A student connects a filament lamp and a thermistor in series with a battery. As the circuit is left switched on, the filament lamp glows brightly and the thermistor warms up. Explain what happens to the current in the circuit as the thermistor warms up, and why this affects the brightness of the filament lamp.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student connects a filament lamp and a thermistor in series with a battery. As the circuit is left switched on, the filament lamp glows brightly and the thermistor warms up.
Model answer (5 marks)
As the thermistor warms up its resistance falls.
Because the lamp and thermistor are in series, the total resistance of the circuit falls.
A lower total resistance allows a larger current to flow from the battery.
In a series circuit the same current flows through every component, so the current through the filament lamp rises.
The power delivered to the lamp is P=I²R, so with a higher current the lamp receives more power and glows brighter.
Because the lamp and thermistor are in series, the total resistance of the circuit falls.
A lower total resistance allows a larger current to flow from the battery.
In a series circuit the same current flows through every component, so the current through the filament lamp rises.
The power delivered to the lamp is P=I²R, so with a higher current the lamp receives more power and glows brighter.
Examiner tips
- Use the command word ‘Explain’ – describe the sequence of changes and give the reason for each.
- Show the relationship P=I²R to link current to brightness.
- Mention that the same current flows through all series components.
Common mistakes
- Saying the resistance of the thermistor increases instead of decreases.
- Confusing the effect of resistance on current with the effect on voltage.
- Forgetting that the lamp’s brightness depends on power, not just voltage.
Mark scheme (5 marks)
- As the thermistor warms up, its resistance decreases
- Because the components are in series, the total resistance of the circuit decreases
- A smaller total resistance means a larger current flows through the circuit
- In a series circuit the same current passes through all components, so the current through the filament lamp increases
- A greater current through the filament lamp means more power is transferred, so the lamp glows more brightly
Key terms in this question
thermistor · series · current
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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