A technician is testing two resistors made from different materials. Resistor A is made from carbon and has a higher resistance than Resistor B, which is made from a metal alloy. Explain why Resistor A has a higher resistance than Resistor B, and describe what would happen to the resistance of Resistor B if its temperature were increased.

WJEC A-Level Physics (Wales) — 2.1 Conduction and resistance · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Resistor A is made from carbon. Resistor B is made from a metal alloy. Resistor A has a higher resistance than Resistor B at room temperature.

Model answer (5 marks)

Resistor A is made from carbon, which has a relatively low number of free electrons (charge carriers). In contrast, the metal alloy of Resistor B contains many free electrons that can move easily through the lattice. Because current flows more readily in a material with many charge carriers, Resistor B has a lower resistance than Resistor A.

If the temperature of Resistor B is increased, the metal ions vibrate more strongly. The increased vibration causes the free electrons to collide more frequently with the ions, which raises the resistance of the metal alloy.

Examiner tips

  • Use the term ‘free electrons’ and ‘charge carriers’ to show understanding of conduction. Explain the effect of temperature on metal resistance by mentioning lattice vibration and collision frequency. Keep the answer concise and directly linked to the marks given.
  • common_mistakes
  • :
  • Confusing the role of electrons with that of ions. Saying that resistance decreases with temperature for metals. Not mentioning the increased collision frequency due to lattice vibration.

Mark scheme (5 marks)

  1. Metals (alloys) contain free electrons that can move through the material
  2. Carbon has fewer free electrons (charge carriers) available, so current flows less easily
  3. Fewer charge carriers means greater resistance to the flow of current in carbon
  4. Increasing the temperature of Resistor B causes the metal ions/atoms to vibrate more
  5. More vibration means free electrons collide more frequently with ions, so resistance increases

Key terms in this question

resistance · temperature

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