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In Physics , thermal expansion is the tendency of matter to increase in Volume or Pressure when heated. For liquids and solids the amount of expansion will normally vary depending on the material's Coefficient Of Thermal Expansion . While for gases the change in volume or pressure is related to the container that the gas is in, this can be easily estimated by the Ideal Gas Law . When things expand Tensile forces are created. When things contract Compressive forces are created. To accurately calculate thermal expansion of a substance a more advanced Equation Of State must be used. This equation would be able to calculate thermal expansion among with many other State Function s. For solid materials with a significant length, like rods or cables, an estimate of the amount of thermal expansion can be described by the ratio of Strain : : is the initial length before the change of temperature and the final length recorded after the change of temperature. For most solids, thermal expansion relates directly with temperature: : Thus, the change in either the Strain or temperature can be estimated by: : where : and : is the coefficient of thermal expansion in ''inverse kelvins''. : is the difference of the temperature between the two recorded strains, measured in Celsius or Kelvin . A number of materials contract on heating within certain temperature ranges; we usually speak of Negative Thermal Expansion , rather than thermal contraction, in such cases. For example, the coefficient of thermal expansion of liquid water is negative below 4 °C, where water has its maximum density, is zero at 4 °C, and is positive above 4 °C. In Materials Engineering , the three primary types of materials have well defined rates of expansion. Polymer s expand as much as 10 times more than Metal s, which expand more than Ceramic s. Thermal expansion generally increases with bond energy. See PVT relation. It should be realized that, in many common materials, changes in size can also be due to water (or other solvents) being absorbed/desorbed, and that many organic materials change size, due to this effect, much more than they do to thermal expansion. Common plastics exposed to water can, in the long term, expand many percent. In general, liquids expand more than solids, and gases expand more than liquids. This is due to the relative amount of energy contained in the molecules in each state. When things expand, they take up more space as they are moving around more vigorously, not because the molecules themselves are growing in size. Heat-induced expansion has to be taken into account for many structures such as railways and bridges, which without the use of Expansion Joint s the structures may buckle. Similar techniques are applied in buildings, water pipes, and road construction. This phenomenon can be beneficial as well, and is used in techniques like Shrink-fitting . Thermometers are an example of thermal expansion. The liquid in them is heated and it expands in the only direction it can, along the tube. EXTERNAL LINKS
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