What's the same for both of these quantities, emf and potential difference? Yeah that is of E M F. What are they both measured in? Votes, votes. So that's what they have in common. And vois energy or work per unit charge, remember that. So both of them are measured in votes. So the emf, the emf is the voltage that you would measure with an open circuit. That means no current is flowing because there's an open switch. I equals zero. So that might be twelve volts, say. But as soon as you close the switch, the voltage across the power supply seems to reduce. So that means energy is lost in the internal resistance of the battery. This can be called the closed. I'll just go to typing its quicker closed circuit voltage or terminal potential difference. So the once current is flowing, this actual energy available to do work in the external part of the circus will probably drop. So this might become 11.7 votes, maybe what you're reading across the power supply. So that means energy is lost within the battery, within the internal resistance of the battery, internal resistance given the symbol little R. So this is how we show it. This circuit shows the internal resistance. This helps to make it clear what is happening in the circuit. The cell produces an emf which is shared between the internal resistance and the lamp. So with 11.6 volts is available to do work in the bulb. That means zero, 0.3 volts must be lost due to heating in the across the internal resistance of the battery, the power supply. So no point three volts, so 11.7 plus 0.3 volts must give the emf. Okay, you're happy with that. So. And you brief in brief, I think maybe in this battery there's internal resistance. Yes. Okay. So some of the energy never escapes from the battery to do work in your external resistor here. So this is big R and this is little R. So this is the equation. It's like conservation of energy. So the energy available from the cell is equal to the voltage drop across your external resistance plus your voltage drop across your internal resistance. So again. It's just law of conservation of energy. If the energy isn't available for work to be done or energy to be transferred to our external resistor, then it must be converted in the internal resistance of the power supply. Okay. So emf is pd plus loss Faso, these are the lost fates. The total voltage supplied in the cell provides the voltage across the external resistor, the voltage across the internal resistance of itself. So emf is energy transferred from other forms to electrical energy per coulum of charge potential difference, energy converted from the electrical energy to other forms per couom in. The external resistors loss folds energy converted in the internal resistance. So we can describe this as open circus voltage equals emf, okay? And the terminal potential difference is energy available. To do work in the external sister, say the bulb. Okay? So we know that it's conservation of energy. If the energy isn't converted here, it's being converted before it gets out of the power supply. And this dotted line indicates the whole power supply. So the terminal potential difference is when the switch is closed. Terminal potential difference, the switch is closed, open circuit voltage. We have the emf measured here. So to if you in a laboratory had to measure the internal resistance of the cell, this is how you would do this. Okay? What do you see in our circuit here, Jackson? 啊，the circuit. Well, this circuit is there's battery, voltmeter, ammeter and variable resistance. Yes. So. Note the amateter and voltmeter readings, adjust the variable resistor and repeat. So you keep changing the value for R and you repeat at least six times, and you plot a graph of the volt meter reading against the amateur reading. Okay? Yeah. So y equals M X plus c. Or a equals bc plus d so what is this value the y intercept equal to? Because we've just rearranged this, e is equal to v plus ir. So v is equal to e minus ir. So y is our y axes, x is our current. I want you to be able to tell me what the y intercept is equal to and what the gradient is equal to. I equal to the voltage that is emf m, mto minus R, minus R, that is. Minus are the. Wait. No, this is not emf. I think the e emf is E C and m is the internal resistance, but that is minus is Yeah negative. So x is current in this circuit and v maybe that is. That is external voltage Yeah v is Yeah v. Okay, so. So, for example, you could get a question like this. A very high resistant voltmeter is connected across a cell and gives a reading of 1.46 volts. When a resistor is connected in series with the cell, the voltmeter falls to 1.3 volts. Calculate the emf of the cell, the internal resistance of the cell. So the emf is the. The voltage across the cell before anything is connected to it. So that's 1.46. The open circuit voltage. Calculate the internal resistance of the cell. So that's e. This is v. So we want to find ir so v is equal to ir, so we can work out the current using I equals V R over R. So we're given the value 1.3 and we know R. So that will give us 1.3 divided by twelve. So that will give us 0.108 amps. So we know the current. But we can work out the voltage drop across our internal resistance point 146 us, 1.3, so 0.16 volts is lost across or the internal resistance. So that's equal to I little R. So we've just worked out current 0.108. So 1.0 point 16 divided by 0.108 gives us 1.48 ohms. So little R, if we know if we've worked out the current if we've worked out the voltage drop across our internal resistor, we can find the value of little R. Okay. I don't think I've ever seen. Any questions like that? Okay. So again, to remind yourselves of the equations were given in the back of each booklet. Voltage is work done per unit charge. Resistance is voltage divided by current, electrical power and energy. So power is current times voltage. Power loss is I squared R. Our losses also be squared over R. So if you had to work out power dissipated as heat. We would use one of these equations. Work is power times, time I, times v, times t, resistivity current is rate of flow of charge and drift velocity