Array ( [0] => [1] => {{Short description|SI unit of temperature}} [2] => {{About|the unit of temperature|the person|Lord Kelvin|other uses}} [3] => {{Use dmy dates|date=December 2020|cs1-dates=y}} [4] => {{Use British English|date=November 2018}} [5] => {{Infobox Unit [6] => | name = kelvin [7] => | image = Temperature-scales-comparison.svg [8] => | caption = Equivalent temperatures in kelvin (K), [[Celsius]] (°C), and Fahrenheit (°F) [9] => | standard = [[SI]] [10] => | quantity = [[temperature]] [11] => | symbol = K [12] => | namedafter = [[William Thomson, 1st Baron Kelvin]] [13] => | convertfromx = yes [14] => | units1 = [[Celsius]] [15] => | inunits1 = ({{Math|''x''}} − 273.15) °C [16] => | units2 = [[Fahrenheit]] [17] => | inunits2 = (1.8 {{Math|''x''}} − 459.67) °F [18] => | units3 = [[Rankine scale|Rankine]] [19] => | inunits3 = 1.8 {{Math|''x''}} °Ra [20] => }} [21] => [22] => The '''kelvin''', symbol '''K''', is the [[base unit of measurement]] for [[temperature]] in the [[International System of Units]] (SI). It is named after [[Belfast]]-born [[University of Glasgow]] scientist [[William Thomson, 1st Baron Kelvin]] (1824–1907). Since the '''Kelvin scale''' is an [[absolute scale|absolute]] [[temperature scale]], 0 K is [[absolute zero]]. [23] => [24] => Historically, the Kelvin scale was developed from the [[Celsius]] scale (symbol °C), such that 273.15 K was 0 °C (the approximate [[melting point]] of [[ice]]) and a change of 1 K was exactly equal to a change of 1 °C. This relationship remains accurate, but the Celsius, [[Fahrenheit]], and [[Rankine scale|Rankine]] scales are now defined in terms of the Kelvin scale. [25] => [26] => The [[2019 redefinition of the SI base units]] now defines the kelvin by setting the [[Boltzmann constant]] to exactly {{physconst|k|ref=no|symbol=no}}. Thus, a 1 K change of [[thermodynamic temperature]] corresponds to a {{val|1.380649|e=−23|u=J}} change of [[thermal energy]]. The [[triple point of water]], which a [[General Conference on Weights and Measures#CGPM meetings|1954 definition]] set to 273.16 K, now has a small uncertainty. [27] => [28] => == History == [29] => {{see also|Thermodynamic temperature#History}} [30] => [31] => === Precursors === [32] => [[File:Melting ice thermometer.jpg|thumb|An [[ice]] water bath offered a practical [[calibration]] point for [[thermometers]] (shown here in [[Celsius]]) before the physical nature of [[heat]] was well understood.]] [33] => [34] => During the [[18th century]], [[Conversion of scales of temperature|multiple temperature scales]] were developed,{{cite journal |title=Kelvin: History |url=https://www.nist.gov/si-redefinition/kelvin-history |journal=NIST |date=14 May 2018 |access-date=21 February 2022}} notably [[Fahrenheit]] and centigrade (later [[Celsius]]). These scales predated much of the modern science of [[thermodynamics]], including [[atomic theory]] and the [[kinetic theory of gases]] 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]] and [[Réaumur scale]]) the melting point of [[water]] served as such a starting point, with Celsius being defined (from the [[1740s]] to the [[1940s]]) by calibrating a thermometer such that: [35] => * Water's [[freezing point]] is 0 °C. [36] => * Water's [[boiling point]] is 100 °C. [37] => [38] => This definition assumes pure water at a specific [[pressure]] chosen to approximate the natural air pressure at [[sea level]]. Thus, an increment of 1 °C equals {{sfrac|1|100}} of the temperature difference between the melting and boiling points. The same temperature interval was later used for the Kelvin scale. [39] => [40] => === Charles's law === [41] => From 1787 to 1802, it was determined by [[Jacques Charles]] (unpublished), [[John Dalton]], and [[Joseph Louis Gay-Lussac]] that, at constant pressure, ideal gases expanded or contracted their volume linearly ([[Charles's law]]) by about 1/273 parts per degree Celsius of temperature's change up or down, between 0 °C and 100 °C. This suggested that the volume of a gas cooled at about −273 °C would reach zero. [42] => [43] => === Lord Kelvin === [44] => [[File:Baron Kelvin 1906.jpg|thumb|upright|[[William Thomson, 1st Baron Kelvin|Lord Kelvin]], the namesake of the unit of measure.]] [45] => In 1848, William Thomson, who was later ennobled as [[Lord Kelvin]], published a paper ''On an Absolute Thermometric Scale''. Using the