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The kelvin, symbol K, is the primary unit of
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
in the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...
(SI), used alongside its prefixed forms and the degree Celsius. It is named after the
Belfast Belfast ( , ; from ga, Béal Feirste , meaning 'mouth of the sand-bank ford') is the capital and largest city of Northern Ireland, standing on the banks of the River Lagan on the east coast. It is the 12th-largest city in the United Kingdom ...
-born and
University of Glasgow The University of Glasgow (abbreviated as ''Glas.'' in Post-nominal letters, post-nominals; ) is a Public university, public research university in Glasgow, Scotland. Founded by papal bull in , it is the List of oldest universities in continuous ope ...
-based
engineer Engineers, as practitioners of engineering, are professionals who Invention, invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considerin ...
and
physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate caus ...
William Thomson, 1st Baron Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, Mathematical physics, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy (Glasgow), Professor of Natural Philoso ...
(1824–1907). The Kelvin scale is an absolute
thermodynamic temperature Thermodynamic temperature is a quantity defined in thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of ...
scale, meaning it uses
absolute zero Absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as zero kelvin. The fundamental particles of nature have minimum vibration ...
as its null (zero) point. Historically, the Kelvin scale was developed by shifting the starting point of the much-older Celsius scale down from the
melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase_(matter), phase exist in Thermodynamic equilibr ...
of
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
to absolute zero, and its increments still closely approximate the historic definition of a degree Celsius, but since 2019 the scale has been defined by fixing the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas ...
to be exactly . Hence, one kelvin is equal to a change in the thermodynamic temperature that results in a change of thermal energy by . The temperature in degree Celsius is now defined as the temperature in kelvins minus 273.15, meaning that a ''change'' or ''difference'' in temperature has the same value when expressed in degrees Celsius as in kelvins, and that 0 °C is equal to 273.15 K. The kelvin is the primary unit of temperature for
engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more specializ ...
and the
physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the "physical sciences". ...
, while in most countries Celsius remains the dominant scale outside of these fields. In the United States, outside of the physical sciences the
Fahrenheit scale The Fahrenheit scale () is a scale of temperature, temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he ...
predominates, with the kelvin or Rankine scale employed for absolute temperature. Those are defined using the kelvin. According to SI convention, the kelvin is never referred to nor written as a ''degree''. The word ''kelvin'' is not capitalised when used as a unit, but is pluralised as appropriate. The unit symbol K is a capital letter. For example, "It is 50 degrees Fahrenheit outside" vs "It is 10 degrees Celsius outside" vs "It is 283 kelvins outside". It is common convention to capitalize Kelvin when referring to the Kelvin scale.


History


Precursors

During the
18th century The 18th century lasted from January 1, 1701 (Roman numerals, MDCCI) to December 31, 1800 (Roman numerals, MDCCC). During the 18th century, elements of Age of Enlightenment, Enlightenment thinking culminated in the American Revolution, Ameri ...
, multiple temperature scales were developed, notably
Fahrenheit The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he originally defined ...
and centigrade (later
Celsius The degree Celsius is the unit of temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated ...
). These scales predated much of the modern science of
thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed b ...
, including
atomic theory Atomic theory is the scientific theory A scientific theory is an explanation of an aspect of the natural science, natural world and universe that has been reproducibility, repeatedly tested and corroborated in accordance with the scientifi ...
and the
kinetic theory of gases Kinetic (Ancient Greek: κίνησις “kinesis”, movement or to move) may refer to: * Kinetic theory, describing a gas as particles in random motion * Kinetic energy, the energy of an object that it possesses due to its motion Art and ent ...
which underpin the concept of absolute zero. Instead, they chose defining points within the range of human experience that could be reproduced easily and with reasonable accuracy, but lacked any deep significance in thermal physics. In the case of the Celsius scale (and the long since defunct
Newton scale The Newton scale is a temperature scale devised by Isaac Newton in 1701. He called his device a "thermometer", but he did not use the term "temperature", speaking of "degrees of heat" (''gradus caloris'') instead. Newton's publication represents ...
and
Réaumur scale __NOTOC__ The Réaumur scale (; °Ré, °Re, °r), also known as the "octogesimal division", is a temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measure ...
) the melting point of
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
served as such a starting point, with Celsius being defined, from the 1740s up until the 1940s, by calibrating a thermometer such that: * The
freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase_(matter), phase exist in Thermodynamic equilibr ...
of water is 0 degrees. * The
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding en ...
of water is 100 degrees. This definition assumes pure water at a specific
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
chosen to approximate the natural air pressure at
sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
. Thus an increment of 1 °C equals of the temperature difference between the melting and boiling points. This temperature interval would go on to become the template for the kelvin.


