Thermal Expansion

When the temperature gets increased, then the volume of the material also gets increased. In general, this is called thermal expansion. We can describe thermal expansion as the fractional change in volume or length per unit change in the temperature. In the case of a solid expansion, normally, the linear expansion coefficient is employed.

In solid thermal expansion, it is expressed in terms of change in height, thickness, and length. However, for both liquid and gas, the volume expansion coefficient is more useful. In general, if the material is a fluid, we can explain it in terms of volume change.

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Among the molecules and atoms, the bonding forces differ from material to material. The characteristics of compounds and elements are known as expansion coefficients. If a crystalline solid has a similar structural configuration throughout (isometric), then the expansion will be uniform in all dimensions of the crystal.

However, if the crystal is not isometric, then the expansion coefficient also varies for different crystallographic directions. As the temperature will change, then resultantly, the crystal will also change the shape. Softer materials have a higher Coefficient of Expansion (CTE), whereas the harder materials like tungsten have a lower CTE.

Types of Expansion

Let us look at the types of expansion.

Linear Expansion

The linear expansion is described as the increase in the length of the solid. For example, if we consider a rod, where the rod's length is l, and suppose that we increase the temperature of a rod by a small amount. Then, the Linear Expansion is given by,

\[\frac{ΔL}{L}\]= 𝛼LΔT

Where,

ΔT is the change in temperature,

ΔL is the change in length

𝛼L is the linear coefficient of thermal expansion

L is the origin length

The Coefficient of the linear expansion of a given solid is represented as ‘a.’ Then, for ‘a,’ the unit is per degree Celsius in the SI and CGS system it is per kelvin i.e. K-1.

Volume Expansion

The volume expansion is described as the increase in the volume of the solid on heating. With a change in temperature of ∆t, the change in volume of a solid can be given by,

∆v = Vy ∆t

Where, ‘y’ is the coefficient of volume expansion.

Area or Superficial Expansion

The Area or Superficial expansion is described as an increase in surface area of the solid on heating. If we consider the area at 0℃ of a solid is A0, then its area at t0c can be given by,

A0 (l + βt)

The Unit of β is given by,

0C-1 or K-1

Where β is called the coefficient of superficial expansion.

6𝛼 = 3β = 2γ

This equation represents the relationship between α as the linear expansion, and β as the superficial expansion, and as the volume expansion. These three coefficients of expansion for a given solid are not given as constant because these values depend totally on the temperature. 

Some examples of thermal expansion in our day-to-day life are riveting, on wooden wheels fixing the metal tires, thermometers, and more.

Factors Affecting The Thermal Expansion

Unlike liquids or gases, solid materials tend to maintain their shape when participating in thermal expansion.

In general, thermal expansion decreases with increasing bond energy, affecting the melting point of solids. So, the high melting point materials are more likely to contain a lower thermal expansion. Generally, liquids expand slightly more to that of solids. The thermal expansion of glasses is higher when compared to crystals. At the given glass transition temperature, the rearrangements in an amorphous material lead to the characteristic discontinuities of the coefficient of specific heat and thermal expansion. These discontinuities allow the glass transition temperature detection, where a supercooled liquid transforms into a glass.

The absorption or desorption of water or other solvents can change many of the common materials; many organic materials change much more, and because of this effect than thermal expansion. Common plastics can be exposed to water, in the long term, expand by more percent.

Applications of Thermal Expansion

Let us look at the Thermal expansion concept used in our daily life.

Thermometers

Thermal expansion in thermometers is used in temperature measurements.

Removal of Tight Lids

To open a bottle cap that is tight enough, immerse it in hot water for a minute or so. Then the metal cap expands and becomes loose, which would become easy to turn it to open.

Riveting

The red hot rivets are forced through holes in the plates to join the steel plates together tightly. Then, the end of hot rivets is hammered. Whereas, on cooling, the rivets contract and bring the plates gripped tightly.