Understanding Thermal Expansion

Thermal expansion refers to the phenomenon whereby the shape, volume, density, or area of a material undergoes changes as a result of temperature change. In most circumstances, phase transitions are excluded under thermal expansion. Before understanding the meaning of thermal expansion, there should be an understanding of what exactly is temperature. In literal terms, temperature stands for the average molecular kinetic energy of an object. When there is an increase in temperature, it implies that molecules move faster on average.

Consequently, upon heating a substance, the particles move at a faster speed, and thereby, take up more space. This is primarily because the molecules tend to move in spaces that were previously empty, causing the size of the object to increase. The coefficient of thermal expression determines how the size of an object undergoes a change following a change in temperature. In the article that follows, we shall cover the different types of thermal expansion, practical applications of the phenomenon, factors affecting thermal expansion, and lastly, the merits and demerits of thermal expansion. 

Types of Thermal Expansion

There are three types of thermal expansion, namely linear expansion, volume expansion, and area expansion. 

Linear Expansion:

It refers to changes in the length of an object primarily due to expansion of its volume. The coefficient of linear expansion indicates the extent of the material expansion upon heating. For example, a rod having length l and suppose that the temperature of the rod undergoes a small increment. Then, the thermal expansion would be as follows:

ΔL/L= αL・L・ΔT

Where,

ΔT stands for temperature change,

ΔL denotes the change in length

αL denotes the linear coefficient of thermal expansion

L is the original length

Here, the SI unit is per degree Celsius. In the CGS system, on the other hand, it is per Kelvin.

Volume Expansion:

When a solid undergoes an increase in volume upon being heated, it is known as volume expansion. If the temperature change is Δt, then volume expansion is denoted by: 

ΔV = V・γ・Δt

Here, ‘γ’ represents the volume expansion coefficient.

Δt is the change of temperature.

Area Expansion:

Also known as superficial expansion, area expansion refers to an increase in the surface area of an object upon being heated. For instance, if the area of a solid is A0 at 0°C, then the area of that solid at t °C is given by A0 (l + βt)

Where β is called the coefficient of area expansion.

Practical Applications

There are multiple examples of the usage of thermal expansion on a daily basis. Consider a situation when you are unable to open the lid of a pickle jar. You try as hard as you can and apply the maximum force possible. However, everything is in vain. Then, you finally try your mother’s trick of running the pickle jar under warm water. The jar opens smoothly. What exactly is the reason behind it?

The answer is thermal expansion. 

Another common application of thermal expansion is observable on a power line. A power line is generally droopy on a day when the temperatures are high. In contrast, it is firm on a day with comparatively low temperatures.

This is precisely because the metal undergoes expansion due when subjected to heat. 

When new structures are designed or land surveys are conducted to measure distances using chains or tapes, thermal expansion of materials is usually taken into consideration.

In almost every area of engineering, the implications of thermal expansion are taken into consideration. Rubber spacers are provided in metal-framed windows. It is generally advisable to not use metal hot water heating pipes in long straight lengths. 

Factors Affecting Thermal Expansion

1. Thermal expansion and bond energy are inversely proportional to one another. This, in turn, affects the melting point. Thus, the higher the melting point, the lower will be the thermal expansion. 
2. Thermal expansion undergoes changes when water or any other solvent is absorbed or absorbed that brings about a change in the common material. 
3. Discontinuities in thermal expansion may be caused by rearrangements in an amorphous material at a particular temperature. 

Merits and Demerits of Thermal Expansion

The merits of thermal expansion are:

1. In a thermometer used to measure temperature, mercury expands as a result of thermal expansion. 
2. A tightly tied metal lid of a bottle or a container can be easily removed by immersing the bottle or container in warm water. This is because the neck of the bottle or the container expands due to thermal expansion, and therefore, the lid can be easily removed. 
3. Metal tires are fixed on wooden wheels of carts with the help of thermal expansion. Here, the iron rims are first heated. Thereafter, thermal expansion enables them to easily slip over the wooden wheel. Lastly, water is poured and the rim contracts and becomes tight over the wheel. 

The demerits of thermal expansion are:

1. Wall collapsing may occur due to bulging.
2. Usually, metal pipes that carry hot water may burst as a result of thermal expansion. 
3. Objects such as doors may undergo a shape change due to thermal expansion. 

Conclusion

Therefore, understanding thermal expansion is crucial in the scientific world. In simple words, thermal expansion is nothing but a change in the shape, size, or volume of a substance caused by temperature change. Thermal expansion is dependent on the material undergoing the temperature change. The coefficient of thermal expansion is the measure of the changes in the size of an object owing to changes in temperature. There are three main types of thermal expansion: linear expansion, volume expansion, and area expansion. Each type has its respective definition and equation. 

There are both pros and cons to the applications of thermal expansion. While on one hand, it can assist you to open tight metal lids, it can result in cracking of glass tumblers due to excessive heating on the other. Thus, thermal expansion stands as a concept of utmost importance today.