Thermal Expansion

The temperature of a body is a state which determines the direction of flow of heat or the degree of hotness of a body. Heat is the cause and temperature is the effect.

Expansion of Solids

Solids expand on heating due to increased atomic spacing. A solid can be considered as periodic arrangement of atoms in the form of lattice. At any particular temperature, the atoms are in a specific state of vibration about a fixed point called as equilibrium position in the lattice.

As the temperature increases, the amplitude of vibration of the atoms increases. If the lattice vibrations are purely harmonic the potential energy curve is a symmetric parabola and there is not thermal expansion. If the lattice vibrations are not harmonic, the potential energy of an oscillator is an asymmetric function of its position and thermal expansion is observed.

Coefficient of linear expansion (α): The ratio of increase in length per one degree rise in temperature to its original length is called coefficient of linear expansion.

Coefficient of area or superficial expansion (β): The increase in area per unit area per one degree rise in temperature is called coefficient of areal expansion.

The coefficient of volume or cubical expansion (γ) is the increase in volume per unit volume per degree rise in temperature.

Expansion of Liquids

Liquids expand on heating except water between 0°C and 4°C. The expansion of liquids is greater than that of solids (about 10 times). Liquids do not possess any definite shape and require a container to hold them. Hence, only cubical expansion is considered.

Since heat effects both the liquid and the container the real expansion of a liquid cannot be detected directly.

Anomalous Expansion of Water

When water at 0°C is heated, its volume decreases up to 4°C and from 4°C its volume increases with the increase of temperature. This peculiar behaviour of water is called anomalous expansion of water. Due to the formation of more number of hydrogen bonds, water has anomalous expansion.

As the temperature increases from 0°C to 4°C, the density increases and as the temperature further increases the density decreases. Hence water has maximum density at 4°C.

Expansion of Gases

Pressure, volume and temperature are the three measurable properties of a gas. Change in one of these factors results in a change in the other two factors.

When a given mass of gas is heated under constant pressure, its volume increases with increase in temperature. When a given mass of gas is heated under constant volume its pressure increases with increase in temperature.