# Coulomb's law

The force between two point charges is F = k q1 q2 / r squared, with k = 8.99e9 N m squared per C squared.

Physics (university) · Part 07: Electric fields and potential · Card 2 of 10 · Last updated 2026-10-19

## Analogy

Gravity's cousin, but it can push as well as pull.

## Example

**Two charges**

F = 8.99e9 x 1e-6 x 1e-6 / 0.1^2
F = 0.899 N

## In practice

Two 1 microcoulomb charges 10 cm apart push each other with about 0.9 N.

## Practice

### Flashcards

- **State Coulomb's law.** F = k q1 q2 / r^2.
- **How does Coulomb force change when distance doubles?** It falls to one quarter.

## Questions people ask

### How does Coulomb's law compare with gravity?

Both fall off with 1/r squared. Electric force can attract or repel, while gravity only attracts. Electric force is far stronger: between a proton and an electron it is about 10 to the power 39 times bigger than gravity. Gravity wins for planets because overall charge cancels.

### How do I add forces from several charges?

The force on a charge is the vector sum of the forces from each other charge, which is the superposition principle. Find each force's size from Coulomb's law, split each into x and y components, then add the components and combine them to get the final size and direction.

## Related concepts

- [Electric charge](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-1)
- [Electric field](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-3)
- [Field lines](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-4)
- [Gauss's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-5)

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