Physics 9702 · AS & A Level · Discharging a capacitor
Discharging a capacitor — practice question
A capacitor with capacitance $470\,\mu\text{F}$ is joined to a battery with electromotive force (e.m.f.) $24\,\text{V}$ in the circuit shown in Fig. 5.1. The two-way switch $S$ is first in position $X$. Wires $P$ and $Q$ are the same long straight wires, each having a resistance of $5.6\,\text{k}\Omega$. They are set close to one another and run parallel. Wire $Q$ is attached to a voltmeter. At $t = 0$, switch $S$ is changed to position $Y$ so that the capacitor discharges through wire $P$.
(a(i))[2]
Calculate the charge $Q_0$ on the capacitor at time $t = 0$.
(a(ii))[1]
Calculate the current $I_0$ in wire $P$ at time $t = 0$.
(a(iii))[2]
Calculate the time constant $\tau$ of the discharge circuit.
(a(iv))[2]
On Fig. 5.2, sketch a line to show the variation with $t$ of the current $I$ in wire $P$ as the capacitor discharges.
(b(i))[3]
Explain why there is an induced e.m.f. across wire $Q$ during the discharge of the capacitor.
(b(ii))[1]
On Fig. 5.3, sketch a line to suggest the variation with $t$ of the voltmeter reading $V$.
Worked solution & mark scheme
This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Applies $Q = CV$” …