State Coulomb’s law, its units, and inverse-square scaling.

Solution & explanation

For stationary point charges in air, F = k abs(q₁q₂)/r², k ≈ 9×10⁹ N m² C⁻². Charges are in coulombs and r in metres. Halving r multiplies force by four; doubling both charge magnitudes also multiplies it by four. Sign determines attraction or repulsion.