Field and Wave Electromagnetics 2nd Edition by David Cheng

Exercise 35

A long wire of radius a 1 lies inside a conducting circular tunnel of radius a 2 , as shown in Fig. 4-10(a). The distance between their axes is D.
a) Find the capacitance per unit length.
b) Determine the force per unit length on the wire if the wire and the tunnel carry equal and opposite line charges of magnitude l.