Video: Measuring resistance

﻿Hi everyone it's Miss White here and welcome to lesson seven of the electricity and magnetism unit, measuring resistance. So before we get started can you make sure you've written the title, which is measuring resistance and you've collected all of the equipment that's on the screen now. So if you need to, pause the video and get yourself prepared. We should now be ready to start. So make sure that you're surrounded by no distractions and that you try your best in every activity that we do in today's lesson. Let's get started. In lesson six, I introduce you to resistance and the relationship between potential difference, current and resistance. Today, we're going to look at measuring resistance before moving on to static next lesson. We're going to start by doing a quick recap to see what you've already learned in the unit so far. So firstly, what is the unit of resistance? That's right. Resistance is measured in ohms. And what is the unit for current? Well done. Current is measured in amps. Third question. What is the unit for potential difference? That's right. Potential difference is measured in volts. And then, what is the relationship between potential difference, current and resistance? So I'll give you a bit longer for this one. So what is the relationship between potential difference, current and resistance? Amazing. So potential difference equals current times resistance. So well done if you got that one correct. In a moment, I'd like to pause the lesson as we apply our knowledge to this question. So using the equation we've just recapped potential difference equals current times resistance. I'd like to calculate the current reading on this ammeter. If you like, you can write down using the scaffold vesro to help you out. So pause video now and attempt this question and press play when you're ready to check your answer. Amazing. So the current is four amps because potential difference equals current times resistance. So potential difference divided by resistance equals our current, and 12 divided by three is four. Next one, calculate the reading of the potential difference on the voltmeter. So pause the video and attempt this one and press play when you're ready to carry on. Amazing work. So if you've got 30 volts well done because current times resistance equals potential difference. So five times six equals 30. So our answer is 30 volts. Amazing if you got that right. And finally, can you calculate the resistance of this fixed resistor? So again, pause the video write down your working out and use the vesro scaffold to help you and tell me what would be the resistance of this fixed resistor? Great work. So you should have got the answer is 4.5 because you've done potential difference divided by current. Nine divided by two and got an answer of 4.5 ohms. So we're going to start by recapping our knowledge of resistance from the last lesson, lesson six. So as I said last lesson, copper wire is made up of lots of copper ions in the form of a giant network called a lattice. This copper lattice has electrons which move through it carrying charge. As we saw in lesson six of this unit, resistance is anything that makes the flow of charge more difficult. There are certain factors which affect resistance and I'm going to through three of them now. So if the temperature is kept constant the following factors will affect resistance. The first one is the length. So the amount of metal ions which the electrons have to pass through is going to affect the resistance within the wire. Secondly, there's the cross section. So the diameter or thickness of the wire. And finally, we've got the material. So how conductive the material is it's going to affect resistance. The more conductive the material is, the more freely the electrons will flow through the wire. Therefore the resistance would be less. I'd now like you to name the three factors that affect resistance so long is temperatures kept the same. So pause the video and see if you can remember all three of the factors that I've just mentioned. Well done. So the factors affecting resistance so long as the temperature is kept constant is the length of the wire, the cross section and the material from which the wire is made from. So why is length so important to resistance? As the electrons move through the wire. They collide with the metal ions. If you increase the length of the wire, the electron has to move through more ions therefore increasing the likelihood of a collision. So what will happen to resistance if there's more collisions? That's right. Resistance is going to increase proportionally to the length of the wire. So the shorter, the wire the less resistance there should be. And the longer this wire, the more resistance there will be. We're now going to go back to Ohm's law. An equation that we can recap at the start of this lesson. And we learned about in lesson six. In lesson six we learned that there's a connection between the potential difference across a component, the current though it and its resistance. And this equation is also important in this lesson today. The relationship if plotted on a graph will look like this. I now like you to pause the lesson and have a look at this graph. Take a moment to read the access and decide what you think is happening, then press play when you're ready to discuss. So here we have a graph to represent Ohm's law. Potential difference is directly proportional to the current at constant temperature. This graph shows a fixed resistance of one ohm and we can check this by using the points that I've indicated on the graph. So you know that Ohm's law is potential difference equals current times resistance. So when you rearrange this to get resistance as the subject, it's potential difference divided by current. So if I do potential difference 12 volts divided by my corresponding current 12 amps I'm going to get a resistance of one ohm. And you can check that this is true throughout if you read up from the X-axis from any of the points and then read across the Y-axis, you should get an answer of one when you divide the potential difference by the current. So the fixed resistance of this graph is one ohm. We're now going to look at how you measure resistance. And this is a very important practical which you're going to redo at Key Stage 4, electricity. Before we look at the practical itself I just want to remind us about the variables. So you should all be aware that there are three variables, the independent variable, the