Okay, so a series circuit consists of two resistors with resistance R one, R2, and a battery of potential difference v, which at the following gives the potential difference across the resistor with resistance R2. Resistance are. Potential difference, of course, are two potential difference. Or two over I choose d very good correct. Remember, you're not given that equation. So it's when you have to memorize. But if you think of it, it's the ratio of the resistances to the ratio of the input voltage be in. So that will be your v eight and this will be R one and R2 and R2. Question 19, again, a multiple choice question. So light dependant resistor and resistor usually connected in series with a six volbattery. As shown, the volmition measures the potential difference across the ldr in daylight. The voltmetia reads three voltes, which reading is most likely if the circuit is now in total darkness. Will it be greater than three volts, equal to three volts or less than three volts? What do you think? Total darkness in total darkness, that is the maximum resistance. So. Maybe that is that has a maximum resistance. The dark lights of voltmeter with revolts in in daylight, daylight and. Which lucly if the circuis now into the darkness, darkness governor said it's a maximum resistance, but. But it just says that the daylight so. I'll be back in one minute. Give me my answer when I come back. Now we know R equals phi over I, so that suggests that R is proportional to v. Okay. Does that help you? So if, as you've correctly said, in total darkness, the resistance will be at a maximum, so what will the output voltage here be? What do you think? I could be over. I but I think this one I think this see a little above three votes. If resistance increases, v increases, doesn't it? Because R is proportional to v over I, so R and v are proportional. V is proportional to R, so the answer will be d. The maximum voltage will be dropping in darkness. So maybe you have this sensor connected to a nightlight in your garden. Okay. So we simply said that resistance is proportional to voltage. So maximum resistance will give nearly maximum voltage. There has to be a little bit of voltage drop across that. So this will take a bigger share of the resistance, a bigger share of the voltage drop. So in and just less than six volts, do you think you understand that? A little about a little below six vows. No, no. So if you think of a light pendant resistor. So. As light falls on it, the resistance decreases Yeah as darkness. It comes. The existence increases. So so in our in our potential divider arrangement, we have a fixed resistor and we have our light dependent resistor. And they are across a sixfold power supply, six volts. So depending on the light, if these are both equal resistances, they have a vote. This will have a voltage drop of three votes across it. The higher. The higher the resistance, the higher the voltage share will be taken by R2 compared with R one. Yeah. Remember what we were learning yesterday about a potential divider circuit? By changing the values of these resistances, we can change the output voltage, and this can be connected to a light or. Make the light switch on or off as you please so in darkness. The light, the voltage will be high here, and this will switch on a light. So. When darkness falls, the light will switch on. Happforce light will switon. Yeah, I agree. But when in darkness is that ldr will see the maximum resistance? Yes. So is that the reason why it is little below six votes? Yes. Okay. And then in light time, in light, in daylight, the voltage across here will be smalls, whereas the voltage drop across here will be high. So no light will switch on. If you say, need four votes to. To switch on this bulb, you need at least four volts across the bulb for it to come on. Okay, so that's a potential divider circuit. Indeed, the dark, the resistor of the ldr will be very large. So practically all the potential difference of six volts will be across it. Okay, have a look at this question. A torch uses a 1.5 volt dry cell cell. Over time, light intensity produced by the torch decreases as the cell goes flat. So we're running down the energy in our cell. Student a sets up the following circuit in an attempt to measure the emf of a cell. Explain why the voltmeter reading will not be the emf of the cell. Enough Yeah of the there's two Marks. Most reading emf with the whole kinetic energy of electrons went through the whole circuit. Yes. But this voltmeter connected the connected each side. The two sides be of a of a bulb of a bulb. So it is potential difference instead of in a emf. Exactly. Yes. So so the potential difference across. The bulb will be less wide. Because they're not going to give away weapons. Oh, video across the Bob will be less because there's internal resistance in the sales ls. Good. So you need when you're asked to explain something, you need to give that much detail. Good. Last, folks. Okay. So resistivity. So we have the equation for resistivity. R equals rho l over a. A student carries out an experiment to termine the resistivity of a coil of nrome wire. He uses an ohhm meter to measure the resistance of the length of nromoir. As shown, the diameter of the wire was measured to be this plus or -0.01 of a millimeter. The length of the wire was measured to be 1.5m plus or -0.05 of medetermine. Which of the measurements introduces the greatest uncertainty into the value for the resistivity? Your answer should include calculations. Okay, so we have the equation R equals rho times l over a. And we know a equals pi R squared. So this is four Marks. We have measured the diameter. So this one also include calculations into the value of resistiity. So. Where will be the biggest source, the biggest uncertainty and why? What is the biggest source of error or uncertainty? Are you linot 5m? But this is 0.01 milmeters. Remember area is radius squared times pi. Okay. Square times pi so not not one square. Times pi. 3.14 times ten to -4 mm. So will our uncertainty come from resistance, from resistivity, from length or from area? Determine