Yesterday we revised a few of these equations. So I'm just going to check that you remember what these equations were. So this one, do you remember where we would use this equation? The equation. So current is charged per unit time. So basically, if you know the charge and the time, you can work out the current flowing I equals nave e. Do you remember what this equation is used for? What does v stand for? We standard for the drift velocity. Good. So this is the drift velocity equation. Do you remember what little n stands for? The number of charge flowing? Yes, good. Per meter cubed. And energy is current times, voltage times, time. So if we think of this equation and if we get a current against time graph, what does the area under the graph, what quantity does the area under the graph give us? Jackson? The area. The area is at the T I I so if we multiply I times t, what quantity do we get looking at this equation? Items, T Q, huge charge. So these are things that come up. And then we have our ohms law equation. The voltage is directly proportional to the current, and as long as the temperature stays the same, so that gives us resistance. And then this equation. What is row rwhat? Is this funny symbol used for? It's something like resistance, but it's it sounds like resistance, but it's not resistance. So resistiity resistivity, excellent, good row. So it's the resistance times the area over the length. And then today we're going to talk more about power potential difference. And we did do a bit about internal resistance, but today we're going to look at potential difference, voltage power. Now are you doing any preparation at home for your a level questions? Are you doing any past paper questions? Well, I haven't done this. I haven't done this in this two days. But Yeah, I have example questions. Good. Excellent. So I mean, we can go over the theory. It's very important you see how these are examined, how what the questions look like for each of these topics. So. Potential difference electromotor force and internal resistance. The difference between electromotor force and potential difference. We know that if you have a nine volt power supply. Nine volts electromotor force. That means that it's the force that pushes the electrons around the circuit. But very often when the current is actually flowing, this may drop to about less so 8.6 volts or something. So this would suggest that there is a voltage drop due to the internal resistance of our power supply. And this is because the electrons jostle around while they're trying to exit the power supply. So a voltmeter there just across the internal resistance should reach 0.4 volts. So this means that in reality, the energy available to do work in the circuit is only 8.4, 8.6 volts. So we can only get 8.6 volts. Across our external resistor in a closed circuit. So there's lost boats. The emf might be nine volts, but 0.4 volts is lost due to heating in the power supply. So this equation internal, this equation, emf is equal to the current times, the external resistance, plus the current times, the internal resistance, which is the same as v vr plus v big little R. So essentially, it's due to the heating that happens inside your power supply. And we can do an experiment. To measure the internal resistance of your power supply, you have what is this symbol in this circus? If you wanted to measure the internal resistance of our power supply, what do we call this component? What does it change? It changes something in the circuit. Change. That is because of the variable resistance, variable resistor. Good. So this is a variable resistor and the current and the voltage. So if we have our equation, emf is equal to ir plus I litr. So if we change ir to v. And rearrange this to make V R Y. This is our m, this is our x plus C This is our C Y equals mx plus c. So we have we can take vthe current, measure the voltage, the current and using. Our y equals mx plus c. We can say v is equal to minus R times I. Plus speak. So the gradient on our graph would look like this, and this would be your emf. The y intercept is your emf of your power supply. Yeah and the gradient is little R, the internal resistance. So you can see here. The gradient is the internal resistance, so you plot a graph and put voltage on the y axes, current on the x axes, and the gradient is your internal resistance. So. So potential divider circuits. You can add different resistors in a circuit to change the resistance. So sometimes we can we prefer to use a circuit like this, a potential divider, to change the resistance across a component. So this variable resistor, which we've just been looking at, actually looks like this in reality, when you slide this contact. You're making the electrons travel different numbers of coils to get to the to exit. So if you want low resistance, this would go in this direction. So low resistance. And if you want high resistance youmake, the electrons travel through many of these coils. So this is a sliding contact and it's what, a variable resistor. So it