Expansivity redirects to here. See also expansivity (dynamical systems).

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 contract tensile forces are created. When things expand 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 functions.

For solid materials with a significant length, like rods or cables, the an estimation of amount of thermal expansion can be described by the $\backslash frac\{\}\{\}\backslash epsilon\_\{thermal\}$ ratio of strain:

$\backslash epsilon\_\{thermal\}\; =\; \backslash frac\{(L\_\{final\}\; -\; L\_\{initial\})\}\; \{L\_\{initial\}\}$

$\backslash frac\{\}\{\}L\_\{initial\}$ is the initial length before the change of temperature and $\backslash frac\{\}\{\}L\_\{final\}$ the final length recored after the change of temperature.

Most solids have an inverse thermal expansion relationship with temperature:

$\backslash epsilon\_\{thermal\}\; \backslash propto\; \backslash frac\{1\}\{\backslash Delta\; T\; \}$

Thus, the change either the stain or change of temperature can be estimated by:

$\backslash frac\{\}\{\}\; \backslash epsilon\_\{thermal\}\; =\; \backslash alpha\; \backslash Delta\; T$

when

$\backslash frac\{\}\{\}\backslash Delta\; T\; =\; (T\_\{final\}\; -\; T\_\{initial\})$

where

$\backslash frac\{\}\{\}\backslash alpha$ is the coefficient of thermal expansion in inverse kelvins.

$\backslash frac\{\}\{\}\backslash Delta\; T$ is the difference of the temperature between the two recored strains, measured in celsius or kelvin.

A number of materials have been discovered to contract on heating; we usually speak of negative thermal expansion, rather than thermal contraction, in such cases.

In materials engineering, the three primary types of materials have well defined rates of expansion. Polymers expand as much as 10 times more than metals, which expand more than ceramics. Thermal expansion generally increases with bond energy. See PVT relation.

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 railways and bridges, without the use of expansion joints 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.

Thermodynamics

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