 In this video we're going to have a look at how energy is conserved in a circuit. So in the diagram we have a long flat battery down the bottom here, which is connected to a light bulb through these wires. Above this is a graph which we will use to show the potential energy of the electrons as they move through the wire, the light bulb, and then through the wire again. So on the y-axis we have the potential energy of the electrons, and on the x-axis we have their position along that top wire. So you may find it helpful to pause the video and have a go completing this graph by yourself before I show you the answer. So let's start with the left side of the wire. So charges at this point are directly connected to the positive terminal, and so they will have a very high potential energy. So let's fill that in. When charges are at the other end of the circuit, they are connected to the negative terminal of the battery, and so they have very low potential energy. So I've represented that with the red lines shown. So here we have our positive terminal of the battery, where all the charges have high potential energy, and here we have our negative terminal of the battery where the charges have low potential energy. So the question is what happens to all this energy when the charges move through the circuit? So in the case of the circuit shown here, the charges energy is converted as they move through the light bulb. So as they move through the wire, originally they have very high potential energy, then they must lose this potential energy going across the light bulb, and then they have low potential energy coming down the other side. So here there's high potential energy, then that potential energy gets converted into light energy, then the charges have low potential energy and they're connected to the negative end of the circuit. So let's look at another example. What would happen to the energy of charges if they travel through a resistor as shown here in the diagram? Well, the charges start at the positive terminal with very high potential energy, so let's draw that in. And they end at the negative terminal with a very low potential energy. Let's draw that in. And so they must have lost their potential energy when they moved through the resistor, and in this case the energy is converted into heat and so the resistor heats up. So there's another example of what happens to the potential energy as charges move through a circuit. So for me, you're very confusing, but also interesting question would be what if I connected the positive and negative terminals of a battery together? Well, I've got a diagram of that here. So on the left-hand side I've got my positive terminal, I've hooked that up with a wire, and then that's going to go all the way to my negative terminal. So let's think what would happen in terms of the potential energy? Well, on the left-hand side, the charges of the positive terminal must still have very high energy, so let's draw that in. And at the negative terminal, the charges must still have very low energy, so let's draw that in. So what happens in between? Well, the charges must lose their potential energy along the wire. Now it so happens that wires have a very small amount of resistance. So if you connected a positive and negative terminal together, the electrical potential energy will be converted into heat, and the entire length of the wire will start to heat up. So warning, don't try this one at home, guys. There's a very good chance that you could burn yourself or give yourself an electric shock, because a lot of electricity or a lot of electrical current is going to move through that wire. I thought at this point I'd better mention that in my first two examples, the light bulb and the resistor, I was pretending that the wires didn't have any resistance, and therefore they wouldn't heat up at all. This is almost true. If a circuit contains a component that has a reasonable resistance, then the resistance of the wire is negligible, and you can pretend that the line at the start and the end of the circuit is completely flat. However, in reality, the areas at the start and end of the circuit would have a tiny slope, and a small amount of potential energy would be lost as the current moves through the wire.