Identical isolated spheres 1 and 2 have equal amounts of charge and are separated by a distance large compared with their diameters. The electrostatic force on sphere 2 due to sphere 1 is (fig.a). Suppose now that a third identical sphere 3, having an insulating handle and initially neutral, is touched first to sphere 1 (fig.b) and then to sphere 2 (fig.c), and finally removed. In terms of , we have to find the force (fig.d) that now acts on sphere 2.

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Let initially the equal charge on identical conducting spheres 1 and 2 be q. By Coulomb’s law the magnitude of force F on 2 due to 1, if the separation d between the two charges which is assumed to be much greater than their size, will be

When the identical conducting sphere 3 which is electrically neutral and is held by an insulating handle is touched with conducting sphere 1, charge of amount will be passed on to sphere 3 from sphere 1. Now the conducting sphere 3 which has charge on it is brought in contact with sphere 2 having charge q, the total charge of amount will be equally divided between conducting spheres 2 and 3. The charge on conducting sphere 2 will therefore be .

Therefore, the magnitude of force on sphere 2 due to conducting sphere 1 by Coulomb’s law will be