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 3b. So distance of centre of cube from each vertex is 3b2Hence potential energy of the given system of charges isU=8×14πε0·(-q)(q)3b/2=-4q23πε0b