Capacitors and capacitance
Cambridge A-Level Physics (9702) · Unit 19: Capacitance · 8 flashcards
Capacitors and capacitance is topic 19.1 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 19 — Capacitance , alongside Energy stored in a capacitor and Discharging a capacitor. In one line: Capacitance (C) is the charge (Q) stored per unit potential difference (V) across a capacitor. Measured in Farads (F), where 1 F = 1 C/V.
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 8 flashcards — 2 definitions, 1 key concept and 5 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.
Capacitance
Capacitance (C) is the charge (Q) stored per unit potential difference (V) across a capacitor. Measured in Farads (F), where 1 F = 1 C/V.
What the Cambridge 9702 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- define capacitance, as applied to both isolated spherical conductors and to parallel plate capacitors
- recall and use C = Q / V
- derive, using C = Q / V, formulae for the combined capacitance of capacitors in series and in parallel
- use the capacitance formulae for capacitors in series and in parallel
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 Capacitors and capacitance
- › Use precise terminology: capacitance is the ratio of the charge on one plate to the potential difference between the plates.
- › Apply the principle of conservation of charge; the total charge before and after redistribution must remain the same.
- › 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.
- › Recall that the time constant (τ = RC) has units of seconds (s).
Define capacitance.
Capacitance (C) is the charge (Q) stored per unit potential difference (V) across a capacitor. Measured in Farads (F), where 1 F = 1 C/V.
State the formula that relates capacitance, charge, and potential difference.
The relationship between capacitance (C), charge (Q), and potential difference (V) is given by the formula: C = Q / V.
Derive the formula for the total capacitance of capacitors connected in parallel.
In parallel, the potential difference across each capacitor is the same. The total charge stored is the sum of charges on each capacitor (Q_total = Q1 + Q2 + ...). Thus, C_total = C1 + C2 + ...
Derive the formula for the total capacitance of capacitors connected in series.
In series, the charge on each capacitor is the same. The total potential difference is the sum of potential differences across each capacitor (V_total = V1 + V2 + ...). Thus, 1/C_total = 1/C1 + 1/C2 + ...
A 5μF capacitor is charged to 10V. What charge does it store?
Using C = Q/V, we have Q = CV = (5 × 10⁻⁶ F)(10 V) = 5 × 10⁻⁵ C or 50 μC.
Two capacitors, 2μF and 4μF, are connected in series. What is the total capacitance?
Using 1/C_total = 1/C1 + 1/C2, we have 1/C_total = 1/(2×10⁻⁶) + 1/(4×10⁻⁶). Therefore, C_total = 1.33 μF.
Two capacitors, 3μF and 6μF, are connected in parallel. What is the total capacitance?
Using C_total = C1 + C2, we have C_total = (3 × 10⁻⁶ F) + (6 × 10⁻⁶ F) = 9 × 10⁻⁶ F or 9 μF.
Describe how an isolated spherical conductor stores charge and relates to its capacitance.
An isolated spherical conductor stores charge uniformly on its surface. The capacitance is proportional to its radius; a larger sphere can store more charge at a given potential. C = 4πε₀r, where r is the radius.
Review the material
Read full revision notes on Capacitors and capacitance — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 19 — Capacitance
Capacitors and capacitance 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 Capacitors and capacitance deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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