A capacitor is connected to a battery and charged to a certain voltage. The battery is then disconnected, and the distance between the plates of the capacitor is increased. What happens to the energy stored in the capacitor?

Core Concept

When the battery is disconnected, the charge Q on the capacitor remains constant. The capacitance C decreases when the plate separation is increased (C = ε₀A/d). The energy stored in a capacitor is given by U = Q²/(2C). Since Q remains constant and C decreases, the energy U increases. This extra energy comes from the work done to separate the plates against the attractive force between them.