A student sets up a series circuit containing a battery, an ammeter, a filament lamp, and a thermistor. As the circuit is left switched on for several minutes, the student notices that the ammeter reading gradually decreases. Explain why the ammeter reading decreases over time, referring to what happens inside both the filament lamp and the thermistor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student sets up a series circuit containing a battery, a filament lamp, and a thermistor. The ammeter reading is recorded immediately after switch-on and then again after several minutes of operation. The reading is noticeably lower after several minutes.
Model answer (5 marks)
As current flows through the filament lamp, electrons collide with atoms in the filament, causing it to heat up.
The filament’s resistance increases as its temperature rises.
The thermistor also warms up over time, its temperature increasing.
Its resistance decreases as its temperature rises.
Because the increase in the filament’s resistance is greater than the decrease in the thermistor’s resistance, the total resistance of the series circuit rises, so the current measured by the ammeter falls.
The filament’s resistance increases as its temperature rises.
The thermistor also warms up over time, its temperature increasing.
Its resistance decreases as its temperature rises.
Because the increase in the filament’s resistance is greater than the decrease in the thermistor’s resistance, the total resistance of the series circuit rises, so the current measured by the ammeter falls.
Examiner tips
- Use the word ‘increase’ for the filament and ‘decrease’ for the thermistor to match the mark scheme.
- Show the chain of cause‑effect: heating → resistance change → current change.
- Keep the answer short and to the point – 5 marks only.
- Mention both components explicitly.
Common mistakes
- Confusing the direction of resistance change for the thermistor (writing ‘increase’ instead of ‘decrease’).
- Omitting the filament’s heating effect or the overall resistance change.
- Writing a long narrative instead of concise, point‑style answer.
Mark scheme (5 marks)
- As current flows through the filament lamp, electrons collide with atoms in the filament / the filament heats up
- The resistance of the filament lamp increases as its temperature increases
- The thermistor also warms up / its temperature increases over time
- The resistance of the thermistor decreases as its temperature increases
- The overall / total resistance in the series circuit increases (because the increase in the filament lamp's resistance is greater than the decrease in the thermistor's resistance), so the current decreases
Key terms in this question
filament lamp · thermistor · series circuit
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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