A student investigates the resistance of a thermistor at different temperatures. The student constructs the circuit shown in Fig. 2.1. [Figure 2.1] The thermistor shown in Fig. 2.1 is placed in an empty beaker and is at room temperature.

1. (a)The student measures the room temperature θR. The thermometer is shown in Fig. 2.2. [Figure 2.2] Record the reading of θR on the answer line. θR= °C [1][1]
2. (b)(i)The student: • closes the switch • records the potential difference across the thermistor VXY while the thermistor is at room temperature θR in the top row of Table 2.1 • opens the switch. The voltmeter is shown in Fig. 2.3. [Figure 2.3] Record the potential difference VXY while the thermistor is at room temperature θR in Table 2.1. Table 2.1 VXY/V | VYZ/V | I/A thermistor at room temperature θR | | 1.4 | thermistor at temperature of hot water θH | 1.2 | 3.3 | [1][1]
3. (b)(ii)The student: • disconnects the voltmeter from points X and Y • reconnects the voltmeter across the 220 Ω resistor between points Y and Z • closes the switch and records the potential difference across the 220 Ω resistor VYZ while the thermistor is at room temperature θR in the top row of Table 2.1. Re-draw the circuit shown in Fig. 2.1 on page 5 to show the voltmeter connected to measure the potential difference across the 220 Ω resistor. [1][1]
4. (c)(i)Explain why the student waits for 30 s before measuring the temperature of the hot water. [1][1]
5. (c)(ii)Explain why the student stirs the water before reading the temperature of the hot water from the thermometer. [1][1]
6. (d)Examine the data in Table 2.1. Compare the reading for the potential difference across the thermistor VXY at room temperature θR with the reading for the potential difference across the thermistor VXY at the temperature of the hot water θH. Suggest what causes the difference in the readings. [1][1]
7. (e)The current I in the circuit is calculated using the equation: I = VYZ / R where R = 220 Ω. Use the measurements recorded in Table 2.1 to calculate the current I at room temperature θR and at the temperature of the hot water θH. Record your answers in Table 2.1. [1][1]
8. (f)The resistance RT of the thermistor is calculated using the equation: RT = VXY / I Use the data in Table 2.1 on page 6 to calculate RT at room temperature θR and at the temperature of the hot water θH. RT at room temperature θR = .............................................. Ω RT at temperature of hot water θH = .............................................. Ω [1][1]
9. (g)Calculate α, the average change in the resistance of the thermistor per degree Celsius, for the thermistor as the temperature of the thermistor rises from room temperature θR to the temperature of the hot water θH. Use the equation shown. α = change in resistance of thermistor / change in temperature Give your answer to 2 significant figures. α = .............................................. Ω/°C [2][2]

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