Temperature

A temperature is a numerical measure of hot and cold in a body that is in its own state of internal thermal equilibrium. Its measurement is by detection of heat radiation or particle velocity or kinetic energy, or by the bulk behavior of a thermometric material. It may be calibrated in any of various temperature scales. The fundamental physical definition of temperature is provided by thermodynamics. Temperature is defined as

T=(dS/dE)^-1.

Within a body that exchanges no energy or matter with its surroundings, temperature tends to become spatially uniform as time passes. When a path permeable only to heat is open between two bodies, energy always transfers spontaneously as heat from a hotter body to a colder one (second law of thermodynamics). The transfer rate depends on the thermal conductivity of the path or boundary between them. Between two bodies with the same temperature, no heat flows. These bodies are said to be in thermal equilibrium.

The kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic systems. It indicates the absolute temperature as proportional to the average kinetic energy of the random microscopic motions of their constituent microscopic particles such as electrons, atoms, and molecules.

The coldest theoretical temperature is called absolute zero. It can be approached but not reached in any actual physical system. It is denoted by 0 K. In matter at absolute zero, the motions of microscopic constituents are minimal.

Temperature is important in all fields of natural science, including physics, geology, chemistry, atmospheric sciences and biology.