A filament lamp and a fixed resistor are connected in series with a battery. Describe how the resistance of the filament lamp changes as it is switched on and reaches its normal operating temperature, and explain why this change occurs.

Cambridge International IGCSE Physics (0625) — 4.3 Electric circuits · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The filament lamp’s resistance rises as it is switched on and warms to its normal operating temperature.

When the lamp is first turned on the current flows through the filament, producing heat. The filament’s temperature increases.

Because the resistance of a metal increases with temperature, the filament’s resistance rises as it gets hotter.

As the resistance increases, the current through the series circuit decreases compared with the initial current when the lamp was cold.

Examiner tips

  • Use the word "rises" for resistance change; mention temperature rise first, then resistance increase, then current decrease.
  • Show the logical sequence: current → heat → temperature ↑ → resistance ↑ → current ↓.
  • Use the phrase "normal operating temperature" to show full completion of the change.
  • Include the reason for resistance change: higher temperature causes greater electron scattering.

Common mistakes

  • Saying the resistance decreases or stays the same; confusing the direction of change.
  • Failing to link the temperature rise to the resistance change; omitting the cause of the resistance increase.
  • Not mentioning that the current falls as the resistance rises.

Mark scheme (4 marks)

  1. The resistance of the filament lamp increases as it reaches normal operating temperature.
  2. The filament's temperature increases as current flows through it / the filament gets hotter.
  3. The higher temperature causes greater resistance (the resistance increases as the filament's temperature increases).
  4. Therefore, the current through the circuit decreases as the lamp warms up / the current is lower at normal operating temperature than when first switched on.

Key terms in this question

filament lamp · resistance · series

Related

More Electric circuits questions

▶ Try answering this question with AI marking (free) →