For the force due to electromagnetism we use Coulomb’s inverse square law:
F=keq1q2r2
(where F is the electrostatic force, q1 & q2 are the two charges, r is the distance between them, and Coulomb’s Constant ke=14πϵ0=8.99×109Nm2C−2, where ϵ0 is the permittivity of free space)