soon-to-be-disused [[caloric theory]], he proposed an "absolute" scale based on the following parameters: [46] => * The melting point of water is 0 degrees. [47] => * The boiling point of water is 100 degrees. [48] => [49] => "The arbitrary points which coincide on the two scales are 0° and 100°" [50] => * Any two [[heat engines]] whose heat source and heat sink are both separated by the same number of degrees will, per [[Carnot's theorem (thermodynamics)|Carnot's theorem]], be capable of producing the same amount of [[work (physics)|mechanical work]] per unit of "caloric" passing through. [51] => [52] => "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 {{mvar|A}} at the temperature {{mvar|T}}° of this scale, to a body {{mvar|B}} at the temperature ({{mvar|T}} − 1)°, would give out the same mechanical effect, whatever be the number {{mvar|T}}. This may justly be termed an absolute scale, since its characteristic is quite independent of the physical properties of any specific substance." [53] => [54] => As Carnot's theorem is understood in modern thermodynamics to simply describe the maximum [[Thermal efficiency|efficiency]] with which [[thermal energy]] can be converted to mechanical [[energy]] and the predicted maximum efficiency is a function of the ''ratio'' between the absolute temperatures of the heat source and heat sink: [55] => [56] => *Efficiency ≤ 1 − {{sfrac|absolute temperate of heat sink|absolute temperature of heat source}} [57] => [58] => 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, {{Math|''T''}}, on the 1848 scale is related to the absolute temperature of the melting point of water, {{Math|''T''mpw}}, and the absolute temperature of the boiling point of water, {{Math|''T''bpw}}, by [59] => * {{Math|''T''}} (1848 scale) = 100 ({{Math|ln {{sfrac|''T''|''T''mpw}}}}) / ({{Math|ln {{sfrac|''T''bpw|''T''mpw}}}}) [60] => [61] => On this scale, an increase of 222 degrees always means an approximate doubling of absolute temperature regardless of the starting temperature. [62] => [63] => In a footnote Thomson calculated that "infinite cold" ([[absolute zero]], which would have a numerical value of negative [[infinity]] 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 [[Thermal expansion#Isobaric expansion in ideal gases|coefficient of thermal expansion]] of an ideal gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value.{{cite web |author-last=Thomson |author-first=William |title=On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations (1881 reprint) |url=https://www3.nd.edu/~powers/ame.20231/kelvin1848.pdf |publisher=Philosophical Magazine |access-date=21 February 2022|quote=If we push the strict principle of graduation, stated above, sufficiently far, we should arrive at a point corresponding to the volume of air being reduced to nothing, which would be marked as −273° of the scale (−100/·366, if ·366 be the coefficient of expansion); and therefore −273° of the air-thermometer is a point which cannot be reached at any finite temperature, however low}} [64] => [65] => Within a decade, Thomson had abandoned caloric theory and superseded the 1848 scale with a new one{{cite web |last1=Thomson |first1=William |title=On the Dynamical Theory of Heat, with numerical results deduced from Mr Joule's equivalent of a Thermal Unit, and M. Regnault's Observations on Steam (Excerpts) |url=https://zapatopi.net/kelvin/papers/on_the_dynamical_theory_of_heat.html |website=Zapatopi.net |publisher=Transactions of the Royal Society of Edinburgh and Philosophical Magazine |access-date=21 February 2022}} based on the 2 features that would characterise all future versions of the Kelvin scale: [66] => * Absolute zero is the null point. [67] => * Increments have the same magnitude as they do in the Celsius scale. [68] => [69] => In 1892, Thomson was awarded the [[Peerages in the United Kingdom|noble title]] 1st [[Baron]] Kelvin of [[Largs]], or more succinctly Lord Kelvin. This name was a reference to the [[River Kelvin]] which flows through the grounds of Glasgow University. [70] => [71] => In the early decades of the 20th century, the Kelvin scale was often called the "absolute [[Celsius scale|Celsius]]" 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.{{efn|For example, ''Encyclopaedia Britannica'' editions from the 1920s and 1950s, one example being the article "Planets".