Coulomb's law states that the force between two point charges is inversely proportional to the square of their distance.
If we have two charges of q1 coulombs and q2 coulombs which are d metres apart, then the Coulomb force, F, can be calculated by F=kq1q2/d2 where k is Coulomb's constant.
If the result is negative then the charges attract each other, otherwise they repel.
Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
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