changes the length of the wire that the electrons have to travel through. Okay. I'm not understandable that this component. Is a variable resistor. So how does this work? The electrons come in here from the power supply carrying energy and. You can add, you can slide a contact this way and this way to make the electrons travel through different lengths of wire. You might have used it in some experiments you did in the laboratory. And they're called Rio stats. Rio stats. So they're variable resistors. So if you think of it, if I make this contact here, Jackson, I'm only making the electrons travel. Through four coils of wire. So the resistance is less. But if I make the contact here, I'm making the electrons travel through many more coils of wire, so the resistance increases. So really it's a variable resistor. Variable resistor. So the variable resistor changes the length of the circuit. That's what it can look like too. So here is your contact point, and you can slide this up and down. So a potential divider circuit with all three connections used means that the voltage can be controlled if this is maybe a fan or a heater and itonly, come on, if there's a certain voltage across here, so the voltage across the lower proportion of the resistance wire will be the same as across the load, because they are in parallel. So if you, for example, want to put a motor fan here. This will come on when a certain, if the temperature gets too high, you want this to switch on, and you can activate it by having a certain voltage going across the, this part of your potential divider. A potential divider will give a voltage from zero to the maximum voltage supply. It's only useful when a small current has been drawn from it. Compared with the current passing through the resistor itself, it is better placed with a simple variable resistor circuit when a large current is needed. So basically, if you have a twelve volt power supply, and you know that after once five volts is across this section of the circus, so that means seven volts must be across there, then the motor will switch on and turn on your fan. So we have an equation for potential divider circuits. One half. This so this can be resistor one and this can be resistor two. So if we divide the voltage by having that is giving V1 and that is giving V2. So those two should add to the total voltage here. So that's twelve volts. So if this is midway, if R one equals R2, these should both be six volts along here. There's the same current flowing through both resistors. The total sum of the voltage is equal to V1 plus V2. And we can use the. The supply voltage is equal to R one over R one. Plus R2 times v in. Be out. It should be. V out is our one plus R2 times v in. So if you know the size of these two resistors, you can. Work out the total voltage across here compared to this is the voltage in this is the v out. All you're doing is changing the potential difference by changing the size of the resistors in ratio. Replace R2 with a sensor. So this is a some sort of variable resistor. It could be a thermistor or a light dependant resistor. A light dependent resithat is I think that is thermistor. Yes, this is a thermister or a light dependent resistor. Light dependent resistor. So again, it's another way of achieving a variable output voltage in dark conditions. You could get a bright libeing switched on here. So you could have a night light been switched on by this type of circuit. If this if R2 was a light dependent resistor. So. Calculate the voltage across each circucalculate the voltage across each of the resistors in this circuit. So. Again, H and six is 14, so v out. Equals R2, so I'll make this R2. This R one. Over R one plus R2. Times v in. So the v out this is v in. So this will be equal to v out of output voltage across six. We can work out and then we can subtract what we get from this value and we can get the v out across this resistor. So R2 is six over 14. Times six. So thatbe six divided by 14 to times 62.57. So I get 2.57. Okay. So this is V2 and therefore, what's a quick way to work out V1? So we can simply take. This -2.57. Equals B1. So -63.42. So this is 3.42. Now, I meant to tell you that in your exam papers, you don't get given every single equation. There are some you need to memorize. So this one I don't think is given. But it's a very useful type of circuit. So instead of having two fixed resistors, we can have a continuous variable, a continuous wire, and the length of the wire will determine the output voltage. So the length of the wire across this will give you the output voltage. So a slide wiso, a potentiometer, this is called a potentiometer, a variable resistor. It could be used to compare the emf of the cells. The length of the resistance war in the circuit is adjusted until the sensitive amnea reads zero. So that means. The push this way will be the same as the push this way. Calculate the emf of b cell a or b. Cell A, I presume this is cell a of emf 1.5 is balanced by a length of 1.2m. Cell b is balanced by a length of 0.98m. Calculate the emf of b so. The emf of one cell and the emf of another cell give a value zero here. So we say it's balanced, which is equal to the current of a plus the current of b. I ate. My b. So. Ratio of emf's is equal to the ratio of the length. So 1.2 over point 98 is 1.5 over the emf of b. So that gives us 1.23 volts. Explain why emfare compared as there is no current in this ammy shard and then it's like an open circuit voltage. So. There's no current flowing here, so the emf's are open circuit voltage. Explain why the driver cell must stay on change, so why does this have to stay the same so current? If the driver cell was changed during the experiment, there would be a different current in the top part of the circuit. It would not be possible to compare the two cells a and b. The experiment relies on the fact that the voltage across length l is equal to the current multiplied by the resistance of the length of the wire, and then the current is constant, the voltage is then proportional to the length of our, we can make comparisons. Okay. So. So remember these papers, electricity comes in paper one, electricity and mechanics, and paper two has waves and quantum physics and materials. So. Which graph shows how current Jackson through a thermister varies with potential difference across the thermister? The Mr, when the temperature increases, when temperature increases, the resistance will decrease. So a assignis going, that will be the graph b good. Well done. Which the following is the si base unit for resistance. Assistance, assistance, remember, if is the voltage divided by the current, and current is the basic units, but the voltage that is different, so the voltage is. Energy per unit charge. So it's the energy that the charges Carry. So our equals be over on it. And if you think of it, base units, it can't be that one because ohms is not a base unit. It can't be this one because klobes is not a base unit. Yeah. It can't be this one because voltage is not a base unit. So R is equal to so this energy. Overcharge. Over current. If youlike. Energy, which is for work, is force times, distance. Hello. So that's what energy is, force times, distance. Charge. Charge is current times time, isn't it? So current is in amps, time is in seconds. This is current. So force times distance, that's MaaS times, acceleration times distance. I know I work these hours in a very muddled way, but essentially, we have MaaS times, acceleration times, distance. Over current times time. All over current. So that's MaaS, so that's kilograms. Acceleration is meters seconds to the minus two. Times, distance. Peram. Or second. Her amp again. Okay, so simplifying this. We have kilograms. Meter squared seconds to the minus three. Amps to the minus two. Okay. But as I say, the quick way to do it is to know your base units. So seconds, meters, kilograms are base units. Amps. Mose and I'm remember that Yeah, I think it's it's in temperature temperature in Kelvin, very good. Yeah. So, so so without working it all out, this would have been a good guess. Yeah, Yeah, Yeah. Okay. So this is a multiple choice question. It's about drift velocity. Yeah trivelocity and R. So I equals N A. Three. Now I, we don't see R here as we know I equals v over R, don't we? Yeah so if we rewrite this v over R. Equals, and that's capital v for voltage. This is lowercase v for drift velocity and a little v drift velocity q. Okay. So basically, we can see relationship between this and this. So one over R is proportional to drift velocity. Do you agree? Yeah. So all we had to do was push resistance instead of current. So we've got our solution there, okay? A variable resistor is connected in a circuit. As shown, the cell has internal resistance, little R. Variable resistor the resistance of the variable resistor is increased, which row of the table is correct, so we increase this resistance. What will happen to the amysia reading and the vote mia reading? Converfirstly internal resistance is unchanged, and the resistance of variable resistance is increased, so the total resistance will increases. When the No Uh Yeah total resistance will increase es, the emitter reading decreases and volmeter increases. Yes, see. Now we didn't do anything about electrical power yet. So. The heating effect of occurrent ce. So basically, we have internal resistance in our batteries, which heats up the battery. And generators give out thermal energy because they're not 100% efficient. The energy that the electrical energy is not converted into is not heat, it's internal energy. Now internal energy is when the potential energy and kinetic energy of the particles increases. So the particles in the material are getting hotter, so they vibrate more, and they stretch and get nearer and further from each other, and they have more kinetic energy. When electrons collide with the material, say, copper atoms oms, they give up kinetic energy to the lattice ions. They vibrate with greater amplitude. So when you think of it. A metal looks like this. You have the ions