Chapter 19, Problem 34PCE

Four point charges are located at the corners of a square with sides of length a. Two of the charges are +q and two are −q. Find the magnitude and direction of the net electric force exerted on a charge +Q located at the center of the square, for each of the following two arrangements of charge: (a) The charges alternate in sign (+q, −q, +q, −q) as you go around the square; (b) the two positive charges are on the top corners, and the two negative charges are on the bottom corners.