Physics 9702 · AS & A Level · Discharging a capacitor

Discharging a capacitor — practice question

Fig. 6.1 illustrates a capacitor with capacitance $C$ in series with a resistor of resistance $R$. At the start, the switch is open and the capacitor has a p.d. of $12\ \text{V}$ across it. When $t = 0$, the switch is closed, producing a current $I$ in the resistor. Fig. 6.2 plots how $I$ changes with $t$.
(a)[3]

Explain why the line in Fig. 6.2 has this form.

(b(i))[2]

Use Fig. 6.2 to find the resistance $R$.

(b(ii))[3]

Use Fig. 6.2 to find the time constant $\tau$ for the circuit shown in Fig. 6.1.

(c)[2]

Use your answers from (b) to find the capacitance $C$.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “p.d. across capacitor directly proportional to charge; p.d. across capacitor equal to p.d. across resistor; current directly proportional to p.d.; current is the rate at which charge decreases (any two)” …

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