Where F is the electrostatic force, k is Coulomb’s constant (8.99 x 109 N•m2/C2), q1 and q2 are the two charges and r is the distance between them.
Coulomb’s law relates the electrostatic force between two particles to their charges and the distance between them. When the distance between charges, increases, the force between them will decrease (i.e. if the distance doubles, F will be reduced by ¼).