Coulomb’s law states that the electrostatic force between two point charges at rest is directly proportional to the magnitude of charges and inversely proportional to the square of distance between them. (The force acts along the line joining the two charges)

F ∝ q_{1}q_{2}

Or, F ∝ 1/r^{2}

F = k q_{1}q_{2}/ r^{2}

where k is proportionality constant and equals to 1/4 π ε_{0}. Here, ε_{0} is the electric permittivity of a vacuum. The value of k is 9 × 10^{9} Nm^{2}/ C^{2} in S.I unit and value of ε_{0} is 8.854 × 10^{-12} C^{2} N^{-1} m^{-2}.