The discussion of the electric field between two parallel conducting plates, in this module states that edge effects are less important if the plates are close together. What does close mean? That is, is the actual plate separation crucial, or is the ratio of plate separation to plate area crucial?

When equal and opposite charges are distributed on the surfaces of two parallel conducting plates, they distribute themselves uniformly over the surface, except the edges. The electric field in the region between the plates is uniform, but at the edges, the field lines bend to produce a non-uniform field. This phenomena was termed as Edge Effect.

If the area of the plates is large when compared to the separation between the plates, the edge of the electric field lines becomes insignificant, since the charge distribution tends to be fairly uniform. And if the plates are close together, the curvature of the lines at the edges reduces, hence the edge effect becomes insignificant.

However, if the area of the plates is very less, the charge tends to concentrate near the edges of the conductors, producing a nonuniform field at the edges. This would happen even if the separation between the plates remains the same.

Therefore, to reduce the edge effects, the area of the plates must be very large as compared to the separation between the plates.