To understand how to calculate forces between charged particles, particularly the dependence on the sign of the charges and the distance between them. coulomb's law describes the force that two charged particles exert on each other (by newton's third law, those two forces must be equal and opposite). the force f21 exerted by particle 2 (with charge q2) on particle 1 (with charge q1) is proportional to the charge of each particle and inversely proportional to the square of the distance r between them: