Thermal Expansion Essay

501 Words3 Pages
Expansion, an increase in the volume of a substance, usually due to the addition of heat to the substance. The heat causes the molecules of the substance to move farther apart, making the substance occupy more space. A loss of heat causes the opposite effect, known as contraction. Although most substances expand when heated, not all expand at the same rate. Aluminum, for example, expands twice as much as iron when both are heated the same amount. Rubber and water are two common substances that differ from most others in their response to heat. Rubber contracts when heated. Water loses volume when its temperature rises from 0° C. to 4° C. (32° F. to 39° F.). It expands when its temperature rises above 4° C. (39° F.). It also expands when it freezes. Expansion of Gases Under equal pressures, all gases expand at the same rate. A gas expands by the same proportion as the temperature rises, provided external pressure remains the same. The effect of heat on the expansion of gases is stated in Charles' Law. A gas occupies 1/273.15 more space for each 1° C. rise in temperature and, conversely, 1/273.15 less space for each 1° C. drop in temperature. This fact enabled scientists to determine absolute zero (-273.15° C.), the temperature at which all molecules theoretically stop moving. A gas can be made to expand without the addition of outside heat if the pressure confining the gas is reduced. This principle is stated in Boyle's Law. Energy for the expansion is drawn from the expanding gas itself, thus causing a lowering of temperature in the gas. Steam engines and turbines, rockets, and internal combustion engines are powered by the expansion of gases. Bread rises in baking because heat expands the carbon dioxide gas it contains. Expansion of Liquids Unlike gases, liquids expand at different rates, depending on their composition. Liquids also expand by
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