Expansion is an increase in the size of a body without the addition of material to the body. Most substances expand when they are heated and contract when they are cooled. Gases naturally expand to fill their containers. But gases also expand when heated and contract when cooled. If a gas is heated in a container that prevents expansion, the pressure of the gas increases (see Gas (How gases behave) (Gas laws)).
Expansion results from a change in energy. An object’s temperature is related to its internal energy. This energy is contained within the vibrations of an object’s atoms and molecules. Raising the temperature increases these vibrations. In a gas, raising the temperature also increases the speed at which the atoms or molecules move about. The increased movement forces the atoms or molecules farther apart, resulting in expansion.
Different materials expand by different amounts in response to the same increase in temperature. For example, aluminum expands twice as much as iron under the same temperature increase.
See also Heat (Changes in size).
