A thermistor and a fixed resistor are connected in series with a battery. Explain what happens to the total resistance of the circuit and the current flowing through it when the temperature of the thermistor increases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A thermistor is used in a temperature-monitoring circuit. As the room warms up, the temperature of the thermistor rises.
Model answer (5 marks)
The resistance of the thermistor falls as its temperature rises.
The total resistance of the series circuit is the sum of the thermistor and the fixed resistor, so it also falls.
Because the battery voltage is constant, Ohm’s law (V=IR) gives I=V/R; a lower R gives a higher I.
In a series circuit the current is the same through every component, so the increased current flows through both the thermistor and the fixed resistor.
The total resistance of the series circuit is the sum of the thermistor and the fixed resistor, so it also falls.
Because the battery voltage is constant, Ohm’s law (V=IR) gives I=V/R; a lower R gives a higher I.
In a series circuit the current is the same through every component, so the increased current flows through both the thermistor and the fixed resistor.
Examiner tips
- Show the thermistor’s resistance decreases with temperature. Explain the series‑circuit rule R_total=R_thermistor+R_fixed. Use V=IR to link lower R to higher I. State that the same current flows through all series elements.
Common mistakes
- Confusing a thermistor with a normal resistor (ignoring its temperature dependence). Forgetting that the battery voltage is constant. Claiming the current changes only in the thermistor, not in the fixed resistor. Not using the V=IR relationship to justify the current change.
Mark scheme (5 marks)
- The resistance of the thermistor decreases as temperature increases
- The total resistance of the series circuit decreases
- In a series circuit the total resistance equals the sum of the individual resistances (so a fall in one reduces the total)
- Current is directly proportional to potential difference / voltage is constant so lower resistance means greater current (correct use of V = IR reasoning)
- Current is the same at every point in the series circuit (so the increased current is the same through both the thermistor and the fixed resistor)
Key terms in this question
thermistor · resistance · series · current
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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