So they're Mr so they are all semiconductors and then a metallic conductor like a copper wire. So what happens to the resistance in a copper wire when you heat it? If you heat a copper wire. Does the resistance go up or does the resistance go down? What do you think? I hit a copper wire, the resistance will increase us. The temperature increases. Yeah. So this is the main thing. You know, if you heat a copper wire, the existence increases as the temperature increases. So remember, an electric current is a general drift of ions in one direction. Your electrons go in the opposite direction, but your positive ions tend to be fixed. So. In a metallic conductor, the electrons, when they're moving, they do hish things. So that's what disrupts their flow conduction in a semiconductor, though. So if we take a thermister. Which is a temperature sensitive resistor. So with her Mr. If it gets poter. As the temperature increases, the electrons are released from their fixed positions more. So here's silicon. So instead of the electrons being attached to the silicon atoms, they get more freed up to flow in a current. To flow in the conducting as they call it. Imagine they're put on a conveyor belt like in an airport. Okay? So that's a key thing. And it's the same. In a light dependent resistor, what do you think the light does when it falls on a light dependent resistor? Does it increase the number of electrons that can flow? Or does it reduce the number of electrons that can reduce electrons? Why do you think that? Because ldr R when the line intensity increasand there is and the resistance will decrease decrease Yeah so in a light dependent resistor, remember about your photoelectric effect. If life shone shines on a metal, electrons can jump off the surface. Well, it's the same with a light dependent resistor and a thermistor. So there's your thermister symbol, there's your light dependent resistor symbol. So in a light dependent resistor, the more. Light that falls on the light depenpendent resistor, the greater the number of electrons eat. To conduct. So. So thermisters, this is actually what a thermistor looks like. So if you were to open up your phone or your laptop, youprobably ly see lots of little thermisters thatwill switch on a fan if the device gets too hot. A light dependent resistor looks like this. Again, my phone. In daylight, the screen gets slightly darker. At night time, it gets brighter. So my screen must have some sort of light sensitive device and strelights. So street lights. Laptops etcetera. So. Resistance against temperature, resistance against light intensity. So these are the key things with a semiconductor. The more light that shines on a semiconductor, the lower the resistance, because the greater the flow of charge. I equals delta Q Q over t. So if you're increasing the charge, you're decreasing the resistance. Same with here. If you're increasing the temperature, you're decreasing the resistance. So this can be called this and negative. Temperature coefficient resistor, the negative temperature coefficient resistor, and this is a negative light coefficient resistor. So uses. If a microwave gets too hot itshut. Down. If our engines and our cars get too hot, they will shut down too. So thermostats can be used to control the temperature of a room. So in this cold weather, if it gets below, say, 16 degrees, your heating should switch on. Mobile foones light dependant resistors, so automatic flash if the room is too dark, high resistance, the electric circuit is designed to switch on the flash if you're trying to take a photograph. And in street lights, at night time, the street lights should come on automatically. So. If the darkness if there's less light, there's more resistance and there's more voltage across your light dependent resistor. And this can switch on a light. So maybe we have a potential divider circuit that looks like this. A fixed resistor. A light dependent resistor. Power supply. And fee out. So the bigger the resistance, the bigger the output voltage. This is vn. And then. This can lead to the strelight being switched on at night to make driving and walking safe. Okay, so. So to summarize, in a wire, the higher the temperature, the higher the light, the greater the resistance, but in a semiconductor, the more the heat, the more the lies, the less the resistance, the better the flow of current. Okay. Do you remember the equation that goes with potential divider circuits? Because this is one that doesn't appear on in the back of your exam? The equation of potential dividers potential dividers. No see house. Equal to R2 over R one plus R2 times v in good. Exactly. It's the ratio of this resistance to the total resistance. And is the equal to the voltage across this resistor compared to the total voltage, R one plus R2. Times v in. It's ratios which you probably prefer to work in. So with that in mind. For these questions, you're required to find the missing value in bold. So I think it's the current they want you to find here. So it's fee out. Yeah. Okay, so try this one, Jackson. Be out equal to once. All 1000 divided by 250 times 52. Votes. Minus the end. So at 15 hundred divide by 20 500 divide times five. Yeah who know it's three. So did you put remember this is R2? This is R one. So they get three votes. So 15 hundred. Divided by 2500. Times five. So that that gives you three volts. Okay. What is the current what is the current? How do we find the current? The current is three divided by 1:15, sorry, 500. So five divided by 2000 thousand minus three correct two Millias. Okay, what is the current in this circus, Jackson? Number three, you're asked to find this current. Thousand 200. Okay, so our. Okay, that may be more complex. Maybe not. So that is four voltes four voltes and four divided by 1020 then is 3.3 times ten two is the minus three and this value, two divided this number that 600 oounds. Yeah. So we've got 600 oes. And what about the current. The current. Four votes. 