Generating Electricity Notes

Oxford AQA IGCSE Physics: Revision notes

Key facts

  • The generator effect induces a pd when a conductor moves relative to a magnetic field or sits in a changing field.
  • If the circuit is complete, the induced pd drives a current whose field opposes the change.
  • Reversing the motion or the magnet's polarity reverses the induced pd and current.
  • Induced pd rises with faster movement, a stronger field, more turns and a larger coil area.
  • An alternator generates ac; a dynamo generates dc.

The generator effect

A pd is induced when there is relative movement between a conductor and a magnetic field, or when the field changes.

The generator effect is the induction of a pd across a conductor moving relative to a magnetic field, or in a changing field. Moving a magnet in or out of a coil induces a pd across the coil; moving the coil past a still magnet does too. What matters is the relative movement. If the circuit is complete, a current flows.

  1. 1

    Magnet moves relative to a coil

    Relative motion

  2. 2

    Pd is induced across the coil

    Relative motion

  3. 3

    Current flows

    If the circuit is complete

The generator effect

A bar magnet is held still inside a coil. Is a pd induced?

Direction of the induced current

The induced current's own field opposes the motion that causes it, and reversing the motion or polarity reverses it.

The magnetic field of the induced current opposes the magnet's motion. This follows from energy conservation.

Reversing the direction of motion, or the magnet's polarity, reverses the induced pd and the current.

1234567−1.5−1−0.50.511.5xyinduced pd
Illustrative: pushing the magnet in gives a pulse of pd one way; pulling it out gives a pulse the opposite way.

Push magnet in

  • Current one way

Pull magnet out

  • Current reverses

Reverse magnet polarity

  • Current reverses

A magnet pushed into a coil induces a current. It is then pulled out. What happens to the direction of the current?

Size of the induced pd

Faster movement, a stronger field, more turns and a larger coil area each increase the induced pd.

The pd is larger when the relative movement is faster, the magnetic field is stronger, the coil has more turns, or the coil area is larger. For example, moving a bar magnet quickly into a coil gives a larger pd than moving it slowly.

0.511.522.533.544.55246810xy100 turns200 turns
Illustrative values: induced pd rises with the speed of movement, and a coil with more turns gives a larger pd at every speed.
  • Faster movementlarger pd
  • Stronger fieldlarger pd
  • More turns, larger arealarger pd

Which change gives a larger induced pd?

Alternators and dynamos

An alternator produces ac that reverses; a dynamo produces dc that keeps one direction.

Power stations use turbines to turn coils between magnets, or magnets in coils, generating electricity by the generator effect.

An alternator gives alternating current: a smooth sine wave that reverses. A dynamo gives direct current: the pd stays on one side of the axis and never reverses.

24681012−1.5−1−0.50.511.5timepdalternator (ac)dynamo (dc)
Alternator (ac) and dynamo (dc) output against time.

How can you tell from a pd-time graph that a supply is dc?

Try an exam question

A magnet is pushed into a coil connected to a meter. Describe what happens, and two ways to make the reading larger.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.