Coulomb's Law

Force between two point charges in vacuum is F = kq₁q₂/r² where k = 1/(4πε₀) = 8.99 × 10⁹ N·m²/C².

Force is along the line joining the charges. Like charges repel, unlike charges attract.

Coulomb force obeys Newton's third law — charges experience equal and opposite forces.

In a medium of relative permittivity εᵣ, force becomes 1/εᵣ times the vacuum value.

Inverse-square dependence: doubling distance reduces force by factor 4.