I am studying Gauss' law and have learned that using symmetry, we need to select a cyclinder in order to calculate electric field of an infinite plane sheet of charge.

[tex]2EA = \frac {\sigma A} {\epsilon} [/tex]

That equation is written using Gauss' law and hence E field is found.

However, why wouldn't we use a cube instead of the cyclinder, for instance ?

Moreover, I am not able to understand why the cyclinder is suitable. (I know it is due to the symmetry arguments, but why ? ) I mean, the normal vector on the surface needs to be parallel to E field if we select the cyclinder. However, I can't see why would E field be parallel to the normal vector.