equation. Okay, so you will have these equations available to you. So. Here are some practice questions on electricity. Are you ready? Do you need to plug in charge something? In charge. Ks, no, I charged for my. Yeah, I have charged for this. IPad or a laptop or a phone, this is okay. Yeah, okay. Tomorrow the water increase, but it's not increase, it comes water. So because the temperature of wine to increase. A rate of energy transfer from conduction electrons to latest ions. B. So the number of conducting electrons per unit volume, so that would be little n. Remember, we have this equation for copper. Little len is the same. So the drift velocity equation does not change if the temperature changes. Okay, let's try this question. Two resistors are connected to a battery. As shown, the terminal potential difference of the battery is v. Which of the following gives the potential difference across the resistor of R one? Actual difference across the resistance resistance are one. So. Special difference. So. I trust be. Three, well done. Remember about question six, why there is I'm not understand. Yeah Yeah selyeah okay, so none of these one. Yeah sewar. My over. I suppose the easiest explanation is that this value is unique for copper. Copper as a material has a certain number of deloccalised electrons per meter cubed. So copper has a certain number of free electrons that can move within the material. It's specific to the copper, and that doesn't change if the copper gets heated up. These just vibrate more. But it doesn't add free. That doesn't make more contion electrons. No. If we were talking about a semiconductor like silicon, remember. Like silicon. Which is made up up of sand. Yes, if we heat a semiconductor to make a thermistor. Then the Husher the semiconductor becomes, the more deloccalzed electrons will be available, but not in a metal. Semiconductors are different to metals in that respect. Sand is silicon dioxide. Silicon dioxide, whereas copper has a fixed number of free electrons per meter cubed volume. Okay. I'd like you to research something about semiconductors and metals and their atomic structure. The structure, a metal bond. A metal bond and a silicon bond. I think silicon is in group four. So. It has four electrons in its oushell. Did you do much chemistry in your past? No, past? No, no. So oxygen is in group six. So it has six electrons in its outer shell. So one, two, three, four, five, six. Thank you. One, two. So each element desires eight electrons in its outer shell. Now, if you heat silicon dioxide, some of these electrons can escape to become conducting. But that is not the case in copper. Copper has a fixed number of electrons available to Carry energy. Okay, so spend some time researching the difference between a semiconductor and a metal conducductor like copper. Okay. Happy with this. So the ratio of the voltage across R one is the same as the ratio of the resistors to the ratio of the voltages. Good. Not that right. Here's a longer question, Jackson. Two resistors are connected as shown, show that the resistance of the combination is about 70. Now they're asking you to show that. You can't just say it's equal to 70, you have to show that. R T one over R T one over R one plus one over R2. Equal to one over 40. 1:18. 71 over 72. So rt equal to 72 ohms. Good. So you wouldn't again, it's like describe, explain. They want you to show that the resistor combination is connected to a battery of emf and internal resistance. The switches closed for five minutes calculate the energy dissipated in the resistor combination. If the emf is nine volts and the internal resistance is 2.5 ohms. So we have an internal resistance here Nicolar. And if we go back to our equations, let's see if you can find out which equation you could use. The equation of power. Calculate the energy dissipated in the resistor combination. So we're given time, we can work out the total resistance because we know that that's 72 ohms. I'll go back to the equation. So we're looking for. So work done is energy. When work is done, energy is converted. So see this equation, energy converted is v times I times t and t must be in seconds. Okay. Iv t so so internal resistance is 2.5 oso, maybe we can so this point b is equal to E F so. So wait. Maybe we can use another equation because we eat no. I equal to U divided by R so that is equal to v square R T. Yes, so R equal. So the capital R equseventy two plus 2.5 and the v is 81 volts. Yeah, maybe that is so there is 20, 74.5 times three, five times six, 300s. And equal to 326.2. 320 very good, well worked out. Okay, some more multiple choice questions. So. Which of the following gives the standard international base units equivalent for the volt? So we know the volt is the unit for emf and for pd. So vote is energy per unit charge, isn't it? And work is force times distance. Charge. Current times time. So can you work it out from there? Remember, we've seven base units. We only tend to use six of them. Yeah. Vote to the vote volequal W divided by q and W equal to m as and q equal to it. So W divided by q then is M A S it's as Yeah times acceleration. W divided by q. One that is F, S and f equal to M A. Oh Yeah, equal to a. I'm writing down my thought processes. This is how I work these out. Yeah. Yeso, I use another v equal to I times R. That is, I is arms arms that is divithat is useful. So. Work. The charge. So from there. Amps to the minus one. Yeah, that is so the unit you want the unit for MaaS. Now this is the object. This is the quantity. Good. Minus two yes. Good. This one. Good. It's a lot of work for one mark. But do it's not difficult if you just work out, you have to work it out. Okay, let's try number seven. Let me give you space. I think we've seen number seven before. It looks familiar. This one that is. Equivalent resistance of this combination, two R and resistance are shown equivalent equal to. So what's rt this? The question is asking you to find the equivalent total resistance of this combination. This one b. Yes. Oh. So let's. See one over two R. Plus one over over R is one over two R. Plus. Two over two R, if we make the common denomination the same. Okay, so that gives me three over two R. And then inverting these because that's one over rt. So. Two. You're a three. Yeah so Yeah we're correct. Sorry, okay. A variable resistor is connected in a circuit, has shown the cell has internal resistance, the resistance of the variable resistor is increased, so little R is going up in value. Which row of the table is correct for what will happen to the amateur reading, and the volt meter reading variable resistor is increased resistance, so. So firstly, volmeter reading voltmeter is the E, F, so that will increase, but a meter decreasing, so that is c. Good. So a circuit was constructed as shown, calculate the total resistance of the resistors in the circuit. This comes up quite a bit, Jackson, just as straightforward, working out the total resistance. Total resistance, so one over 30. Plus all over 40. 