where F is the force exerted by particle 1 on particle 2 (and vice versa), k is the Coulomb/electric constant, 8.99 x 109
is the charge of particle 1, q2
is the charge of particle 2 and r is the distance between the centres of the particles/charges.
is the permittivity of free space. We usually just use k.
We assume the charges are point-like and all the charge is concentrated in the center. An approximation is to assume the separation of the charges, r, is much greater than their radii.
The law is an inverse-square-law,