how it is attached in a circuit so if i draw a simple circuit up here and we want to include something which will determine the current so we want to include an ammeter or we add it in series so it's part of the main circuit and i'm going to finish off this circuit by adding a resistor so an ammeter is always added in series notice these can be either analog or digital
• 175:00 - 175:30 and that means whether it reads using an arrow which wobbles from one number to another or digital means that it's something like 2.75 amps so it's far more precise so now what is the difference between ac and dc because we hear these when applied to an electrical circuit so first of all remember what they stand for ac is an alternating current dc is a direct current and then it's a simple matter of learning their definitions
• 175:30 - 176:00 so an alternating current continuously changes direction and your example here is mains electricity so the stuff that comes into your home whereas in direct current the current flows in one direction only and you tend to find this inside cells and batteries
• 176:00 - 176:30 now conventional current this is where it gets a little bit confusing because we're looking at historically how current was determined now it's depicted conventionally as flown from positive to negative so what that means if we draw a simple cell here a circuit containing a light bulb now that's the positive terminal the
• 176:30 - 177:00 tool one of the cell the short stubby one is the negative one so the conventional current would therefore mean that it would flow in this direction however in actuality electrons flow in the opposite direction but if they ask you what a conventional current is you're saying that it flows from positive to negative you might have already realized but voltage and potential difference are pretty much the same thing
• 177:00 - 177:30 and we kind of use these terms interchangeably but what does it actually mean well it is a force which is measured in volts as such it is measured by obviously a voltmeter and notice when you add a voltmeter into a circuit such as one just containing a bulb like this it must be added in parallel which means it comes off as a separate branch
• 177:30 - 178:00 now what does potential difference of voltage actually mean well it describes the energy given to electrons pushed out and defines how much energy is given to each coulomb of charge going back to our voltmeter temporarily i've already said that it gets added in parallel now again like the ammeter it can either be an analog or digital
• 178:00 - 178:30 voltmeter and do notice that one volt is the equivalent to one joule per coulomb which actually makes sense because if we take this equation triangle which is e v key which stands for energy voltage and charge well if we look at voltage we know that it is energy divided by charge well what's the unit of energy well it's joules what's
• 178:30 - 179:00 the unit of charge well it's coulombs and look these units match which is why they're equivalent units now think about what the word resistance means in english it means if you're a resistant person it means that you don't like suggestions or you're resistant to change which means you don't want to do it so resistance in an electrical circuit is all to do with how easily a current flows in a circuit or in a component or material
• 179:00 - 179:30 the units of resistance is the ohm and the important equations you need to know about come about from this formula triangle which states that resistance equals voltage divided by current now notice that increased resistance reduces the current
• 179:30 - 180:00 now how could we determine the resistance of a component in a circuit let's take an example circuit here's our battery got a bulb we have a resistor now we see from the resistance formula triangle that it's calculated by doing v divided by i well how do we find out voltage by adding a voltmeter into the circuit how do we find out current by adding an ammeter into the circuit as i've just taught you the ammeter needs to go in series which is why i'm going to add it
• 180:00 - 180:30 as part of the main circuit and i'm interested in the resistance of the light bulb which is why the voltmeter is being added in parallel over here so once you get your readings from your ammeter and your voltmeter then you can substitute them into this equation to work out the resistance making sure you give your final answer with the unit ohm so now we need to look at how the resistance of a wire changes dependent on both its diameter
• 180:30 - 181:00 so if we take a cross-section of the wire from this distance to this distance and also its lamp so obviously how long the wire is now notice that resistance is directly proportional to the wire's length so if you double the length you double the resistance now if you look at how the resistance
• 181:00 - 181:30 the wire is affected by the diameter you see a different relationship which is that the resistance is inversely proportional to diameter of the wire and that means if you halve the cross-sectional area you get a doubling of the resistance so let's think about it ideally if you want low resistance you want a nice wide
• 181:30 - 182:00 wire and you want it to be short if you take a very skinny wire and you make it very long then you're going to have a high resistance there are a few graphs i want to talk through now now do remember ohm's law and that states that the current through a resistor constant temperature is
• 182:00 - 182:30 directly proportional to the potential difference across the resistor and various components are bake ohm's law and some do not obey ohm's law now a wire obeys ohm's law and you'll see that because you can see it's a straight line through the origin so clearly you can see that as the current increases the voltage increases and that's at constant temperature so a wire a base ohm's law you get a very different shape when you show a filament bulb so a light bulb this is
• 182:30 - 183:00 not a proportional relationship we've got a sigmoidal shape an s shape and the bulb therefore does not obey ohm's law because there's no direct relationship between the two so a four volt battery you can supply current at five amps for 20 minutes before it needs recharging calculate how much charge the battery can provide before it needs recharging at this point what i always do is i draw out my
• 183:00 - 183:30 formula triangles because why not it's good to get your head um nice and into the question and knowing that you actually know stuff that you can use so the first one is voltage is current times resistance the next one is charge is current times time and then finally the third electrical triangle that i um use is work done is voltage times charge and just remember that the units of charge are coulombs so at the end of
• 183:30 - 184:00 a question write a capital c and they're really fussy about how you write the units to make sure it's a capital c rather than a lowercase c in order to get the mark and remember i just said that resistance is measured in ohms and the sign for an ohm is like this which i've drawn really badly anyway let's look at the question then calculate how much charge the battery can provide before it needs recharging so we're looking for a triangle that has q in it so there's two of those and what else have i been told well i've got the current because i've been told five amps
• 184:00 - 184:30 and i've been given the time which is 20 minutes so what i'm going to use is the qit triangle so i'm going to write out because it's always good practice to do this q equals i times t i is 5. okay t is 20 minutes but remember that's not very um scientific so we're going to convert that into seconds by doing 20 times 60 and the answer here is 6 000 and like i just said the units of charge are coulombs so i'm going to write a capital c here b each coulomb of charge from the
• 184:30 - 185:00 battery can carry three joules of energy calculate how much work the battery can do before it needs recharging okay so for part b you're gonna use the formula w equals v times q okay v has been given as four and q is something that i just worked out which is six thousand so what's four times six thousand well it is twenty
• 185:00 - 185:30 four thousand and work done is measured in joules um so there's the answer right there okay let's look at another type of question okay so i've drawn a horrendous um circuit here but hopefully it will still manage to help you at least some way so this is an example of sort of thing you can get in the exam um we can see that we have a series circuit with a battery made up of three cells an ammeter with a reading of 0.5 amps and three regular
• 185:30 - 186:00 resistance in series we've been given two of their resistances we don't know the third one and a voltmeter has been added in parallel around resistance number two so calculate the total potential difference across the battery okay so all you have to do here because remember potential difference is the same as voltage is add up those three readings so that's two plus two plus two which is six volts nice and straightforward okay work out the total resistance so it'd be tempting here to try and add up all the resistances of the individual resistors
• 186:00 - 186:30 you can't do that because you don't know the resistance of resistor three so we're going to have to do this a different way and we're going to use this equation triangle so resistance is b divided by i the v is 6 which i just worked out i is 0.5 given by the ammeter and therefore the answer is 12 and the units are ohms okay finally calculate the resistance of r3 so if we know that the total resistance is
• 186:30 - 187:00 12 ohms but we've been given r1 and r2 that's 6 ohms that tells us the difference between those two values will be r3's value so 12 takes 6 is obviously six and that is six ohms three the photograph shows an electrical appliance called a toaster the toaster has a power of 1800 watts when operating a voltage of 230 volts state the equation linking power current and voltage so i'm going to put my formula triangle nice and out of the way over here it's piv
• 187:00 - 187:30 so p at the top current and voltage in the bottom so what is the equation it is power equals current times voltage part two show that the current in the toaster is about eight amps so current equals power divided by voltage i always write out the equation i'm using we know that power is 1 800 dividing it by 230 to give seven point eight three to three sig fig which is approximately eight amps
• 187:30 - 188:00 which fusivating would be suitable for the toaster remember the fuse rating needs to be slightly higher than that calculated current and so it's not ideal because 13 is a lot higher but you can't say one three or seven you can't say seven because it's lower which means it would constantly have melted when the toaster was in use which is why d is the correct answer here b the toaster uses main electricity mains electricity provides alternating