17.1 thousand plus 20 plus ten. Okay 47.1. The graph shows how the resistance of a light dependant resistor varies with the light intensity measured in lux. So the brighter it is, the lower the resistance. The darker it is, the higher the resistance. Yeah remember is a light dependent resistor a metal or a semiconductor? A dependent resistor, well, the dependent resist, the more light, a bigger, large intensity, less resistance. Do you think it behaves like a metal wire or like a semiconductor? Is the light energy making more very good? Exactly. So it's a semiconductor. So it's arranged as a potential divider circuit, the voltage across here, which could be drawn in different ways. So this is just like your. That's what they mean by that nine volt. This is a variable resistor, this is your light dependent resistor, and this is your vout. The student increased the resistance of the variable resistor. While the light intensity was constant. The voltmeter recorded the potential difference across the variable resistor. Explain what happened to them reading on the voltmeter. So they're increasing the resistance here. So you have to explain what you would see happening to the reading on this fameter. If the light intensity stayed the same, light intensity was constant. So explain, remember why and how. Why something happens, how something happens. What happened to reading off the 5m blokay? Increase the resistance of vable resistant stor. So this resistance increases what we're you seeing happening to the reading on the vote meter? Reading on the multimeter sufffacthe resistance of variable resistance so resistance. So the resistance of the whole circuit will increase. So the emf will increase, but that, that is recording the potential difference across available able resistance. So increase, then the v out will increase. Yeah, the will increase and variable resistor. So basically R is proportional to v, isn't it? So the if you increase this v ouwill increase good. Explain why, how. Where is the voltage coming from and how is a shared voltage coming from the nine volts side? Yep. So that is fixed. Yeah. So this. He's taking a bigger share of the nine votes compared to this. And they want three Marks here. So resistance is proportional to v. Current in the circuit decreases, the potential difference across the ldr decreases, the volmeter reading increases. So we didn't think of talking about miss. The current will decrease, so the current decreases. Yeah, we should have. Here the current decreases. And the ldr will take. The ldr. This voltage will take a smaller share wanted it. So this if we had to vote a vote measure across our ldr, it will decrease. So the current will decrease and potential difference in ldr R will decrease will decrease as well and reading in the meter will increase. Yes. Okay. Do you want to write that down? Well, Yeah, I'm writing down. I mean, Yeah, I have already writing it down. Okay. The variable resistor was set at a resistance of 4.5 kiloohms. Determine the light intensity when the potential difference shown on the volmeter was 2.5 volts. So I'm going to just put it with miss. Is there a graph about this? Yes. Yeah, the graph. So maybe we can calculate the resistance and then find the light intensity. So if we've set this to. Four point. Was it 74.5 kiloohms? Killer ohms. And dv out is equal to. 2.5 volts. So if we can work out the resistance of this and then read off the graph. Yeah determine the light intensity. So if we can work out this R, then we can use the graph to find the light intensity correct. Where do we go from here? Resistance. The l is 2.5 volts yes. Remember, this is the ratio of voltages to resistances. So if we want R one and we know R2. Now we know output voltage that's 2.5 and we know input voltage is nine. So we know this is 4.5k. I'll just put A K over 4.5. 1.5k plus R one. So that's our one. Just make a pleasure. Okay. So we've just won our noso. We should be able to work it out. We add R2R one plus R2 could be in. So. So this one so firstly 4.5 times nine. 40.5. And then it's 2.5R one times 2.5 times 4.5 equal to 11.25Yeah I see 11.25k are one equal to 40.5k. So I equforty point five. 11.25 40.5. Divide 11.25. 3.6. 3.6. Kilo kilms. So if we go. So it's about 40 400 lux. They get a very different. 6.5. Hundred and 80, 185 looks. 4.5 by ten to the three. Hmm. So they've worked out the current and then used the current and the voltage across the ldr to find the resistance. They get 11700 ohms, 11.7 killer ohms. You have that numbered in you. 11.7. So they're getting that value. What have we done wrong? I equals we over R. So they've found the current through both resistors using the information from our variable resistor. So 2.5 divided by. 4.5 by ten to the three. See, that is fine, but the answer is 6.5. And then but if we know 2.59-2.5. Is 6.5 votes across our ldr. Yeah. Ldr? Is the same current 6.5 by ten to the -6.6 by ten to the minus four. So we can get the same answer. I'll go back and check why we didn't get it the other way. We should have got the same answer the other way. Okay. Jackson, what do you want to do tomorrow? The energy? Yes, something about energy, kind energy, power and work. Okay, right. We'll leave it there. Bye, bye, bye. About miss.