Thermal expansion of solids, liquids and gases

Understanding how substances respond to changes in temperature is a fundamental aspect of physics. One such response is thermal expansion. In general terms, thermal expansion refers to the tendency of a substance to change its shape, area, volume, and density in response to changes in temperature.

What is thermal expansion?

Thermal expansion is the increase in the size of a substance when it is heated. When substances become hotter, their molecules move more rapidly. This motion causes the particles to require more space, which leads to an increase in the dimensions of the substance.

Real-life examples

• Metal lids on glass jars may be easier to open if the lid is run under warm water. This is because metal expands more than a glass jar, causing the lid to loosen.
• Bridges often have gaps, called expansion joints. These gaps allow the material to expand in the heat and keep the bridge from bending or cracking.

Thermal expansion in solids

Solids have fixed shapes and volumes because of the strong forces between their particles. However, even in solids, the particles can vibrate in place. When a solid is heated, these vibrations increase, causing the solid to expand.

Linear expansion

Linear expansion refers to the change in one dimension (length) of a solid material when it is heated. The expansion behavior can be described mathematically by the formula:

ΔL = α * L₀ * ΔT

Where:

• ΔL = change in length
• α = coefficient of linear expansion
• L₀ = original length
• ΔT = change in temperature

Visual example of linear expansion

In the above graphic, the blue rectangle represents the solid object at its original length, while the red rectangle represents the same solid object after heating, showing the increase in length.

Thermal expansion in liquids

Liquids have no definite shape, but they do have a definite volume. Unlike solids, the particles in a liquid move freely. When heated, these particles move faster and push each other apart, causing the liquid to expand.

Volumetric expansion

Expansion in fluids is usually described in terms of volume. The volume expansion formula is:

ΔV = β * V₀ * ΔT

Where:

• ΔV = change in volume
• β = coefficient of volume expansion
• V₀ = original volume
• ΔT = change in temperature

Text example of fluid expansion

Imagine filling a glass bottle with water and sealing it tightly. If the bottle is left in a hot car, you may see some water leaking out of the bottle. This happens because the water expands more than the bottle when both are heated, causing the bottle to overflow.

Thermal expansion in gases

Gases expand more effectively than solids and liquids. This is because gas particles are far apart and have minimal forces of attraction. When gases are heated, their particles move faster and expand more rapidly.

Charles's law

The behaviour of gases under thermal expansion is primarily governed by Charles’s law. It states that the volume of a gas is directly proportional to its absolute temperature, assuming the pressure remains constant:

V₁ / T₁ = V₂ / T₂

Where:

• V₁ and V₂ are the initial and final volumes
• T₁ and T₂ are the initial and final temperatures in Kelvin

Text example of gas expansion

A balloon released into the sun will begin to expand, as the gas particles inside the balloon will move faster and further away from each other at the higher temperature.

Conclusion

Thermal expansion is an important concept in understanding how materials behave under changes in temperature. It plays a vital role in everyday life and has many practical applications in various engineering and construction projects. By considering thermal expansion, engineers can design structures that accommodate fluctuations in temperature.

Comments