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 both the filament lamp and the thermistor in your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A series circuit contains a battery, an ammeter, a filament lamp, and a thermistor connected end to end. The circuit is left switched on and the ammeter reading is observed to fall over several minutes.
Model answer (5 marks)
The filament lamp and the thermistor both heat up as current flows through them, so their temperatures rise.
The filament lamp has a positive temperature coefficient of resistance; as its temperature increases its resistance rises.
The thermistor also has a positive temperature coefficient, so its resistance increases with temperature.
In a series circuit the total resistance is the sum of the individual resistances, so the overall resistance of the circuit increases.
According to Ohm’s law (V = I R) the battery voltage is fixed, so an increase in total resistance causes the current to decrease, which is why the ammeter reading falls.
The filament lamp has a positive temperature coefficient of resistance; as its temperature increases its resistance rises.
The thermistor also has a positive temperature coefficient, so its resistance increases with temperature.
In a series circuit the total resistance is the sum of the individual resistances, so the overall resistance of the circuit increases.
According to Ohm’s law (V = I R) the battery voltage is fixed, so an increase in total resistance causes the current to decrease, which is why the ammeter reading falls.
Examiner tips
- Show the chain of cause and effect: heating → resistance ↑ → total R ↑ → I ↓
- Use the exact terms ‘temperature coefficient of resistance’ and ‘Ohm’s law’
- Keep the answer concise and in order of the marks
Common mistakes
- Confusing the filament lamp’s resistance with the thermistor’s; both increase Forgetting that the battery voltage is constant and using V = IR incorrectly
- Not mentioning that the thermistor also has a positive temperature coefficient
Mark scheme (5 marks)
- As current flows, the filament lamp (and thermistor) heat up / temperature of both components increases
- The resistance of the filament lamp increases as its temperature increases
- The resistance of the thermistor also increases as its temperature increases
- In a series circuit the total resistance is the sum of the individual resistances, so total resistance increases
- Greater total resistance means less current flows, so the ammeter reading decreases (using voltage = current × resistance / Ohm's law reasoning)
Key terms in this question
filament lamp · thermistor · series
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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