 In the previous video, I showed you a few different circuits and how these could be used to convert electrical energy into various forms of useful energy. These circuits are fairly straightforward. They consisted of a power source which has provided the electrical energy and some sort of load which converted the electrical energy into a useful form. However, it's possible for circuits to be much more complicated than this. Pictures of circuits can be complicated and it's unclear how all the components are connected. In order to be able to clearly and accurately describe circuits, physicists, electricians and engineers use circuit diagrams. In a circuit diagram, different symbols are used to represent different electronic components. Some of the commonly used symbols are here. So first up, we have a battery or a power supply or sometimes it's known as a cell. So for these, there's a big line and also there's a smaller line. And the larger of the two lines represents the positive terminal. For a battery that's composed of multiple cells, sometimes they'll draw several lines together like this. The key thing is the tall line represents the positive terminal and the short line represents the negative terminal. Another component is a resistor. So a resistor converts electrical potential energy into heat or some sort of other useful energy. So the next symbol here, so we've got these two symbols here, they represent a light bulb. So a light bulb is a special type of resistor that converts electrical potential energy into light and also some of the energy is wasted as heat. Here we have an LED, so that stands for light emitting diode. So this is actually a type of diode, but it converts electrical energy into light energy. And finally, down the bottom here we have a switch. A switch is a component which allows a point in the circuit to be disconnected or reconnected easily. It lets you turn the circuit or part of a circuit on or off. Straight lines are used to represent wires which connect these different components together. So let's have a go at drawing a circuit together. Specifically, let's try and draw a circuit that contains a battery, a switch and a light bulb. So that way by flicking the switch we should be able to turn the light on and off. So first of all, we're gonna need some sort of battery. So remember the long line is the positive terminal. So then we'll have our positive terminal. We use a straight line to represent a wire which connects it to our light bulb. So then I'm gonna have a switch. So represent the switch with a dot like that. And now the switch joins back to the negative terminal of the battery. And that there is our circuit diagram. If we were to turn on the light on or off, we'd do that just by adjusting this switch here. And rather than having it in the open position we could have it in a closed position like this. So in that case the light would be on and in this case the light would be off. So note, to indicate that wires are joined it's traditional to use a filled in black circle like these three shown here. This shows that the wires are actually connected together. So this second symbol, the bridge, is used for two wires that cross over one another but are not actually joined. In the past people have used this bottom symbol to indicate that wires are unjoined but it's easy to confuse it with the wires joined symbol. So the bridge symbol, this second one is preferable.