Energy stored in a capacitor
Cambridge A-Level Physics (9702) · Unit 19: Capacitance · 7 flashcards
Energy stored in a capacitor is topic 19.2 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 19 — Capacitance , alongside Capacitors and capacitance and Discharging a capacitor. In one line: The energy stored in a capacitor is given by the formula W = (1/2)QV, where W is the energy in Joules, Q is the charge in Coulombs, and V is the potential difference in Volts.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 2 definitions, 3 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Formula that relates the energy (W) stored in a capacitor to its charge (Q) and potential difference (V)
The energy stored in a capacitor is given by the formula W = (1/2)QV, where W is the energy in Joules, Q is the charge in Coulombs, and V is the potential difference in Volts.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Determine, using the area under a graph of potential against charge, how much electric potential energy a capacitor stores.
- Recall the energy stored by a capacitor, W = ½QV = ½CV², and use it.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Energy stored in a capacitor
- › Calculate energy stored using (1/2)CV^2 for each state separately, then find the difference. Do not use (1/2)C(delta V)^2.
How can the energy stored in a capacitor be determined from a potential-charge graph?
The electric potential energy stored in a capacitor is equal to the area under the potential-charge (V-Q) graph. This area represents the work done to charge the capacitor.
State the formula that relates the energy (W) stored in a capacitor to its charge (Q) and potential difference (V).
The energy stored in a capacitor is given by the formula W = (1/2)QV, where W is the energy in Joules, Q is the charge in Coulombs, and V is the potential difference in Volts.
State the formula that relates the energy (W) stored in a capacitor to its capacitance (C) and potential difference (V).
The energy stored in a capacitor can also be calculated using the formula W = (1/2)CV², where W is the energy in Joules, C is the capacitance in Farads, and V is the potential difference in Volts.
A 100μF capacitor is charged to a potential difference of 10V. Calculate the energy stored in the capacitor.
Using the formula W = (1/2)CV², W = (1/2) * (100 × 10⁻⁶ F) * (10 V)² = 0.005 J. The energy stored in the capacitor is 0.005 Joules.
A capacitor stores 0.02 J of energy when charged to 200V. Calculate the charge on the capacitor.
Using the formula W = (1/2)QV, rearrange to find Q = 2W/V. Therefore, Q = (2 * 0.02 J) / 200 V = 2 × 10⁻⁴ C. The charge on the capacitor is 0.2 mC.
How does increasing the potential difference across a capacitor affect the energy stored?
Increasing the potential difference across a capacitor increases the energy stored. Since energy is proportional to V² (W = 1/2 CV²), doubling the voltage quadruples the energy stored.
A parallel-plate capacitor is fully charged and then disconnected from the power supply. If the plate separation is doubled, how does the stored energy change?
Doubling the plate separation halves the capacitance (C=ε₀A/d). Since the charge remains constant (isolated system), energy increases (W=Q²/2C). Halving C, doubles W, resulting in double the energy.
Review the material
Read full revision notes on Energy stored in a capacitor — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 19 — Capacitance
Energy stored in a capacitor sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Energy stored in a capacitor deck
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