A capacitor is connected to a power supply and becomes fully charged. The power supply is then replaced with one of higher voltage, and the capacitor charges again to the new voltage. Explain how the charge stored and the capacitance of the capacitor change, if at all, when the voltage is increased.

AQA A-Level Physics (7408) — 3.7.4 Capacitance · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Capacitance is defined as the charge stored per unit of potential difference. A capacitor stores charge on its plates when connected to a voltage source.

Model answer (5 marks)

1. The charge stored on the plates is given by Q = CV.
2. When the supply voltage is increased, V rises.
3. Because C is a fixed property of the capacitor, Q must increase to satisfy Q = CV.
4. Therefore the charge stored increases in proportion to the new voltage.
5. The capacitance C does not change; it depends only on the geometry and dielectric, not on the applied voltage.

Examiner tips

  • Use the formula Q = CV to link Q and V. Show that C is constant, so Q∝V. Mention that C is a property of the capacitor, not the voltage.
  • common_mistakes
  • :
  • Saying the capacitance increases with voltage. Confusing Q with C or V. Forgetting to state that C is constant.

Mark scheme (5 marks)

  1. Charge stored increases when voltage increases
  2. Charge is proportional to (or depends on) voltage / potential difference
  3. Capacitance remains the same / does not change
  4. Capacitance is a property of the capacitor itself (not the voltage applied)
  5. Correct use of the relationship C = Q/V to support reasoning (e.g. both Q and V increase proportionally so C = Q/V stays the same)

Key terms in this question

capacitance · charge · capacitor

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