charged plane

$E(x) = \frac{\iint_S \mathbf{E} \mathrm{d} \mathbf{S}}{2 S} = \frac{\frac{\sigma S}{\epsilon_0}}{2 S} = \frac{\sigma}{2 \epsilon_0} = konst$

$\varphi = \frac{\sigma}{2 \epsilon_0} x$