Electromagnetic Induction

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CognitoElectromagnetic induction and generators

Key facts & equations

Electromagnetic induction

Moving a conductor through a magnetic field induces an EMF (electromotive force).
If circuit is complete, an induced current flows.
Faraday's law: magnitude of induced EMF proportional to rate of change of magnetic flux linkage.
Ways to induce EMF: move conductor in field, move field past conductor, rotate conductor in field.
Induced EMF is greater if: magnetic field stronger, conductor moves faster, more turns of wire.

Generators and alternators

Generator: converts kinetic energy to electrical energy using electromagnetic induction.
Coil rotates in magnetic field → flux linkage changes → EMF induced → current flows.
Alternator: produces AC. Slip rings and brushes maintain continuous connection as coil rotates.
Dynamo: produces DC. Uses split-ring commutator to maintain current direction.
One full rotation → one complete AC cycle. Higher rotation rate → higher frequency and higher peak EMF.

Factors affecting induced EMF

Stronger magnet → greater EMF.
More turns of wire in coil → greater EMF.
Faster rotation → greater EMF and higher frequency.
Using a soft iron core in the coil → greater flux linkage → greater EMF.
When coil is perpendicular to field: maximum rate of flux change → maximum EMF.
When coil is parallel to field: zero rate of flux change → zero EMF.

Lenz's law

The direction of the induced current opposes the change causing it (conservation of energy).
Moving N pole into coil: induced current creates N pole at near end → repels magnet.
This is why work must be done to move the magnet — the generator converts work to electrical energy.
Fleming's right-hand rule: used to find direction of induced current in a straight conductor.

Exam questions — 4 questions · 10 marks · AQA 8463 style

1 markHow to increase induced EMFAQA 8463 style

Which change would NOT increase the induced EMF in a generator?

A Using a stronger magnet

B Rotating the coil faster

C Using a coil with more turns

D Increasing the resistance of the external circuit

3 marksGenerator vs alternatorAQA 8463 style

Explain the difference between an alternator and a dynamo, including the type of current each produces and how this is achieved. (3 marks)

Hint: Two devices, two outputs. Key difference: how the rotating coil connects to the external circuit (slip rings vs split-ring commutator).

+30 XP

1 markZero EMF positionAQA 8463 style

When is the induced EMF in a generator coil at zero?

A When the coil is perpendicular to the magnetic field

B When the coil is parallel to the magnetic field (coil plane contains field lines)

C When the coil rotates at its maximum speed

D When more turns are added to the coil

3 marksLenz's law explanationAQA 8463 Higher style

A student pushes a bar magnet into a coil. The coil opposes the motion. Explain this observation in terms of Lenz's law and conservation of energy. (3 marks)