Thermal expansion is the affinity of matter to change volume in response to temperature changes. The anharmonic oscillations introduced in the matter due to an increase in temperature have an effect on the mean separation of atoms which results in thermal expansion. Atoms and molecules in a solid, for example, oscillate continuously around the equilibrium point. Thermal motion is a phrase used to refer to this type of stimulation. When a material is heated, its constituent particles migrate more, leading to a more significant average splitting among them and their neighbours. The coefficient of thermal expansion of a material is defined as the degree of expansion divided by the change in temperature; it usually changes with temperature. Thermal expansion is used in various applications such as railroad track buckling, engine coolants, mercury in thermometers, joint expansion, etc. This article will cover the thermal expansion of solids, liquids, and gasses and their applications.

What causes matter to expand when heated? 

The shape of particle potential in matter is the primary reason for expansion. An increase in the temperature also raises the kinetic energy of the atoms, causing the particles to vibrate and move, leading to greater average atom separation and thus thermal expansion, i.e., the vibrational origin of thermal expansion. 

• Particles in solids and liquids are constantly aware of the presence of other particles. This interaction can be mathematically represented as a potential curve. 
• The inter-particle potential works in an asymmetric format as a function of the distance between the particles.
• It is worth noting that the potential curve is steeper for shorter distances. 
• The equilibrium or the average particle-particle distance grows when a substance heats up.
• It is uncommon to find materials that contract or retain their shape as the temperature rises. 
• This effect is limited in scope and occurs only within specific temperature ranges.

Linear Thermal Expansion

The alteration in length measurements of an element due to the thermal volumetric dimension is associated with temperature alteration by a linear expansion coefficient. It is the fractional variation in length per degree change in temperature. 

Assuming that pressure has no effect,

ΔL=LαΔT

Where ΔL = change in length, L = initial length, α=the coefficient of linear expansion, and ΔT= change in temperature.

This equation calculates the change in linear expansion rather than the new length. If the object was trying to expand, we added L to the initial length L to get a new size.

The alteration in the linear dimension L is defined by the expansion coefficient across a temperature range and can be calculated to be,

ΔLL= ∆TαL

If the linear expansion coefficient does not change significantly with the change in temperature, this equation will give substantial results. 

Area Thermal Expansion

Objects grow in all directions. Their areas, volumes, and lengths all increase with temperature. The area thermal expansion coefficient relays an alteration in the dimensions of a material to a temperature change. It’s defined as,

ΔA=αA AΔT

The relationship between area and linear thermal expansion coefficients is:

αA = 2αL

Volume Thermal Expansion

The expansion and contraction depend upon the temperature deviation of a substance. Isotropic substances are those that develop at the same rate in all orders. In the case of a gas, growth is determined by how the pressure changes during the process due to the volume of a gas varying significantly with both pressure and temperature.

We can overlook the effects of pressure on a solid and calculate the volumetric thermal expansion coefficient as,

ΔV=αV VΔT

In the case of isotropic materials,

αV = 3αL

Examples of thermal expansion of solids, liquids, and gases in daily life and their applications are as follows:

• Try the expansion effect if you’ve ever tried unscrewing a stuck lid from a glass jar and found it difficult to open. Pour some hot water on the lid of the glass jar. The heat will cause the lead of the pot to expand slightly, allowing it to open easily.
• Water contracts as it cools from room temperature to its freezing point. It expands at just four degrees Celsius above its freezing point.
• Liquid expands on heating and can be made to rise into a tube. The expansion of a liquid, for instance, mercury, is used by thermometers to measure temperature using a calibrated scale.
• When air is heated, it expands and becomes less dense than the surrounding air, which employs an upward force on the hot air, causing steam and smoke to rise and hot air balloons to float.
• Railroad tracks and bridges have expansion joints that allow them to expand and contract freely in response to temperature changes.

Applications of thermal expansion:

1. Coolant for engines

The thermal expansion can be found inside a car’s radiator. On a cold day, if the radiator is topped off with coolant, increasing temperature may cause the coolant to expand until it overflows. The latest vehicles include a surplus space to accumulate the fluid displaced by volume expansion. The effective recycling of the excess liquid starts when the engine starts to cool down. New cars are much less likely to overheat than older vehicles. 

1. Gas thermometers with volume

The expansion coefficients of solids and liquids vary significantly, and gases show the maximum pattern of expansion in response to temperature increases. The constant gas thermometer was developed due to the predictable behaviour of gases in these situations. An empty container is connected to a glass tube containing mercury in a volume gas thermometer. The mercury column moves upward as gas is released into the empty container. 

Conclusion

Thermal expansion is generally defined as the increase in the volume of a material as its temperature rises, which is usually expressed as a fractional change in length or volume per unit temperature change; a linear expansion coefficient is generally used for solid expansion, whereas a volume expansion coefficient is used for liquid or gas expansion.