As shown in the figure, a square has sides of d = 5.0 cm with a charge of Q = -18.0 µC at one corner (a) and with charges of q = +6.0 µC at the remaining three corners (b, c, and d). Find the electric field at the center of the square. magnitude N/C direction towards ---Select--- the top corner b the right corner c the bottom corner d the left corner a . Find the potential at the center of the square.