dependent variable and control variables. So please pause the video now and match up the name of the variable to the correct description. Press play when you're ready to go through your answers. So you should have gotten them matched up like this. The independent variable is the variable we choose to change to see its effect on the outcome. whilst the dependent variable is the variable that is measured to judge the outcome of the test. And finally, the control variables are the quantities we need to keep the same because if we didn't keep them same, they would affect the outcome of our investigation. So the student's investigation was the length of the wire affects resistance. So I'd like you to have a going to at answering these two questions. So what is the unit for electrical resistance and then can you identify the variables? So what is the student changing, what are they measuring and what should they really keep the same? So pause the video now and have a go at these two questions and press play when you're ready to go through the answers. Amazing work. So you should have noted that the resistance is measured in ohms. And then you should have identified the variables. So the independent variable is the length of the wire because that is what the student is changing. The dependent variable is the resistance of the wire because that is what will be measured. And finally, the control variables are things that we mentioned earlier, all the factors that affect resistance. So they include the diameter of the wire, the material of the wire and the current through the wire. And that's really important as that allows us to keep the temperature constant. If we keep the current constant at about 0.5 amps, the temperature will not change as this will not affect resistance. What do you think would be a suitable range and interval for independent variable, the length of the wire? Great. So suitable range would be from about 20 to a 100 centimetres and the suitable intervals would be 10 centimetres. Should be 20, 30, 40, 50, 60, 70, 80, 90, a 100. So our range is 20 to a 100 and our intervals are 10 centimetres. So that's the space between each data point. So here we have our results table. And in the left hand column, you usually have the independent variable. So that's the variable that you're changing. So in our case, it's the length of the wire. So that's going to go in the left hand column. Normally in a table, we have the dependent variable on the right hand side. Here, the dependent variable is the resistance. But in order to find the resistance, we have to calculate it from the potential difference on the current. So we need to include those in our table as well. Resistance equals potential difference divided by current. So that you should remember that from our rearrangement of the Ohm's law equation in the last lesson. So also important to note that we're going to have to keep our current constant in order to stop the temperature from changing. So what are we going to use to measure the potential difference? Brilliant. It is the voltmeter, well done. And how is the voltmeter going to be connected? That's right. Across the ends of the component that you're trying to measure the potential difference across. What we're then going to use to measure the current through the component? Brilliant. It is the ammeter, well done. So we're going to use the ammeter to measure the current through the component. And how is the ammeter to connected? Amazing, well remembered. The ammeter is connected in series with the component. Here is a circuit setup that we're going to use no investigation. So put your pens down and listen as I run through the method. We firstly set up our circuit using a power park and ammeter which is placed in series and two wires with crocodile clips on the end, we will then attach our crocodile clips one metre apart on the wire which we want to measure the potential difference of. We're going to connect the voltmeter in parallel across the wire and make sure it's one metre apart otherwise we're not going to get accurate readings. Once we've done that, we'll switch the power pack on and adjust the dial upwards from zero so the current reading is 0.5 amps. Once we've done this, we need to take some readings. So firstly, we need to make sure we've recorded the length of the wire. We then need to measure the current through the wire and the potential difference across the wire. Step four is to switch off the power park whilst we write down our readings. And then we're going to repeat this practical again but decreasing the length of the wire by 10 centimetres each time until we reached 20 centimetres. Before we go for our results, we're just going to have a look at some of the science behind the investigation. As we looked at previously in this lesson, as the electrons move through the wire they collide with the metal ions and a transfer of energy takes place. You can see this now if you rub your hands together. What do you notice? That's right. They get hotter because the friction has caused there to be a transfer of energy. And the same thing happens inside the wire. Every time the electrons and the metal ions touch, the metal ions are going to get hotter due to this transfer of energy. And that's exactly how filament lumps work. The filament or wire inside gets hotter and grows inside the glass bulb. Now, I'd like you to think back to the unit on particles. what will ions do if they are hotter. That's right. They're going to vibrate more. And if they vibrate more will this make it easier or harder for the electron to move through? That's right. It's going to make it a lot harder if the ions are vibrating more for electrons to get through. So what's going to happen to the resistance? It is going to increase, brilliant. So this is why we need to make sure we turn off the power pack between readings. One, because it's going to affect our results as it will increase the resistance the longer it's turned on for. And two, it's also for safety because it's going to glow very, very hot. So that wire will get very hot very fast. So we need to make sure when we're writing our readings down that we switch off our power packs, one, to stop the resistance from getting higher and two, for safety reasons. So I've just gone through the method. What I'd like you to do is pause the lesson and have a go at putting the method in the correct order. So read all five steps and then decide what order are they going to go in. Press play when you're ready to continue