Temperature, Thermistors & LDRs
Temperature, Thermistors & LDRs
- The resistance of a metal increases when its temperature increases.
- The syllabus uses an NTC thermistor: its resistance decreases when temperature increases. An LDR's resistance decreases when light intensity increases.
Same input, different material response
- In a metal, higher temperature produces stronger lattice vibrations. Electrons collide more often, so resistance increases.
- In an NTC thermistor, heating releases many more mobile charge carriers. This effect dominates, so resistance decreases.
- A filament lamp is metal: as current heats its filament, its resistance rises and the I–V graph becomes less steep.
Light changes an LDR
- Increasing light intensity releases more mobile carriers in the semiconductor, so resistance decreases.
- For both an NTC thermistor and an LDR, identify the input first, then state the resistance change. Circuit output is covered in the potential-divider lesson of Topic 10.
Your turn9702/12/F/M/25 Q32
An NTC thermistor is connected to a constant p.d. Its temperature increases. State how its resistance and power change. [2] (9702/12/F/M/25 Q32)
Show worked answer
Its resistance decreases. At constant p.d.,, so its power increases.