2.6 LDRs and thermistors
9 exam questions worth 19 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What happens to the resistance of an LDR as the light intensity increases?
Choose one answer.
Correct Not quite: the answer is It decreases
Mark scheme
B: it decreases
Model answer
It decreases (B).
State how the resistance of a thermistor changes as its temperature increases.
Mark scheme
decreases / gets smaller
Model answer
The resistance decreases.
An automatic street light uses an LDR. Describe how the resistance of the LDR changes when it gets dark and what this does to the lamp.
Mark scheme
the resistance of the LDR increases (becomes high) in the dark (1); this causes the circuit to switch the lamp on (1)
Model answer
In the dark the resistance of the LDR becomes high. This changes the current so that the circuit switches the lamp on.
A thermistor is used in a fire alarm. Explain how the alarm is triggered when there is a fire.
Mark scheme
the temperature rises so the resistance of the thermistor falls (1); the current increases and this triggers the alarm (1)
Model answer
The temperature rises, so the resistance of the thermistor falls. The current increases, and this triggers the alarm.
A thermistor is connected to a 6.0 V supply. At 60 °C the current is 0.012 A. Calculate the resistance of the thermistor at 60 °C.
Mark scheme
R = V / I = 6.0 / 0.012 (1); 500 Ω (1)
Model answer
R = V / I = 6.0 / 0.012 = 500 Ω.
Describe how you could investigate how the resistance of a thermistor changes with temperature.
Mark scheme
put the thermistor in water in a beaker and connect it to a power supply with an ammeter and voltmeter (or an ohmmeter) (1); heat the water, measuring the temperature with a thermometer, and record the current and potential difference at several temperatures (1); calculate resistance using R = V / I and plot resistance against temperature (1)
Model answer
Put the thermistor in a beaker of water and connect it in a circuit with a power supply, an ammeter and a voltmeter across it. Heat the water and record the temperature with a thermometer. At several temperatures, record the current and potential difference and calculate the resistance using R = V / I. Plot a graph of resistance against temperature.
An LDR is connected to a 6.0 V supply. In bright light the current is 4.0 mA, and in the dark it is 0.050 mA. Calculate the resistance in each case.
Mark scheme
bright: R = 6.0 ÷ 0.0040 (1); 1500 Ω (1); dark: R = 6.0 ÷ 0.000 050 (1); 120 000 Ω (1)
Model answer
In bright light R = 6.0 / 0.0040 = 1500 Ω. In the dark R = 6.0 / 0.000 050 = 120 000 Ω.
A thermistor is used in a circuit that switches on a fan when an engine becomes too hot. Explain how the thermistor makes the circuit work.
Mark scheme
as the engine gets hotter the resistance of the thermistor decreases (1); so the current in the circuit increases (1); this switches on the fan / is detected by the circuit (1)
Model answer
As the engine heats up the resistance of the thermistor falls, so the current increases. The circuit detects this and switches on the fan.
Which of these is a use of an LDR?
Choose one answer.
Correct Not quite: the answer is An automatic street light
Mark scheme
B: An automatic street light
Model answer
An automatic street light (B).
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