 So, in this video, I wanted to introduce you to a new word called resistivity. Resistivity allows us to compare the resistance of different materials. You might ask, why don't we just use resistance to compare materials? I'll show you why. Let's suppose I had three wires. All the wires have the same thickness. They have a cross-sectional area of one square millimetre, which is one millionth of a square metre. The first is 10 metres long and made of copper. The next two wires are only one metre long. One is made of aluminium, and the other one is made of copper. So I can measure their resistance using what's called an ohmetre. So I measure the resistance of the first copper wire, and I get 0.172 ohms. Then I measure the resistance of the first aluminium wire, and I get 0.027 ohms. Well, if I look at those two numbers, I could say aluminium has a lower resistance in copper, and end my experiment there. But hopefully, all of you are good scientists and realise that I've cheated, and it isn't a fair test because the piece of copper was longer than the piece of aluminium. So in order for it to be a valid experiment, I need to try and keep all the other variables the same. So I can measure the third wire, which is made of copper, and is one metre long. Its resistance is 0.017 ohms. So this is lower than the 0.027 for the aluminium wire, so now I can say that coppers are better conductor than aluminium. So if we want to compare different materials, it so happens that resistance is not good enough. We need to take the length into account as well, because the resistance is directly proportional to the length. I could write this as an equation. But then, why can't we just compare materials by talking about their resistance per metre? That would be fair, right? Well, it so happens that there's another variable which affects the resistance of material. Suppose that I had a fourth wire that was still one metre long, except this time it had a cross-sectional area that is ten times larger, so it's ten millimetres squared. So the wire is ten times thicker. This means it has a lot more electrons in it that are able to move. So the resistance is lower, ten times lower in fact. So the resistance of this wire would be 0.0017 ohms. However, our ohm metre can't read that accurately, so it just says 0.002 ohms. So resistance is also inversely proportional to area. So if we took both of these equations into account, then we would get that resistance has to be proportional to length and inversely proportional to area. But it's not necessarily equal, so we have to add in a constant, which I'm going to represent with the Greek letter rho. So the term that I've just introduced here, rho, it kind of looks like a little bit across between e and p, is the resistivity. And it's the most accurate way to compare the resistance of different materials. So we can figure out the units of resistivity using this equation. First, let's rearrange. We know that the units on both sides of the equation must be the same. So what are the units on the left-hand side? We don't know. But on the right-hand side, we have resistances ohms. Area is metres squared, and that's all divided by metres. So if I cancel out the units, then I get the units on the right-hand side are ohms times metres. Therefore, the units on the left-hand side of the equation are also ohm metres, which means that the resistivity has units of ohm metres. So if you find it difficult to memorise the units of resistivity, there's no need to worry, because as long as you can remember the equation above, you can always work them out.