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.

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:

**Linear expansion.****Areal expansion.****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.

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.

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.

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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