A-LevelPhysicsD.C. circuits9702/42 · Oct/Nov 2018 · Q78 Marks

A circuit incorporating an ideal operational amplifier (op-amp) is shown in Fig. 7.1. [Figure 7.1] The variation with temperature θ of the resistance RT of the thermistor is shown in Fig. 7.2. [Figure 7.2]

(a)The output potential VOUT of the op-amp circuit changes sign when the temperature of the thermistor is 4.0°C. Calculate the resistance R.[2]
(b)State and explain whether the output potential VOUT is +5.0V or −5.0V for a thermistor temperature of 2.5°C.[3]
(c)The output of the op-amp is to be displayed using two light-emitting diodes (LEDs) labelled G and B. When the temperature of the thermistor is below 4.0°C, only the LED labelled G emits light. The LED labelled B emits light only when the temperature of the thermistor is above 4.0 °C. On Fig. 7.1, draw and label the symbols for the two LEDs.[3]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

[C1] R/RT = 2.4/1.8 or at 4.0°C, RT = 3.2 kΩ
[A1] hence R/3.2 = 2.4/1.8 R = 4.3 kΩ
[B1] RT = 3.37 kΩ or RT is greater (than 3.2 kΩ)
[M1] V+ > V⁻
[A1] hence output is +5.0 V
[B1] correct LED symbol
[M1] two diodes shown connected, in parallel and with opposite polarities, between VOUT and earth
[A1] diodes labelled to show correct polarities consistent with (b) (G pointing from VOUT to earth and B pointing from earth to VOUT if (b) correct)

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