1. The problem statement, all variables and given/known data 1. A portion of an infinitely long, concentric cable. The inner conductor carries a charge of 6 nC/m; the outer conductor is uncharged. The radius of the inner conductor is 1.5 cm, of the outer conductor is 6.5cm and of the inner surface of the outer conductor is 4.5 cm. Find the surface charge densities on the inside and the outside surface of the outer conductor 2. Relevant equations density= Q/A 3. The attempt at a solution 1. So I have found that the Electric Field in 1.5<r<4.5 and r>6.5 is E= 108/r (N/C) and Electric Field in 0<r<1.5 and 4.5<r<6.5 is E=0. So area of the cylinder is 2*pi*L. Thus Density= Q/A = Q/(2*pi*r*L). However, since this cable is infinitely long, how do I get L then ?