Resistance changes in thermistors and LDRsEdexcel International A Level Physics: Subtopic test

1. 1
   A technician heats a metal wire and a negative temperature coefficient (NTC) thermistor, one at a time, from 20 °C to 80 °C. Each is connected to a constant 5.0 V supply of negligible internal resistance through an ammeter.

   (a)

   Why does the resistance of the metal wire increase as it is heated?

   [1 mark]

   • AThe number of conduction electrons increases
   • BThe wire contracts and becomes thinner
   • CThe lattice ions vibrate with larger amplitude, so conduction electrons collide with them more often
   • DThe conduction electrons are released from their atoms more easily

   (b)

   What happens to the current in the thermistor as it is heated, and why?

   [1 mark]

   • AIt increases, because more electrons are released and the number of conduction electrons rises
   • BIt decreases, because the lattice ions vibrate more
   • CIt stays the same, because the potential difference is constant
   • DIt increases, because the lattice ions vibrate less

   (c)

   The thermistor current is 2.0 mA at 20 °C and 12.5 mA at 80 °C. Calculate the resistance of the thermistor at each temperature.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A light-dependent resistor (LDR) is connected to a 6.0 V supply of negligible internal resistance through an ammeter. The ammeter reads 12 mA in bright light and 0.10 mA in darkness.

   (a)

   Calculate the resistance of the LDR in bright light.

   [1 mark]

   • A2.0 Ω
   • B500 Ω
   • C72 Ω
   • D0.50 Ω

   (b)

   What is the best explanation for the fall in resistance of the LDR in bright light?

   [1 mark]

   • AThe lattice ions vibrate more, so the resistance falls
   • BThe LDR is warmed by the light, so its length decreases
   • CLight makes the conduction electrons move faster
   • DPhotons release electrons from atoms, increasing the number of conduction electrons

   (c)

   Calculate the resistance of the LDR in darkness and how many times larger it is than in bright light.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A thermistor is connected across a 5.0 V supply of negligible internal resistance. Its resistance is 2.0 kΩ at 25 °C and 0.30 kΩ at 75 °C.

   (a)

   Calculate the current in the thermistor at each temperature and the percentage increase in the current between 25 °C and 75 °C.

   [3 marks]

   (b)

   Explain, using a model of conduction, why heating a metal conductor increases its resistance whereas heating this thermistor decreases its resistance.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A greenhouse controller uses a light-dependent resistor (LDR) to sense daylight and a negative temperature coefficient (NTC) thermistor to sense the temperature.

   (a)

   Explain how the resistance of the LDR changes with illumination, in terms of the conduction electrons in the LDR.

   [6 marks]

   (b)

   The designer is deciding whether the thermistor or a platinum wire coil is the better temperature sensor between 0 °C and 100 °C. The coil has a resistance of 100 Ω at 0 °C and 139 Ω at 100 °C. The thermistor has a resistance of 8.0 kΩ at 0 °C and 0.50 kΩ at 100 °C. Evaluate which should be chosen.

   [6 marks]

   Total for question 4: 12 marks

End of questions