Fundamentals of Engineering Electromagnetics 1st Edition by David Cheng

Exercise 11

For a coaxial capacitor of length L , use the method of virtual displacement to find the force between the inner conductor (radius a ) and the outer one (radius b ) that carry charges +Q and Q, respectively. The permittivity of the insulating material is .
Hint: First assume the inner and outer radii to be a and a + r, respectively; then differentiate with respect to r.