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.