Increase in volume of a material as its temperature is increased, usually expressed as a fractional change in dimensions per unit temperature change. When the material is a solid, thermal expansion is usually described in terms of change in length, height, or thickness. If a crystalline solid has the same structural configuration throughout, the expansion will be uniform in all dimensions. Otherwise, there may be different expansion coefficients and the solid will change shape as the temperature increases. If the material is a fluid, it is more useful to describe the expansion in terms of a change in volume. Because the bonding forces among atoms and molecules vary from material to material, expansion coefficients are characteristic of elements and compounds.
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