}} [72] => [73] => === Triple point standard === [74] => [[Image:Phase-diag2.svg|thumb|upright=1.5|A typical [[phase diagram]]. The solid green line applies to most substances; the dashed green line gives the anomalous behavior of water. The boiling line (solid blue) runs from the triple point to the [[Critical point (thermodynamics)|critical point]], beyond which further increases in temperature and pressure produce a [[supercritical fluid]].]] [75] => In 1873, William Thomson's older brother [[James Thomson (engineer)|James]] coined the term ''[[triple point]]''{{Cite journal|last=Thomson|first=James|date=1873|title=A quantitative investigation of certain relations between the gaseous, the liquid, and the solid states of water-substance|journal=Proceedings of the Royal Society of London|url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044106377062&view=1up&seq=48|volume=22|page=28|bibcode=1873RSPS...22...27T |issn=0370-1662|quote=and consequently that the three curves would meet or cross each other in one point, which I have called the ''triple point''.}} to describe the combination of temperature and [[pressure]] at which the solid, liquid, and gas [[Phase (matter)|phases]] of a substance were capable of coexisting in [[thermodynamic equilibrium]]. While any two phases could coexist along a range of temperature-pressure combinations (e.g. the [[boiling point]] of [[water]] 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 atmosphere (unit)|standard atmospheric pressure]] and very close to 0.01 °C per the historical definition of Celsius then in use. [76] => [77] => In 1948, the Celsius scale was recalibrated by assigning the triple point temperature of water the value of 0.01 °C exactly{{Cite journal |last=Swinton |first=F. L. |date=September 1967 |title=The triplet point of water |url=https://pubs.acs.org/doi/abs/10.1021/ed044p541 |journal=Journal of Chemical Education |language=en |volume=44 |issue=9 |pages=541 |doi=10.1021/ed044p541 |bibcode=1967JChEd..44..541S |issn=0021-9584}} and allowing the [[melting point]] at standard atmospheric pressure to have an empirically determined value (and the actual melting point at ambient pressure to have a [[Low-pressure area|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.{{cite web |title=Resolution 3 of the 9th CGPM (1948) |url=https://www.bipm.org/en/committees/cg/cgpm/9-1948/resolution-3 |publisher=BIPM |access-date=21 February 2022}} The triple point could be measured with ±0.0001 °C accuracy, while the melting point just to ±0.001 °C. [78] => [79] => 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]] (CGPM) introduced a new internationally standardized Kelvin scale which defined the triple point as exactly 273.15 + 0.01 = 273.16 degrees Kelvin.{{cite web |title=Resolution 3 of the 10th CGPM (1954) |url=https://www.bipm.org/en/committees/cg/cgpm/10-1954/resolution-3 |publisher=BIPM |access-date=21 February 2022}}{{cite web |title=Resolution 3: Definition of the thermodynamic temperature scale |work=Resolutions of the 10th CGPM |publisher=Bureau International des Poids et Mesures |url=http://www.bipm.fr/en/CGPM/db/10/3/ |year=1954 |access-date=2008-02-06 |url-status=dead |archive-url=https://web.archive.org/web/20070623215318/http://www.bipm.fr/en/CGPM/db/10/3/ |archive-date=23 June 2007}} [80] => [81] => In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol {{not a typo|°K}}.{{cite web |title=Resolution 3: SI unit of thermodynamic temperature (kelvin) |work=Resolutions of the 13th CGPM |url=http://www.bipm.fr/en/CGPM/db/13/3/ |publisher=Bureau International des Poids et Mesures |year=1967 |access-date=2008-02-06 |url-status=dead |archive-url=https://web.archive.org/web/20070421013852/http://www.bipm.fr/en/CGPM/db/13/3/ |archive-date=21 April 2007}} The 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction {{sfrac|273.16}} of the thermodynamic temperature of the triple point of water."{{cite web |title=Resolution 4 of the 13th CGPM (1967) |url=https://www.bipm.org/en/committees/cg/cgpm/13-1967/resolution-4 |publisher=BIPM |access-date=21 February 2022}}{{cite web|url=https://www.bipm.org/en/CGPM/db/13/4/|title=Resolution 4: Definition of the SI unit of thermodynamic temperature (kelvin)|year=1967|work=Resolutions of the 13th CGPM|publisher=Bureau International des Poids et Mesures|archive-url=https://web.archive.org/web/20070615125646/http://www.bipm.fr/en/CGPM/db/13/4/|archive-date=15 June 2007|url-status=dead |access-date=2008-02-06}} [82] => [83] => After [[metre#Speed of light definition|the 1983 redefinition of the metre]], this left the kelvin, the [[second]], and the [[kilogram]] as the only SI units not defined with reference to any other unit. [84] => [85] => 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]] (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]].