Problem 3.200

Between the plates of a parallel-plate capacitor there is a metallic plate whose thickness takes up $\eta=0.60$ of the capacitor gap. When that plate is absent the capacitor has a capacity $C$ $=20$ nF. The capacitor is connected to a dc voltage source $V$ $=100 \mathrm{~V}$. The metallic plate is slowly extracted from the gap. Find: (a) the energy increment of the capacitor; (b) the mechanical work performed in the process of plate extraction.