• 188:00 - 188:30 comment describe the difference between alternating current and direct current so learn these definitions off by heart an alternating current is one where the current continuously changes direction whereas the direct current is one where the current flows in one direction only the photograph shows an electric heater the power of the heater is 2 watts the heater is connected to a 230 volts main supply state the equation linking power current and voltage and i can remember that is piv so therefore power equals current times
• 188:30 - 189:00 voltage calculate the current in the heater so rearrange so that current is power divided by voltage power is 2 000 watts divide that by 230 volts and you get an answer which is 8.7 amps which of these fuses should be used with the heater so you're looking for one which is higher and not too high compared with the current you've calculated and that will have to therefore be 13 amps deep because it's the only current higher than the one we've calculated
• 189:00 - 189:30 the two heating elements can be connected in series or parallel describe an advantage of each method so with parallel the obvious advantage is that you have independent control so you can turn on and off the heating elements independent of each other so you could just have one on or both on at the same time the series one is a bit more of a state the obvious here you can use a single switch only to control both heating elements some electrical appliances are fitted with an earth wire describe how an earth
• 189:30 - 190:00 wire acts as a safety feature that is worth four marks so state that first of all the earth wire is connected to the metal casing and that if the casing becomes live the earth wire provides a low resistance path to obviously the earth and then you can talk about the use of a fuse which is attached to the earth wire so that increasing current will cause the fuse to melt breaking the circuit which cuts off the supply explain why this heater should be fitted
• 190:00 - 190:30 with an earth wire and the obvious thing here is because it has a metal casing and moment and remember metals are good conductors of electricity now we're going to discuss kilowatt hours and the cost of using electrical appliances so first of all we'll start by defining kilowatt hour and it's the electrical energy converted by a one kilowatt appliance used for one hour a quick note on units which i know people don't like how do you get 2 000
• 190:30 - 191:00 watts into kilowatts well you divide by a thousand to get two kilowatts and then quite often we have to convert from seconds into hours so for example if you had 1800 seconds and you needed that in hours you need to first of all divide it by 60 to get it into minutes and then divide by 60 again to get it into hours and so that answer here is 0.5 hours so seconds into minutes into hours
• 191:00 - 191:30 divide by 60 twice so electricity meters measure the number of units of electricity being used within the home obviously the more units used the greater the cost and how that cost is calculated is done using this equation so there's your kilowatt hour unit and we're going to do power which remember is in watts or kilowatts times time in hours
• 191:30 - 192:00 in terms of the cost you want to do the number of units used and multiply it by the cost per unit so here's our first example an electric fire needs two kilowatts it is switched on for three hours if each kilowatt cost 15p how much does it cost to run the fire so let's work out the units used and we need to do power times time we know that our power is 2 kilowatts our time was 3 hours so we've used 6 units so
• 192:00 - 192:30 that's six kilowatt hours so we know the number of units used we now need to work out the cost so we do units used times cost per unit so we know the answer was six units used 15p a unit so that's 90p or 0.9 pounds question 2 a tv set needs 250 watts it
• 192:30 - 193:00 is switched on for 30 minutes if each unit's kilowatt hour costs 16p how much does it cost to run the tv so let's work out how many units it's used first of all so you're going to do power times time remember we need that power in kilowatt so i'm going to do 250 divided by a thousand and we need our time in hours so to get from minutes hours you divide by 60. once you pop that into your
• 193:00 - 193:30 calculator the number of kilowatt hours is 0.125 and then finally we do cost is number of units multiplied by cost per unit so that's 0.125 times 16p and that gives a final answer of 2p 100 what light is left on for three days if each unit of electricity costs 12p how much does it cost to run the light we'll
• 193:30 - 194:00 start by working out the number of units used in kilowatt hours so remember we need to do power times time be very careful with the units here our power is in watts we need that in kilowatts so i'm dividing that by a thousand and then in terms of time we need that in hours so remember to get days into hours you need to multiply by 24 because there are 24 hours in one day so we know 7.2 kilowatt hours have been used and then finally the cost is the number
• 194:00 - 194:30 of units times cost per unit so we do 7.2 times 12 to get 86.4 p which rounds down to 86 p or 0.86 pounds right long list of electrical symbols you need to know so let's start with the easiest most straightforward ones we have the cell if we connect two or more cells together then we have a battery
• 194:30 - 195:00 next up this indicates a power supply and if you draw the same symbol with a squiggle between the two connections then you have a ac or an alternating current power supply this gets more complicated now this particular symbol is the junction
• 195:00 - 195:30 of conductors a lamp is just a circle with a cross in it moving on to the family of resistors so a simple rectangle is just a regular resistor one that has an arrow going through it is a variable resistor i always call the thermistor hockey stick because of its shape so you have a
• 195:30 - 196:00 regular resistor with what i call the hockey stick going through it that is a temperature dependent resistor or a thermistor as it is better known when arrows approach it means that something is dependent and the arrows represent light so you have a light dependent resistor here one ldr
• 196:00 - 196:30 leaving the resistors behind now so we're going to draw another rectangle but we're going to break it up this time to represent a heater if we draw what looks like a resistor with an arrow coming into it in this particular direction then you have a potential divider this is a relay coil
• 196:30 - 197:00 a transformer more basic now is a switch and this one is open remember that if you draw like this you then have a closed switch
• 197:00 - 197:30 the symbol which looks like this represents the earth or ground here's an electric bow looks a bit like a jellyfish the same symbol but sort of upside down is a buzzer which makes a noise this is a microphone which remember you
• 197:30 - 198:00 talk into and it converts your sound signals into electrical signals a loud speaker takes electrical signals and turns them into sound waves goodness i'm so bad at drawing circle with an m is a motor a circle with an a we've already looked at is an ammeter which measures current
• 198:00 - 198:30 the v with a circle is a voltmeter which measures voltage circle with an arrow going upwards is a galvanometer something which looks a bit like the play button is a diode which remember only allows current to flow in one direction only
• 198:30 - 199:00 arrows coming out of it then you have a light emitting diode or an led and then last but not least a rectangle with a line going through it is a fuse which remembers a safety device because it melts when the current is too high cutting off the circuit we just need to discuss current and voltage rules for both a series and parallel circuit so notice that in a series circuit the current is
• 199:00 - 199:30 the same everywhere whereas the voltage of individual components adds up to the total voltage and i'm going to show you what that actually means now so i'm drawing a battery here which let's say it has eight volts across it then we've got a light bulb here a regular resistor here and i'm just going to complete the circuit so if i tell you that the current here
• 199:30 - 200:00 is 12 amps because i've said that the current is the same everywhere it doesn't matter if an ammeter got added here here or here all those readings would be 12 amps however with the voltage if you added voltmeters here and here their readings would not be 8 volts let's say that this was 5 volts the
• 200:00 - 200:30 reading that means that this reading here must be 3 volts and that's due to that second point that the voltage of the individual components adds up to the total voltage the opposite is true in a parallel circuit here you find that the voltage is the same everywhere whereas the current of the individual components adds up to the total current that means that each of these light bulbs here will have 12 volts going through them and that explains why with a parallel circuit how if you keep
• 200:30 - 201:00 adding extra bulbs the brightness remains the same and that's because they receive the same voltage however it will mean that your battery will run out three times as quickly as if there was only one in terms of the current pretend that was a 2 amp bulb this was a 2 amp bulb well we know because it has to add up to the total current this lamp down here is slightly different in that it has a current of one amps
• 201:00 - 201:30 a few bitty parts now to cover to do with electricity which is first of all what does a variable potential divider or a potentiometer g it uses a variable resistor so let's draw that circuit symbol quickly to take a portion of the cell or battery's voltage and it enables that voltage to be delivered
• 201:30 - 202:00 to a separate circuit and obviously the amount of voltage delivered to the separate circuit can be controlled by varying the resistor touching on rectifiers now these are simply devices which can
• 202:00 - 202:30 change an alternating current so ac into a direct current dc you must remember the definitions of alternating current and direct current an alternating current is one which continuously changes direction as seen in mains electricity a direct current is one where the current flows in one direction only we already looked at the circuit symbol for a diode going into slightly more detail so we know that a diode allows current to flow in one
• 202:30 - 203:00 direction only in terms of their use while they're used to produce a direct current from an alternating current and when used in this way clearly they can act as a rectifier remember that a relay switch is basically an a safety device and what happens when you turn it on is it switches on the current causes current to flow and that's
• 203:00 - 203:30 connected to a separate circuit which contains much greater voltage so in this way you can turn on that much greater voltage circuit without physically touching yourself and being at risk of electrocution and we just need to talk about it a little bit here in electricity so a switch on a large powerful circuit is operated by a separate small circuit
• 203:30 - 204:00 an electromagnet is switched on in the first circuit which attracts the contact of the second circuit causing a closure of the switch looking more closely at light dependent resistors remember this is the symbol diagram depends on light which is why the arrows come towards that resistor so these are otherwise known as ldrs
• 204:00 - 204:30 now notice that as light intensity increases resistance decreases it's often used in circuits as an input transducer it can be used as part of a light sensitive switch which would make sense so that if it is placed in a potential divider to deliver voltage to a lamp the
• 204:30 - 205:00 lamp will come on when it's dark and that's due to an increase in resistance when it gets dark because of the low light intensity and finally thermistors remember these are temperature dependent resistors their circuit symbol is similar to a hockey stick so remember that the resistance of
• 205:00 - 205:30 thermistor decreases as temperature increases it can obviously be used as an input transducer in circuits needing to be sensitive to temperature such as in fire alarms let's look more closely how the fire alarm might work so the thermistor is
• 205:30 - 206:00 placed in a potential divider to deliver high potential difference in high temperatures when attached to a relay which we've already mentioned this high voltage is used to turn on the electromagnet
• 206:00 - 206:30 the electromagnet in turns turns on a switch and the alarm bell rings
• 206:30 - 207:00 now we're going to look at electromagnetic induction or basically how voltage or current may be induced it's a very similar concept we just need to look at it from a slightly different angle so i hope you realize that with your force and your magnetic field and your current it's a bit like a physics formula triangle which is that if you put in magnetic field and you put in current then you get force so it makes sense therefore that if you put in the
• 207:00 - 207:30 force you put in the magnetic field then you should be able to make current and indeed that's what electromagnetic induction is all about if a wire is moved into a magnetic field at right angles then you find that a voltage will be induced and if it's connected up to a complete circuit that's where your current comes from do notice that you must move the wire or the magnets it doesn't matter which way around you do it you must move them at 90 degrees to each other if they're parallel then you won't induce your voltage
• 207:30 - 208:00 now in terms of working out which direction everything's going to move in this time you use fleming's right hand rule so you find again that the thumb shows the direction of the force or the motion first finger is the magnetic field second finger is the current but you have to use your right hand because otherwise it won't line up properly if you use your left hand and this is really how a simple generator works because if you're creating current then clearly you could make electricity from that and all a generator is is a machine which creates electricity so how could
• 208:00 - 208:30 you increase the size of that induced current well clearly you could use stronger magnets you could use have a stronger magnetic field you could move the wire more quickly and you could wrap the y into a coil so linked to this is what dictates the direction of the induced emf and the key point to notice here is that it opposes the change causing it well what does that mean so say you have a bar magnet moving towards that coil which contains the current we know that that coil containing the current will
• 208:30 - 209:00 have a temporary magnetic field around it so we'll act as a temporary magnet and what that will actually do is repel the approaching magnet and i'll make you some notes which to be honest if you're not really understanding what i'm saying you can literally just learn these looking at the use of generators in everyday life you can use the example of a bicycle dynamo and just to explain what this is so some bikes i don't know
• 209:00 - 209:30 if you've ever used the boris bikes in london or any of those bikes you can rent in other countries don't if you've ever cycled them in the night time but they have lights and it's not because they have batteries that need replacing it's because they contain simple dynamos which uses your motion of pedaling the bike to actually power the lights so we need to explain from an electromagnetic point of view how that works so as you turn your pedals clearly the bicycle wheel turns this turns a magnet which is located within a coil and this
• 209:30 - 210:00 magnetic field of the magnet cuts the surrounding coil which induces a current so within a bicycle dynamo you find that there is indeed a simple generator fairly niche part of the specification now so we need to relate the position of the generator coil to the peaks and zeros of voltage output and this simply states that peaks in the voltage output correlates to the coil which is at 90 degrees
• 210:00 - 210:30 to the magnetic field and a zero relates to the coil being parallel to the magnetic field so how is a simple electromagnetic constructed and that's simply by connecting a wire to a circuit and running a current through it how can we increase the strength of the magnetic field of that wire carrying a
• 210:30 - 211:00 current well the most basic thing we can do is obviously increase the current and we can also wrap the wire into a solenoid so if we coil it up that will also increase its magnetic field strength we're now interested increasing the magnetic field of a solenoid because it's already in a coil what you can do now is add more turns of coil or you could add an iron ore which is just like an iron bar which you thread through the middle of the solenoid you can also increase the current as i've already specified
• 211:00 - 211:30 now we need to describe an experiment to show the force acting on a wire in a magnetic field so what you want to do here is firstly you want to place a wire between the north and south poles of a magnet then you want to run a current through that wire [Music] and you will see it move
• 211:30 - 212:00 notice that reversing the current reverses the direction of movement of the wire and the same if you also sew three part b really if you reverse the magnetic field you'll also reverse the direction of movement of the wire
• 212:00 - 212:30 and obviously the way in which you actually work out which way the wire is moving is using fleming's left hand rule where don't forget that your thumb your left thumb shows the direction of the force your first finger shows the direction of the magnetic field
• 212:30 - 213:00 and your second finger shows the direction of the current similarly but slightly different we need to describe an experiment to show the corresponding force but this time on beams of charged particles where will those charged particles come from well it's an electron gun and what the electron gun does is it fires a beam of electrons across a fluorescent screen which will show their path
• 213:00 - 213:30 once you've done that you put a magnetic field perpendicular to the screen so right angles that's the sign for a right angle and you observe the path of electrons on the screen we're now looking at the motor effect which is the part of this topic which people really aren't a fan of but if i talk you through the overview first of all and then we'll look a bit more closely about how it all works so let's
• 213:30 - 214:00 start by looking at what the motor effect really is an automotive effect is know that a motor is a piece of wire that spins so that's what's happening in a motor a wire is spinning and we're trying to work out how we can create that and that is via the motor effect and you might have seen a simple motor being built at school so what you have here is you have two permanent magnets you have a wire in between them which is coiled up and then that is attached to an electrical circuit so it can carry a current and the point here is that when the wire
• 214:00 - 214:30 carrying the current is placed within this magnetic field of the two permanent bar magnets you find that there's a phenomenon which we call the motor effect and what will happen is that wire will start spinning and you have a simple motor that's only if you've got a coil of wire placed within the magnetic field of two permanent magnets and it needs to carry a current and you've probably heard of flemmi's left hand rule and that's what i'm going to talk about now and that's just a way of working out whether the coil moves up on one side or
• 214:30 - 215:00 whether it moves down because obviously has to move up to begin its spin then it moves down moves up it moves down so that's what's happening with our motor so looking at fleming's left hand rule you've got to hold your thumb forefinger and second finger at right angles to each other now the thumb represents the force for the motion and that will show the direction in which one side of that coil will be moving so taking the left hand side for example in this instance it would be moving upwards
• 215:00 - 215:30 the first finger magnetic field the first finger shows the direction of the magnetic field and remember that the magnetic field always runs from north to south so based on that diagram you get given in the exam or in your textbook have a look at those two permanent bar magnets look at the north pole look at the south pole and make sure that your first finger matches up with that your second finger shows the direction of the current and on your textbook it should show an arrow showing which way the current's running if it's running
• 215:30 - 216:00 clockwise or anti-clockwise just make sure that that finger is lined up there so once you've lined up your magnetic field which is your first finger your current which is your second finger you will find that your thumb either points out or it points down and so it will ask you which direction will the coil move and you will say upwards or you will say downwards just make sure you've got them all in 90 degree planes to each other my way of remembering what finger stands for what is that the first finger is your magnetic field first field second
• 216:00 - 216:30 it contains a c means that it is showing you the direction of the current so in terms of answering the five markers which tends to be something like a wire is placed within a magnetic field and a force is felt disgust it's worth five marks so it doesn't matter how they word up these questions the answer is always the same you want to start by saying the why carrying the current so your coil has a temporary magnetic field and what that does is it
