The force of attraction between the plates of air filled capacitor having charge Q and area of each plate A is given by

Let x be the separation between the plates of the condenser. The energy stored in the capacitor is given by U=12CV2=ε0AV22x ∵C=ε0Ax ∴ dUdx=−ε0AV22x2So,  F=−dUdx=ε0AV22x2                      (1)Further,   Q=CV=ε0AVxor              V=Qxε0A                         ...(2)From eqs. (1) and (2), we getF=ε0A2x2×Qxε0A2=Q22ε0A