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.