Problem 3.146

A capacitor consists of two stationary plates shaped as a semi-circle of radius $R$ and a movable plate made of dielectric with permittivity $\varepsilon$ and capable of rotating about an axis $O$ between the stationary plates (Fig.3.34). The thickness of the movable plate is equal to $d$ which is practically the separation between the stationary plates. A potential difference $V$ is applied to the capacitor. Find the magnitude of the moment of forces relative to the axis $O$ acting on the movable plate in the position shown in the figure.