# Magnetic field

A region where moving charges and magnets feel force. Measured in tesla. Field lines form closed loops.

Physics (university) · Part 08: Circuits and magnetism · Card 8 of 12 · Last updated 2026-10-19

## Analogy

An invisible landscape that a compass needle follows.

## Example

**Field size**

Earth: 5e-5 T
Fridge magnet: 5e-3 T
MRI scanner: 1.5 T

## In practice

Earth's magnetic field is about 5e-5 T. A fridge magnet is about 5e-3 T.

## Practice

### Flashcards

- **What is the SI unit of magnetic field?** The tesla (T).

## Questions people ask

### Can you have a magnet with only one pole?

No one has ever found one. If you cut a bar magnet in half, you get two smaller magnets, each with a north and south pole. Field lines have no start or end, so magnetic field lines always form closed loops.

### What makes a magnet magnetic?

Spinning electrons inside atoms act like tiny magnets. In ordinary materials they point in random directions and cancel. In iron, groups of atoms called domains line up, so the effect adds up. A magnet has most domains pointing the same way.

## Related concepts

- [Field of a long straight wire](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-9)
- [Magnetic force on a charge](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-10)
- [Circular motion in a field](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-11)
- [Force on a current-carrying wire](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-12)

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