Coulomb's law and electric fields
Electric force follows an inverse square law, and the field at a point is the force per unit charge placed there.
A worked example
Two 5.0 microcoulomb charges 0.20 m apart repel with F = 8.99 x 10^9 x (5.0 x 10^-6) squared / 0.20 squared = 5.6 N.
The mistake people actually make
Convert microcoulombs to coulombs and square the separation, not just the number in front of it. Tripling the distance cuts the force to a ninth, not a third.