• 216:30 - 217:00 and it interacts with the permanent magnetic field of the bar magnets this creates a force causing the wire to turn and believe it or not this applies as well when you're talking about a loudspeaker because anything that moves in and out in this way involving magnets will use the motor effect even though it doesn't seem particularly obvious so taking the speaker cone for example we start by saying that the wire carrying the current has a temporary magnetic field this interacts with the permanent magnetic field of the bar magnets found
• 217:00 - 217:30 in the speaker this creates a force and this force moves the speaker cone upwards vibrating air particles causing the sound that reaches your ears so you're using the same answer here and honestly as long as there's a wire moving or the speaker cone moving your answer remains the same the photograph shows a small electric motor explain why the coil starts to spin when the switch is closed remember this is a magnetism question so this is
• 217:30 - 218:00 an answer which you can definitely vote learn so what you want to say is when the switch is closed that the current flows around the circuit this creates a temporary magnetic field around the magnet which you can see is near the coil it's it's stuck in the coil so right here is where the magnet is so this temporary magnetic field interacts with the permanent magnetic field of the magnet which creates a force and that force is what is used to turn the coil
• 218:00 - 218:30 and you can mention fleming's left hand rule just for an extra mark suggest how to make the core spin in the opposite direction the obvious thing here is to switch the direction of the current or you could have swapped the magnets over suggest how to make the coil spin more slowly so how do we weaken that magnetic field well we can reduce the current you could also reduce the voltage or had a weaker magnetic field which is harder to quantify
• 218:30 - 219:00 the photographs show how an electric toothbrush fits on its charger the charge and the toothbrush each have a coil of wire inside them the diagram shows how the two coils are linked by a u-shaped core this arrangement of core and coil acts
• 219:00 - 219:30 as a transformer that reduces voltage name the type of transformer that reduces voltage well that would be a step down transformer explain why the coil is made of soft magnetic material such as iron remember it's whereas iron is softly magnetizing and therefore it loses its magnetism easily and also you need to state the fact that the magnetic field in the core can change state the equation linking the input primary and output secondary voltages and the turns ratio of a transformer this is something you're just going to have to learn
• 219:30 - 220:00 so write input voltage divided by output voltage equals primary turns divided by secondary turns please just land that off by heart the transformer has 520 primary turns and 30 secondary turns the input voltage to the transformer is 44 volts calculate the output voltage so let's just substitute those numbers in so that calculation will look like this the input voltage is 44. we're looking at the output so i'm going to put x here primary turns it's 520 second g is 30. it's up to you what
• 220:00 - 220:30 matter you want to use to do this what i tend to do is flip the whole lot so it becomes x over 44 equals 30 over 5 20 and then all you need to do is use your calculator to do 30 divided by 520 and then times it by 44 to get x by itself and x will equal 2.5 volts diagram shows parts of a transformer the input voltage to the transform is 230
• 220:30 - 221:00 volts the output is 25 there are 100 turns on the secondary coil name the type of transformer shown in the diagram well it's a step down because the output voltage is lower so right step down there state the equation linking input primary voltage output secondary voltage primary tens and secondary terms we really need to learn all these equations so this is what the equation is remember that it's input primary voltage over output which is secondary voltage equals number of primary turns divided by secondary terms so now we're calculating
• 221:00 - 221:30 the number of turns on the primary coil so i'm going to be using that equation and x is therefore np here and then let's substitute what we know we know that there are 100 turns on the secondary coil oh my gosh the gardeners are so noisy and then on the output of the transformer we've got 25 volts so that's voltage on the secondary and then if you scroll up you'll see that the input voltage in the primary side is 230 and now you need to solve that for x so just
• 221:30 - 222:00 do 230 divided by 25 times it by 100 and you'll have 920 turns b explain how transformer works and your answer should include the reasons for using two coils the iron core and alternating supply don't stress too much if you're like i don't know how to crowbar those things into my answer just write what you would write normally and you'll find that they'll just fit in so first of all say that transform either steps up or steps down the voltage say that the current in the primary coil produces the magnetic field for the
• 222:00 - 222:30 third month say that this current is changing which causes a changing magnetic field in the core you need to say that the core strengthens the magnetic field then state that the field lines interact with the secondary coil and that this induces a voltage in the secondary coil um if you think i said that quite fast just rewind this video and listen again but you do need to learn all the steps it's a nightmare and i hate magnets too you're not alone we start by looking back in history and
• 222:30 - 223:00 look at the original structure of the atom so we can actually understand all our findings that we know to be true today so originally there was the thompson plum pudding model now you don't need to know too much about this but just know that a plum pudding this was in the 1800s by the way so you can imagine a christmas pudding if you don't know what a plum pudding is i don't know what one is so a big sphere of sponge embedded with different types of fruit and in the case of the plumpeting those were plumbed and thompson stated that
• 223:00 - 223:30 the sponge was made out of positive charge and that those plums embedded within that sphere of sponge were the electrons now we know that this is false because we now know the modern day structure of the atom with its nucleus and its shells with the electrons circling and we're going to talk about the gold foil experiment to help us understand why this new atom became the accepted model so rutherford fired alpha particles and we'll talk about particles soon at gold foil now he found that most of them passed straight through and this was strange
• 223:30 - 224:00 because really if the atom was structured like a plum pudding there's no way these alpha particles should have passed straight through but because they passed straight through it told him that the atom is largely empty space which we know to be true some alpha particles were deflected and because an alpha particle is positive it told him that they had hit something also positive and that they had been repelled and that made him understand that the nucleus was positively charged which we know to be true because that's where the protons are found
• 224:00 - 224:30 lastly very few of the alpha particles were deflected in this way and this told him that the nucleus was very small so do link together what he did with what findings he found out and the conclusions he drew from those findings and that will help you score really highly so we now have our structure of the atom we know that it has a nucleus containing protons and neutrons remember this is also known as the nucleon number and that's just the name given to all the particles found within the nucleus and surrounding the nucleus are the
• 224:30 - 225:00 shells of electrons where the electrons orbit so just to remind ourselves that the mass of a neutron and a proton is one a mass of an electron is very small so one divided by 2000 or 1800 depends what your teacher's taught you and that the neutrons because they're neutral have no charge protons have a positive one charge and electrons have a negative one charge now looking at the periodic table just remember that the atomic number is the number of protons and the mass number is the number of
• 225:00 - 225:30 protons plus the number of neutrons and this will become really important when we now come to look at isotopes you've probably met isotopes in chemistry so things like carbon 12 carbon 14 remember that these are atoms of the same element with the same number of protons but different number of neutrons if we look at carbon 12 and carbon 14 in the periodic table they both have an atomic number of six which makes sense because they're the same element which means that they must have the same atomic number their mass numbers are
• 225:30 - 226:00 different though carbon 14 has two extra neutrons when compared with carbon 12. so when we look at radioactive isotopes we're just talking about isotopes which are unstable and tend to give off radiation so nuclear fission let's be very specific with our definition remember that it is the splitting of an atomic nuclei do say atomic nuclear if you don't say that you don't get the mark so where is this carried out it's carried out artificially within a nuclear reactor which means we need to add a
• 226:00 - 226:30 fuel and we tend to use uranium-235 now we're going to be moving on to looking at half-life and background radiation so let's first of all state what is background radiation and it is radiation which is always present in our surroundings and so you may need to list several sources so that could be cosmic rays from space it could be radioactive rocks like granite found in cornwall could be food and drink and medical
• 226:30 - 227:00 sources such as x-rays now what is the unit for measuring radiation it is the becquerel capital b little q and what instruments do we use to measure the level of radiation whether that's background or not we use the geiger miller detector or counter so ionizing radiation may be given off an atom and this is a random process and it may give off alpha beta or gamma radiation and that's what we need to talk about now so i'm going to start by chatting with you what alpha beta and gamma radiation is and then i'll show you some decay equations so alpha
• 227:00 - 227:30 radiation first of all so what happens when alpha radiation happens is that two protons and two neutrons are lost from a particular atom and clearly therefore you will have a new element whose atomic number remember that's the proton number is two fewer than it was before and the mass number because it's lost two protons and two neutrons will have gone down by four with beta radiation what's happened this time is that a neutron has turned into a
