P7.4 Induced potential and the generator effect

What is electromagnetic induction?

When a conductor moves through a magnetic field (or when the magnetic field through a coil changes), a potential difference (EMF) is induced across the conductor. If the conductor is part of a closed circuit, this drives a current. This is called the generator effect.

Faraday's law (conceptual)

The magnitude of the induced EMF is proportional to the rate of change of magnetic flux through the coil. In practice:

• Move the conductor faster → larger EMF.
• Use a stronger magnetic field (higher flux density) → larger EMF.
• Use a coil with more turns → larger EMF (each turn contributes).

Lenz's law

The direction of the induced current is such that it opposes the change that caused it. This is conservation of energy in action — you have to do work against the opposing force to keep the conductor moving.

The alternator (AC generator)

A rectangular coil rotates in a magnetic field. As it spins, each side alternately moves up then down through the field — the induced current reverses every half-turn. The output is an alternating current AC. Slip rings (not split-ring commutator) allow the current to flow out without reversing it back.

The dynamo (DC generator)

Same principle as alternator but uses a split-ring commutator — it reverses the external circuit connections every half-turn to keep the output current flowing in one direction: direct current (DC).

The microphone

Works on the same principle as the generator effect. Sound waves vibrate a diaphragm attached to a coil in a permanent magnetic field. The coil moves → changing flux → induced AC current → electrical signal.

Exam tips

• The generator uses the right-hand rule (or think: opposite to Fleming's left-hand rule for motors).
• Alternator uses slip rings; motor and dynamo use split-ring commutator — know the difference.