Resistance changes with temperature and lightEdexcel A-Level Physics: Subtopic test

1. 1
   A platinum resistance thermometer uses a coil of platinum wire. The resistance of the coil is 100 Ω at 0 °C and increases steadily as the temperature rises.

   (a)

   Which statement best explains why the resistance of the platinum coil increases as its temperature rises?

   [1 mark]

   • AThe number of conduction electrons per unit volume increases greatly
   • BThe metal ions vibrate with larger amplitude, so collisions with conduction electrons are more frequent
   • CThe charge on each conduction electron decreases
   • DThe conduction electrons are pulled back towards the nuclei

   (b)

   What happens to the number of conduction electrons per unit volume in the platinum as its temperature rises?

   [1 mark]

   • AIt stays almost constant
   • BIt increases greatly
   • CIt decreases
   • DIt falls to zero

   (c)

   The same p.d. is applied across the coil at a higher temperature. Using I = nqvA, explain what happens to the mean drift velocity of the conduction electrons.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A light-dependent resistor (LDR) has a resistance of 1.5 kΩ in bright light and 90 kΩ in darkness. It is connected across a 3.0 V cell of negligible internal resistance.

   (a)

   What is the current in the LDR in bright light?

   [1 mark]

   • A2.0 A
   • B33 μA
   • C2.0 mA
   • D4.5 mA

   (b)

   What is the current in the LDR in darkness?

   [1 mark]

   • A2.0 mA
   • B3.3 mA
   • C0.33 mA
   • D33 μA

   (c)

   Explain, in terms of conduction electrons, why the resistance of the LDR is lower in bright light.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An NTC thermistor has a resistance of 8.0 kΩ at 15 °C and 1.2 kΩ at 60 °C. It is connected across a 5.0 V supply of negligible internal resistance.

   (a)

   Calculate the power dissipated in the thermistor at 15 °C and at 60 °C.

   [3 marks]

   (b)

   Explain, in terms of charge carriers and lattice vibrations, why the resistance of the thermistor falls as it is heated.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician compares the behaviour of three components as their surroundings change: a coil of copper wire and an NTC thermistor, which are heated from 20 °C to 80 °C, and a light-dependent resistor (LDR), which is moved from a dim room into bright sunlight. Each is connected in turn across a constant 6.0 V supply.

   (a)

   Compare the changes in resistance of the copper coil and the thermistor as they are heated, and explain the differences.

   [6 marks]

   (b)

   In a dim room the current in the LDR is 0.30 mA, and in bright sunlight it is 15 mA. Evaluate whether the change in the number of conduction electrons could account for this change in resistance.

   [6 marks]

   Total for question 4: 12 marks

End of questions