posted by CLaire .
You have three identical charged spheres, each with a charge of 2.80 µC. You have arranged them so they are each at the corners of a square that measures 4m on a side. Find the magnitude of the electric field at the "empty" corner. (The corner with no charged sphere.)
Apply Coulomb's law for the individual E-fields; then add them as vectors. The resultant will be directed along a diagonal.