A2 Daily A Level Physics question

2026-03-26 OCR A Capacitance I (A2) OCR-A 6.2.1 Capacitors: C = Q/V; energy (1/2 CV^2, description); dielectrics (qualitative)

In a lab, a parallel-plate capacitor is kept connected to a DC supply that holds the p.d. at a steady 12 V. A plastic sheet fully fills the gap between the plates. Which statement must be true?

A The energy stored in the capacitor increases because the plastic allows more charge to be stored at the same voltage. (correct)
B The energy stored in the capacitor decreases because the plastic weakens the electric field between the plates.
C The voltage across the capacitor rises above 12 V as the plastic is inserted.
D The charge on the capacitor falls because the plastic blocks charge flow onto the plates.

Answer

The correct answer is A.

Correct: A — The energy stored in the capacitor increases because the plastic allows more charge to be stored at the same voltage. A battery fixes the p.d., and inserting a dielectric increases capacitance so extra charge flows on; with fixed voltage, greater capacitance means more stored energy. B is wrong because although the field inside the dielectric is reduced, the battery keeps the voltage constant and the increased capacitance means energy rises, not falls. C is wrong because the supply holds the p.d. at 12 V; it does not increase when the dielectric is inserted. D is wrong because the dielectric does not block charging; instead, capacitance increases so the charge on the plates increases while connected to the supply.

Try today's guest Daily MCQ A Level Physics MCQ practice Problem-solving previews