CBSE Class 12-science Answered

If there were two infinite parallel planes of opposite charge, there would be a field inside them, but not outside them. You can tell this because, assuming you know the derivation for a single plane of charge, you can find the field for two planes by superposition of the solutions, and the fields of oppositely charged plates cancel outside, but reinforce each other between the plates.

In reality, there is a nonzero field outside the plates of a capacitor because the plates are not infinite. A charged particle near the plates would experience a stronger force from the closer plate that is not totally canceled out by the farther one.

And to answer your other question, edge effects are ignored when deriving the simple expression for the field between parallel plates of charge you are probably referring to, but they can also be taken into account to give a more complicated expression.