The figure shows eight-point charges arranged at the corners of a cube with sides of length d. The values of the charges are +q and -q,as shown . This is a model of one cell of a cubic ionic crystal. In sodium chloride (NaCl),for instance,the positive ions are Na+ and the negative ions are Cl− .Calculate the potential energy Uof this arrangement.(Take as zero the potential energy of the eight charges when they are infinityly far apart.)