3.33 times ten to the minus three pares. Good, fine, okay. Do you feel happy with these questions? Yeah. Yeah again, Jackson, it's good to learn this equation. You're given most of the equations. I've shown you the equation. You're she's in the exams, but you're not given this equation. So it's when you have to memorize. Okay, good. So we've done light dependant resistors. We've done potential divider circuits already. Now we're going to look at internal resistance questions. Do you remember the equation that goes with this? You are given this one in e equals ir plus my little R. Remember, so you're given this equation. So let's try finding little R for this First Circuit. So we know the emf. We know the current. So this one emf equal to ir on plus ir. So the capital R is 300 ohms. And the internal energy, we don't know, and the emf is ten volts. And that is 25. That is 25 milliares. That is 25 times ten to the minus three. So ten votes. 4000 and that is -300. You are equal to 100, that's what cash. Hundred domes, cave. Again. Let's try question five. Five. This one. There's two parallels. Well, five. So how to do this, I'm not sure. Work out rt first for this combination Yeah big R. And then she was exactly the same way as the other one. Okay, got it. One over 300. Plus one over 500. 187.5 oz. Good. So we know big R, we know I, so we know e is three is 14 out, ten to the minus three. Times one. 嗯，没有。And then so. Three. So this one times are so so firstly three divided by. 14 times ten to the minus three. And that 224 214.3. And then -187.5. And that is 26.8. 26.8. Good. We'll try question with a graph. Remember, if you were doing an experiment to find the resistance, the internal resistance of a battery, little R, you would have to set up a circuit like this. Variable resistor and aateure. And a vote major. Sure. And you would plot potential difference against current. And you would get a line like this, this is an experiment you should know. So the graph below was collected by a student investigating a battery. Jackson, from the data collected, determinmine the internal resistance of the battery and its emf. From the graph, we can find the internal resistance and the emf of the battery. Do you remember how. Determine the internal resistance of every f okay, so Yeah, not potential difference. And this graph shows the potential difference and current and so the internal resistance and its E M F. You can get both of these from this graph, and it's very easy once you learn it. The. Emf is the point where current equal to zero. Good, excellent. So and then we can use the equation to calculate the Yeah the R, the little R Yeah we can we can use the equation minus R times I plus b. I've rearranged this equation. So y equals. X plus c. You probably call this something different in maths, a is equal to B, C plus d or something. And essentially that is the emf good and the gradient. Is the internal resistance. So this is our y intercept. Good. So what is it? What is die? What is the value? What I'm you're asked to determine the value. Five, 6.25. Good. No, no, no. Two, 6.2 maybe 6.2 is fine. Yeah and what about little R, the internal resistance emf equal to 6.2 and the resistance resistance. So remember. Y equals mx plus c. What does m stand for? The resistance, capital resistance. So we should calculate the the gradient gradient exactly. Yeah. Do you want to do that quickly? Resiokay. 6.2-3.65. 2.55 and that is from. Zero to 16. So. 60. 2.55 divided by 16. Yeah. 0.16. Turned right. So we didn't spot that these are milliamps. So a power of but essentially the method is right. So. -1.5 over 9.4 by ten to the -13, minus three. So power of ten. So you've got 1.51 point. So if we convert the current two milliamps webe there. See, knowing these tricks, you can save a lot of time in an exam. So here's potential difference. Again, current is in mili amps. So. Determine the internal resistance and the emf. One more of these questions and then we look at kurtchov slaws. So again, what is the electromotor force? Emf that is a okay. Let me see this this. So 3.1, I think you can 3.1, I agree, it's hard to read these off the screen. And then the internal resistance, knowing that this x axis in milliamps. So the internal resistance. They always like you to use more than half the graph b, the line to work out your gradient. So delta y over delta x. Equals 3.1 minlius two point. 25. Over. Ten by ten minus three. Okay, so that's your gradient. 0.85. Divided by ten x minus three. 