Imagin a parallel plate capacitor with a charge `+ Q` on one plate and `-Q` on the other. Initially, the plates are almost, but not quite, touching. The plates are gradually pulled apart to make the separation d. The separation d is small compared to dimensions of the plates and we can maintain that field between the plates is uniform. Area of each plate is A.
(a) Write the energy stored in the electric field between the plates when separation between then is d.
(b) Assuming that the energy calculated in part (a) can be attributed to the work done by the external agent in pulling the plates apart, calculate the electrostatic attraction force between the plates.

Answer

(a) `(Q^(2)d)/(2 in_(0)A)
(b) `(Q^(2))/(2 in_(0)A)`