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Thermal expansion
Thermal Expansion of Solids , Liquids and Gases
When matter is heated it expands and when cooled it contracts.
According to the kinetic theory the molecules of solid and liquids are in constant vibration.
When heated, they vibrate faster and force each other a little further apart.
Expansion results , and this is greater for liquid , gases expand even more.
Precautions against thermal expansion
1) Gaps must be left between lengths of railway track to allow the expansion of the steel rails in the hot seasons.
2) Metal bridges must be constructed to allow for expansion. One end of the bridge is usually fixed and the other end rests on rollers to allow movement due to expansion.
Uses of expansion
If equal lengths of two different metals, e.g. copper and iron , are fixed together so that they cannot move separately , they form a bimetallic strip.
when the strip is heated it bends , copper expands more than iron and to allow this the strip bends with copper on the out side.
Bimetallic strips have many uses, few of them are mentioned here.
1) Fire alarm:- Heat from the fire makes the bimetallic strip bend and complete the electrical circuit , so ringing the alarm bill.
2) Thermostat:- A thermostat keeps the temperature of a room or an appliance constant. Bimetallic strip can be used in thermostat to control the temperature in a heating system such as an electric iron.
Unusual expansion of water
As water is cooled to 4oC it contracts as we would expect. However between 4oC – 0oC, it expands surprisingly. Water has a maximum density at 4oC.
The unusual expansion of water between 4oC – 0oC explains why fish survives in a frozen pond.
Measurement of temperature
The temperature of a body tells us how hot the body is.
It is measured in degree Celsius (oC).
The molecular kinetic theory explains that the temperature is a measure of average kinetic energy of the molecules of the body.