85 oves. 1:10 to the minus three. Good, okay, let's see if we got it. Nice. 85.7, 0.6. So okay, always show block points you're using on your graph, Jackson, and always use more than half of the graph line because that's what they like you to do. Okay, internal resistance and then kurthoslaws kurchoslows remember kurch of slows, there's two laws. Do you remember kurcho's laws? What are the two laws which allow us to work out current in any part of a circuit if we know the emf and if we know the resistance? So kchov laws ws. What's the first law? Do you remember? How of the parents entering a junction equal to sum of? Currents leaving a junction. So in brief, that is charge conconversation conservation. Good. Yeah. Another way we can rise it. Some of the currents equals zero because if we say currents entering are positive, currents leaving are negative. We end up with a net value of zero. Okay? So that's two ways of defining khurchav's. First law, the second. So that's conservation of charge. Conservation of charge is the first law. So. Charge is conserved. In other words, the electrons coming into a junction must the electrons leaving a junction. And then the second law, energy, is conserved in a circuit. Energy is sponsored in a circus. So this can be. The sum of the. Equals the sum of the I are. Yeah. Pds. So the sum of the emf is equal to the sum of the pds. Equals I R. So we could write that the sum of the emf's is equal to the sum of the. Some of the emfs is some of the iOS. So energy is conserved, charge is conserved. Do you want to do another lesson tomorrow on electricity, Jackson, or would you prefer to start looking at something else? Something else? Okay. So we have mechanics, we have electricity, we have materials, we have fluids, we have density, we have energy, we have waves, we have quantum physics. And then we have paper three, the practical skill test, where it's all about uncertainties and percentage errors and things. So. Have a think, and by the end of this lesson, tell me what youlike to look at. Remember, in paper one, it's mainly mechanics and electricity. And paper two is materials and waves and quantum physics. Paper for three is the practical element. Okay, so let's use kurchaslaws to find the current here. Flowing of okay the current is flowing. Six divided by 51 point. Yeah. And with that, another one. Let's try number two. This is, Oh, my God, two cells, two cells. And remembering you, you have to be. Two amps. Now be careful with number three. There's something different. What's the catching number three? Because number is three, six votes and two votes still osix ables and four votes. And the current is that way. Oh God, this this cell, listen, this cell is pushing electrons in this direction, but this cell is pushing it in the opposite direction. So in this circuit, see, we have the plus end next to the plus end. So we have to subtract the emf's. Okay, so yousay six minus four gives two volts. In this one, we have plus going to minus. That's fine. And plus going to minus. So the electrons are all going in the same direction. The energy they're carrying is fine. But in this circuit. The electrons will be pushed in this direction but also pushed in this direction. So we have to say six minus four for the emf. Do you see why? Six minus four. Now let me redraw this circuit. Do you see what I've drawn? Do you see? Jackson, these two cells are pushing electrons in the opposite direction. So which one has more force? I mean, which one? Which which electrons has more? Which electrons have more connected entry? The total circuit will flow in that direction. All. So the total emf is two. So six minus four is three I plus two I five I is two so I is two divided by five, which is 0.4 amps. Okay. If you end up with a negative current, that's okay, just means the current is flowing in the opposite direction. Right? So for. Circuit five, let's work out what I one would be, I two and I three. Using both kkchoslaws. So this is the one we're doing, the circuit we're doing. So the first thing we can say is the currents are the same. To solve this one, we have to use both of kurcho's laws. So we can say I one is equal to I two plus I three. Do you agree I one equals I two plus I three because they're both flowing into I one. I two plus I three. That's our first equation. And then the sum of the emf's is equal to the sum of the ir's. So the next thing we can do is make a couple of simultaneous equations to solve for I three or I two or I one. Okay? So we have to do a continuous loop. So this is the first loop I'm going to do. In this loop, I only have six folds. So six voltes emf is equal to I one times three. Three I one because I one will be flowing around this bit. Plus two I two. Okay. I'm just going to redraw this three high one. Those our equation and then I can make another loop. I could do this loop. She got to six folks. Minus. Plus four votes. So that's ten volts. Emf is equal to three I one. Yes. Yeah six volts ten volts is three I one. Yeah and plus two I two. I didn't use, I could have done this as a Luke. I could just make this mine, a Looi chose to make life a bit difficult. But we know three I one is ten, so I one must be 0.3I one is. Zero ught point three recurring amps. So I can put substitute that into this equation. Six is equal to one plus two, I, two, so five. So I, two must be two and a half. 2.5. So 2.5. Point three, so I can continue substituting. Anyway, have you another lesson after this? Yeah at well o'clock at 1:00 okay, so. Would you like to have a go at this question and finish it for tomorrow? Okay. You can take different loops and different ways of doing it, but you have to find values for I one, I two and I three. And shall we look at fluids tomorrow? Or materials? I prefer material. Okay, let's go back to materials. Okay, Jackson, very good work today. I know you can do it. What am I doing that helps you? I think maybe I can learn these questions more clearly. Yeah. Take a photograph of question five and finish it today so you can show it to me tomorrow. Okay. You've got a snapshot of it. Yeah, good. Okay. I'll talk to you tomorrow. Bye miss bye.