Paper 4 DC Circuits Drills

Q1 Potential divider calibration

A student uses an NTC thermistor as the lower component of a potential divider. The output p.d. is measured across the thermistor. The calibration data are:

$\theta /{}^{\circ }\mathrm{C}$	20	30	40	50	60	70
$V_{\mathrm{out}}/\mathrm{V}$	3.57	3.18	2.78	2.37	1.97	1.67

1. Plot $V_{\mathrm{out}}$ against $\theta$ and draw a smooth best-fit curve. [5]

2. Use the graph to estimate the temperature when $V_{\mathrm{out}}=2.55\mathrm{V}$. [2]
3. Explain why the output should be measured with a high-resistance voltmeter. [2]
4. Explain why the thermistor should be allowed to reach thermal equilibrium after the bath temperature changes. [2]
5. Suggest why the calibration curve should not be assumed valid outside the measured temperature range. [1]

Q2 Potentiometer internal resistance

A potentiometer is used to compare a cell’s emf with its terminal p.d. when supplying a load.

1. Explain the purpose of finding a balance length. [2]
2. State how internal resistance could be found if current through the load is measured. [2]
3. State one practical precaution. [1]