A filament lamp is connected to a battery in a simple series circuit. As the lamp is switched on and left running, the current through the lamp decreases over time, even though the potential difference of the battery stays the same. Explain why this happens.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A filament lamp is connected to a battery in a simple series circuit. When the circuit is first switched on, the current is relatively large. After a short time, the current decreases and settles at a lower steady value.
Model answer (4 marks)
The filament is initially cold, so its resistance is low. When the lamp is switched on, current flows and electrical energy is converted into heat, raising the filament’s temperature. As the temperature rises, the ions in the metal vibrate more strongly, increasing the likelihood of electron‑ion collisions and making it harder for electrons to move. Consequently the resistance of the filament increases. With the battery voltage unchanged, Ohm’s law (I = V/R) shows that the current must decrease as the resistance rises.
Examiner tips
- Use the sequence: heating → increased ion vibration → higher resistance → lower current; include all four points in order.
- Mention that the battery voltage remains constant to justify the current drop.
Common mistakes
- Saying the lamp’s resistance stays the same; forgetting the temperature dependence of resistance.
- Confusing the cause of heating with the effect of heating; e.g., claiming the current causes the resistance to drop.
Mark scheme (4 marks)
- As current flows, electrical energy is transferred to thermal energy in the filament, causing it to heat up / the filament's temperature increases
- Electrons collide with ions in the filament, and the ions vibrate more as temperature rises, making it harder for electrons to flow
- The resistance of the filament lamp increases as its temperature increases
- Because resistance increases (and potential difference stays the same), the current through the lamp decreases
Key terms in this question
filament lamp · current · potential difference
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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