# Faraday's law

A changing magnetic flux creates a voltage (emf) in a loop: emf = N x change in flux / change in time.

Physics (university) · Part 09: Induction, light and optics · Card 2 of 11 · Last updated 2026-10-19

## Analogy

The faster you swing a magnet through a coil, the brighter the bulb.

## Example

**Coil emf**

emf = N x change in flux / time
= 100 x 0.01 / 0.1
= 10 V

## In practice

A 100-turn coil whose flux drops by 0.01 Wb in 0.1 s makes 10 V.

## Practice

### Flashcards

- **State Faraday's law.** Induced emf equals the rate of change of magnetic flux (times the number of turns).

### Interview questions

**Explain how a generator makes electricity using Faraday's and Lenz's laws.**

- A coil rotates in a magnetic field
- Flux changes so emf is induced
- Lenz's law explains why more work is needed when current flows

## Questions people ask

### Why does a static magnet not make current?

A steady flux gives zero rate of change, so no emf. Only changing flux induces voltage. Holding a magnet still inside a coil does nothing. Moving the magnet, switching the field on or off, or rotating the coil makes the flux change.

### How do induction cookers work?

An alternating current in a coil under the pan creates a changing magnetic field. This induces swirling currents, called eddy currents, in the metal base of the pan. The pan's own resistance turns them into heat, so the pan warms while the hob stays cool.

## Related concepts

- [Magnetic flux](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-1)
- [Lenz's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-3)
- [Inductance](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-4)
- [Transformer](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-5)

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