Lord Kelvin

In 1848, William Thomson, who was later ennobled as
Lord Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, Mathematical physics, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy (Glasgow), Professor of Natural Philoso ...
, published a paper ''On an Absolute Thermometric Scale''. Using the soon-to-be-defunct
caloric theory The caloric theory is an obsolete scientific theory that heat consists of a self-repellent fluid called caloric that flows from hotter bodies to colder bodies. Caloric was also thought of as a weightless gas that could pass in and out of pores in ...
, he proposed an "absolute" scale based on the following parameters: * The melting point of water is 0 degrees. * The boiling point of water is 100 degrees. "The arbitrary points which coincide on the two scales are 0° and 100°" * Any two heat engines whose heat source and heat sink are both separated by the same number of degrees will, per Carnot's theorem, be capable of producing the same amount of
mechanical work In physics, work is the energy transferred to or from an object via the application of force along a Displacement (vector), displacement. In its simplest form, for a constant force aligned with the direction of motion, the work equals the pro ...
per unit of "caloric" passing through. "The characteristic property of the scale which I now propose is, that all degrees have the same value; that is, that a unit of heat descending from a body at the temperature ° of this scale, to a body at the temperature ( − 1)°, would give out the same mechanical effect, whatever be the number . This may justly be termed an absolute scale, since its characteristic is quite independent of the physical properties of any specific substance." As Carnot's theorem is understood in modern thermodynamics to simply describe the maximum
efficiency Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
with which
thermal energy The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, de ...
can be converted to mechanical
energy In physics, energy (from Ancient Greek: wikt:ἐνέργεια#Ancient_Greek, ἐνέργεια, ''enérgeia'', “activity”) is the physical quantity, quantitative physical property, property that is #Energy transfer, transferred to a phy ...
and the predicted maximum efficiency is a function of the ''ratio'' between the absolute temperatures of the heat source and heat sink: *Efficiency ≤ 1 − It follows that increments of equal numbers of degrees on this scale must always represent equal ''proportional'' increases in absolute temperature. The numerical value of an absolute temperature, , on the 1848 scale is related to the absolute temperature of the melting point of water, , and the absolute temperature of the boiling point of water, , by: * (1848 scale) = 100 () / () On this scale, an increase of 222 degrees always means an approximate doubling of absolute temperature regardless of the starting temperature. In a footnote Thomson calculated that "infinite cold" (
absolute zero Absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as zero kelvin. The fundamental particles of nature have minimum vibration ...
, which would have a numerical value of negative
infinity Infinity is that which is boundless, endless, or larger than any natural number. It is often denoted by the infinity symbol . Since the time of the Greek mathematics, ancient Greeks, the Infinity (philosophy), philosophical nature of infinit ...
on this scale) was equivalent to −273 °C using the air thermometers of the time. This value of "−273" was the negative reciprocal of 0.00366—the accepted
coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...
of an ideal gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value. Within a decade, Thomson had abandoned caloric theory and superseded the 1848 scale with a new one based on the 2 features that would characterise all future versions of the kelvin scale: * Absolute zero is the null point. * Increments have the same magnitude as they do in the Celsius scale. In 1892, Thomson was awarded the noble title 1st
Baron Baron is a rank of nobility Nobility is a social class found in many societies that have an aristocracy (class), aristocracy. It is normally ranked immediately below Royal family, royalty. Nobility has often been an Estates of the rea ...
Kelvin of
Largs Largs ( gd, An Leargaidh Ghallda) is a town A town is a human settlement In geography, statistics and archaeology, a settlement, locality or populated place is a community in which people live. The complexity of a settlement can ...
, or more succinctly Lord Kelvin. This name was a reference to the
River Kelvin The River Kelvin (Scottish Gaelic: ''Abhainn Cheilbhinn'') is a tributary of the River Clyde in northern and northeastern Glasgow, Scotland. It rises on the moor south east of the village of Banton, Scotland, Banton, east of Kilsyth. At almo ...
which flows through the grounds of Glasgow University. In the early decades of the 20th century, the Kelvin scale was often called the "absolute
Celsius The degree Celsius is the unit of temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated ...
" scale, indicating Celsius degrees counted from absolute zero rather than the freezing point of water, and using the same symbol for regular Celsius degrees, °C.


