Example #2 (cont.)
Start with the definition of capacitance:
C = Q/V . Then relate the voltage to the charge that has been moved. In the coaxial case, we know that the electric field between the wire and the cylinder is that due to the wire itself: E = 2kl/r, where l=Q/L . From this we can get the voltage, V = -?E?dr = -??(2kl/r)dr = 2k(Q/L) ln(rcyl/rwire).