1. The problem statement, all variables and given/known data Two cylindrical conductors, of distance between them d and radius a (a<<d), have dielectric layer of relative permitivitty εr and thickness a. Calculate capacitance per unit length of this system. 2. Relevant equations Capacitance per unit length, C'=Q'/U Gauss law, cylindrical symmetry, E=Q'/(2πεr2) 3. The attempt at a solution I have started with the equation C'=Q'/U. Voltage integration limits: (a - 2a) + (2a - (d-2a)) + ((d-2a) - (d-a)) After calculating the partial integrals, voltage U is: U=Q'(ln2/εr+ln((d-2a)/(2a))+ln((d-a)/(d-2a))/εr) Applying voltage in the expression for capacitance per unit length gives C'=2πε0/(ln2/εr+ln((d-2a)/(2a))+ln((d-a)/(d-2a))/εr) In my books solution, in the numerator there is πε0 for C'. Are my limits of integration for voltage correct? Maybe it is a mistake in books solution. Thanks for replies.