If $V(x, y)$ is the electric potential at a point $(x, y)$ in the
$x y$ -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.