Physics 9702 · AS & A Level · Discharging a capacitor
Discharging a capacitor — practice question
Figure 5.1 shows how the charge Q on one plate of a capacitor varies with potential difference V. In Fig. 5.2, the capacitor is connected to an 8.0\,\text{V} power supply together with two resistors R and S. R has resistance 25\,\text{k}\Omega and S has resistance 220\,\text{k}\Omega. The switch may be set to either position X or position Y.
(a)[2]
With the switch in position X, the capacitor is fully charged. Calculate the energy E stored in the capacitor.
(b)
The switch is then moved to position Y.
(b(i))[2]
Show that the time constant of the discharge circuit is $3.3\,\text{s}$.
(b(ii))[4]
The fully charged capacitor in (a) stores energy E. Determine the time t taken for the stored energy to fall from E to $\frac{E}{9}$.
(c)[2]
A second identical capacitor is joined in parallel with the first capacitor. State and explain how, if at all, the time constant of the discharge circuit changes.
Worked solution & mark scheme
This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Equivalent to area under graph OR $\frac12 QV$” …