Question

Four identical point charges are placed at the corners of a square. A fifth point charge placed at the centre of the square experiences zero net force. Is this s stable equilibrium for the fifth charge? Explain.

Answer 1

No. Only for very special displacements, the electrostatic force acts in a direction that points back toward the equilibrium position. For a general displacement, the electrostatic force does not point toward the equilibrium position, and the fifth charge moves further from equilibrium, making the system unstable.