\begin{equation} \vec{F} = k \frac{q_1 q_2}{r^2} \hat{r} \end{equation} \begin{equation} \vec{F} = \frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r^2} \hat{r} \end{equation}
\begin{equation} \vec{E} = \frac{F}{q_{\, test}} \hat{r} \end{equation} \begin{equation} \vec{E} = \frac{1}{4 \pi \epsilon_0} \frac{q_{\, source}}{r^2} \hat{r} \end{equation}
\begin{equation} \vec{\tau} = \vec{p} \times \vec{E} \end{equation} \begin{equation} U = - \vec{p} \cdot \vec{E} \end{equation}