and then you have the free electrons that are free to move around. Energy transferred is proportional to current squared, so the current is the dominant factor here. So power is equal to I squared R. So if we think of electrical power. It's equal to current. Times voltage. Which is equal to. Q over t. Q over t times. Energy or unit charge. So power is equal to. Now we can cancel the cues here, q and q, which is equal to work done over time taken. So power, if we have the equation, power is current times. Tage or potential difference. It's the same as power. Is work done over time, okay. Now if if we combine this equation with resistance. Equals voltage divided by parent. We can say loss, folks. Is equal to resistance times current ge. Okay, I've just rearranged this. So power loss. Is equal to. Current times lost voltage. Which is equal to I times I which gives me. I squared R, so the power loss in an electrical circuit is equal to I squared R, so that means current. The size of the current determines the size of the power loss. Power is measured in Walts energies and Jews when comparing devices, power compare power rather than total energy used, because over a very long period of time, all devices will transfer transfer a lot of energy. It's better to compare 100 watt bulb with a 60 watt bulb. So what's the difference between a 60 watt bulb and a 100 watt bulb? Which converts more energy each second. Electrical energy? Yes. A 100 watt bulb or a 60 watt bulb? A one watts. Yeah. So a 100 watt bulb converts 100 jles of energy per second from electrical energy to light and heat energy. Electrical energy is measured in kilowatt hours. So electrical energy. Is power times time. And the power is measured in kilowatts, times, hours. Otherwise it would be too small if we use time as seconds and power as watts, wehave huge numbers of values. Yeah. Potential difference. Okay, we look at questions. Four. A twelve volt battery, a twelve volt car battery delivers 30 amps for 10s. How many coulobes of charge flow out of the battery? Look at the type of graph we have. Okay, how many charcharwell? Q equal to I over T Q equal to it t. 30 times ten, 300, yes. Good. How much energy does each coulum Carry? Is coulm Carry? Twelve voltes times 30 ampairs 360. 3600Yeah what is the total energy converted? Go to energy. Total energy is the power, is the, which the power times 300, 360 times 300, 108000. So. That was twelve jewels. That was 36, not north. The total energy converted, so energy is voltage times charge. So the voltage is twelve, the charge is. 300 so. We get twelve times 300 is 3600 joules. What is the power of operation of the battery? What is the power of the battery? 306, 3300, 3600Jews divided by 10s. 360 volts. Good. Yeah. 360 watts. Okay, let's try question nine. An X -ray machine operates at 90 kilovolts. Remember, an X -ray machine has electrons that hit a metal and excite electrons, and the electrons discharge x rays. Only 1% of the power supplied produces x rays, so it's only 1% efficient. The rest is wasted as heat calculate the power drawn from the supply. So we have a current of 100 milliamps and a power a voltage of 90 kilovolts. So the power drone forms the supply. So offer is at nine times ten, two os of four volts, and the current is not 0.1 ampaere. So the power drawn from the supply, but is mentioned, is that only 100, sorry, only 1% of the power supplied, produced, produces x rays. Yes, but that's not in the question yet. You're right. So that should the power should times 1% and that is the power drone for the supply. The useful power drawn from the supply is 1% of that which is 90 watts, but the power drawn from the supply is 9000 watts. 90 by ten to the three multiply by point. One gives me 9000 watts from the supply, the rate at which heat is produced. So of 1% produces x rays, the other 99% produces thermal energy. So what you work out 99% of this, so multiply this by point 99. And you get ace 91 zero bots dissipaated as heat. The heat is heso. The heat is 99% of this of the total power. Yes. Very inefficient. Yes. Yeah. Okay, so. Do you, do you know where to get past paper questions? What book? Where are you getting your questions from? Jackson to practice? Well, I guess three exam papers from school. Good. Which year have you? Oh, that name is. 2019. Oh 2019 since day ten thurthis one is tenth January, fourteenth may and tenth October. Are they paper one or paper two? Maybe paper one because there is paper reference, because paper reference is is zero one, maybe then is paper one. So they look like this. Okay, so I'd like you to choose a paper, maybe do the dune, I can find the dunpaper, find the dpaper and try all the electricity questions. Yeah. June 2019. Try all the multiple choice electricity questions only and the longer questions. Okay, electricity. Okay, I will find it. That's electricity. So start working on them and we'll check on how you're getting on tomorrow and I expect you to have some questions for me. Okay, I talked to you. Okay, Jackson. Bye, bye, bye, miss.