1. The problem statement, all variables and given/known data Given a coaxial cable with outer conductor of radius b, and inner conductor of radius a. Prove that maximum potential is obtained when a = b /e, e is base of natural log. 2. Relevant equations gauss law, ... 3. The attempt at a solution By using Gauss Law, for a < r < b, i have E = 2*k*[tex]\lambda[/tex] / r, where [tex]\lambda[/tex] = Q/L then the potential difference, V= [tex]\int[/tex] E dr V = 2*k*[tex]\lambda[/tex] ln (b/a), but i am not sure how to continue by differentiating V wrt a. Help me pls... Thank you.