Thermal Expansion of Solids, Liquids and Gases

2.2.1 Everyday Applications and Consequences

Introduction

Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. When a substance is heated, its particles move more and thus maintain a greater average separation. Because the particles are more spread out, the substance expands. Conversely, when a substance is cooled, its particles move less and the substance contracts.

Applications of Thermal Expansion

1. Bimetallic Strips

Bimetallic strips are made of two different metals with different coefficients of thermal expansion bonded together. When heated, the metal with the higher coefficient expands more, causing the strip to bend. This principle is used in:

• Thermostats: In household and industrial heating systems, bimetallic strips control the flow of electricity to maintain a desired temperature.
• Circuit Breakers: Bimetallic strips are used in circuit breakers to interrupt the flow of electricity if the current becomes too high, preventing overheating and potential fires.
• Oven Controls: Bimetallic strips regulate the temperature in ovens.

2. Expansion Joints in Bridges and Roads

Bridges and roads are designed with expansion joints to accommodate the expansion and contraction of the materials due to temperature changes. Without these joints, the stress caused by expansion and contraction could lead to cracking and structural failure.

Application	Description
Bridges	Expansion joints allow the bridge deck to expand and contract without causing stress on the structure.
Roads	Similar to bridges, expansion joints in roads prevent cracking due to temperature fluctuations.

3. Thermos Flasks

Thermos flasks utilize the principle of thermal expansion and contraction to minimize heat loss or gain. The double-walled construction creates a vacuum between the walls. The inner wall is typically silvered to reduce radiation. The difference in the thermal expansion of the inner and outer walls minimizes the amount of heat conducted between the two walls.

4. Hot Air Balloons

Hot air balloons work because the air inside the balloon is heated. Heating the air causes it to expand, making it less dense than the surrounding cooler air. This difference in density creates buoyancy, lifting the balloon and its payload.

Consequences of Thermal Expansion

1. Stress on Materials

If thermal expansion is constrained, it can cause significant stress on materials. This can lead to:

• Cracking: In structures like bridges and buildings, if expansion is not accommodated, the stress can cause cracks to form.
• Deformation: Materials can deform permanently under stress.
• Failure: In extreme cases, the stress can lead to structural failure.

2. Railway Tracks

Railway tracks are laid with small gaps between the rails to allow for expansion in hot weather. Without these gaps, the rails would buckle under the expansion, causing derailments.

3. Glassware

Glassware with uneven thickness can be susceptible to thermal shock. When heated rapidly, the outer layer expands faster than the inner layer. When cooled rapidly, the outer layer contracts faster than the inner layer. This differential expansion can create internal stresses that cause the glass to crack or shatter.

4. Rivets and Fasteners

Rivets and other fasteners are often designed with a small gap to allow for thermal expansion. This prevents the fasteners from being stressed and potentially failing due to expansion and contraction.

Conclusion

Thermal expansion is a fundamental property of matter with numerous practical applications and potential consequences. Understanding this principle is crucial in the design and construction of many everyday objects and structures.