{{cite web |title=Resolution 10 of the 23rd CGPM (2007) |url=https://www.bipm.org/en/committees/cg/cgpm/23-2007/resolution-10 |publisher=BIPM |access-date=21 February 2022}}{{cite web|title=Unit of thermodynamic temperature (kelvin) |work=SI Brochure, 8th edition |at="Section 2.1.1.5" |url=http://www1.bipm.org/en/si/si_brochure/chapter2/2-1/2-1-1/kelvin.html |publisher=Bureau International des Poids et Mesures |year=1967 |access-date=2008-02-06 |url-status=dead |archive-url=https://web.archive.org/web/20070926215600/http://www1.bipm.org/en/si/si_brochure/chapter2/2-1/2-1-1/kelvin.html |archive-date=26 September 2007 }} [86] => [87] => === 2019 redefinition === [88] => {{main|2019 redefinition of the SI base units}} [89] => [[File:Unit relations in the new SI.svg|thumb|upright=1.5|The kelvin is now fixed in terms of the Boltzmann constant and the [[joule]], itself defined by the [[Caesium standard|caesium-133 hyperfine transition frequency]] and the [[Planck constant]]. Both ''k'' and ''k''B are accepted shorthand for the Boltzmann constant.]] [90] => In 2005, the [[International Committee for Weights and Measures|CIPM]] began a programme to redefine the kelvin (along with the other SI units) using a more experimentally rigorous method. In particular, the committee proposed [[2019 redefinition of the SI base units#Kelvin|redefining the kelvin]] such that the [[Boltzmann constant]] takes the exact value {{val|1.3806505|e=-23|u=J|up=K}}. [91] => {{cite web [92] => |url=http://www.bipm.org/utils/en/pdf/si_brochure_draft_ch2.pdf [93] => |title=Draft Chapter 2 for SI Brochure, following redefinitions of the base units [94] => |author=Ian Mills [95] => |publisher=CCU [96] => |website=BIPM [97] => |date=29 September 2010 [98] => |access-date=2011-01-01 [99] => |url-status=dead [100] => |archive-url=https://web.archive.org/web/20110110104615/http://www.bipm.org/utils/en/pdf/si_brochure_draft_ch2.pdf [101] => |archive-date=10 January 2011 [102] => }} 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 [[2019 redefinition of the SI base units|larger program]]. [103] => {{cite press release [104] => | url = http://www.bipm.org/utils/en/pdf/Press_release_resolution_1_CGPM.pdf [105] => | title = General Conference on Weights and Measures approves possible changes to the International System of Units, including redefinition of the kilogram. [106] => | publisher = [[General Conference on Weights and Measures]] [107] => | location = Sèvres, France [108] => | date = 23 October 2011 [109] => | access-date = 25 October 2011 [110] => | url-status = dead [111] => | archive-url = https://web.archive.org/web/20120209175127/http://www.bipm.org/utils/en/pdf/Press_release_resolution_1_CGPM.pdf [112] => | archive-date = 9 February 2012 [113] => }} [114] => [115] => The redefinition was further postponed in 2014, pending more accurate measurements of the Boltzmann constant in terms of the current definition, [116] => {{Cite web [117] => |url=http://www.bipm.org/cc/TGFC/Allowed/Minutes/CODATA_Minutes_14-BIPM-public.pdf [118] => |title=Report on the Meeting of the CODATA Task Group on Fundamental Constants [119] => |date=3–4 November 2014 [120] => |place=[[BIPM]] [121] => |first=B. [122] => |last=Wood [123] => |page=7 [124] => |quote=[BIPM director Martin] Milton responded to a question about what would happen if ... the CIPM or the CGPM voted not to move forward with the redefinition of the SI. He responded that he felt that by that time the decision to move forward should be seen as a foregone conclusion. [125] => |url-status=dead [126] => |archive-url=https://web.archive.org/web/20151013174929/http://www.bipm.org/cc/TGFC/Allowed/Minutes/CODATA_Minutes_14-BIPM-public.pdf [127] => |archive-date=13 October 2015 [128] => }} but was finally adopted at the 26th CGPM in late 2018, with a value of {{mvar|k}} = {{physconst|k|after=.