Chapter 2. ElectrostaticsCapacitors: Problem 2.40
Suppose the plates of a parallelplate capacitor move closer together by an infinitesimal distance ?, as a result of their mutual attraction.
 Use equation (2.45) of Griffiths to express the amount of work done by electrostatic forces, in terms of the field E and the area of the plates A.

 Use equation (2.40) of Griffiths to express the energy lost by the field in this process.