Question

Four equal-magnitude point charges (3.0 C) are placed at the corners of a square that is 40 cm on a side. Two, diagonally opposite each other, are positive, and the other two are negative. Find the force on either negative charge.

Answer 1

$F=2F_1+F_2=2\frac{kq^2}{r^2}-\frac{kq^2}{2r^2}=\frac{3kq^2}{2r^2}=760\cdot 10^9~N.$