8.3 Exercise 12

Suppose that the capacitance of a variable capacitor can be manually changed from 100 pF to 800 pF by turning a dial, connected to one set of plates by a shaft, from $0 °$ to $180 °$. With the dial set at $180 °$ (corresponding to $C = 800 \text{pF}$), the capacitor is connected to a 500-V source. After charging, the capacitor is disconnected from the source, and the dial is turned to $0 °$. If friction is negligible, how much work is required to turn the dial from $180 °$ to $0 °$ ?