A capacitor is fully charged and then disconnected from the power supply. The capacitor is then connected to a resistor and allowed to discharge completely. Describe what happens to the charge on the capacitor plates and the current in the circuit during the discharge process.

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

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

Model answer (5 marks)

The charge on the two plates decreases as the capacitor discharges.
The decrease is gradual – the rate of loss of charge slows as the voltage falls.
Consequently the current in the circuit also decreases.
At the instant the resistor is connected the current is at its maximum.
Both the charge and the current fall to zero when the capacitor is fully discharged.

Examiner tips

  • Use the word "decreases" for charge and current; mention the rate slows. Show the initial maximum current and final zero. Keep the answer concise – 5 marks can be earned with 5 short points.
  • common_mistakes
  • :
  • Saying the charge or current disappears instantly. Forgetting to mention that the current starts high. Using vague terms like "changes" instead of "decreases".

Mark scheme (5 marks)

  1. The charge on the capacitor plates decreases (during discharge)
  2. Charge decreases at a decreasing rate / charge decreases gradually / not instantly
  3. Current in the circuit decreases (during discharge)
  4. Current is initially at its maximum (at the start of discharge) / current starts high
  5. Current (and charge) both reach zero when the capacitor is fully discharged

Key terms in this question

capacitor · discharge · charge · current

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