Coulomb's law (C. de Coulomb): The basic law for electrostatics, equivalent to Newton's law of universal gravitation. It defines that the force between two point charges is proportional to the arithmetical product of their respective charges and also proportional to the inverse square of the distance among them; mathematically,
F = 1/(4 pi epsilon0) (q Q/r2) e,
Here q and Q are the strengths of the two charges, r is the distance between two, and e is the unit vector directed from the test charge to second.