The term thermal expansion refers to the increase in size of an object as that object is heated. With relatively few exceptions, all objects expand when they are heated and contract when they are cooled. Perhaps the most important exception to this rule is water. Water contracts as it cools from its boiling point to about 39.2°F (4°C). At that point, it begins to expand as it cools further to its freezing point. This unusual effect explains the fact that ice is less dense than water.
Different materials expand or contract at different rates. In general, gases expand more than liquids, and liquids expand more than solids.
When an object is heated or cooled, it expands or contracts in all dimensions. However, for practical reasons, scientists and engineers often focus on two different kinds of expansion, or expansivity: linear expansivity (expansion in one direction only) and volume expansivity (expansion in all three dimensions). The amount by which any given material
expands in either way is known as its coefficient of linear (or volume) expansivity.
The choice of these two variables is a practical one. Scientists and engineers often want to know the amount by which some pipe, bar, wire, or other long object will expand. For example, how much longer will a line of telephone wire be on a hot summer day when the temperature is 86°F (30°C) compared to a cold winter day when the temperature is 14°F (−10°C)? The fact is that the wire expands in all three directions, but it is only the linear direction (the length) that is of interest in a real-life situation.
Only solids have a coefficient of linear expansion. They differ from each other widely, with the coefficient of linear expansion of aluminum having a value nearly 50 times as great as that of fused quartz.
Volume expansivity also has its practical applications. Suppose that someone wants to know how much a balloon will expand as its temperature increases. The answer to that question depends on the volume expansivity of the gas used. The volume expansivity of gases ranges from a relatively low value for air to a relatively high value for carbon dioxide and sulfur dioxide.
A great many practical devices and systems depend on the thermal expansion of materials. An example is the bimetallic strip. A bimetallic strip consists of two metals of different thermal expansivities welded to each other. When the strip is heated, one metal expands more rapidly than the other. The strip bends in the direction of the metal with the lower thermal expansivity.
Perhaps the most common use of the bimetallic strip is in a thermostat. When a room becomes cold, the two metals in the strip contract, one more than the other. At some point, the strip bends enough to come into contact with a metal button that closes an electrical circuit, turning on the furnace. As the room warms up, the bimetallic strip begins to bend in the opposite direction. Eventually it pulls away from the contact button, the circuit is broken, and the furnace turns off.
The thermal expansion of objects in the real world often requires the attention of scientists and engineers. For example, the joints used to hold a bridge together have to be designed to provide space for expansion and contraction of the bridge deck. And railroad tracks are built so that they can slide toward and away from each other on hot and cold days, making sure that they do not bend out of shape because of overheating.