Question

The two plates of a parallel-plate capacitor are a distance d apart and are mounted on insulating supports. A battery is connected across the capacitor to charge it and is then disconnected. The distance between the insulated plates is then increased to 2d. If fringing of the field is still negligible, which of the following quantities is doubled?

(A) The capacitance of the capacitor

(B) The total charge on the capacitor

(C) The surface density of the charge on the plates of the capacitor

(D) The energy stored in the capacitor

(E) The intensity of the electric field between the plates of the capacitor

Answer 1

The correct option is: (D) The energy stored in the capacitor

Explanation:

Since the battery is removed, the charge remains constant. If the distance is increased, the capacitance will decrease (C ∝ A/d) and since Q = CV, the potential difference must increase by the same factor that the distance increases and U_C = ½ QV.