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.