coefficient of thermal expansion

All materials change their size when subjected to a temperature change as long as the pressure is held constant. In the special case of solid materials, the pressure does not appreciably affect the size of an object, and so for solids, it usually not necessary to specify that the pressure be held constant. The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in volume per degree change in temperature at a constant pressure.

The volumetric thermal expansion coefficient is the most basic thermal expansion coefficient. All substances expand or contract when their temperature changes, and the expansion or contraction always occurs in all directions. Substances that expand at the same rate in any direction are called isotropic. Unlike gases or liquids, solid materials tend to keep their shape. For solids, one might only be concerned with the change along a length, or over some area. Expansion coefficients are specially defined for these cases, and they are known as the linear and area expansion coefficients. However, they all come from the volume expansion coefficient, which explains how the substance expands in any direction.

Some substances expand when cooled, such as freezing water, so they have negative thermal expansion coefficients.