# Electric field between capacitor plates

When we try to find the electric field between the capacitor plates, what is the right way to do it? This is one of the ways I've seen and I don't understand why:

Using a Gaussian cylinder on the positive metal plate, E inside is zero while E outside is $$\frac { \sigma}{ \epsilon_0}$$

But from what I know about conductors is that the charge uniformly spreads itself out on the surface, not only on the bottom, so why do we take a cylinder that only takes into account the charge on the bottom and not on the top as well?

• So the electric field due to one metal plate without taking into account the other metal plate, should be $E=\frac{\sigma}{2\epsilon_0}$ , correct? Jul 9, 2019 at 19:18
• @cazanova yes it is correct for a isolated metal plate Jul 9, 2019 at 19:47
• Yes, that is correct. I have deleted my comment and have posted it as an answer as I think it pretty much answers your question.
– user87745
Jul 9, 2019 at 20:29

You are implicitly taking care of the fact that you have charges on the other plate as well. You are doing it when you assume that $$E$$ is zero outside (i.e., below the bottom plate). Otherwise, you would have $$E=\frac{\sigma}{2ϵ_0}$$ both above the bottom plate and below the bottom plate.