PX275 -G11b - other examples

\begin{matrix} \vec{\nabla} \cdot \vec{E} = \frac{ρ}{ε} \\ \vec{E} = - \nabla ϕ \\ \nabla^{2} ϕ = - \frac{ρ}{ε} \end{matrix}

\nabla^{2} ϕ = 0

\vec{\nabla} \cdot \vec{E} = \frac{δ (\vec{r})}{ε}