Identical +7.99 ?C charges are fixed to adjacent corners of a square. What charge (magnitude and algebraic sign) should be fixed to one of the empty corners, so that the total potential at the remaining empty corner is 0 V? Solution The potential due to a point charge is kq/r where k is a constant equal to 9x10^9Nm^2/C^2 in MKS If we call the length of each side D, the potential due to the two positive charges is V = 7.99k/D(1/1 + 1/1.414) = 13.64k/D The 3 rd charge will be at a distance D. Let us call it q. Potential due to it = qk/D For total potential to be zero: 13.64k/D + qk/D = 0 => qk/D = -13.64k/D => q = -13.64 uC .