1. The problem There are four charges at the corners of a square of side L. Three of the charges are -q and one is -Q(upper left corner). A) Find the force on -Q, assuming -Q< 0. B) If -Q=-q, find the electric field at the center of the square. 2. Relevant equations Coulomb's Law E = k (Q1)(Q2)/r2 E = E1 + E2 + ... 3. The attempt A) I basically did E = k [(2q)(-Q)/ (L)2 + (q)(-Q)/(2L2)^1/4] or something like that on the test. In my textbook it says to use the superposition principle, when there's multiple charges.