A voltmeter is connected in parallel with a filament lamp in a circuit. As the lamp gets brighter, the voltmeter reading increases. Explain why the resistance of the filament lamp increases as its brightness increases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A filament lamp is connected in a simple circuit. As more current passes through the lamp, it glows more brightly and becomes hotter.
Model answer (4 marks)
The filament lamp is a metal wire. When more current flows through it, the power dissipated (P=I²R) increases, so the filament heats up. As the temperature rises, the metal ions vibrate more strongly. These increased vibrations scatter the free electrons more, so the electrons encounter greater resistance to their flow. Consequently the resistance of the filament rises as its brightness (and temperature) increases.
Examiner tips
- Use the phrase "metal wire" to show knowledge of the material. Explain the heating via I²R and link it to temperature. Mention electron scattering by vibrating ions. Keep answer concise – 4 marks can be earned with 3–4 short sentences.
Common mistakes
- Confusing resistance with voltage or current. Saying resistance decreases with temperature. Omitting the role of electron scattering by vibrating ions.
Mark scheme (4 marks)
- The filament lamp's resistance increases because its temperature increases
- Greater current through the filament causes the filament to become hotter / the filament heats up as more current passes through it
- The filament is a metal, so it has free electrons that carry charge / current flows via free electrons
- At higher temperatures the ions/atoms in the filament vibrate more, making it harder for free electrons to flow / increasing opposition to current flow
Key terms in this question
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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