# Field of a long straight wire

A current makes circular field lines around the wire. Its size is B = mu0 I / (2 pi r).

Physics (university) · Part 08: Circuits and magnetism · Card 9 of 12 · Last updated 2026-10-19

## Analogy

Ripples spreading around a stick in water.

## Example

**Wire field**

B = 2e-7 x I / r
B = 2e-7 x 10 / 0.05
B = 4e-5 T

## In practice

10 A at 5 cm gives a field of 4e-5 T, close to Earth's own field.

## Practice

### Flashcards

- **How does a straight wire's field vary with distance?** B falls as 1/r.

## Questions people ask

### How do I find the field direction around a wire?

Use the right-hand rule: point your right thumb along the current and your curled fingers show the field direction. The field circles the wire. Reverse the current and the circle reverses. Field lines never point toward or away from the wire.

### What is mu zero?

Mu zero is the permeability of free space, 4 pi times 10 to the minus 7 tesla metres per ampere. It sets how strongly currents create magnetic fields in a vacuum. It appears in the wire formula and in the equations for solenoids and light speed.

## Related concepts

- [Magnetic field](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-8)
- [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)

Web page: https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-9
Study this card in the app: https://studyvisually.com/app/learn?category=physics-university&card=physics-university-part-8-concept-9