Problem 3.201

A glass plate totally fills up the gap between the electrodes of a parallel-plate capacitor whose capacitance in the absence of that glass plate is equal to $C=20 \mathrm{nF}$. The capacitor is connected to a dc voltage source $V=100 \mathrm{~V}$. The plate is slowly, and without friction, extracted from the gap. Find the capacitor energy increment and the mechanical work performed in the process of plate extraction.