If identical charges (−q) are placed at each corner of a cube of side b, then electric potential energy of charge (+q) which is placed at centre of the cube will be

Length of the diagonal of a cube having each side b is 3 b.So distance of centre of cube from each vertex is 3 b2.Hence potential energy of the given system of charge is U=8×14πε0.(−q) (q)3 b/2=−4q23πε0b