The formula
F = k · q₁ · q₂ / r²
| Symbol | Quantity | Unit |
|---|---|---|
| F | Electric force | N |
| q₁ | First charge | C |
| q₂ | Second charge | C |
| r | Distance between charges | m |
What it means
Coulomb's law gives the force between two point charges: proportional to both charges and inversely proportional to the square of the distance between them. Like charges repel, opposite charges attract.
How to work it out by hand
- Multiply the two charges together
- Divide by the square of the distance
- Multiply by Coulomb's constant, about 8.99 × 10⁹
What is worth knowing
The inverse-square form is identical to Newton's law of gravitation, but the strengths are not remotely comparable: the electrical repulsion between two protons is about 10³⁶ times their gravitational attraction. That is why gravity is irrelevant inside an atom and dominant between planets — matter is electrically neutral in bulk, so the far stronger force cancels itself out and the weak one is left to shape the universe. Note also that the force acts along the line joining the charges, and that superposition holds: with several charges, add the individual forces as vectors.