E(\vec{r})=\int_{\vec{r}_0}^{\vec{r}}\vec{F}(\vec{r'})\cdot d\vec{r'}=\int_{\vec{r}_0}^{\vec{r}} k (\vec{r}_0-\vec{r'})\cdot d \vec{r'}=\frac{1}{2}k |\vec{r}_0-\vec{r}|^2