When matter is heated, it normally expands and when cooled, it normally contracts. The atoms in a solid vibrate about their mean position. When heated, they vibrate faster and force each other to move a little farther apart. This results into expansion.

Thermal Expansion and its types
The molecules in liquid or gas move with certain speed. When heated, move faster and force each other to move a little farther apart. This results in expansion of liquids and gases on heating. The expansion is more in liquids than in solids; gases expand even more.

A change in temperature of a body causes change in its dimensions. The increase in the dimensions of the body due to an increase in its temperature is called thermal expansion.

There are thre types of thermal expansion:
  1. Linear expansion.
  2. Areal expansion.
  3. Volume expansion.     
Linear expansion: 

The expansion in length due to thermal energy is called linear expansion. It takes place in one dimension only. 

Calculating change in lenght ( ΔL) due to application of heat:

The change in length ΔL is proportional to length L. The dependence of thermal expansion on temperature, substance, and length is summarized in the equation ΔL = αLΔT,where ΔL is the change in length L, ΔT is the change in temperature, and α is the coefficient of linear expansion, which varies slightly with temperature.

Areal expansion: 

The increase in surface area, on heating is called areal expansion or superficial expansion. This type of expansion takes place in 2 dimensions . Thus its related to area.

Calculating change in area ( ΔA) due to application of heat:

For small temperature changes, the change in area ΔA is given by ΔA = 2αAΔT, where ΔA is the change in area A, ΔT is the change in temperature, and α is the coefficient of linear expansion, which varies slightly with temperature.

Volume expansion: 

The increase in the volume due to heating is called volume expansion or cubical expansion.The expansion takes place in 3 dimensions.

Calculating change in volume(ΔV ) due to application of heat :

The change in volume ΔV is very nearly ΔV = 3αVΔT. This equation is usually written as ΔV = βVΔT, where β is the coefficient of volume expansion and β ≈ 3α.

Applications of thermal expansion:

  • Linear expansion property of mercury is used in thermometers.
  • Ever thought that in winter when a cap of metal container gets jammed, and how it's losened up again after appication of heat, that's an example of areal expansion.
  • The thermostat is a heat-regulating device which works on the principle of thermal expansion.
  • Cracks in the road when the road expands on heating.
  • Sags in electrical power lines.
  • Windows of metal-framed need rubber spacers to avoid thermal expansion.
  • Expansion joints (like joint of two railway tracks).
  • The length of the metal bar getting longer on heating.
  • Tire bursts in hot days when filled full of air due to thermal expansion.
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