H62: Discrete charge distributions Four charges q, +3q, 3 q, +q are located at positions z = 3 a, a, +a, +3 a respectively along the z axis. (21) Find the exterior Cartesian multipole moments up to the lowest non-vanishing order. (b) Thus nd an approximate form for the faraway potential, Vex(r),for |r| :s' 3 a