Triple point standard

In 1873, William Thomson's older brother James coined the term ''
triple point In thermodynamics, the triple point of a substance is the temperature and pressure at which the three Phase (matter), phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at w ...
'' to describe the combination of temperature and
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
at which the solid, liquid, and gas phases of a substance were capable of coexisting in
thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal State variables, state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable ...
. While any two phases could coexist along a range of temperature-pressure combinations (e.g. the
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding en ...
of
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
can be affected quite dramatically by raising or lowering the pressure), the triple point condition for a given substance can occur only at a single pressure and only at a single temperature. By the 1940s, the triple point of water had been experimentally measured to be about 0.6% of
standard atmospheric pressure The standard atmosphere (symbol: atm) is a unit of pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ...
and very close to 0.01 °C per the historical definition of Celsius then in use. In 1948, the Celsius scale was recalibrated by assigning the triple point temperature of water the value of 0.01 °C exactly and allowing the
melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase_(matter), phase exist in Thermodynamic equilibr ...
at standard atmospheric pressure to have an empirically determined value (and the actual melting point at ambient pressure to have a fluctuating value) close to 0 °C. This was justified on the grounds that the triple point was judged to give a more accurately reproducible reference temperature than the melting point. In 1954, with absolute zero having been experimentally determined to be about −273.15 °C per the definition of °C then in use, Resolution 3 of the 10th
General Conference on Weights and Measures The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established i ...
(CGPM) introduced a new internationally standardised Kelvin scale which defined the triple point as exactly: 273.15 + 0.01 = 273.16 "degrees Kelvin". In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K. The 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction of the thermodynamic temperature of the triple point of water." After the 1983 redefinition of the metre, this left the kelvin, the
second The second (symbol: s) is the unit of Time in physics, time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally t ...
, and the
kilogram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. ...
as the only SI units not defined with reference to any other unit. In 2005, noting that the triple point could be influenced by the isotopic ratio of the hydrogen and oxygen making up a water sample and that this was "now one of the major sources of the observed variability between different realizations of the water triple point", the
International Committee for Weights and Measures The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established i ...
(CIPM), a committee of the CGPM, affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the kelvin would refer to water having the isotopic composition specified for
Vienna Standard Mean Ocean Water Vienna Standard Mean Ocean Water (VSMOW) is an Reference materials for stable isotope analysis, isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulg ...
.