}} [129] => [130] => 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]], [[metre]] and [[second]] are defined in terms of the [[Planck constant]], the [[speed of light]], and the duration of the [[caesium-133]] ground-state [[hyperfine transition]] respectively. Thus, this definition depends only on [[Universal Constants|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),{{cite web |url=http://www.bipm.org/wg/CCT/TG-SI/Allowed/Documents/Updating_the_definition_of_the_kelvin2.pdf |title=Updating the definition of the kelvin |publisher=[[BIPM]] |access-date=2010-02-23 |url-status=dead |archive-url=https://web.archive.org/web/20081123043044/http://www.bipm.org/wg/CCT/TG-SI/Allowed/Documents/Updating_the_definition_of_the_kelvin2.pdf |archive-date=23 November 2008}} and the triple point of water continues to be a commonly used laboratory reference temperature. [131] => [132] => The difference is that, before the redefinition, the triple point of water was exact and the Boltzmann constant had a measured value of {{val|1.38064903|(51)|e=-23|u=J|up=K}}, with a relative standard uncertainty of {{val|3.7|e=-7}}. [133] => {{cite journal [134] => |title=The CODATA 2017 values of ''h'', ''e'', ''k'', and ''N''A for the revision of the SI [135] => |collaboration=Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants [136] => |first1=D B |last1=Newell |first2=F |last2=Cabiati |first3=J |last3=Fischer |first4=K |last4=Fujii [137] => |first5=S G |last5=Karshenboim |first6=H S |last6=Margolis |first7=E |last7=de Mirandés [138] => |first8=P J |last8=Mohr |first9=F |last9=Nez |first10=K |last10=Pachucki |first11=T J |last11=Quinn [139] => |first12=B N |last12=Taylor |first13=M |last13=Wang |first14=B M |last14=Wood |first15=Z |last15=Zhang [140] => |journal=Metrologia |volume=55 |issue=1 |pages=L13–L16 [141] => |date=29 January 2018 [142] => |doi=10.1088/1681-7575/aa950a |bibcode=2018Metro..55L..13N [143] => |doi-access=free |bibcode-access=free [144] => }} Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now {{val|273.1600|(1)|u=K}}. [145] => [146] => The new definition officially came into force on 20 May 2019, the 144th anniversary of the [[Metre Convention]].{{cite web |title=Resolution 1 of the 26th CGPM (2018) |url=https://www.bipm.org/en/committees/cg/cgpm/26-2018/resolution-1 |publisher=BIPM |access-date=21 February 2022}} [147] => [148] => == Practical uses == [149] => [[File:Kelvin Temperature Chart Vertical tightened.svg|thumb|390x390px|[[Colour temperature]] (right) of various light sources (left)]] [150] => [151] => === Colour temperature === [152] => {{see also|Stefan–Boltzmann constant}} [153] => 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]] radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about {{val|4000|u=K}} appear reddish, whereas those above about {{val|7500|u=K}} appear bluish. Colour temperature is important in the fields of image projection and [[photography]], where a colour temperature of approximately {{val|5600|u=K}} is required to match "daylight" film emulsions. [154] => [155] => In [[astronomy]], the [[stellar classification]] of stars and their place on the [[Hertzsprung–Russell diagram]] are based, in part, upon their surface temperature, known as [[effective temperature]]. The photosphere of the [[Sun]], for instance, has an effective temperature of {{val|5772|u=K}} [https://books.google.com/books?id=zT2HEAAAQBAJ&pg=PA216][https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html][https://books.google.com/books?id=2QBBEAAAQBAJ&dq=5772+K&pg=PA51][https://arxiv.org/pdf/1605.09788.pdf#] as adopted by IAU 2015 Resolution B3. [156] => [157] => 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, [[Color theory#Warm vs. cool colors|"warmer" colours]]. [158] => [159] => === Kelvin as a unit of noise temperature === [160] => {{main|Noise figure}} [161] => For electronics, the kelvin is used as an indicator of how [[electronic noise|noisy]] a circuit is in relation to an ultimate [[noise floor]], i.e. the [[noise temperature]]. The so-called [[Johnson–Nyquist noise]] of discrete resistors and capacitors is a type of thermal noise derived from the [[Boltzmann constant]] and can be used to determine the noise temperature of a circuit using the [[Friis formulas for noise]]. [162] => [163] => == Derived units and SI multiples == [164] => {{main|Orders of magnitude (temperature)}} [165] => The only [[SI derived unit#Derived units with special names|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]] that multiplies it by a [[power of 10]]: [166] => {{SI multiples [167] => | unit=kelvin [168] => | symbol=K [169] => }} [170] => [171] => == Orthography == [172] => According to SI convention, the kelvin is never referred to nor written as a [[Degree (temperature)|''degree'']]. The word "kelvin" is not capitalized when used as a unit. It may be in plural form as appropriate (for example, "it is 283 kelvins outside", as for "it is 50 degrees Fahrenheit" and "10 degrees Celsius").