Trick We would like to first calculate v from this expression, and then to obtain e=Vv, instead of directly doing E(r)=40 r—r r—r even though the two ways yield the same result Equipotential surface r defines a 2-dim surface in space called the equipoten- tial surface Example: an equipotential surface about a point charge is a concentric sphere For a general distribution of charges, the resultant equipotent surtace can be quite complex