# Gravitational, electric and magnetic fields: Unit 3.6(a)

> state Faraday law and Lenz law for electromagnetic induction

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Specification statement (Unit 3.6(a)): state Faraday law and Lenz law for electromagnetic induction

## Explanation

1. **Magnetic flux** Φ = BA (Wb) for a field perpendicular to an area A. **Flux linkage** = NΦ for a coil of N turns.
2. **Faraday's law:** the induced emf is equal to the rate of change of magnetic flux linkage: ε = −Δ(NΦ)/Δt.
3. **Lenz's law:** the direction of the induced current is such that it **opposes the change** that produces it. This is energy conservation: you must do work to keep the change going.

## Worked example

A magnet is pushed north-pole first into a coil. Which pole does the near end of the coil become, and why?

1. The flux through the coil is increasing.
2. By Lenz's law the induced current opposes the increase, so it repels the approaching magnet.
3. The near end becomes a north pole.

Answer: North pole, repelling the magnet.

## Check

Which change does NOT induce an emf in a coil?

- Moving a magnet into the coil
- Holding a strong magnet stationary inside the coil
- Rotating the coil in a field
- Switching on a current in a nearby coil

The answer and its explanation are shown in the app once you choose an option.

## Apply it

Explain, using Lenz's law, why it is harder to push a magnet into a coil when the coil is connected to a resistor than when the circuit is open.

The model answer is shown in the app after you have tried it.

## Related

- [Gravitational, electric and magnetic fields lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.fields)
- [Gravitational, electric and magnetic fields in the library](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.fields)