AP Physics Submitting
Schedules Course Info Ch. Reviews Guides Problems Labs Videos AP Info AP Physics
Announcements Contact Shortcuts WebAssign Canvas Equations Lab FAQ Software IWP

The Generator

player is loading....
The device that I have here is a generator, let's see what parts it has. I can turn the crank here. When I turn the crank what it does is it turns an armature inside. Now an armature is a coil of wire that's specially wound and it turns in a magnetic field. And the purpose of course is, you know from reading about generators, is to generate electrical current. Now you can't see the armature inside there but I'm just telling you about it. Also inside here, are a series of gears, and the purpose of the gears are to increase the rate of rotation of the armature compared to the rate of cranking so I can crank slowly and I can get a very rapid rotation inside the armature. The output of the generator comes through these two wires so if I clip this to something external such as a light bulb, I'll get electrical current going through the light bulb. Before I do that, I want to show you another function of this device. This can also act as a motor which basically acts as a reverse of the generator. As a motor, as you know, what we do is we take an electrical energy or electrical current and we convert it to mechanical energy of rotation. So, to make it work as a motor I just take my clips and I connect it to our battery. And what I do the current will pass through the armature inside here, the armature is in a magnetic field so the magnetic field so the magnetic field exerts magnetic force on the current and that produces a torque which turns the crank, so let's see that happen. Alright so that's the motor, alright now let's put the battery pack aside. Now we're going to see it in reverse as a generator. So this time I will be inputting mechanical energy of rotation and the result will be electrical energy. So we'll connect this to the light bulb. Now with a little bit of cranking we get light in the bulb, a lot of cranking the bulb is very bright. So as I crank faster I will be generating greater current. And as you know the power dissipated by the bulb is the square of the current times the resistance of the bulb. There is also something else interesting going on that you didn't see but I can feel it and I want to talk to you about that. When I turn this I feel a mechanical position when I'm turning, something that pushes back on my hand. And that depends on what I have connected to the circuit. If, for example, there is nothing connected to the circuit, this is very easy to turn. If I have the light bulb aside and the too clips connected, it is very hard to turn. The difference of these three situations is the resistance of the external circuit. In here the external circuit has essentially zero resistance it's only the wires (when the clips are attached to each other). Here we had infinite resistance (when the clips were not attached to anything). And here we have maybe about 10 ohms of resistance for that light bulb (when the clips were attached to the light bulb). Alright, so as you know the amount of resistance is going to influence the amount of current in the circuit. So, we had more current when there was less resistance. Now, let's examine this cause for the mechanical resistance to turning from the view point of something you know about induced electro magnetic fields and that is Lenz's Law . Lenz's Law tells us that an induced EMF gives rise to a current that opposes the original change in flux. Alright now let's see how that applies to our generator right here. As I turn this I am producing a current and that current is passing through the armature. Alright so we have a current that is being moved in a wire through this magnetic field. The result of that is induced EMF, that induced EMF will generate a current which actually opposes the current I am producing while turning it. Alright and the result of that opposition is to create a force on the armature, which opposes the force which I am applying to the crank. That appears in the term of a torque which is called a counter torque. So there is a counter torque opposing the torque or the forward torque of my turning. And that's going to be greater when there is more current in the external circuit. So that's greatest when there's no resistance in the circuit, or since there's zero resistance in the current, so that there's the greatest amount of current. And that resistance, that counter torque, is leashed when the resistance is infinite and there is not current in the outer circuit.

© North Carolina School of Science and Mathematics, All Rights Reserved. These materials may not be reproduced without permission of NCSSM.