Coulomb’s law · Year 13
Coulomb’s law gives the size of the electric force between two point charges. It depends on the product of the charges and gets weaker with the square of the distance – just like gravity, but it can attract or repel and is enormously stronger. It explains how atoms hold together and how charged particles push each other around.
Key facts
- F = kQ₁Q₂ ÷ r², with k = 9.0 × 10⁹ N m²/C².
- Like charges repel; unlike charges attract. The forces on the two charges are equal and opposite.
- Inverse square: doubling r makes F a quarter; tripling r makes it a ninth.
- Always convert: µC → × 10⁻⁶ C, nC → × 10⁻⁹ C, cm → ÷ 100 m.
- For several charges in a line, find each force separately (with its direction) and add them as vectors.