TEMPERATURE is a physical quantity that is a measure of hotness and coldness on a numerical scale. It is a measure of the thermal energy per particle of matter or radiation; it is measured by a thermometer, which may be calibrated in any of various temperature scales, Celsius, Fahrenheit, Kelvin, etc.
Temperature is an intensive property, which means it is independent of the amount of material present; in contrast to energy, an extensive property, which is proportional to the amount of material in the system. For example a spark may well be (very briefly!) as hot as the Sun.
Empirically it is found that an isolated system, one that exchanges no energy or material with its environment, tends to a spatially uniform temperature 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. 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.
In kinetic theory and in statistical mechanics, temperature is the effect of the thermal energy arising from the motion of microscopic particles such as atoms, molecules and photons. The relation is proportional as given by the Boltzmann constant.
Thermal vibration of a segment of protein alpha helix. The amplitude of the vibrations increases with temperature.
The lowest theoretical temperature is called absolute zero, however it cannot be achieved in any actual physical device. It is denoted by 0 K on the Kelvin scale, −273.15 °C on the Celsius scale. In matter at absolute zero, the motions of microscopic