The kelvin (the unit name is spelled with a lower-case 'k') is the unit of temperature in the International System of Units (SI). It is known as the Kelvin scale, widely used in science and technology. Apart from the absolute zero, it does not have a reference temperature. ![]() Referring to the Boltzmann constant, to the Maxwell–Boltzmann distribution, and to the Boltzmann statistical mechanical definition of entropy, as distinct from the Gibbs definition, for independently moving microscopic particles, disregarding interparticle potential energy, by international agreement, a temperature scale is defined and said to be absolute because it is independent of the characteristics of particular thermometric substances and thermometer mechanisms. The absolute zero, defined as 0 K, is exactly equal to −273.15 ☌, or −459.67 ☏. Theoretically, in a body at a temperature of absolute zero, all classical motion of its particles has ceased and they are at complete rest in this classical sense. Experimentally, absolute zero can be approached only very closely it can never be reached (the lowest temperature attained by experiment is 100 pK). At this temperature, matter contains no macroscopic thermal energy, but still has quantum-mechanical zero-point energy as predicted by the uncertainty principle, although this does not enter into the definition of absolute temperature. Water freezes at 32 ☏ and boils at 212 ☏ at sea-level atmospheric pressure.Īt the absolute zero of temperature, no energy can be removed from matter as heat, a fact expressed in the third law of thermodynamics. The Fahrenheit scale is in common use in the United States. Since the standardization of the kelvin in the International System of Units, it has subsequently been redefined in terms of the equivalent fixing points on the Kelvin scale, and so that a temperature increment of one degree Celsius is the same as an increment of one kelvin, though numerically they differ by an additive offset of exactly 273.15. Because of the 100-degree interval, it was called a centigrade scale. ![]() It is an empirical scale that developed historically, which led to its zero point 0 ☌ being defined as the freezing point of water, and 100 ☌ as the boiling point of water, both at atmospheric pressure at sea level. The Celsius scale (☌) is used for common temperature measurements in most of the world. Temperature scales differ in two ways: the point chosen as zero degrees and the magnitudes of the incremental unit of temperature. ( January 2021) ( Learn how and when to remove this template message) Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This section needs additional citations for verification. 9.5 Generalized temperature from single-particle statistics.9.4 Definition from statistical mechanics.6.4 Thermodynamic equilibrium axiomatics.6.2 Bodies in a steady state but not in thermodynamic equilibrium.6.1 Bodies in thermodynamic equilibrium. ![]() 3.2.1 Microscopic statistical mechanical scale.2.5 Statistical mechanical versus thermodynamic temperature scales.Temperature is important in all fields of natural science, including physics, chemistry, Earth science, astronomy, medicine, biology, ecology, material science, metallurgy, mechanical engineering and geography as well as most aspects of daily life. Experimentally, it can only be approached very closely, but not reached, which is recognized in the third law of thermodynamics. The lowest theoretical temperature is absolute zero, at which no more thermal energy can be extracted from a body. The most common scales are the Celsius scale (formerly called "centigrade", with the unit ☌), the Fahrenheit scale (with the unit ☏), and the Kelvin scale (with the unit K), the latter being used predominantly for scientific purposes and is the primary temperature scale defined by the International System of Units (SI). ![]() It is calibrated in various temperature scales that historically have relied on various reference points and thermometric substances for definition. Temperature is measured with a thermometer. It is the detectable portion of the molar thermal energy of matter a temperature difference allows heat transfer to occur, as energy flows from a hotter body to a colder body. Temperature is a physical quantity that expresses how hot matter is, or as a measure of the average translational kinetic energy per atom or molecule in the system.
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