which of the three measurements introduces the greatest uncertainty into the value for resistivity. So we have a no ometer measuring resistance in ohms. We have. Probably a caliper measuring the diameter of the wire here. The diameter of the. Remember one of these, the diameter of the. So resistance, let's calculate row first. The resistance has an uncertainty of 0.1. So we can measure resistance using a oometer to the nearest tenth of an ome length is to the nearest. Point zero five of a meter, so to the nearest 5 cm. The diameter is to the nearest hundredth of a millimeter. But because it' S R squared, what do we have to do with the uncertainty if we have a quantity that's measured that is squared? What happens to uncertainty if we have I know it' S D, we measure the diameter, but we divide that by two, but it's still a squared quantity. So if it's a squared quantity, Jackson, we have to double the uncertainty. So we calculate the uncertainty and we double it. R equals rho times length 1.25. Over. Hi. Times. Point 27. I tend to the minus three. Okay, I I R squared so we can find a value for resistivity. Now do you want to help me do that? Hi. Rediity times. 1.25 and divided by pine times, not point 27 divided by two times, ten to the minus three. Yeah that is millimeter square. And we can calculate the resistivity. 1.04 times ten to minus six. Did you square resistance? Okay, good. And then to see where the most uncertainty. Comes from the uncertainty of resistance is to the tenth of a noso. That's 0.55%, 0.1 over that times 100. The uncertainty for the length is 0.05 over the length of the wire, times 100, which is 4%. So the uncertainty for d is 0.01m over 2.7 times. So. So point zero one over 0.27 times 100 is 3.7 but it's multiplied by two. Because it's pi R squared. So the measuring of the diameter gives the highest amount of uncertainty, okay? Okay. Another multiple choice question, Jackson. Then we look at a real PaaS paper. Wait a minute. At last question. So what's the uncertainty? Noone zero de one meters divided by zero, one, two, seven. Now if we look at the question. If you remember how to find uncertainties. So the length is the uncertainty over what we measure times 100. So for length. Percentage on certainty. Is. 0.05 divided by 1.25. Times 100. Point zero five divide by 1.25 times 100. So that gives us 4% uncertainty. For length, okay. For resistance, what is the uncertainty using our ometer for measuring resistance? We can measure it. Is that a resistiity? No, no, that's the resistance. Remember, uncertainty. What do we mean by uncertainty? This is an ohm measure. And it measures things to the nearest point, one of an ohm. So it's instead of using an amateure and a vote measure in your circuit, so a no meter takes the place of using an amateter and a vote meter in your circuit. Because it's the same as putting an amitter in series and a voltmeter in parallel across your circuit. Okay, an ohm meter, but it can only measure to the nearest. 0.1 of no. How do I know that, Jackson? Know what? This meter is only measuring to the nearest tenth of an ode, so that is the uncertainty in resistance. Okay, how do I know that? Use 0.01 divided by 0.05. No. To get the uncertainty for resistance, it's 0.1. Over 18.2. Times 100. 1.11, 18.2, 0.1. How do I know the uncertainty is point one ohms? How do I know I think that is 18.2. So the minimum unit is not 0.1. Exactly good. So that's what we're using here. So point one divided by 18.2 times 100 is 0.54. No point five. Five. Resent uncertainty. So we've got uncertainty for length, uncertainty for resistance. The last thing we have to worry about is uncertainty for cross section tional area. So 0.0 ught 27 plus or -0.01. A equals 0.01 over not point 27. Times a honindridge. Let's see if they use diameter. Yeah, so that gives me 3.7%. However, because area Jackson is equal to ir squared. So this is the diameter, not the area, but the area. So the area is calculated. By finding pi R squared. So we have to multiply this by two. So we get 74, 7.4% uncertainty in total for the cross sectional area because we have to square this quantity. Yeah so that is 0.01Yeah and divided by 0.27 times 100Yeah which gives me this. So that's for a diameter. But the equation demands that we square this quantity when we're using it. So we have to multiply the uncertainty by two. So the diameter. The diameter uncertainty is doubled, giving the greatest amount of uncertainty into the value for the resistivity. Okay. So that's 7.4. This type of thing comes up in paper three, if you remember, okay? Right? The two resistors shown each have resistance R, which expression gives the correct resistance between the two, the point x and y? From points A, X and y there is each hab resistance, each have resistance that is. R. I choose a. The first the what I choose. A good well ton. The current potential difference graphs for a resistor and a thermior show the resistor and thermister are connected in series to a six volt battery. What is the current in amps in the resistor? This one, the current in empires, in this resistance, so 96 votes, battery, six votes. And the current empress, so potential semester resistor, so in the stor, six votes. Apires 1.5. I choose, say. A the potential difference across resistors add up to the potential difference across the thermistor must equal six files. This occurs when current is 0.5 amps 1.5. So the potential I understand the sum is six volts, not the single one Yeah what's the current. Okay. Where would you add an amneture and vote measure to the circus that would be able so if you wanted to plot a graph of current against potential difference for your light emitting diode. 