Expansion, Thermal
Thermal expansion is the change in size of an object as the temperature changes. Normally, as the temperature increases, the size of an object also increases. Conversely, the object will shrink as the temperature drops. Look at electrical power lines on a hot summer day. They will sag more than on a cold winter day. If you look at many bridges, they have interlocking metal fingers forming an expansion joint, where the bridge joins the road. This is to accommodate thermal expansion. (One notable exception to this general rule is water. It usually undergoes a normal thermal expansion, but as it approaches its freezing point, it will expand instead of shrink.) As an object expands or contracts with a temperature change, its change in length depends on three quantities: the original length, the temperature change, and the thermal properties of the material composing the object. Think about a metal rod that is 3 ft (1 m) long and has its temperature increased to the point where it expands by one hundredth of a millimeter. Now consider two such identical rods placed end to end.
They are the equivalent of a single rod 6 ft (2 m) long that will expand by two hundredths of a millimeter for the same temperature increase. Hence a larger object will have a greater change in length with temperature than a smaller object, simply because the larger object has more material to expand. Secondly, the change in length also depends on the temperature change. An object will expand twice as much for a 20° temperature change as for a 10° temperature change. In a third variable, different materials also expand at different rates. For example, with a given temperature change, a block of wood will not expand as much as a similar block of metal. The coefficient of thermal expansion, found by experimentation, is a property of the material that accounts for the different expansion rates for different materials. Using these three factors, the change in length of an object is equal to the coefficient of thermal expansion multiplied by the original length multiplied by the temperature change.
The length of an object is not the only dimension that expands or contracts. The height and width also change. As all the dimensions of the object change in size, the volume of the object changes. The change in volume is equal to the coefficient of volume expansion times the original volume times the temperature change. Notice that the change in volume is similar to the change in length, but the coefficient of volume expansion is roughly three times the coefficient of expansion used for the length because an object expands in three dimensions.
When designing bridges, power lines, or anything else that might be subjected to wide temperature variations, engineers must take into account the effects of thermal expansion. Depending on the temperature range in a particular area, they may choose building materials that expand with heat or not, different types of joint structures, and what other kind of reinforcements might be needed.
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