Problem 3.145

A cylindrical layer of dielectric with permittivity $\varepsilon$ is inserted into a cylindrical capacitor to fill up all the space between the electrodes. The mean radius of the electrodes equals $R,$ the gap between them is equal to $d,$ with $d \ll R .$ The constant voltage $\hat{V}$ is applied across the electrodes of the capacitor. Find the magnitude of the electric force pulling the dielectric into the capacitor.