{{citation |work=NIST SP 811 |title=NIST Guide to the SI {{!}} Chapter 9: Rules and Style Conventions for Spelling Unit Names |url=https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-9-rules-and-style-conventions-spelling-unit-names#97 |quote=A derived unit is usually singular in English, for example, the value 3 m2·K/W is usually spelled out as 'three square meter kelvin per watt', and the value 3 C·m2/V is usually spelled out as 'three coulomb meter squared per volt'. However, a 'single' unit may be plural; for example, the value 5 kPa is spelled out as 'five kilopascals', although 'five kilopascal' is acceptable. If in such a single-unit case the number is less than one, the unit is always singular when spelled out; for example, 0.5 kPa is spelled out as 'five-tenths kilopascal'. }}{{Cite web |title=Definition of KELVIN |url=https://www.merriam-webster.com/dictionary/kelvin |access-date=2023-08-21 |website=www.merriam-webster.com |language=en}}{{Cite book |url=https://translation-council-support-group.web.cern.ch/sites/default/files/styles/large/CERN%20TM%20English%20language%20style%20guide.pdf |title=CERN English Language Style Guide |publisher=[[CERN]] |year=2022 |pages=64}} The unit's symbol K is a capital letter,{{cite web |title=Resolution 3 of the 13th CGPM (1967) |url=https://www.bipm.org/en/committees/cg/cgpm/13-1967/resolution-3 |publisher=BIPM |access-date=2022-02-21}} per the SI convention to capitalize symbols of units derived from the name of a person.{{Cite journal |date=2010-01-13 |title=Writing with SI (Metric System) Units |url=https://www.nist.gov/pml/owm/writing-si-metric-system-units |journal=NIST |language=en}} It is common convention to capitalize Kelvin when referring to Lord Kelvin or the Kelvin scale.{{cite book |last1=Brady |first1=James E. |last2=Senese |first2=Fred |title=Chemistry, Student Study Guide: The Study of Matter and Its Changes |date=28 January 2008 |publisher=John Wiley & Sons |isbn=978-0-470-18464-6 |page=15 |url=https://books.google.com/books?id=zS1EX-e7kPQC&pg=PA15 |language=en}} [173] => [174] => The unit symbol K is encoded in [[Unicode]] at code point {{unichar|212A|kelvin sign}}. However, this is a [[Unicode compatibility characters|compatibility character]] provided for compatibility with legacy encodings. The Unicode standard recommends using {{unichar|004B|latin capital letter [[k]]}} instead; that is, a normal capital [[K]]. "Three letterlike symbols have been given canonical equivalence to regular letters: {{unichar|2126|ohm sign}}, {{unichar|212A|kelvin sign}}, and {{unichar|212B|angstrom sign}}. In all three instances, the regular letter should be used."{{cite book|title=The Unicode Standard, Version 8.0|date=August 2015|publisher=The Unicode Consortium|location=Mountain View, CA, USA|isbn=978-1-936213-10-8|section=22.2|url=https://www.unicode.org/versions/Unicode8.0.0/ch22.pdf|access-date=6 September 2015|url-status=live|archive-url=https://web.archive.org/web/20161206230132/http://www.unicode.org/versions/Unicode8.0.0/ch22.pdf|archive-date=6 December 2016}} [175] => [176] => == See also == [177] => {{Portal|Energy}} [178] => * [[Outline of metrology and measurement]] [179] => * [[Comparison of temperature scales]] [180] => * [[International Temperature Scale of 1990]] [181] => * [[Negative temperature]] [182] => [183] => == Notes == [184] => {{notelist}} [185] => [186] => == References == [187] => {{Reflist|refs= [188] => {{cite web |title=SI Brochure: The International System of Units (SI) – 9th edition (updated in 2022) |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507 |publisher=BIPM |access-date=7 September 2022}} [189] => {{cite web |title=SI base unit: kelvin (K) |url=https://www.bipm.org/en/si-base-units/kelvin |publisher=BIPM |access-date=5 March 2022}} [190] => {{cite journal |title=A Turning Point for Humanity: Redefining the World's