10m and voltmeter. A meter and good meter able data to collect it, the current and potential difference of a light potential difference. Okay. Potential difference. Good. I meet her. The current against current against. There. I think I would push it there because you just want the current going through the. Led okay. Our Meare in series with led and volt meter in perallel with that. Because some of the current can flow in this direction and some can flow in this direction. So you want just the current in the flight emitting diode. A light depenpension resistor. And the thermister are connected in series. Okay, so has complained. Highest combined resistance highest so I choose be dark and hot. I mean, this one. Yeah. Now remember, these are not metals, Jackson. Dear Mr is a semi conductor. It's not like a metal conductor. In a metal conductor, we have deloccalzed electrons that can Carry the energy and drift in this direction. But in a semiconductor, a semiconductor needs heat energy or light energy to actually free up electrons. So they are freed up to the conducting belt to conduct a current. Okay, so heat. And lice. Energy are needed to free up electrons to be conducting in a semiconductor as opposed to a metallic conducductor. Do you understand that? The difference. So a metal. Has electrons that can roam around, Carry, drifting around, carrying electrical energy if a voltage is put across the metal. As a semiconductor, these electrons tend to be attracted to the atom closely, and heat and light energy, if heat or light energy land on the. Semiconductor, it frees up electrons so that they can conduct a current. A semester. I agree with a semiconductor, but why when the temperature increases, the resistance will decrease because it normally when the temperature increase, the resistance will increase as well in a metal, in copper, in nprbe. So what we're saying is that conditions for a metal is. A metal is not the same as a semiconductor. So a semiconductor is made of things like silicon dioxide. Silicon dioxide holds onto its electrons unless it's heated or if light shines on it. And then we know electrons can escape from the surface of the atom. Okay. So this is a straightforward question. Now Jackson, just using your formula. I is q over t okay? The charge flows in this time, I equal to Q Q equal to it. So 180. Times six. 1080 kilns. Good. That went with the previous question. Okay. You did very well, Jackson, but we've got to get better with what these questions want. And you know the stuff, your numeracy is very excellent. It's just a matter of learning the rules. So this is a paper from 2020Yeah 2020. Which of the following gives the standard international base units equivalent to the vote? Okay vote v equal W over q. Herum quorems can divided by it is current times t and that is W W is walk down, walk down, that is F S good. And f is wj yes. And knows. Let's start. So. We have kilograms. Measure squared seconds to the minus three pumps to the minus one. Okay. You can see the style of the questions. They're similar to what we've just been doing. So this is similar to one we've just looked at, except it's two R and R. Which of the following is the equivalent resistor of this combination? This combination, that is one over two R plus one over R. So that is one over two R plus two over two R. So that is two R over three b. Excellent. Good. The graph shows the variation of current with potential difference V E for a diode. Okay. Which of the following statements is correct? The diode has zero resistance when connected in the forward direction. The diode has zero resistance when connected in the reverse direction. The diode starts to conduct when the potential difference is about 0.7 volts. The diode stops conducting when the potential difference is about negative. Point seven. Which do you think is there? The diohas sera resistance when connected in the forward direction. Well, that's not strictly because of that. So we can say that one low direction and in the inverse reverse direction, adadult start to conduct嗯。It has very high resistance in the reverse direction, doesn't it? So that's clearly wrong. So then we have one of the next two to choose from. Difference is about a noseven noone seven and that is the potential difference starto conduct and is -0.7 when the potential difference is. Grout, good. Yeah. Remember, paper one has lots of mechanics moments, etcetera. We want to get to the electricity questions. Okay, a student wants to investigate how the resistance of a negative temperature coefficient thermester varies with its temperature. So a thermister. Write a set of instructions that the student could follow and include one safety precaution. So there's six Marks for this. Now, do you get to do any experiments? Have you ever done an experiment with a thermistor? Mr, no, no. Okay. So what they suggest you do is you get a beaker. And you heat it. And you lower your thermistor into it. Into the hot water. We need to measure resistance, so we can either have an amitter and a volmeter, or we can have an ohm meter. Connected to our power supply. What else should we have with our experiment if we're investigating how resistance there I will, about this. This space is at the this space is at the water. Yes. Okay. And the water. So okay. Okay, so we should put this semester into the water. Yes, and we should see how resistance changes with temperature. So what else do I need to do this experiment? What else we need we need to make sure about the temperature before around the semester. How do we measure temperature? Mometer thermometer good so we add a thermometer. And maybe a stopwatch. So every minute we measure the temperature and the voltage and current. So b and three current and amps. Calculate resistance, who knows? And temperature in degree es cenrate. Temperature decent. And then we plot a graph. Of resistance against temperature, what do we expect our results to look like, bearing in mind our thermister is a semiconductor? It's a semiconductor. It's not a metal. Will resistance increase as temperature increases or will resistance decrease as temperature decreases? No. All right. I know our time is up. A negative temperature coefficient thermistor, okay, Jackson, for tomorrow, you're going to tell me everything you know about a negative temperature coefficient thermister, okay, okay, Jackson, good work. Today. You worked hard. Today. We're getting there. No one said physics was easy, but with your right toes, you're doing well. Okay? Okay. Bye bye.