thermal expansion

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.

thermal expansion [′thər·məl ik′span·chən]

(physics)

The dimensional changes exhibited by solids, liquids, and gases for changes in temperature while pressure is held constant.

Thermal expansion

Solids, liquids, and gases all exhibit dimensional changes for changes in temperature while pressure is held constant. The molecular mechanisms at work and the methods of data presentation are quite different for the three cases.

The temperature coefficient of linear expansion α_l is defined by Eq. (1),

(1)

where l is the length of the specimen, t is the temperature, and p is the pressure. For each solid there is a Debye characteristic temperature &THgr;, below which α_l is strongly dependent upon temperature and above which α_l is practically constant. Many common substances are near or above &THgr; at room temperature and follow approximate equation (2),

(2)

where l₀ is the length at 0°C and t is the temperature in °C. The total change in length from absolute zero to the melting point has a range of approximately 2% for most substances.

So-called perfect gases follow the relation in Eq. (3),

(3)

where p is absolute pressure, v is specific volume, T is absolute temperature, and R is the so-called gas constant. Real gases often follow this equation closely. See Gas constant

The coefficient of cubic expansion α_v is defined by Eq.(4)

(4)

, and for a perfect gas this is found to be 1/T. The behavior of real gases is largely accounted for by the van der Waals equation. See Kinetic theory of matter

For liquids, α_v is somewhat a function of pressure but is largely determined by temperature. Though α_v may often be taken as constant over a sizable range of temperature (as in the liquid expansion thermometer), generally some variation must be accounted for. For example, water contracts with temperature rise from 32 to 39°F (0 to 4°C), above which it expands at an increasing rate. See Thermometer