2019 redefinition

In 2005, the
CIPM The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established i ...
began a programme to redefine the kelvin (along with the other SI units) using a more experimentally rigorous method. In particular, the committee proposed redefining the kelvin such that the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas ...
takes the exact value . The committee had hoped that the program would be completed in time for its adoption by the CGPM at its 2011 meeting, but at the 2011 meeting the decision was postponed to the 2014 meeting when it would be considered as part of a larger program. The redefinition was further postponed in 2014, pending more accurate measurements of the Boltzmann constant in terms of the current definition, but was finally adopted at the 26th CGPM in late 2018, with a value of  =  For scientific purposes, the main advantage is that this allows measurements at very low and very high temperatures to be made more accurately, as the techniques used depend on the Boltzmann constant. It also has the philosophical advantage of being independent of any particular substance. The unit J/K is equal to kg⋅m2⋅s−2⋅K−1, where the
kilogram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. ...
,
metre The metre (British English, British spelling) or meter (American English, American spelling; American and British English spelling differences#-re, -er, see spelling differences) (from the French unit , from the Greek language, Greek noun , "m ...
and
second The second (symbol: s) is the unit of Time in physics, time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally t ...
are defined in terms of the
Planck constant The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivalenc ...
, the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special relativity, special theory of relativity, is ...
, and the duration of the
caesium-133 Caesium (55Cs) has 40 known isotopes, making it, along with barium and mercury (element), mercury, one of the elements with the most isotopes. The atomic masses of these isotopes range from 112 to 151. Only one isotope, 133Cs, is stable. The longe ...
ground-state
hyperfine transition In atomic physics Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. Atomic physics typically refers to the study of atomic structure and the interaction between atoms. It is prim ...
respectively. Thus, this definition depends only on universal constants, and not on any physical artifacts as practiced previously. The challenge was to avoid degrading the accuracy of measurements close to the triple point. For practical purposes, the redefinition was unnoticed; water still freezes at 273.15 K (0 °C), and the triple point of water continues to be a commonly used laboratory reference temperature. The difference is that, before the redefinition, the triple point of water was exact and the Boltzmann constant had a measured value of , with a relative standard uncertainty of . Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now . The new definition officially came into force on 20 May 2019, the 144th anniversary of the
Metre Convention The Metre Convention (french: link=no, Convention du Mètre), also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations (Argentina, Austria-Hungary, Belgium, Brazi ...
.


Practical uses


Colour temperature

The kelvin is often used as a measure of the colour temperature of light sources. Colour temperature is based upon the principle that a
black body A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about appear reddish, whereas those above about appear bluish. Colour temperature is important in the fields of image projection and
photography Photography is the visual art, art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It i ...
, where a colour temperature of approximately is required to match "daylight" film emulsions. In
astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
, the
stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...
of stars and their place on the
Hertzsprung–Russell diagram The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosity, luminosities versus their stellar classifications or eff ...
are based, in part, upon their surface temperature, known as
effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperatur ...
. The photosphere of the
Sun The Sun is the star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other st ...
, for instance, has an effective temperature of . Digital cameras and photographic software often use colour temperature in K in edit and setup menus. The simple guide is that higher colour temperature produces an image with enhanced white and blue hues. The reduction in colour temperature produces an image more dominated by reddish, "warmer" colours.


Kelvin as a unit of noise temperature

For electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to an ultimate
noise floor In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where noise is defined as any signal other than the one being monitored. In radio com ...
, i.e. the
noise temperature In electronics, noise temperature is one way of expressing the level of available noise power introduced by a component or source. (This is to be distinguished from Temperature Noise in Thermodynamics or Principal Interferrometric Analysis Over C ...
. The so-called
Johnson–Nyquist noise Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the electronic noise generated by the Thermal energy, thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, whic ...
of discrete resistors and capacitors is a type of thermal noise derived from the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas ...
and can be used to determine the noise temperature of a circuit using the
Friis formulas for noise Friis formula or Friis's formula (sometimes Friis' formula), named after Danish-American electrical engineer Harald T. Friis, is either of two formulas used in telecommunications engineering to calculate the signal-to-noise ratio of a multistage a ...
.


Derived units and SI multiples

The only SI derived unit with a special name derived from the kelvin is the degree Celsius. Like other SI units, the kelvin can also be modified by adding a
metric prefix A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pr ...
that multiplies it by a power of 10:


Unicode character

The symbol is encoded in
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, ...
at code point . However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using instead; that is, a normal capital K. "Three letterlike symbols have been given canonical equivalence to regular letters: , , and . In all three instances, the regular letter should be used."


See also

*
Comparison of temperature scales This is a collection of temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Con ...
*
International Temperature Scale of 1990 The International Temperature Scale of 1990 (ITS-90) is an equipment calibration standard specified by the CIPM, International Committee of Weights and Measures (CIPM) for making measurements on the Kelvin and Degree Celsius, Celsius temperature sc ...
* Negative temperature


References


Bibliography

*


External links

{{CGS units 1848 introductions Scottish inventions SI base units William Thomson, 1st Baron Kelvin Scales of temperature Scales in meteorology