with the lesson. So it's now time for you to check your answers. So pause the lesson and make sure you've got the five steps in the correct order before pressing play to resume the lesson. We're now going to have a look at risk assessments. So in any risk assessment, the first thing you do is to identify the hazard. So that is what is dangerous in your practical. You then need to decide the risk. So why is it dangerous? Why is your hazard a risk? And finally, what measures are you going to take in order to reduce the risk? So these would be your safety precautions. So considering our method and what I've already mentioned, let's have a look at our risk assessment for this practical. So what could be a possible hazard. That's right. The hot wire super. So our hazard is the fact that the wire is going to heat up. And how is that a risk? Excellent. It's a risk because it could lead to burns if touched. And if it also melts, it could end up catching failure if it's around flammable materials. So we've got a hazard that the wire heats up and this is at risk of burning someone. So what safety precaution can we take? That's right. Limit the current to 0.5 every time and always start from zero on the power pack dial and increase it up to 0.5 amps. Don't leave it there. So well done if you've got these three correct. So here's some example data from the practical. So what I'd like you to do is pause the video and have a go at calculating resistance. So I've told you the potential difference and I've told you the current. What you need to do is make resistance the subject like we did in lesson six and calculate resistance. Press play when you're ready to go through the answers. Amazing. So please check your answers now. So before we move on I just want to check our understanding of variables. So what was our independent variable? That's right. The length of the wire because that's what we changed. And what was one of our control variables that you can see in this table? Brilliant. The current. So we kept the current the same at 0.5 amps throughout. And finally, what was our dependent variable? What were we measuring? Great work. It was the resistance, well done. Now some of you may have also noticed that we probably should have kept the potential difference the same. However, our potential difference had to be adjusted in order for us to ensure we kept the same current. By keeping the same current, this allowed us to keep the temperature the same and also allowed us to calculate the resistance. So here's a graph of those results. What I'd like you to do is complete the concluding sentence. So what is the relationship between length and resistance of the wire. In order to do this, we need to make sure we've drawn a line of best fit. By drawing a line of best fit, this will check for what? That's right, anomalies. So by drawing the line of best fit, it allows us to clearly identify any anomalies, which we need to omit from our conclusion. So as we look at this line of best fit here we can say, that as the length of the wire increases, the resistance increases. Well done. In a moment I'll ask you to write a conclusion for our data. But first I want to show you some examples for us to discuss. So here we've got two conclusions. So I want you to pause the video and read both answers. What is good about each of them? Then tell me, which is best and why? So pause the video, read the two conclusion answers and then decide which one is better and why? So you should have noted the answer one has made a general statement about the relationship and used data. However, answer two has stated what type of mathematical relationship this is and given examples of where the values are directly proportional. Therefore, answer two is better. What I mean by direct proportionality is that when one variable is multiplied or divided by constant factor, the other will be multiplied or divided by that same factor. So if you have a look at answer number two, they stated that if you double the length of the wire, the resistance also doubles. So what happens to one variable will also happen to the other. In a moment I'm going to ask you to describe the relationship between the length of wire and resistance for our results. In your answer, it's really important that you use data to back up your statements. However, you may have learned in other units that using a table to write a conclusion is a good idea. However, from a table it's really difficult to identify relationships and also to spot anomalies which you need to omit. Therefore, you always make your conclusions by drawing a line of best fit on a graph and not from raw data in a results table. Because like I've said, some of this data may be anomalous. So here I've drawn a graph of our results. So what I'd like you to do is describe the relationship between the length of a wire and the resistance and using data from the graph. Because from our table it was hard to see direct proportionality like we can on this graph. So pause the video now and have a go at answering this question and press play when you're ready to go through your answer. Here's the conclusion that you should have written. The length is directly proportional to the resistance. So if we double the length of the wire, the resistance also doubles. An example of this is when the length is 40 centimetres. The resistance is two ohms. And when the length is 80 centimetres, the resistance is four ohms. So by doubling the length from 40 to 80 centimetres, my resistance has also doubled from two to four ohms. What I'd now like you to do is pause the video and see how many more directly proportional relationships you can see from this graph. What I mean by that is, are there any other cases where the length of the wire increases by particular multiple factor and the resistance increases by the same factor. Press play when you're ready to continue. That was a really tough lesson. So I'm really impressed that you've made it to the end and you're all still smiling. Thank you for working so hard once again. It's now time for you to have a go at the exit quiz and not see it for lesson eight. Bye.

Worksheet: Measuring resistance

Exit Quiz: Measuring resistance

Completed: Measuring resistance

This lesson focuses on identifying variables in the measuring resistance practical and how we can appropriately display our results. We will then describe and explain the effect of a length of wire on resistance

Science

Measuring resistance

Video

Worksheet

Final Quiz

Lesson Complete