I want to find the potential of a conducting disk in space whithout laplace equation.(i know how to do that).I want to use the potential of a uniformly charged rod.because its equipotentials are ellipsoids(ellipses rotated around the rod).now a disk is a limiting case of an ellipsoid. I cant get through the math part.can someone write and simplyfy it? because a disk is an ellipsoid where semi major axis approaches zero and semi minor axis approaches R then c approes iR !!