Verify that each of the following force fields is conservative. Then find, for each, a scalar potential $\phi$ such that $\mathbf{F}=-\nabla \phi$.
$$\mathbf{F}=-k \mathbf{r}, \mathbf{r}=\mathbf{i} x+\mathbf{j} y+\mathbf{k} z, \quad k=\mathrm{const.}$$