The kelvin (the word 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 absolute zero temperature, 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 (least 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. The United States commonly uses the Fahrenheit scale, on which 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. 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 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 was developed by historical progress, which led to its zero point 0 ☌ being defined by the freezing point of water, and additional degrees defined so that 100 ☌ was the boiling point of water, both at sea-level atmospheric pressure. 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 incremental units or degrees on the scale. ( 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 (100 pK), 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, denoted as ☌), the Fahrenheit scale (denoted as ☏), and the Kelvin scale (denoted as K), the last of which is predominantly used for scientific purposes by conventions of the International System of Units (SI). Thermometers are calibrated in various temperature scales that historically have used various reference points and thermometric substances for definition. ![]() Temperature is measured with a thermometer. Temperature should not be confused with heat. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow of energy, when a body is in contact with another that is colder or hotter. Temperature is a physical quantity that expresses hot and cold or a measure of the average kinetic energy of the atoms or molecules in the system.
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