• 227:30 - 228:00 proton and stayed within the nucleus of that particular atom so because the neutron proton has the same charge clearly the mass number will be unchanged but the proton number will have gone up by one so you will have a new element now gamma radiation is very different because it's an electromagnetic wave so you see no change in mass or atomic number do notice that alpha radiation is the largest it has the largest mass because it is made up of two protons and two neutrons
• 228:00 - 228:30 and it is the most ionizing whereas gamma is the least ionizing and ionizing is just the ability to cause something else to become an ion which is a charged particle so if we compare the properties of alpha beta and gamma notice that alpha as i've already said is the most ionizing b2 is in the middle gamma is the least ionizing in terms of their penetrating powers that's how easily they pass through substances you notice that alpha is stopped by air bc is stopped by aluminium foil and
• 228:30 - 229:00 gamma is stopped by several centimeters of lead or several meters of concrete in terms of their range in air alpha has a range of only about five to ten centimeters beta has a few meters range whereas gamma has an indefinite infinite range in air so gamma rays are very different from alpha and beta because gamma remember is an electromagnetic wave so it doesn't have any mass at all so no protons and no neutrons
• 229:00 - 229:30 now it's emitted after an alpha beta particle has been emitted and that's to the use of what's called the geiger muller counter or tube so what you need is a method for actually stopping other rays obstructing and getting in the way so you use paper because we know that alpha is not very
• 229:30 - 230:00 penetrating so paper is used to stop alpha particles aluminium foil remember stops beta particles and the detection actually occurs within the geiger molar tube because the radiation ionizes gas held within that tube so the gigamolar tube detects current
• 230:00 - 230:30 as the radiation in question ionizes gas held within the tube we use a slightly different method for detecting alpha particles this time you're going to use a cloud chamber and remember that contains cold alcohol vapor in air and what you see is that alpha particles moving through that cloud make visible trails of condensed alcohol
• 230:30 - 231:00 do notice though that as with the beta and the gamma you can actually detect them using a geiger muller tube also which is probably an easier experiment to describe a tiny point now which is what if they ask you what the nature of radioactive emission is so how is it characterized well that is that it happens spontaneously so it could happen at any time
• 231:00 - 231:30 and also randomly you can't predict the decay that will occur or the direction in which it will occur and lastly it's unaffected by factors which which would usually alter the rate of reaction so it's unaffected by changes in temperature and pressure the table shows the nature of our from beta particles so alpha we know is a helium nucleus made up of two protons and two neutrons and beta is a fast
• 231:30 - 232:00 moving electron explain why alpha particles and beta particles have different penetrating powers so basically let's start by looking at the difference so state first of all that alphas are much larger because they're made up of two protons and two neutrons so they have a heavier mass they have a higher charge this means that they cause more ionization which means they can't penetrate as far and that's due to all the energy which has been lost as they've caused lots of ionization and also point out that the alpha
• 232:00 - 232:30 particles are more likely to collide with the atoms because they are bigger or you could give the converse argument for beta so you could say they're smaller they have less charge they cause less ionization they have a lower mass so in this first alpha decay equation let's have a look we need to work out the new mass number and the new atomic number of thorium so we're starting with uranium-238 that is its mass number well i've just told you that you lose two protons and two neutrons in alpha decay which means the
• 232:30 - 233:00 mass number must have therefore decreased by four which is why its new mass number is two three four because you've lost two protons it means that the atomic number decreases by two to ninety and that is your answer in the second example i'm after the new mass number of lead so we've lost two protons two neutrons again which is why the mass number will have decreased by four to become two one two and this time we're looking for the original atomic number or proton number of polonium
• 233:00 - 233:30 so we know that we've lost two protons to get to 82 which means the original atomic number must have been two more than 82 so the answer here is 84. in this beta equation we're looking at what happens to sodium so i've already told you that in beta decay a neutron turns into a proton and stays within the nucleus of an atom because of that the mass number is unchanged so we're going to stay having a mass number of 22. the neutron turns into a proton and stays
• 233:30 - 234:00 within the nucleus and because of that we have an increase of 1 on the atomic number so that turns into 12 and because we've increased our atomic number we clearly have a new element and if you look in the periodic table you'll see that the element with the atomic number 12 is magnesium so in this question we're looking for the new mass number the new proton number again so we have a neutron turning into a proton meaning that the mass number is unchanged which is why i'm going to write a 14 here because we've gained an extra proton it means
• 234:00 - 234:30 the atomic number will have gone up by one forming seven and because you have a new atomic number we have a new element and that element according to the periodic table is nitrogen so half-life now let's define it first of all remember this is the time taken for half the radioactive nuclei to decay and you might have used coin tossing at school to help you model this the reason why that works really well is because radioactive decay is a random process and as is tossing the coin and with
• 234:30 - 235:00 tossing coins you've no idea you can't predict whether or land on heads or tails and it's the same with radioactive decay you have no idea which of the radioactive atoms will decay first in terms of using tossing a coin as a model it does have several limitations and this is really the number of times you can do it i mean realistically probably at most about a thousand times whereas with radioactive decay you're looking at millions of atoms which need to decay and now i'm going to show you a question involving half-life and i'll talk you through the half-life calculations
• 235:00 - 235:30 because they're probably the most difficult part of this topic a sample of sodium 24 has an activity of 1400 becquerels on the access sketch a graph to show how the activity of this sample changes over the next 40 hours the half-life of sodium 24 is 15 hours so at time equals zero we're going to have an activity which is 1400 we know that the half-life is 15 hours so look along 15 hours half of 1400 is 700
• 235:30 - 236:00 so we need across there when another 15 hours has taken place so at 30 hours that 700 will have hard again so 350 so we need a mark here and now you can attempt to draw a curved line which i'm appalling up oh it's not too bad granite is a rock it contains a radioactive isotope of uranium that decays very slowly explain how scientists can use this radioactivity to find the age of a piece of granite
• 236:00 - 236:30 so first of all make a note that there is a known proportion of activity when the rocks are formed this means that you can now measure the proportion of uranium now and then you compare the activity of the original uranium value with the activity now and this can help you determine the number of half-lives which have elapsed and then you calculate the age from reference to half-life suggests why the age of a piece of granite could not be found using uranium isotope with a half-life for 15 hours
• 236:30 - 237:00 and that's obvious because the half-life is too short 15 hours has meant that the decay has occurred far too quickly and so the activity now would be far too small to measure um 25 has a half-life of one minute what fraction of it remains after three minutes so what you have to do here is work out how many half-lifes have occurred and because it's three minutes and one goes into three minutes three times three half lives have occurred and then all you have to do because you're finding out a fraction is do a
• 237:00 - 237:30 half times a half times a half to find that 1 8 remains after three minutes question three zenon 133 is a radioactive gas used for diagnosing lung problems in 15 days its activity falls to one eighth of its original value what is its half-life so we need to work out how many half-lives occur to get to one-eighth so what you need to do is you might have to do trial and error but you need to just do as many half-lives as you need to in order to get to one
• 237:30 - 238:00 eighth and the answer is actually going to be three of them so three half life's occurred in 15 days so how long did it take for one half life to occur well you just need to do 15 divided by three and the answer is five so five days is it's half life question four the half-life of the radioactive isotope sodium 24 is 15 hours a sample has account rate of 240 counts per minute its count rate 60 hours later will be okay so this time again we need to find out how many half-lives have occurred so
• 238:00 - 238:30 i'm going to do 60 hours divided by 15 and i see that four half-lifes have occurred so therefore after one half-life 120 counts would remain but there were four of them so i'm going to times it by a half four times and that is the same as 240 times answer and that is 15 as my answer so it's
• 238:30 - 239:00 count rate after four half lives have occurred so i've divided 240 by two or times them by half four times and i have got 15 as my answer so that's 15 counts per minute question five a radioactive isotope of silver has a half-life of 20 minutes a sample gives a rate of 6400 counts per second at nine o'clock at what time will the count rate be about 200 counts per second okay this sounds hard but again just use trial and error to work out how many half-lives have occurred so i'm just gonna do
• 239:00 - 239:30 six thousand four hundred times a half and i've got three thousand two hundred so i'm gonna times it by half again and i'm gonna keep going until i get to two hundred and actually what i found here is that it has taken five half-lives in order to get to 200 counts so all i did was times six thousand four hundred by a half five times and i got to two hundred so like i said that means
