Temperature

Temperature is a measure of the average energy of motion, or kinetic energy, of particles in matter. When particles of matter, whether in solids, liquids, gases, or elementary plasmas, move faster or have greater mass, they carry more kinetic energy, and the material appears warmer than a material with slower or less massive particles. Kinetic energy, a concept of mechanics, is the product of mass and the square of a particle's velocity. In the context of thermodynamics, it is also referred to as thermal energy and the transfer of thermal energy is commonly referred to as heat. Heat always flows from regions of higher temperature to regions of lower temperature.

English language speakers refer to an object of low temperature as being cold, and associate various degrees of higher temperature to terms such as luke-warm, warm, hot, and others.

Historically, two equivalent concepts of temperature have developed, the thermodynamic description and a microscopic explanation based on statistical physics. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature,

The temperature of an ideal monatomic gas is related to the average kinetic energy of its atoms as they move. In this animation, the size of helium atoms relative to their spacing is shown to scale under 1950 atmospheres of pressure. These room-temperature atoms have a certain, average speed (slowed down here two trillion fold).

first stated by Lord Kelvin, is stated entirely in empirical, measurable variables. Statistical physics provides a deeper understanding of thermodynamics by describing matter as a collection of a large number of particles, and derives thermodynamic (i.e. macroscopic) parameters as statistical averages of the microscopic parameters of the particles.

In statistical physics, it is shown that the thermodynamic definition of temperature can be interpreted as a measure of the average energy in each degree of freedom of the particles in the thermodynamic system. Because its temperature is seen as a statistical property, a system must contain a large number of particles for temperature to have a useful meaning. For a solid, this energy is found primarily in the vibrations of its atoms about their equilibrium positions. In an ideal monatomic gas, energy is found in the translational motions of the particles; with molecular gases, vibrational and rotational motions also provide thermodynamic degrees of freedom.

Temperature is a physical property that underlies the common notions of hot and cold. Something that feels hotter generally has a higher temperature, though temperature is not a direct measurement of heat. Temperature is one of the principal parameters of thermodynamics. If no net heat flow occurs between two objects, the objects have the same temperature; otherwise, heat flows from the object with the higher temperature to the object with the lower one. This is a consequence of the laws of thermodynamics.

Temperature is measured with thermometers that may be calibrated to a variety of temperature scales. In most of the world (except for Belize, Myanmar, Liberia and the United States), the Celsius scale is used for most temperature measuring purposes. The entire scientific world (these countries included) measures temperature using the Celsius scale and thermodynamic temperature using the Kelvin scale, which is just the Celsius scale shifted downwards so that 0 K^[1]= −273.15 °C, or absolute zero. Many engineering fields in the U.S., notably high-tech and US federal specifications (civil and military), also use the Kelvin and Celsius scales. Other engineering fields in the U.S. also rely upon the Rankine scale (a shifted Fahrenheit scale) when working in thermodynamic-related disciplines such as combustion.

For a system in thermal equilibrium at a constant volume, temperature is thermodynamically defined in terms of its energy (E) and entropy (S) as: