The force is measured by following formula-
                                                                                          F = k * Q1 * Q2 / r²   (Coulomb’s inverse square law)
Where k – Constant of proportionality.
F – Force (attractive/repulsive) on each charged particle.
Q1 – Point charge 1.
Q2 – Point charge 2.
r – Distance between the two particles.
There are three conditions to be fulfilled for the validity of Coulomb’s law:
  1. The charges must have a spherically symmetric distribution (e.g. point charges, or a charged metal sphere).
  2. The charges must not overlap (e.g. be distinct point charges).
  3. The charges must be stationary with respect to each other.