# Electric field

The force per unit of positive test charge at a point, E = F/q. For a point charge E = k q / r squared.

Physics (university) · Part 07: Electric fields and potential · Card 3 of 10 · Last updated 2026-10-19

## Analogy

A weather map that shows how strongly a charge would be pushed at each spot.

## Example

**Field of a point charge**

E = k q / r^2
E = 8.99e9 x 2e-9 / 0.25
E = 71.9 N/C

## In practice

A 2 nC charge makes a field of about 72 N/C at 0.5 m.

## Practice

### Flashcards

- **What is the SI unit of electric field?** Newtons per coulomb (N/C), the same as volts per metre.

### Interview questions

**Use Gauss's law to find the field outside a charged conducting sphere of charge Q and radius R.**

- Choose a spherical surface
- Field is the same size everywhere on it
- E x 4 pi r^2 = Q / epsilon0 gives E = kQ/r^2

## Questions people ask

### What is a test charge?

A test charge is a tiny positive charge used to probe a field. It is small enough that it does not disturb the charges creating the field. The field at a point is the force on the test charge divided by its charge, so it does not depend on the test charge.

### What does the field of a dipole look like?

A dipole is a positive and a negative charge close together. Field lines leave the positive charge and curve around to enter the negative one. The field falls off faster than for a single charge, as 1/r cubed far away, because the opposite charges nearly cancel.

## Related concepts

- [Gauss's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-5)
- [Electric charge](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-1)
- [Coulomb's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-2)
- [Field lines](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-4)

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