Three charged particles lie on a straight line and are separated by a distance d as shown in the Figure. Charges and are held fixed. Charge , which is free to move, is found to be in equilibrium under the action of electric forces. We have to find in terms of .

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Applying Coulomb’s law we note that the force on charge due to charges and will be

Direction of forces on due to and as are being determined by the product of their signs, we therefore have not shown them by affixing vector signs. As the charge is in equilibrium, the net force on it has to be zero.

Therefore,

This equation gives

.