Two electric charges always exert force on each other.
The magnitude of the force exerted by two charged particles, is inversely proportional with the square of the distance between the particles.
One is able to calculate the magnitude by using Coulomb's law.
In the above formula are q1 and q2 the charges of the particles.
The k is the Coulomb constant, also known as the electrostatic constant. The ε0 is the electric permittivity.