Measurement System |url=https://www.nist.gov/si-redefinition/turning-point-humanity-redefining-worlds-measurement-system |journal=NIST |date=12 May 2018 |access-date=21 February 2022}} [191] => {{cite web |last=BIPM |date=20 May 2019 |title=Mise en pratique for the definition of the kelvin in the SI |url=https://www.bipm.org/documents/20126/41489682/SI-App2-kelvin.pdf/cd36cb68-3f00-05fd-339e-452df0b6215e?version=1.5&t=1637237805352&download=false |access-date=18 February 2022 |website=BIPM.org}} [192] => {{cite web |title=Kelvin: Introduction |url=https://www.nist.gov/si-redefinition/kelvin-introduction |website=NIST |access-date=2 September 2022 |language=en |date=14 May 2018}} [193] => {{cite web |title=Handbook 44 – 2022 – Appendix C – General Tables of Units of Measurement |url=https://www.nist.gov/system/files/documents/2021/11/30/2022-HB44-Section-Appendix-C.pdf |website=nist.gov |publisher=NIST |access-date=21 February 2022}} [194] => {{cite journal |author-last=Benham |author-first=Elizabeth |title=Busting Myths about the Metric System |url=https://www.nist.gov/blogs/taking-measure/busting-myths-about-metric-system |journal=NIST |date=6 October 2020 |publisher=Taking Measure (official blog of the NIST) |access-date=21 February 2022}} [195] => {{Cite journal |author-first=John |author-last=Dalton |date=1801 |url=https://books.google.com/books?id=3qdJAAAAYAAJ&pg=PA595 |title=Essay II. On the force of steam or vapour from water and various other liquids, both in vacuum and in air |journal=Memoirs of the Literary and Philosophical Society of Manchester |volume=5 part 2 |pages=550–574}} [196] => {{Cite journal |author-first=John |author-last=Dalton |date=1801 |url=https://books.google.com/books?id=3qdJAAAAYAAJ&pg=PA595 |title=Essay IV. On the expansion of elastic fluids by heat |journal=Memoirs of the Literary and Philosophical Society of Manchester |volume=5 part 2 |pages=595–602}} [197] => {{citation |author-last=Gay-Lussac |author-first=Joseph Louis |author-link=Joseph Louis Gay-Lussac |date=1802 |title=Recherches sur la dilatation des gaz et des vapeurs |journal=Annales de Chimie |volume=XLIII |page=137}}. [http://web.lemoyne.edu/~giunta/gaygas.html English translation (extract).] [198] => {{cite web |author-last=Thomson |author-first=William |title=On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations |url=https://zapatopi.net/kelvin/papers/on_an_absolute_thermometric_scale.html |website=zapatopi.net |publisher=Philosophical Magazine |access-date=21 February 2022}} [199] => {{cite web |author-last=Thomson |author-first=William |title=On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations (1881 reprint) |url=https://www3.nd.edu/~powers/ame.20231/kelvin1848.pdf |publisher=Philosophical Magazine |access-date=21 February 2022}} [200] => {{cite journal |author-last=Kelvin |author-first=William |title=On an Absolute Thermometric Scale |journal=Philosophical Magazine |date=October 1848 |url=http://zapatopi.net/kelvin/papers/on_an_absolute_thermometric_scale.html |access-date=2008-02-06 |url-status=dead |archive-url=https://web.archive.org/web/20080201095927/http://zapatopi.net/kelvin/papers/on_an_absolute_thermometric_scale.html |archive-date=1 February 2008}} [201] => {{cite book |title=Physikalisches Wörterbuch |language=de |trans-title= |chapter=Nox, Dunkelleuchtdichte, Skot |author-first=Wilhelm Heinrich |author-last=Westphal |editor-first1=Wilhelm H. |editor-last1=Westphal |author-link=Wilhelm Heinrich Westphal |date=1952 |edition=1 |publisher=[[Springer-Verlag OHG]] |publication-place=Berlin / Göttingen / Heidelberg, Germany |isbn=978-3-662-12707-0 |doi=10.1007/978-3-662-12706-3 |pages=125, 271, 389 |chapter-url=https://books.google.com/books?id=QaCFBwAAQBAJ&pg=RA1-PA125 |access-date=2023-03-16 |quote-pages=271, 389 |quote=Dunkelleuchtdichte. […] Unter Zugrundelegung dieser Empfindlichkeitskurve hat man 1940 in Deutschland die Dunkelleuchtdichte mit der Einheit [[Skot (unit)|Skot]] (sk) so festgesetzt, daß bei einem Licht der Farbtemperatur 2360 {{not a typo|°[[Kelvin (unit)|K]]}} 1 sk = 10−3 asb gilt. 1948 ist von der [[Internationale Beleuchtungskommission|Internationalen Beleuchtungskommission]] (IBK) die Bezugstemperatur auf 2046 {{not a typo|°K}}, die Erstarrungstemperatur des [[Platinum|Platin]]s, festgesetzt worden. Die Bezeichnung Skot wurde von der IBK nicht übernommen, dafür soll "skotopisches