Measurement of temperatureCopy

Learning Objectives

• Appreciate how a physical property that varies with temperature may be used for the measurement of temperature, and state examples of such properties
• Demonstrate understanding of sensitivity, range and linearity
• Recognise the need for and identify fixed points
• Describe the structure of a thermocouple and show understanding of its use as a thermometer for measuring high temperatures and those that vary rapidly
• Describe and explain the structure and action of liquid-in-glass thermometers
• Describe and explain how the structure of a liquid-in-glass thermometer relates to its sensitivity, range and linearity

Contents to learn

Measurement of temperature

The temperature of a body tells us how hot the body is.

It is measured in degree Celsius (oC) or Kelvin (Base unit).

The molecular kinetic theory explains that the temperature is a measure of average kinetic energy of the molecules of the body.

Temperature can be measured with the help of thermometer.

There are different types of thermometers.

1. liquid – in – glass thermometer.
2. thermocouple thermometer.
3. constant – volume gas thermometer.
4. resistance thermometer.

Thermometer – thermocouple

A thermocouple consists of wires of two different materials, e.g. copper and iron , joined together.

When one junction is at a higher temperature than the other an electric current flows and produces a reading on a sensitive voltmeter which depends on the temperature difference between the two junctions (hot junction and the cold junction).

A voltmeter can measure the voltage that develops by the thermocouple, and a table of calculated values can be looked at to determine the corresponding temperature.

It is used in industries to measure a wide range of temperatures from -250oC up to 1500oC.

Another major advantage of using thermocouple is that it can be made very small, means that it will respond very quickly to a change in temperature.

Liquid in glass thermometer

In this type, the liquid in a glass bulb expands up a capillary tube when the bulb is heated.

Mercury and coloured alcohol are in common use.

Mercury freezes at -39oC and boils at 357oC and is suitable for high temperature.

Alcohol freezes at -115oC and boils at 78oC and is therefore more suitable for low temperature.