Measuring Temperature
Physics: Cambridge International A Level, topic 9.2.2
Thermocouples
A thermocouple can measure high and fast-changing temperatures.
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Structure
- Two different metals are joined at different points into a strip.
- The temperature is fixed at one point and if the temperature at the other point is different, a voltage is induced.
- The size of the voltage depends on temperature difference.
Advantages
- Thermocouples are made of metal, so they can measure very high temperatures. Unlike thermometers which use ethanol that boils at 78°C.
- Thermocouples are quick to respond to temperature changes.
Disadvantages
- Thermocouples are non-linear.
Liquid-in-glass Thermometers
We usually measure room temperatures using a liquid-in-glass thermometer.
Structure
- Thermometers today usually contain alcohol, but they used to contain mercury.
- A thinly-walled bulb of alcohol (called the reservoir) is attached to a capillary tube with a narrow diameter.
- When the alcohol expands it is pushed up the capillary tube.
- The distance the liquid moves is a way of measuring temperature.
Disadvantages
- Because glass insulates the bulb of alcohol (reservoir), thermometers can be slow to respond.
Thermistors
Another way of measuring the temperature of an object is to use a thermistor.
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Thermistor
- A thermistor is a semiconductor with a resistance that changes with its temperature.
- Usually, increasing temperature decreases the resistance.
- We can use a thermistor to turn a heater off when a house reaches certain temperatures.
Advantages
- Small size
- Quickly comes into thermal equilibrium with the object being measured.
- Thermistors are very efficient.
- Can produce a temperature reading at a distance from the object.
Disadvantages
- Thermistors require recalibrating often.
- Thermistors are non-linear.
Test yourself on Measuring Temperature
What is a potential disadvantage of a thermocouple?
How does a thermometer measure temperature?
What is the boiling point of ethanol and why is this a drawback for its use in thermometers?
Which of the following is a disadvantage of using a thermistor?