# Thermal Expansion

## Expansion, Thermal (Encyclopedia of Science)

Thermal expansion is the change in size of an object as its temperature changes. Normally, as the temperature increases, the size of an object also increases. Conversely, most objects shrink as the temperature drops. On a hot summer day, electrical power lines sag between power poles. The sag occurs because the wires grow longer as the temperature increases. Long bridges often have interlocking metal fingers along the joints where sections of the bridge are joined to each other. The metal fingers allow the bridge sections to expand and contract with changes in the temperature.

A relatively small number of substances contract when they are heated and expand when they are cooled. Water is the most common example. As water is cooled from room temperature to its freezing point, it contracts, like most other substances. However, just four degrees Celsius above its freezing point, it begins to expand. At its freezing point a gram of ice takes up more space than does a gram of liquid water. This change explains the fact that ice floats on top of water.

Factors affecting thermal expansion

Imagine that a long, thin metal wire is heated. The wire expands. The amount by which it expands depends on three factors: its original length, the temperature change, and the thermal (heat) properties of the metal itself.

Some substances...

(The entire section is 605 words.)

## Thermal Expansion (Encyclopedia of Science)

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.

General trends

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

Joints such as this one are used in bridges to accommodate thermal expansion. (Reproduced by permission of JLM Visuals.)

expands in either way is known...

(The entire section is 610 words.)