Paper 4 DC Circuits Answers

Q1 Potential divider calibration

1. Axes and units [1]; suitable scale [1]; accurate plots [2]; smooth best-fit curve [1]. [5]

2. Interpolation from the best-fit curve gives approximately $45$–$46{}^{\circ }\mathrm{C}$. Accept a value consistent with the candidate’s curve. [2]
3. A high-resistance voltmeter draws negligible current, so it does not significantly load the divider or alter the output ratio. [2]
4. Before equilibrium, the thermistor temperature differs from the bath thermometer reading, so the plotted pair would not represent one common temperature. Waiting removes this thermal-lag error. [2]
5. The NTC response is nonlinear and may follow a different curvature outside the calibrated interval; extrapolation lacks supporting data. [1]

Q2 Potentiometer internal resistance

1. Balance length gives the unknown p.d. by comparison with the calibrated potential gradient, with no current through the detector at balance. [2]
2. Find emf $\mathcal{E}$ with no load and terminal p.d. $V$ under load. Lost volts $=\mathcal{E}-V=Ir$, so $r=(\mathcal{E}-V)/I$. [2]
3. Keep driver cell current steady, avoid heating the wire, make brief contact with the jockey, or check zero/end connections. [1]