Back Ford
page 12
CDA logo
Electromagnetic induction Electricity from movement
What is a generator?
Photo of wind turbine
Picture 4.1 A wind turbine turns a generator to generate electricity. The viewing platform shows how big the generator is.

In picture 4.1, the wind turbine turns a generator to produce electricity at the rate of 80 kW (that's 80 kJ every second). The turbine is the ‘windmill’ part at the front. The wind makes it turn round.

In turn, the turbine turns the generator; inside the generator is a coil of wire which is made to turn in a magnetic field. Making the coil turn produces a voltage in the coil. The voltage can drive a current along the cables of the National Grid to light up our homes.

The generators in a power station are similar, but much bigger. They are capable of producing many megawatts of power.

A generator is a bit like a motor in reverse. We put in movement and can take out an electric current.

This is an example of electromagnetic induction – a voltage is induced in the coil when it moves in a magnetic field.

Part of generator animation Part of generator animation Part of generator animation
Part of generator animation
4.2 Use the controls to spin the coil slowly, fast or step through it. (Note that the step button represents the slow spin of the coil using freeze frame.)
The meter is showing zero when it is in the middle.
You can download an animation of a wind turbine.

Windows users: right click here and choose 'Save target as" from the menu to save it to your hard drive. This is a safe, 800 kB executable file. Once it has downloaded, choose "Run" from the options.

Mac users: control/click here and choose 'Save as" from the menu to save it to your hard drive. This is a 1.3 MB flash movie which will open in the Flash player if you have it. Otherwise, open it your browser with the flash plug-in.

Inside a generator
The inside of a simple generator is very similar to the inside of a simple electric motor. There is a coil that is free to rotate between two magnets. The magnets are linked by a steel former and the coil is connected to wires using brushes. However, instead of a commutator, the generator uses slip rings. So the contacts are not swapped over - each brush maintains contact with the one end of the coil all the way through the cycle.

A voltage is induced when the coil turns in the magnetic field.

Use the controls to run the animation at different speeds or step through it.

Notice:

  • the position of the coil when the induced voltage reaches its maximum value
  • the change of direction of the current during a cycle
  • the effects of turning faster.

What makes a generator work?

A generator generates a voltage. It will deliver a current when we connect it to a load (such as a light bulb). The current lights up the bulb. However, it also makes it harder to turn the generator.

We have to work harder to keep the generator turning once a current is being taken. The more current we take from the generator, the harder it is to turn it.

This makes sense: we don't get something for nothing. As soon as we get the generator to do work for us, we have to put more work into it. And the more work we get it to do, the more work we have to put in. If this weren't the case, we'd be getting something for nothing. And this would go against the ideas of the conservation of energy.

There is a good physical reason why it gets harder to turn the generator when it is delivering a current: it starts to behave like a motor. The current is flowing in the coils. Therefore there is a force on the coils – as though it were a motor. And this force will oppose the motion of the generator and make it harder to turn it. This is the physical origin of Lenz's Law (see page 13). The force on the induced current opposes the force you apply to make the current flow.

Interactive graphic of cyclist
Picture 4.3 A bicycle dynamo generates a voltage to light a lamp. The bigger the current that it delivers, the harder it becomes to pedal.
Work in, electricity out
When you pedal a bicycle, it's a bit harder when the dynamo is working to power the lights. It isn't simply that there is more friction. You have to do work to make the dynamo supply electricity to the lights. And the more current the lamp takes, the harder it is to pedal. Whenever we take a current from a generator or dynamo, there has to be some mechanical driving force:
  • A cyclist does work pedalling to turn the dynamo (using up the chemical energy from their food).
  • The wind does work to turn the turbine; the wind slows down.
  • Moving steam in a thermal power station turns the turbines which turn the generators (we have to burn more fuel to make more steam).

In each case, we don't get something for nothing. In order to deliver an electric current, we need to do mechanical work.

Question 11
Look at the simple generator in picture 4.2.

a) Is the output voltage a.c. or d.c.?

b) What is the effect on the voltage of reversing the direction the coil rotates?

c) When the coil turns faster, two features of the output voltage change. What are they?

d) When the output voltage is greatest,
i. what is the position of the coil?

ii. what is the direction of movement of the sides of the coil compared with the magnetic field?

back top next