Four point charges have the same magnitude of 3.3 x 10^-12 C and are fixed to the corners of a square that?

Question

Four point charges have the same magnitude of 3.3 x 10^-12 C and are fixed to the corners of a square that is 7.2 cm on a side. Three of the charges are positive and one is negative. Determine the magnitude of the net electric field that exists at the center of the square.

Answer 1

The flieds due to two positive charge on opposite corners cancel each other out, so the only two charge that matter are the posivite charge and negative one. The distance to the center ofthe square from a corner is:
d = l/sqrt(2) where l = length of a side of the square

Then field is E = qk/d^2 -(-qk/d^2) = 4qk/d^2 where k = 1/(4*pi*e0)

You can run the numbers.