Trick We would like to first calculate v from this expression, and then to obtain e=-Vv, instead of directly doing E(r)= r r 7E r—r even though the two ways yield the same result Equipotential surface r defines a 2-dim surface in space, called the equipotent tial surface Example: an equipotential surface about a point charge is a concentric sphere. For a general distribution of charges, the resultant equipotential surface can be quite complex