# Magnetic flux

How much magnetic field passes through a surface, flux = B A cos theta. The unit is the weber.

Physics (university) · Part 09: Induction, light and optics · Card 1 of 11 · Last updated 2026-10-19

## Analogy

Rain falling through a hoop: tilt the hoop and less rain goes through.

## Example

**Loop flux**

flux = B A cos 0
= 0.5 x 0.02 x 1
= 0.01 Wb

## In practice

A 0.5 T field through a 0.02 square metre loop, face-on, gives 0.01 Wb.

## Practice

### Flashcards

- **What is the unit of magnetic flux?** The weber (Wb), equal to one tesla square metre.

### 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 the angle matter in flux?

Only the part of the field perpendicular to the surface passes through it. A field parallel to the loop's plane slides past and gives zero flux, while a field at right angles gives maximum. That is why flux contains cos theta, with theta measured from the loop's normal.

### What changes flux in a loop?

Three things: the field strength B, the loop's area A, and the angle between the field and the loop. Moving a magnet nearer, stretching the loop or rotating it all change the flux. Any of these changes can induce a voltage in the loop.

## Related concepts

- [Faraday's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-2)
- [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)

Web page: https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-1
Study this card in the app: https://studyvisually.com/app/learn?category=physics-university&card=physics-university-part-9-concept-1