# Electromagnetic induction and transformers

> Explaining induced emf with magnetic flux, Faraday's law and Lenz's law, then applying the model to generators, transformers and efficient power transmission.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.electromagnetic-induction
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 4.5
- Difficulty: 5/5
- Assessment objectives: AO1, AO2, AO3
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 4.5(a) | calculate magnetic flux and induced emf from a changing flux linkage | AO2 |
| Unit 4.5(b) | use Lenz's law to determine the direction of induced current | AO1, AO2 |
| Unit 4.5(c) | explain generators and the effect of slip rings or a commutator | AO1, AO2 |
| Unit 4.5(d) | apply the transformer equation and evaluate transmission losses | AO2, AO3 |

## Summary

Explaining induced emf with magnetic flux, Faraday's law and Lenz's law, then applying the model to generators, transformers and efficient power transmission.

## What earns marks

- Magnetic flux through a coil is Φ = BA cos θ, and induced emf is proportional to the rate of change of flux linkage NΦ; no change means no induced emf.
- Lenz's law gives the direction: the induced current produces a magnetic effect that opposes the change in flux, expressing conservation of energy rather than a mysterious extra force.
- A generator changes mechanical work into electrical energy by rotating a coil or magnet; the output waveform and use of slip rings or a commutator depend on the design.
- For an ideal transformer Vp/Vs = Np/Ns and power is conserved; step-up transmission reduces current and therefore reduces I²R loss in cables, while real transformers have heating and flux losses.

## Where marks are lost

- Using the magnetic field itself rather than the rate of change of flux when deciding whether an emf is induced.
- Quoting Lenz's law as 'the current opposes the motion' without identifying the change in flux it opposes.
- Assuming a transformer works with steady direct current; changing magnetic flux is required and the windings must share a changing field.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Electromagnetic induction and transformers lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.electromagnetic-induction)