• 239:30 - 240:00 five half lives have occurred and each half life took 20 minutes so therefore five half-lives takes a hundred minutes so all you need to do now is work out what 100 minutes past 9 o'clock is and remember those 60 minutes an hour so that brings us up to 10 40. that one was quite hard but you can work it out like i said just use trial and error to work out how many half-lives have occurred practice doing as many questions as these as possible in order to get good at answering the different types of question
• 240:00 - 240:30 remember when you're reading off graphs to look at you can read off the half-life by looking at how long it took for the counts to reduce by half and then you need to read across to see how long that took take into account background radiation because you'll see that the graph line will never actually touch the x-axis and that's because background radiation exists and remember but that comes from things like cosmic rays x-rays rocks just general stuff really and remember half-life is actually the amount of time
• 240:30 - 241:00 taken for half the radioactive nuclei to decay so after 42 days the activity of a sample of phosphorus 32 has decreased from 400 becquerels to 50 baccarats what is the half-life of phosphorus 32 so we've gone from having an activity of 400 becquerels to 50 so we need to work out how many half-lives took place so there's the first one which would have taken us down to 200 becquerels the second one would have taken us down to 100 becquerels
• 241:00 - 241:30 and then the third one would have taken us to 50 baccarats so we can see here that three half lifes have taken place and what was our time frame well three half lifes must have occurred in 42 days so then it's a simple expedient of doing 42 divided by 3 to work out how long each half-life took and that answer is therefore 14 days looking at uses of radiation alpha radiation is used in smoke alarms
• 241:30 - 242:00 i'm going to chat to you how this works so you have an alpha source in a smoke alarm which is giving off alpha particles and what happens is those upper particles collide with air particles in the air and they ionize them creating a small electric current which is picked up by the detector now i've already told you that alpha's ranginer is very limited so the moment there's a fire there's now smoke particles in the air and they obstruct the alpha and they stop it reaching the detector so the
• 242:00 - 242:30 detector gets zero reading and therefore the alarm goes off beta radiation is used in aluminium foil thickness a kitchen floor that you rapture sandwiches in so there were two rollers and there's a detector either side so you've got a source of beta detector on the other side and there's a certain amount that should be picked up and if that detector reading is too low it tells you that the aluminium foil is too thick so the rollers press together to make the foil thinner and so the detector can pick up the correct amount
• 242:30 - 243:00 in terms of in use in medicine do you remember that we use radioactive sources in medicine such as radioactive iodine and we call this a medical tracer and that's because it's used to diagnose things like kidney problems so the patient takes a sample of radioactive iodine it flows through their body and there's a detector which picks up how much radiation there is and you'll see a characteristic graph reading where the reading stays high and that tells you that there's a blockage within the kidney so they do make great diagnostic
• 243:00 - 243:30 tools do notice some crucial properties though and that is that you've got to have a an isotope which decays into a stable product which makes sense because you don't want it giving off radiation it has a medium-sized half-life you don't want it to have such a short half-life that you can't actually pick up how much there is because it's already decayed away but you also don't want it to be so long that you're staying radioactive for many years to come because that is dangerous and this is because of all the dangers relating to radiation so remember ionizing radiation causes mutation within our
• 243:30 - 244:00 cells and mutation is the first step leading towards cancer so it's pretty nasty stuff we can reduce our risk and limit our exposure to radiation by wearing protective clothing by standing behind lead shields by using tongs to handle the radioactive material and for using things like photographic film you see these in little medical badges that radiographers wear and that will show up if they've been exposed to too much radiation looking at use of radioactive carbon so carbon 14 in
• 244:00 - 244:30 carbon datings that's when you determine how old plant material is for example so carbon-14 is radioactive and during a plant's lifetime remember when it photosynthesizes it takes in carbon dioxide so a certain proportion of the plant will contain carbon 14. now once it dies clearly it won't be photosynthesizing anymore and over time that radioactivity decreases and by comparing the radioactivity of a sample with the radioactivity of a living version of that particular plant you can
• 244:30 - 245:00 work out how old it is that's a very clever dating technique now the first thing to point out is that the earth rotates on its axis and that rotation takes 24 hours and that therefore explains why one day which is 24 hours is defined as the time taken for the earth to rotate on its axis once now if you check out the sun over here obviously the side of the earth facing
• 245:00 - 245:30 the sun will be in its daytime and then the other side of the earth will be in its night time and then as the earth rotates then that other side will come back round and be facing the sun and therefore experience daytime whilst the other side experiences night time now one thing i do need you to be really aware of is that the earth is on a tilt can you see here that it sits at an angle it's not sitting directly upwards and this 23.5 degree tilt is incredibly important
• 245:30 - 246:00 and this is responsible for the seasons now if you live in a more equatorial country the earthquake will have little or no effect on you but if you live in the north or south hemisphere this will have a huge effect because as you see here the northern hemisphere is tilted towards the sun and that means that it will be summer the southern hemisphere is tilted away from the sun and that means it will be winter because it will receive less of the sun's energy as the earth continues
• 246:00 - 246:30 its motion around the sun then the seasons change through spring and autumn so when the northern hemisphere is tilted towards the sun you have summer and when the southern hemisphere is tilted away from the sun you get winter when that earth rotates so effectively it almost appears like the sun is on this side then you can see that you'd
• 246:30 - 247:00 have the winter in the north and summer in the south we've already mentioned that a day is 24 hours and that's to do with how long it takes for the earth to rotate on its axis but what about a year what is a year well a year is the time taken for the earth to orbit the sun once and that takes approximately 365 days now we're discussing what a month is now this is the time taken for a moon to orbit a planet
• 247:00 - 247:30 and for the moon of the earth that takes around 31 days remember lots of other planets especially ones like jupiter have lots of different moons and they'll have different month lengths and that's because the differences in how long it takes for that moon to orbit those larger planets so what are we referring to when we look at the phases of the moon well the first thing you need to be aware of is that planet earth is rotating on its axis so remember the time taken for a single rotation on the earth's axis equals a day
• 247:30 - 248:00 and then orbiting the earth is the moon and so obviously when we're stood here we'll see this side of the moon when we're stood here we'll see this side of the moon what makes it even more complicated however is that the earth itself is orbiting the sun it's really important that you remember here that neither the moon or the earth give off light the sun gives off light and then objects reflect that light and that's how we see them
• 248:00 - 248:30 so when we see the moon it's because light from the sun is reflecting off of it into our eyes and so fundamentally what all of this means is that at any one particular point and it just totally depends on where we're standing on the earth's surface you're going to see a different amount of the moon and that's what the phases of the moon is all about and i'm going to talk to you as to why sometimes you don't see any moon at all and that's what's called a new moon sometimes when you look out into the night sky you see the whole moon and we call that a full moon and then at other times of the
• 248:30 - 249:00 month you'll see varying other amounts of the moon and that's why this is called the phases of the moon so if we start in position one over here and pretend that the sun is here and so those light rays are coming across here and so they're just hitting this side of the moon and then being bounced straight back out into space and that means that none of the rays reflecting of the moon are reaching us on earth and so therefore we can't see it it's basically invisible to us and we call that a new moon sun's
• 249:00 - 249:30 rays reflected away from earth so i've drawn that moon there sorry it's not very circular but that's basically trying to tell you that you wouldn't be able to see it so now let's add an arrow just to tell you that as we move around the cycle in an anti-clockwise direction then the moon is becoming more visible and we'll explain why now so now we're going to look at position two the waxing crescent now because we're looking at how the
• 249:30 - 250:00 earth and moon move relative to the sun it almost appears that the sun stays in one position in the sky so if we look at position two now because of the change in angle it means that some of those sun's rays which are reaching the moon are being reflected in such a way that they can be seen from the earth and as that moon comes more into show we therefore call it a waxing crescent so more of the sun's rays are reflected off the moon towards the earth so moving to position three you can see
• 250:00 - 250:30 that even more of the light is being reflected towards the earth we have what's known as a first quarter moon the waxing gibbous basically means you can see three quarters of the moon's surface we say waxing because that's when the moon's tending to show us more of its surface waning means that it's fading away now the full moon is the most exciting because imagine all those sun's rays hitting the moon's surface and then being reflected straight back hence you
