A2 Daily A Level Physics question

2026-02-21 OCR A Capacitance I (M6.1) OCR A Physics A Module 6.1 Capacitors: C = Q/V; energy stored (qualitative ½CV²); dielectrics (qual.)

In a lab, a metal-plate capacitor is connected to a steady DC power supply. A thin plastic sheet (dielectric) is slid fully between the plates. Which statement must be true?

A The charge on the plates decreases and the stored energy decreases, because the dielectric reduces the electric field.
B The charge on the plates is unchanged and the stored energy is unchanged, because the supply fixes the circuit conditions.
C The charge on the plates increases but the stored energy decreases, because the dielectric lowers the potential difference across the plates.
D The charge on the plates increases and the stored energy increases, because the dielectric increases the capacitance while the supply holds the potential difference fixed. (correct)

Answer

The correct answer is D.

Correct: D — The charge on the plates increases and the stored energy increases, because the dielectric increases the capacitance while the supply holds the potential difference fixed. A The field inside the dielectric is reduced, but with the voltage fixed by the supply the capacitance rises, so both charge and stored energy increase rather than decrease. B With the voltage fixed, increasing capacitance draws more charge from the supply and raises the stored energy, so neither remains unchanged. C The supply fixes the potential difference, so it does not drop; with higher capacitance at the same voltage, the stored energy does not decrease. D With a constant applied voltage, a higher capacitance means more charge is stored and the energy in the capacitor is greater.

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