A-LevelPhysicsD.C. circuits9702/42 · Feb/Mar 2020 · Q77 Marks

7 (a) On Fig. 7.1, sketch the temperature characteristic of a negative temperature coefficient (n.t.c.) thermistor. Label the axes with quantity and unit. [Figure 7.1] (b) An n.t.c. thermistor and a resistor are connected as shown in Fig. 7.2. [Figure 7.2]

(a)On Fig. 7.1, sketch the temperature characteristic of a negative temperature coefficient (n.t.c.) thermistor. Label the axes with quantity and unit. [2][2]
(b)The temperature of the thermistor is increased. State and explain the change, if any, to the reading on the voltmeter. [2][2]
(c)The variation with the fractional change in length Δx/x of the fractional change in resistance ΔR/R for a strain gauge is shown in Fig. 7.3. [Figure 7.3] The unstrained resistance of the gauge is 120Ω. Calculate the new resistance of the gauge when it is extended to a strain of 0.020. resistance = ................................................................ Ω [3][3]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

[1] axes labelled with resistance and temperature and line with negative gradient throughout
[1] concave curve not touching temperature axis
[1] resistance of thermistor decreases
[1] total circuit resistance decreases so voltmeter reading increases / current increases so voltmeter reading increases / greater proportion of resistance in fixed resistor so voltmeter reading increases / p.d. across thermistor decreases so voltmeter reading increases
[1] (0.020 strain means) ΔR / R = 0.090
[1] ΔR = 0.090 × 120 = 10.8 Ω
[1] resistance = 120 + 10.8 = 130 Ω

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