Grade 11 → Thermal physics → Heat and temperature ↓
Thermal expansion of solids, liquids and gases
In thermal physics, the fundamental concept we explore is how substances change when heat is added. This often involves not only a change in their internal energy, but also a change in their physical dimensions. This phenomenon is known as thermal expansion. As the temperature of an object increases, its particles move more rapidly and take up more space, causing the substance to expand. This is universally observed in solids, liquids, and gases. Each state of matter reacts differently to temperature changes, and they expand to different extents.
Thermal expansion of solids
Solids by nature have a definite shape and definite volume because of the relatively strong forces between their atoms or molecules. However, solids also undergo thermal expansion. When heated, the atoms in a solid vibrate more rapidly, increasing the distance between them. This expansion is usually linear, volumetric, or area-related in nature.
Linear expansion
Linear expansion refers to the increase in the length of a solid object when it is heated. The change in length is proportional to the original length and the change in temperature. The formula for linear expansion is expressed as:
ΔL = α × L₀ × ΔT
Where:
ΔL
is the change in length,α
is the coefficient of linear expansion (specific for the material),L₀
is the original length,ΔT
is the change in temperature.
For example, consider a metal rod 1 m long at an initial temperature of 20°C. If the coefficient of linear expansion is 12 × 10-6 /°C, and the temperature is increased to 100°C, the expansion in length can be calculated as:
ΔL = 12 × 10-6 /°C × 1 m × (100°C - 20°C) = 0.00096 m
The rod extends to 0.96 mm.
Visual example
Imagine this Represents a solid bar. When heated, the bar can expand , indicates increase in length.
Thermal expansion of liquids
Liquids have no definite shape, but they have definite volume. They also expand on heating. Since liquids have no definite shape, we mainly discuss volume expansion. The formula for volume expansion of a liquid is given as:
ΔV = β × V₀ × ΔT
Where:
ΔV
is the change in volume,β
is the coefficient of volume expansion (specific to a liquid),V₀
is the original volume,ΔT
is the change in temperature.
For example, if you have a container with 2 liters of water at 25°C and the coefficient of volume expansion for water is about 210 × 10-6 /°C. When the water is heated to 75°C, the volume change is calculated as:
ΔV = 210 × 10-6 /°C × 2 L × (75°C - 25°C) = 0.021 L
The volume increases by 21 ml.
Visual example
Imagine this Represents a container filled with liquid. After heating, it can expand , indicating an increase in quantity.
Thermal expansion of gases
Gases are quite different from solids and liquids in terms of expansion. They expand much more than solids and liquids when heated. This is because the forces between particles in a gas are extremely weak compared to the latter two states of matter. The behaviour of gases under thermal conditions is often described using the ideal gas law and its related principles, such as Charles's law, which states that at constant pressure, the volume of a gas is directly proportional to its temperature (measured in Kelvin).
To describe the volume expansion of gases at constant pressure we use:
V₁/T₁ = V₂/T₂
Where:
V₁
andV₂
are the initial and final volumes, respectively,T₁
andT₂
are the initial and final temperatures in Kelvin, respectively.
Let us consider a gas sample in a balloon whose initial volume is 1.5 liters at 300 K. If the temperature is increased to 400 K without changing the pressure, the new volume can be calculated as:
V₂ = V₁ × (T₂/T₁) = 1.5 L × (400K / 300K) = 2.0 L
Gas volume increases to 2.0 liters.
Visual example
Imagine this
Practical implications of thermal expansion
Understanding thermal expansion is important in everyday life and in many technological applications. Engineers and designers must consider it when building everything from bridges to thermometers because of the potential impact of thermal changes. Examples include:
- Bridges and railways: Expansion joints are incorporated in the construction of bridges and railway tracks to take into account the expansion and contraction of metal due to temperature changes.
- Thermometer: Most liquid thermometers work on the principle of volumetric expansion of the liquid inside the thermometer.
- Bimetallic strips: These are used in many devices such as thermostats. They consist of two different metals bonded together, which expand at different rates to bend when heated. This bending can open or close a circuit. Visualization note: Imagine two bonded strips, where the red strip (higher expansion coefficient) bends more when heated, causing the strip to coil.
Special cases and considerations
Thermal expansion has some peculiar effects, such as when water approaches its freezing point. Unlike most substances, water expands when it freezes, which is why ice floats on water. This unusual expansion is important for aquatic life in cold climates, as it keeps the water underneath from freezing.
With gases, deviations from ideal behavior occur at high pressures and low temperatures. Real gases do not always obey the ideal gas law because of interactions between gas particles and the volume they occupy, which becomes more important under these conditions.
In addition, some materials have negative coefficients of expansion over certain temperature ranges, meaning they shrink when heated. Engineers and scientists must be aware of these unique properties to avoid serious structural or mechanical failures.
Applications and wider impact
The principles of thermal expansion extend beyond Earth-based applications. They are important in space technology such as satellites and the International Space Station, which experience significant temperature fluctuations. Materials used in these environments must be carefully selected to withstand such conditions without degradation or failure.
In the electronics industry, thermal expansion is an important consideration due to the miniaturization of components. The expansion and contraction of electronic materials can create stress and cause failures in circuits and microchips, which requires precise design and material selection.
Conclusion
Thermal expansion is a fascinating and complex phenomenon that has wide-ranging implications across a variety of fields. From everyday uses such as cooking thermometers to essential components of infrastructure and advanced scientific applications, it is vital to understand how different states of matter react to temperature changes. This knowledge not only helps us build safer and more efficient systems but also connects with our deep scientific curiosity about the natural world.