Coulomb’s law · Grade 9
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.