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A2 Daily A Level Physics question

2026-04-07 OCR A Capacitance I (M6.1) 6.1.1 Capacitors and capacitance (C = Q/V; effect of dielectrics, qualitative) 6.1.2 Energy stored in a capacitor (E = 1/2 C V^2)

In a lab, a parallel-plate capacitor is connected to a steady 9.0 V DC supply. A plastic sheet with relative permittivity 3.0 is fully inserted between the plates without changing the plate area or separation. Which statement about the energy stored in the capacitor after insertion is correct?

  1. A It decreases to one-third of its original value.
  2. B It remains the same as before.
  3. C It increases to three times its original value. (correct)
  4. D It increases to nine times its original value.

Answer

The correct answer is C.

Correct: C — It increases to three times its original value. A is the disconnected (constant-charge) case: inserting the dielectric would reduce voltage and make the energy one-third, but here the supply fixes the voltage. B ignores that increasing capacitance at fixed voltage increases energy. C is correct because the dielectric makes the capacitance three times larger, so with V fixed, E' = 1/2 (3C) V^2 = 3 × (1/2 C V^2). D over-squares the effect; the energy scales linearly with capacitance at fixed voltage, not with its square.

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