18.3.2—Coulomb’s law F = Q1Q2 / (4πε0 r 2) for the force between two point charges

Two point charges exert forces of magnitude F=|Q₁Q₂|/(4πε₀r²), along their joining line; like charges repel and unlike charges attract.

Use centre separation r and treat the force as a vector when multiple charges act. Include signs to determine direction.

Doubling one charge doubles force, while doubling separation quarters it.

The inverse-square separation is not the distance from a charge’s surface, and equal force magnitudes act on both charges.