I'm studying for the physics GRE and am fairly poor with EM. 1. The problem statement, all variables and given/known data What is the work needed to assemble four point charges q into a regular tetrahedron of side length a? 2. Relevant equations W = 1/2ΣqiV(ri) 3. The attempt at a solution Assume that the origin is at one of the points, then the potential between each pair (there will be 3 pairs) is V = q/(4πεa). Since there are three pairs the V shows up 3 times in the summation. Also in the summation according to the work equation I wrote above you must multiple by q. Hence W = 1/2 * 3q2/(4πεa). I get W = 1/2 * [ 3q2/(4πεa) ] , but the solutions says the work is 3q2 / (2πεa). Why is there not a 2 * 4 *πεa in the denominator? Am I missing something? Thanks!