Chapter 14.1, Problem 56E

If V(x, y) is the electric potential at a point (x, y) in the xy-plane, then the level curves of V are called equipotential curves because at all points on such a curve the electric potential is the same. Sketch some equipotential curves if $V(x,y)=c/\sqrt{r^2-x^2-y^2}$, where c is a positive constant.