Question

Four charges +q, -q, +q, -q are arranged alternately at corners A, B, C, D of a square of side d. With +q already at A, the work done in bringing -q from infinity to corner B is:

Options

• \[+\frac{kq^2}{d\sqrt{2}}\]

• \[-\frac{kq^2}{d\sqrt{2}}\]

• \[+\frac{kq^2}{d}\]

• \[-\frac{kq^2}{d}\]

Answer 1

With +q at A, the potential at B (distance d away) due to it is kq/d. Bringing -q there gives \[W_2 = (-q)\times\frac{kq}{d} = -\frac{kq^2}{d}\], which is negative because the \[+q\] & \[-q\] interaction is attractive.