Explain how a capacitor stores energy and describe the factors that affect the energy stored in a capacitor.

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).

Model answer (5 marks)

1. A capacitor stores charge on its two plates, with equal and opposite charges on each plate.
2. The separated charges create an electric field and a potential difference between the plates.
3. Energy is stored in this electric field.
4. For a given potential difference, a larger capacitance means more energy is stored.
5. The energy stored is proportional to the square of the potential difference, so it increases rapidly as the voltage rises.

Examiner tips

  • Use the exact phrase "equal and opposite charges" for point 1. Show the relationship U = ½CV² or U = ½CV² to justify point 5. Mention that increasing C or V increases U to satisfy the mark scheme.
  • common_mistakes
  • :
  • Confusing capacitance with charge; forgetting that C = Q/V. Using U = CV instead of U = ½CV². Not recognising that energy depends on the square of the potential difference.

Mark scheme (5 marks)

  1. A capacitor stores charge on its plates (equal and opposite charges on each plate)
  2. The separated charges create a potential difference / electric field between the plates, and energy is stored in this electric field
  3. Greater capacitance means more energy is stored (for the same potential difference)
  4. Greater potential difference means more energy is stored
  5. Energy stored is proportional to the square of the potential difference (energy increases rapidly as pd increases)

Key terms in this question

energy stored

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