Physics 9702 · AS & A Level · Energy stored in a capacitor

Energy stored in a capacitor — practice question

A capacitor is formed from two metal plates with an insulator between them, as illustrated in Fig. 3.1. The potential difference across the plates is $V$. Fig. 3.2 shows how the magnitude of the charge $Q$ on one plate varies with $V$.
(a)[3]

Explain why the capacitor stores energy and not charge.

(b(i))[2]

Use Fig. 3.2 to find the capacitance of the capacitor.

(b(ii))[2]

Use Fig. 3.2 to find the reduction in energy stored in the capacitor when the potential difference $V$ is reduced from $10.0\,\text{V}$ to $7.5\,\text{V}$.

(c)[3]

Capacitors X, Y and Z, each with capacitance $10\,\mu\text{F}$, are arranged as shown in Fig. 3.3. At the start, the capacitors are uncharged. A potential difference of $12\,\text{V}$ is applied between points A and B. Determine the magnitude of the charge on one plate of capacitor X.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “The charges on the plates are equal in magnitude and opposite in sign, so there is no resultant charge” …

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