• 250:30 - 251:00 can see the moon in its full entirety in its circular shape obviously you still won't be able to see the back of the moon but before math it will appear in the sky as if it's a circle and then at position six seven and eight you're seeing the opposite of what was taking place in two three and four so you're seeing less of the moon's surface each time and that's why we described this part of the moon as the waning gibbous and the waning crescent so let's just highlight a few things the new moon
• 251:00 - 251:30 is when we can't see anything at all as i said we're moving around the faces of the moon in an anti-clockwise direction so as more of the moon surface is exposed to us we say that it is waxing the waxing crescent followed by the waxing give us when you see the moon as a full circle we say that a full moon has been reached and then as it begins to fade away we say that we have a waning gibbous moving into the third quarter followed by the waning crescent before the whole process starts again now we need to look at orbital speeds
• 251:30 - 252:00 and this is given by the equation b equals 2 pi r over t so v equals orbital speed r is the radius of the orbit and t is the orbital period otherwise known as the time taken to complete one orbit now a note on units because they're going to be quite annoying about this they're probably going to give you certain units in the question and then ask for different ones in the answer so you do need to be comfortable converting between hours minutes seconds days kilometers
• 252:00 - 252:30 meters so a quick reminder for you to go from meters to kilometers divide by a thousand kilometers to meters you're going to times by a thousand seconds into minutes divide by 60 into hours divide by 60 into days divide by 24 and then obviously use the reverse to go from days to hours
• 252:30 - 253:00 to minutes to seconds you want to times by 24 times by 60 times by 60. and now we're ready to actually answer a question which is going to be the best way of showing you how to answer these questions so the earth orbits the sun once in 365 days the radius of the earth's orbit is 150 million kilometers calculate the orbital speed of the earth in kilometers per hour so let's start by writing out the equation we need b equals 2 pi r
• 253:00 - 253:30 over t hopefully you recognize the use of 2 pi r for maths when you're doing your circle calculations so we need to do 2 pi times the radius and which you know is 150 million kilometers luckily our final answer is doing kilometers so we keep that number as it is then in terms of time we've got 365 days we need to convert those into hours so you need to multiply by 24 and your final answer looks like this
• 253:30 - 254:00 and we're going to give that answer to three significant figures we now need to talk about the solar system remember that the solar system consists of a single star known as the sun and has several planets orbiting it you need to know the order of these planets so the planet closest to the sun is mercury followed by venus earth mars jupiter saturn uranus and neptune
• 254:00 - 254:30 and then pluto is a dwarf planet it's too small to be counted as a major planet in terms of remembering the order of the planets use the mnemonic my very easy method just speeds up naming and this is useful because my stands for mercury very stands for venus easy as earth method is mars just as jupiter speeds is saturn
• 254:30 - 255:00 up is uranus and naming is neptune we'll just add an additional label here pointing out that pluto is a dwarf planet in addition to the planets notice we have an asteroid belt that sits between mars and jupiter and asteroids are lumps of rock which also orbit the sun other celestial objects include comets you can see these here now comets are made out of dust and ice they have very elliptical orbits and that's really
• 255:00 - 255:30 important that word elliptical it means that their orbits are oval shaped as opposed to circular and that's actually really important because notice that all the planets have this elliptical orbit it's a common misconception that people think that the orbit of planets is circular the other thing to be aware of with comets is that their tails always point away from the sun and in addition they travel fastest when they're closest to the sun and that's due to the increased gravitational field strength
• 255:30 - 256:00 going back to the fact that there are elliptical orbits one thing to notice that the sun only sits at the center of a planet's orbits if the planet's orbit is approximately circular the more elliptical the planets orbit the less that the sun will sit in the center of its orbit now you'll all be aware that the earth has its own moon and the moon orbits the earth and that single orbit takes approximately 30 days or 31 days
• 256:00 - 256:30 which is the length of a month because the moon orbits the earth it is a satellite and so we can now define a satellite satellites are objects which orbit other celestial objects and so the easiest way to say that is of objects which orbit the sun and planets you have both natural and artificial satellites now natural satellites are moons artificial ones are ones which we place into space
• 256:30 - 257:00 we now need to talk about the accretion model and in order to do that we're just going to consider the solar system again and the eight planets which orbit the sun now looking more closely at this diagram you can see that the planets are two different types the four inner planets those closest to the sun mercury venus earth and mars well they're much smaller and they're actually composed mainly of metals and rocks the four outer planets jupiter saturn uranus and neptune are much larger and composed mostly of gases
• 257:00 - 257:30 and we're now going to understand through use of the accretion model why this is the case we'll start by making a few notes so we've just made a quick note of those two things in terms of the inner planets being much smaller being rocky and made of metals the outer planets being much larger and composed of gases but to understand how the planets were formed we need to first look at how the sun was
• 257:30 - 258:00 born and this was over 5 billion years ago a nebula which is a giant cloud of dust and gases which were mostly hydrogen and helium with a small percentage of heavy atoms began to contract and collapse in on itself so five billion years ago a nebula began to collapse in on itself and a nebula is a giant cloud of dust and gas so when that nebula began to collapse in on itself and start to contract the
• 258:00 - 258:30 atoms contained within it collided together generating heat and eventually they reached a temperature where nuclear fusion could take place and our sun came into being so the sun was born let's remind ourselves quickly about nuclear fusion remember this is when two hydrogen nucleo join and the reason
• 258:30 - 259:00 why you need so much heat in order to do that is because remember those nuclei are positively charged and therefore they have a propensity to repel now the material in the nebula that was not absorbed into the sun swirled around in a flat disk of dust and gas held in orbit by the sun's gravity and this is known as an accretion disk so the material in the nebula are not
• 259:00 - 259:30 absorbed by the sun swirled around in a flat disk of dust and gas known as an accretion disk now each planet began as microscopic grains of dust in the accretion disk then the atoms and molecules began to stick together or accrete into large particles and then by gentle collisions some grains built up into balls and then into objects a mile in diameter called planetesimals
• 259:30 - 260:00 and these objects were large enough to attract other objects by gravity rather than by chance whilst the planets were forming the sun sent out energy and particles in a steady stream known as stellar winds and these winds proved so strong that they blew off most of the gases of the four planets closest to the sun leaving them smaller with only their rocks and metals intact hence the smaller rocky planets
• 260:00 - 260:30 were formed now the four outer planets were so far from the sun that the stellar winds could not blow away their ice and gases and therefore they remained gaseous with only a small rocky core so the sun's stellar winds could not blow off the gases of the four planets furthest from the sun hence jupiter saturn uranus and neptune are gaseous
• 260:30 - 261:00 and large with a small rocky core and these gaseous planets were primarily made out of lighter elements such as hydrogen and helium to begin with as the sun's gravity had pulled the heavier elements closer to the center in the original accretion disk now we're all aware that gravity is really important it's what keeps us
• 261:00 - 261:30 standing on the earth's surface but what actually affects the strength of the gravitational field number one the mass of a planet so jupiter will have a larger gravitational field strength compared with earth and that's because it has a larger mass don't talk about it in terms of size you have to talk about it in terms of mass and secondly distance between two objects or two celestial bodies if you have an increasing distance
• 261:30 - 262:00 then you have a decreasing gravitational field strength just to point out a few extra points remember that sun is the largest object in our solar system and because of that it therefore has the largest mass and consequently largest gravitational field strength and that explains why all the planets orbit the sun now if we look more closely at this point that when the distance between two
• 262:00 - 262:30 objects increases the gravitational field strength decreases that also goes a way to explain why planets further from the sun travel at lower speeds and that's because they have a reduced gravitational field strength let's now talk more about the sun so our first fact is that the sun is in fact a medium-sized star it's really not that big if you compare
• 262:30 - 263:00 it to other stars in our universe well what's it made up of two elements and those are hydrogen and helium which remember sit at the start of the periodic table they're the elements with the lowest mass and the fewest number of subatomic particles we know that it radiates energy for what form does that energy take well most of that energy falls within the infrared visible and ultraviolet regions of the electromagnetic spectrum
• 263:00 - 263:30 but how does the sun release this energy well that's through nuclear fusion reactions looking more closely what these two words mean nuclear means relating to nuclei fusion means joining so what could we possibly be joining well it's joining two hydrogen nuclei in order to form a helium nuclei now we're going to broaden our topics to
