Multiple choiceQuestion 1 · Recall
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By the end you can explain why the resistance of an NTC thermistor falls as its temperature rises and why the resistance of an LDR falls as the light intensity rises, read values from their characteristic data tables, and predict how the voltage across each device changes in a potential divider.
The main symbols used in this lesson:
| Symbol | Meaning |
|---|---|
| resistance |
A metal wire and a thermistor look similar, yet they respond to heating in opposite ways. The reason is what limits the current in each.
In a metal, the number of free electrons is fixed. Heating makes the positive ions vibrate more strongly, so the electrons collide with them more often and the resistance rises.
In a semiconductor — the material a thermistor or an LDR is made from — there are very few free charge carriers when it is cool. Heating, or absorbing light, frees many more of them. That increase in the number of charge carriers outweighs the extra scattering, so the resistance falls.
Resistance depends on two things: how many charge carriers there are, and how easily they move. In a metal only the movement changes. In a thermistor or an LDR the number changes, and that dominates.
Thermistor (NTC)
A semiconductor component whose resistance falls as its temperature rises. The common negative temperature coefficient (NTC) thermistor is used as a temperature sensor: a rise from 20 °C to 60 °C can reduce its resistance by a factor of four.
Worked example: Reading a thermistor's characteristic
A student measures the resistance of an NTC thermistor as it warms.
| Temperature / °C | 0 | 20 | 40 | 60 | 80 |
|---|---|---|---|---|---|
| Resistance / kΩ | 12.0 | 6.0 | 3.0 | 1.5 | 0.75 |
(a) By what factor does the resistance fall between 20 °C and 60 °C?
(b) What is the percentage decrease in resistance between 20 °C and 40 °C?