Uniform-charge estimate at a capacitor corner ($|E_n|\simeq V/(4d)$)

Four uniform square charge-sheet pairs, rotated around a common corner, contribute equal normal fields there. Their superposition is a large central plate pair, so each contributes one quarter of its field. This gives $|E_n|\simeq \sigma /(4{ϵ}_0)\simeq V/(4d)$ when the bulk charge-voltage relation is used. It is an approximation for prescribed uniform sheets, not an exact equipotential-conductor result. Reflection in the mid-plane does give zero tangential field for either model.

A conducting capacitor plate must remain an equipotential surface in electrostatic equilibrium. Near a finite plate's edge or corner, the surrounding geometry differs from that at its center, so the surface charge generally redistributes rather than remaining uniform.

This redistribution supplies the local electric field needed to maintain the constant surface electric potential. A model with prescribed uniform sheets can estimate a fringing field of a capacitor, but its corner field need not be the field of actual conducting plates at the specified voltage. Solving the appropriate boundary value problem determines both field and surface charge; ideal sharp corners can introduce singular limiting charge densities.