Coulomb's law states that the magnitude of the force that one charged particle exerts on a second charged particle is proportional to the magnitude of the charges and inversely proportional to the square of the distance between them. It is written as F = kQQ/r2 , where F is the force, Q is the charge on the first particle, Q is the charge on the second particle, r is the distance between the two particles, and k is a proportionality constant. The direction of the force is along the line joining the two particles. Coulombic force is one of the principal forces involved in atomic reactions.

The proportionality constant k in Coulomb's law is associated with the environment of the charged particles. In a vacuum the proportionality constant is written as 1/4pe0 where e0 is the permittivity of vacuum and is obtained experimentally. In SI units k...