An equipotential surface is a surface on which the potential, or voltage, is constant. Electric field lines are always perpendicular to these surfaces, and the electric field points from surfaces of high potential to surfaces of low potential. Suppose, for example, that a set of surfaces has been chosen so that their voltages are 5 V, 4 V, 3 V, 2 V, etc.. Then since the voltage difference between neighboring sheets is constant ( V) we can estimate the magnitude of the electric field between surfaces by the formula

where is the perpendicular distance between neighboring surfaces. (This formula is really just an approximate version of the path-integral definition of the voltage difference given above.) Note that this means that the electric field is strong where the equipotential surfaces are close together and weak where they are far apart.

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