 So it turns out that Faraday's law of induction is actually very important to our technology. For example, it's how we do a conversion of energy from mechanical energy into electrical energy. Let's look at a schematic version of the simplest version of one of those devices, which is an AC generator. So you start with a magnet or magnets that produce a permanent magnetic field. And then we're going to put in a coil. And that's just a coil of wire. And what we're going to do is we're going to rotate it around its axis. And because we've got this area of coil and we're turning it through the magnetic field, and what that's going to do is change the magnetic flux that's going through that coil. The magnetic field is going to be constant. The actual area of the coil is of course constant, but the angle is changing and therefore the perpendicular area is changing. Indeed, it's changing sinusoidally. So this A loop is the total area of our loop here, but it's changing sinusoidally because the angle is changing. The angle is going to go through two pi radians every time we go through a period. And so that cosine has to be two pi radians every time we go through a period. So time divided by period. So obviously that flux is changing. And so obviously it's going to be time dependent and therefore it's going to produce a voltage in that coil. And if you put many turns on that coil, then you'll get a stronger voltage in that coil. If we look at the flux as a function of time, we start at time equals zero, cos of zero is one. And so we start at the maximum of the magnetic field times the total area of the loop. And then cos just goes like this. Now, if we want to see how that flux changes in time, we have to look at the slope of this graph. And we can see that initially it doesn't have any slope at all. And so this will be zero. So while the coil is in this position, there will be no voltage through the coil. But then when we get to here, we'll have quite a high slope. And then at this point, we'll have none again. And then a strong slope going other way. And then none and so on. And so we can see at the point where the slope is the maximum in the negative direction. We've got a maximum slope there. And when it's flattened out, we have zero change in our magnetic flux. And then we've got our maximal change and so on. And of course, the change in the magnetic flux is precisely just proportional to the voltage. And therefore, this is a picture of the voltage we're going to get out. If we do that mathematics properly, we'll find that that's also a sinusoid. And you'll see that it's alternating. And that's why this is called an AC generator. It's also known as an alternating current generator. This is actually how all our power is transmitted. In Australia, the frequency at which this is oscillating is 50 hertz, 50 times a second. Now some devices don't like the potential to oscillate like that. They're like a nice constant potential. And those are called direct current devices, DC. And to make a DC generator, what you do is you basically take an AC generator and cheat. So all the business end of the generator with the magnetic fields and the coils and whatnot is basically the same. But then once these wires come off here, what we can do is we can construct clever devices that switch which wire these cords are connected to every half turn. And so what that's going to do is it's going to take our old alternating current and it's going to make it look like this. And there you can see you've got a much flatter looking voltage, although it does have these dips here. And the way you can fix that is you can put in extra coils. So you might have back in your generator, you might have another coil that's at right angles. And the currents are going to come out of phase with each other. So they're going to look like this. And if you're really careful, you can organize them to add together such that you get a very smooth current indeed.