• 263:30 - 264:00 now consider stars as a whole rather than just the sun so firstly let's consider the definition of a galaxy well the galaxy is a collection of billions of stars and a good example of a galaxy is the milky way it's the one we find ourselves in and therefore the milky way must contain our solar system and therefore our star the sun to give you an idea of scale the other stars the other billions of stars which make up our milky way are much further away from the earth than
• 264:00 - 264:30 our sun is from us and because these distances are so huge we can no longer talk in terms of thousands of miles or thousands of kilometers we need to use a new unit known as the light year so contrary to what you might think a light year is not a unit of time it's a unit of distance and one light year is the distance traveled by light in one year through space
• 264:30 - 265:00 and what is exactly one light year well it's 9.5 times 10 to the 15 meters so a very very long way indeed now we're going to consider the life cycle of stars so first of all there are two types of star we need to consider small stars like our sun and large stars now what both types of star are formed from is pretty much the same and that is
• 265:00 - 265:30 interstellar clouds of gas and dust which contain hydrogen so we've said what a star is but how does a star come into being well the first step is the formation of a protostar now a protostar is an interstellar cloud collapsing and increasing in temperature as a result of its internal gravitational attraction
• 265:30 - 266:00 so you can think of a protostar as almost a child in its infancy and then how does that proto star become the adult star the stable star and that's really the situation we find our sun in at the moment the crucial thing to notice with a stable star is that the forces are balanced here so the inward force of gravitational attraction equals the outward force due to the high temperature in the center of the star
• 266:00 - 266:30 now we're going to consider the life cycles of both a small and large star so we've already said that they both begin life as a protostar then they enter the adult portion of their lives which we can call a stable star its proper name really is the main sequence and as we've already said the forces are balanced and that's very important in terms of
• 266:30 - 267:00 releasing energy remember that the stars carry out nuclear fusion with hydrogen so everything's the same at the moment at one particular point in time that hydrogen fuel runs out the forces become unbalanced and the situation of small stars that it expands to form a red giant in the case of a large star it forms a red supergiant so the forces are unbalanced
• 267:00 - 267:30 and at this point the star's using helium as a fuel source going back to our small star after a red giant a planetary nebula is formed that has a white dwarf at its center whereas a red supergiant explodes in what's called a supernova and in a supernova heavier elements are formed it's the ones which are heavier than
• 267:30 - 268:00 iron and then finally to finish off our white dwarf eventually turns into a black dwarf with a small star whereas our supernova can form either a neutron star which as the name suggests is made up of neutrons or if the star's massive enough it forms a terrifying black hole where nothing can escape not even light so i've just zoomed out so you can see a real comparison so in both situations we know we're starting out with a protostar now when that small star enters its main sequence the adult part of its life when
• 268:00 - 268:30 it's stable remember that the forces are balanced and we're carrying out nuclear fusion of hydrogen to release energy with the large starts a similar situation the star enters its main sequence again the forces are balanced and we have nuclear fusion of hydrogen now that hydrogen fuel will eventually run out notice it runs out much more quickly in the large star situation now when the hydrogen fuel runs out in a small star the forces become unbalanced and the star expands to form a red giant in the case of a large star it forms a
• 268:30 - 269:00 red supergiant and then the next step for the small star is that a planetary nebula is formed consisting of a white dwarf in its center which eventually turns into a black dwarf excitingly the situation is different with a large star after the red supergiant you get a massive explosion known as a supernova where heavier elements are formed and then the final step in its life cycle is it becomes either a neutron star or a black hole now we move on to the topic of the universe we've already mentioned the
• 269:00 - 269:30 term the milky way remember that's the galaxy we find ourselves in but the milky way is just one of the many billions of galaxies which make up our universe so in terms of the scale of what we're talking about it's unimaginable this is huge now here's an image of the milky way one thing you do need to know is the diameter the distance from one side to the other and remember we use light years in order to work out this distance because meters kilometers and miles are just ridiculous they're far too small
• 269:30 - 270:00 and the diameter of the milky way is a hundred thousand light years because remember a light year is the distance traveled by light through space in one year now we've talked a lot about various different things to do with space the solar system the milky way galaxies etcetera what about the start of time how did the universe come into being and the most widely accepted theory is the big bang theory now the big bang theory states that the universe began at a single point in a huge explosion and that time and mata were created at this
• 270:00 - 270:30 point what is our evidence supporting the big bang theory number one redshift and number two cmbr which stands for cosmic microwave background radiation now in order to understand how redshift supports the big bang theory you also need to know that this theory therefore states that the universe is expanding everything started out in one particular point and it becomes really useful if we look at this diagram here so here we are
• 270:30 - 271:00 on planet earth and we can see light from distant galaxies now if the stars in those galaxies remained in one place i.e the universe wasn't expanding they weren't moving away from us then the light given off by those stars would remain the same in terms of wavelength and as you can see here the wavelength on this side is the same as here and so therefore the light given off will remain the particular color that it is looking at the second model here
• 271:00 - 271:30 this is saying that those galaxies those stars are moving away from us i.e the universe is expanding now what you would find in that situation if you compare with the light in the first example is the wavelength has increased and then if you look at all the different colors on the visible spectrum remember for me that the colors that sit at the red end of the spectrum have a much longer wavelength than the colors that sit at the violet end of the spectrum so we can therefore say that light given off those distant galaxies
• 271:30 - 272:00 is red shifted it has a longer wavelength a third option is maybe the inverse is becoming smaller it's shrinking and that means that those galaxies and those stars would be moving closer to us so what would that mean in terms of wavelength of light well it would get squashed it would become shorter which would mean that that light would appear more blue to us and therefore be blue shifted however we know that that light is red-shifted and we're going to write an explanation for that now
• 272:00 - 272:30 so let's first of all state that redshift provides evidence that the universe is expanding and therefore that everything started out in one particular point so it therefore provides evidence for the big bang theory so we're going to write that because the universe is expanding it therefore means that galaxies are moving away from us and therefore the light given off by those galaxies has a longer wavelength and as such has been red-shifted because remember that the red light has a longer
• 272:30 - 273:00 wavelength than the violet light so that's our first piece of evidence for the big bang theory red shift the second piece of evidence we need to contemplate is cmbr which i've already mentioned cosmic microwave background radiation now cmbr is microwave radiation of a specific frequency which is observed at all points in space around us and how was cmbr produced well it was produced shortly after the universe was formed and then that radiation has
• 273:00 - 273:30 expanded and therefore been distributed evenly across the whole of the universe so again why does cmbr provide evidence for the big bang theory because we think that that cmbr was produced at one point in time at the big bang and then as the universe expanded it got spread out and evenly distributed
• 273:30 - 274:00 throughout space and that's what you can see from this picture here that says looking at the cmbr and our scientists were wondering where did it come from how did it become so evenly distributed and that's because we believe that the universe is expanding from a single point now let's talk a little bit more about the fact that a the universe is expanding and that b galaxies are moving away from us as a result of that now we can work out the speed at which the
• 274:00 - 274:30 galaxy is moving away from us by looking at the change in wavelength of the galaxy starlight due to red shift next up we can know the distance that that galaxy is away from us but that can only be determined using the brightness of a supernova in that galaxy and remember a supernova is a massive explosion of a very large star now we're going to allocate v for the
• 274:30 - 275:00 speed and d for the distance and then together we can calculate the hubble constant h o as being the speed divided by distance so if we were to define the hubble constant we'd say it was the ratio of the speed at which the galaxy is moving away from earth to its distance from the earth and you need to know what h0 is currently thought to be and it is just an estimate and that hubble constant is
• 275:00 - 275:30 thought to be 2.2 times 10 to the minus 18 per second and then lastly if we take this equation now which is distance divided by speed equals one over the hubble constant this is actually used to provide an estimate for the age of the universe and this is actually evidence for the idea that all matter in the universe was present at a single point so it provides further evidence for the big bang so we're done well done for staying all
• 275:30 - 276:00 this way i'm really impressed if you manage to watch the video all in one don't forget about my revision guides my perfect answer revision guides are available on my website right now and you can click on this card to buy yourself a copy of my revision guide which makes the perfect accompaniment to these videos [Music]
• 276:00 - 276:30 you