The Uniform Field

A large charged plate makes a uniform field: the same everywhere near it. With $\sigma = Q/A$ its charge per square metre:

$$E = \frac{\sigma}{2\varepsilon_0}$$

on each side, away from a positive plate. $\varepsilon_0 = 8.85\times10^{-12}\ \mathrm{C^2/N\cdot m^2}$.

Two plates with equal and opposite charges: between them the two fields add, outside they cancel.

$$E = \frac{\sigma}{\varepsilon_0}$$