1. The problem statement, all variables and given/known data A cone made of an insulating material has a total charge Q spread uniformly over its sloping surface. Calculate the energy required to bring a small test charge q from infinity to the apex A of the cone. The cone has a slope of length L. 2. Relevant equations 3. The attempt at a solution Since work done equals change in P.E so I have to find the P.E of the system. Suppose at a distance x from A I take an elementary cylinder of length dx. Let charge density be σ. dQ=σdV Wait! I don't know PE of disc and this seems an extremely time consuming process to me. Is there any other method?