The diagram below shows three charges at the corners of a square of sides d = 3.90 m. Here, q1 = q2 = −q and q3 = −1.5 q where q = 7.00 nC. (a) What is the magnitude of the electric field at the center of the square due to these three charges? N/C (b) You now replace q1 in the square with another point charge q4, while leaving q2 and q3 alone. What value of q4 will produce an electric field at the center directed vertically down? nC
