Thermal Expansion

Thermal expansion takes place in all states of matter. The gases expand more than liquids and liquids expand more than solids for the same amount of heat. Thermal expansion plays an important role in many engineering applications.

When an object is heated its molecules vibrate more violently because they have more kinetic energy. They also need more space around them. This causes the material to expand.

Increase in length due to heating is called linear expansion. Increase in area as superficial expansion and that of volume as volume expansion or cubical expansion. The thermal expansion is different for different substances.

Every day applications of thermal expansion of solids

• If we find difficult to remove the stopper from a glass bottle, we can heat the neck of the bottle. Now the neck of the bottle expands and the stopper comes out easily.
• The principle of thermal expansion is used in fixing iron rim with the wooden wheel firmly.
• Rivets are used to hold steel plates together very tightly. A very hot rivet is pushed through the two plates and its end is hammered over. When the rivets cools down it pulls the two plates together very tightly.
• To avoid bursting of soft drink bottles containing gas, due to thermal expansion, their walls are made very thick.

Bimetallic Strip

A bimetallic strip consists of two different metals such as brass and iron joined together.

At normal temperature the bimetallic strip is straight. As it is heated the brass expands more than the iron. So the brass forms the outside of a curve with the iron on the inside. Such a bimetallic strip can be used in a thermostat to break an electrical circuit. A thermostat is used to maintain a steady temperature in a system. As the temperature increases the strip bends and breaks electrical contact in the heater circuit. When the temperature decreases, the bimetallic strip returns to its original position and shape. Thus contact is restored.

Some of the problems created by thermal expansion is the changing of shape and dimensions of objects such as doors, Wall collapsing due to bulging, cracking of glass tumbler due to heating and bursting of metal pipes carrying hot water or steam.

Thermal Expansion and Railway Lines

Rails are made of steel which expands on heating and contracts on cooling. A gap is left between two ends of the rails at the joint. If no gaps are left, due to expansion in summer the rails get distorted causing derailment. For the same reason, gaps are left in the concrete slabs of bridges and Highways.

Clock Pendulums

The period of oscillation of a pendulum in a clock depends on its length. When the temperature changes, the length also changes. Hence the clock loses time in summer and gains it in winter. This can be compensated by using a bimetallic pendulum against the effect of thermal expansion.