
\item Example: Calculate the electric field $\vec{E}$ from a
given central potential:
\begin{eqnarray}
  \vec{E}=\nabla\,\Phi(r)\,,~~\Phi(r)=\frac{q}{4\pi\,\epsilon_0\,r}\,.
\end{eqnarray} 

Describe the solutions for $\vec{r}$ in words.
