Between the plates of a parallel-plate capacitor there is a metallic plate whose thickness takes up η = 0.60 of the capacitor gap. When that plate is absent the capacitor has a capacity C = 20nF. The capacitor is connected to a dc voltage source V = 100 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.