Thermal Expansion

Every material (except water which contracts on heating from 0°C to 4°C) expands on heating. The increase in the size of a body on heating is called thermal expansion.

The expansivity of different materials is normally different. The fact can be easily noticed with the help of a bimetallic strip. A bimetallic strip is a strip having two layers of two different metals one over the other. Consider the bimetallic strip made of steel and aluminium. When we clamp one end of the strip and heat it uniformly with the help of a Bunsen burner, it bends with aluminium layer outward. This clearly shows that aluminium has increased in length more than steel and caused bending.

Increase in length of a metallic bar will be more for a longer bar and also for a greater rise in temperature of the same bar. Consider a metallic bar of length L0 at temperature 0° C. Increase in its length ΔL at a temperature Δt is given by:

ΔL ∝ L0Δt

ΔL = αL0Δt

α = ΔL/L0Δt

Here α is a constant for the material of the bar and is called as the Linear expansivity of the bar. The Linear expansivity (or Coefficient of Linear expansion) of a material is defined as the change in length per unit original length per degree celsius rise in temperature. The SI Unit of coefficient of expansion is per kelvin (which is same as per degree celsius in magnitude).

A piece of solid may expand along length, breadth and height simultaneously hence there will be an increase in its volume with temperature. The Volume expansivity of a material may be defined as change in volume per unit original volume per degree celsius rise in temperature.

γ = ΔV/VΔt

Expansivity of solids is very small therefore we cannot see and measure expansion of solids easily. But liquids expand much more than solids and gases many times more than liquids and so we can see expansion of liquids and gases easily. However, since liquids and gases do not have a definite shape, it will be volume expansivity only relevant for fluids.

Uses of thermal expansion in day to day life

1. The property of thermal expansion is used in the construction of thermometers.

2. A tightly closed metallic cap of a bottle may be opened by using thermal expansion. The cap on heating expands, becomes loose and may be opened easily.

3. A horse-cart (Tanga) has wooden wheels on the rims of which iron rings are mounted. The iron ring is made of a radius slightly less than the radius of wheel. Then the ring is heated so that its radius becomes slightly more than the radius of the wheel. The ring is than slipped on the rim of the wheel while hot. Subsequently on cooling, it contracts and firmly holds the rim of the wheel.

4. Thermostats used in heating and cooling devices make use of a bimetallic strip to automatically switch off the heating cooling circuit when the temperature rises or falls beyond a certain value. After some time when the temperature returns below or above a certain value the bimetallic strip resumes its original position and the circuit again becomes on.

5. We have to take care of thermal expansion while making big structures or otherwise these structures may collapse. For example:

  • Gaps are left at the joints of a railway tracks or else during summer due to thermal expansion the rails will bend and derail the train.
  • The iron bridges are not made of continuous structures. At one end the girders are left open and placed over rollers.

6. While pouring hot tea in a glass tumbler it is suggested that a metallic spoon be first placed in the tumbler and the tea be poured over it. In case the tea is directly poured in the tumbler it may get cracked due to uneven expansion of its different parts.

Anomalous Expansion of Water

Generally all the liquids expand in volume when they are heated but water decreases in volume when heated from 0°C to 4°C and then after 4°C, it increases in volume on further heating. The volume of a fixed mass of water is minimum at 4°C, i.e. its density is maximum at 4°C with further rise in temperature its density decreases.