We have to show that the plates of a parallel plate capacitor attract each other with a force given by

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Let the separation between the plates of a capacitor, area A, be x. The capacitance of a parallel plate capacitor is

The electric energy of a capacitor is

Therefore, energy of a capacitor having charge q when the separation between its plates is will be

And,

Therefore, an additional amount of work equal to will have to be done by some external agency in increasing the separation between the plates from x to . This implies that plates of a charged parallel plate capacitor attract each other with a force