Explain light-dependent resistors and thermistors
Predict how the resistance of an LDR and a negative-temperature-coefficient thermistor changes with conditions.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 9. Circuit calculations and component sketches are original study practice; full timed assessment, supervised practical competence and human subject-teacher review remain outstanding.
Before you begin
- Use charge, time, energy, power, area and consistent SI units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How does LDR resistance respond to greater light intensity?
What you will learn
- Understand that an LDR's resistance decreases as light intensity increases
- Understand that an NTC thermistor's resistance decreases as temperature increases
- Interpret the stated sensor trends without assuming a linear response
An LDR responds to light intensity
A light-dependent resistor (LDR) has lower resistance when the incident light intensity is greater. In a simple potential-divider context, this changing resistance can alter the share of supply potential difference across the LDR and a fixed resistor.
The syllabus outcome specifies the direction of the change, not a linear resistance-versus-light law. Use the trend to predict what happens when illumination changes; do not calculate a numerical resistance without supplied data or a model.
The stated thermistor has a negative temperature coefficient
For the thermistors assumed in this syllabus, resistance decreases as temperature increases. This is a negative temperature coefficient (NTC) response.
The relationship is generally non-linear. Distinguish the material response from current and potential difference in the surrounding circuit, which also depend on how the sensor is connected.
Pause and explain
What is assumed about thermistors in this syllabus?
Worked example
An LDR moves from a dim room into brighter light. State the resistance change. A stated NTC thermistor is then warmed; state its resistance change.
Show the worked solution
- The LDR resistance decreases as light intensity increases.
- The NTC thermistor resistance decreases as temperature increases.
- No numerical change can be calculated without a supplied response model.
Answer Both resistances decrease; no numerical amount is specified.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Rank the resistance of the same LDR in a dark box and under a bright lamp.
Give me a hint
Compare light intensity, not voltage or current.
Compare my reasoning
- The bright lamp gives greater incident light intensity.
- LDR resistance decreases when light intensity increases.
- Therefore the bright condition has lower resistance.
The LDR has lower resistance under the bright lamp than in the dark box.
Look for these in your work
- Light conditions compared
- Correct trend used
- Ranking stated
A negative-temperature-coefficient thermistor is heated from 20 °C to 50 °C. State its resistance trend and whether the amount of change can be calculated from the syllabus statement alone.
Give me a hint
Use the NTC direction, then check if a numerical response curve was supplied.
Compare my reasoning
- Temperature increases.
- For an NTC thermistor, resistance decreases.
- The qualitative statement gives no numerical curve or amount.
Its resistance decreases; the numerical change cannot be found without response data or a model.
Look for these in your work
- Temperature change identified
- Resistance trend correct
- Numerical limit stated
A temperature sensor circuit's current increases after its NTC thermistor is warmed. A learner concludes the thermistor resistance must have decreased. State what is supported and what additional circuit information matters.
Give me a hint
Separate the known sensor trend from the complete circuit response.
Compare my reasoning
- The stated NTC thermistor's resistance decreases as it warms.
- The whole-circuit current also depends on the supply and other component connections and values.
- A current observation alone does not specify the divider output or every component voltage.
The resistance trend is supported by the NTC property, but predicting circuit current or output voltage also requires the supply and circuit arrangement.
Look for these in your work
- Sensor trend used
- Circuit context distinguished
- Conclusion bounded
Common mistakes
- Claiming an LDR resistance rises with brighter illumination
- Treating all thermistors as positive-temperature-coefficient devices here
- Assuming either sensor response is linear without data
What can you explain now?
How does LDR resistance respond to greater light intensity?
Compare with the explanation
It decreases
That is the stated LDR trend.
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Make it stick
Why does a sensor's resistance trend alone not determine the output voltage of its whole circuit?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.