Difficult# The Potential Due to Four Charges At an Arbitrary Point

EANDM-BMORP7

Figure 1 depicts a square of side length $s = 20.0$ mm on whose corners are arrayed four charges, each having a charge of the same magnitude $q = 1.00$ nC. Three of the charges are positive and one of them is negative, so that $q_1 = q_2 = q_4 = +q$ and $q_3 = -q$. Within the square there is a point $A$ whose coordinates are given by the vector $\vec{A} = \cfrac{3}{5}s\hat{x} + \cfrac{2}{3}s\hat{y}$.