Stilb" gesagt werden. Als höchstzulässiger Grenzwert für die Dunkelleuchtdichte ist in Deutschland 10 Skot festgesetzt worden, um eine Verwendung der Dunkelleuchtdichte im Gebiet des gemischten [[Photopic vision|Zapfen]]- und [[Scotopic vision|Stäbchensehen]]s zu vermeiden, da in diesem Bereich die photometrischen Maßgrößen wegen der allmählich gleitenden Augenempfindlichkeitskurve ihren Sinn verlieren. [...] Skot, abgek[ürzt] sk, Einheit für die Dunkelleuchtdichte, welche für zahlenmäßige Angaben und zum Anschluß der Dunkelleuchtdichte an die normale Leuchtdichte 1940 von der {{ill|German Lighting Society|de|Deutsche Lichttechnische Gesellschaft|lt=Deutschen Lichttechnischen Gesellschaft}} geschaffen wurde. Für diesen Anschluß wurde die Strahlung des [[Black body|schwarzen Körper]]s bei ''T'' = 2360 {{not a typo|°K}}, d.h. eine Strahlung der Farbtemperatur ''T''1 = 2360 {{not a typo|°K}} vereinbart. Eine Lichtquelle strahlt mit der Dunkelleuchtdichte 1 sk, wenn sie photometrisch gleich einer Strahlung der Farbtemperatur ''T''2 = 2360 {{not a typo|°K}} und der Leuchtdichte von 10−3 asb (Apostilb) ist. Bei der Farbtemperatur ''T''1 = 2360 {{not a typo|°K}} gilt also die Relation: 1 sk = 10−3 asb = 10−7/π sb. |trans-quote=}} [202] => }} [203] => [204] => == Bibliography == [205] => * {{cite web |author=Bureau International des Poids et Mesures |title=The International System of Units (SI) Brochure |version=9th Edition |publisher=International Committee for Weights and Measures |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf |date=2019 |access-date=2022-04-28}} [206] => [207] => == External links == [208] => {{wiktionary}} [209] => [210] => {{Scales of temperature}} [211] => {{SI units}} [212] => {{CGS units}} [213] => [214] => [[Category:1848 introductions]] [215] => [[Category:Scottish inventions]] [216] => [[Category:SI base units]] [217] => [[Category:William Thomson, 1st Baron Kelvin]] [218] => [[Category:Scales of temperature]] [219] => [[Category:Scales in meteorology]] [] => )
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Kelvin

Kelvin is a unit of temperature widely used in the scientific field. It is named after the Scottish physicist William Thomson, who was also known as Lord Kelvin.

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It is named after the Scottish physicist William Thomson, who was also known as Lord Kelvin. The kelvin scale is an absolute temperature scale that starts at absolute zero, the point at which all molecular motion ceases. It is based on the fundamental unit of the International System of Units (SI), the kilogram, and is defined as 1/273. 16th of the triple point of water. The kelvin scale is used in areas such as thermodynamics, meteorology, and physics, where precise measurements of temperature are crucial. It is especially preferred in scientific research and engineering applications because it avoids the negative values associated with the Celsius and Fahrenheit scales. The use of kelvin also simplifies various calculations and allows for more accurate comparisons between different temperature scales. The Wikipedia page about Kelvin covers various aspects related to this temperature scale. It provides a historical background, highlighting Lord Kelvin's contributions and his role in the development of the kelvin scale. It also delves into the technical details of the scale, including its definition and relation to other temperature units. The page explains the conversion formulas between kelvin and other temperature scales such as Celsius, Fahrenheit, and Rankine. It also describes the practical applications of the kelvin scale in different fields, such as in the measurement of absolute zero, thermodynamic calculations, and climate research. Additionally, the Wikipedia page delves into the kelvin's significance in the measurement of color temperature, where it is used to define the perceived color of light sources. It also addresses the limitations and criticisms associated with the use of the kelvin scale, including its reliance on the triple point of water and potential measurement errors. Overall, the Wikipedia page provides comprehensive and detailed information about the kelvin scale, making it a valuable resource for those seeking to understand this fundamental unit of temperature measurement.

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