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.
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 simply expand more easily than others. If you heat wires of aluminum, iron, and tungsten metals—all the wires being the same size and heated to the same temperature—each wire will expand by a different amount. The ease with which a substance expands is given by its coefficient of expansion. For comparison, the coefficients of expansion for aluminum, iron, and tungsten are 23 × 10 −6 , 12 × 10 −6 , and 5 × 10 −6 per degree Celsius, respectively.
The values given in the previous sentence actually refer to the coefficients of linear expansion. They measure how much a substance expands in only one direction. But suppose the above experiment were done with blocks of aluminum, iron, and tungsten rather than wires. In that case, the expansion would occur in all three directions: length, width, and depth. The measure of expansion in all three directions is called the coefficient of volume expansion.
Length and temperature. Suppose this discussion is limited to a single kind of material, say an iron wire. The amount by which that wire expands when heated depends on only two factors: its original length and the temperature to which it is heated. An iron bar that is 16 feet (5 meters) long will expand more than a bar that is 3 feet (1 meter) long. And a 16-foot (5-meter) bar will expand more if heated by 68°F (20°C) than a 16-foot (5-meter) bar that is heated by 50°F (10°C).
Engineers and architects must always take into consideration the fact that objects usually expand when they are heated. As a result, they have to design buildings, bridges, power lines, and other structures to compensate for expansion and contraction.
Thermal expansion also is used in the construction of certain appliances and devices in homes and industry. One example is the bimetallic strip. A bimetallic strip consists of two pieces of metal welded to each other. The two metals are chosen to have different coefficients of expansion. When the bimetallic strip is heated, it bends one way or the other as one metal expands faster than the other.
One use of bimetallic strips is in thermostats used to control room temperatures. As a room warms up or cools down, the bimetallic strip in the thermostat bends one way or the other. If it bends far enough, it comes into contact with a button that turns a furnace on or off.