Kelvin 16620 226048517 2008-07-16T16:27:50Z Prosfilaes 49272 iws go in alphabetical order {{otheruses}} {{temperature}} The '''kelvin''' (symbol: '''K''') is a [[Units of measurement|unit increment]] of [[temperature]] and is one of the seven [[SI base unit]]s. The '''Kelvin scale''' is a [[Thermodynamic temperature|thermodynamic (absolute) temperature]] scale where [[absolute zero]], the theoretical absence of all thermal energy, is zero (0 K). The Kelvin scale and the kelvin are named after the [[United Kingdom|British]] physicist and engineer [[William Thomson, 1st Baron Kelvin]] (1824–1907), who wrote of the need for an “absolute thermometric scale”. == Definition of kelvin == The kelvin unit and its scale, by [[international agreement]], are defined by two points: [[absolute zero]], and the [[triple point]] of [[Vienna Standard Mean Ocean Water]] (VSMOW).<ref name=sib2115>{{cite web | title=Unit of thermodynamic temperature (kelvin) | work=SI Brochure, 8th edition | pages=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 | date=1967 | accessdate=2008-02-06}}</ref> This definition also exactly relates the Kelvin scale to the [[Celsius]] scale. Absolute zero—the temperature at which nothing could be colder and no [[heat]] energy remains in a substance—is, by definition, exactly 0 K ''and'' −273.15 °C. The [[triple point]] of water is, by definition, exactly 273.16 K ''and'' 0.01 °C. This definition does three things: #It fixes the magnitude of the kelvin unit as being exactly 1 part in 273.16 of the difference between absolute zero and the triple point of water; #It establishes that one kelvin has exactly the same magnitude as a one-degree increment on the Celsius scale; and #It establishes the difference between the two scales’ null points as being exactly 273.15 kelvins (0 K ≡ −273.15 °C and 273.16 K ≡ 0.01 °C). Temperatures in kelvin can be converted to other units per the table at bottom left. ===Temperature equivalents=== {| border="1" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; background: #f5f5f0; border: 2px #52524d solid; border-collapse: collapse; font-size: 95%;" |- |style="background:#d4d4d4" align="center"|''' ''' |style="background:#d4d4d4" align="center"|'''Kelvin''' |style="background:#d4d4d4" align="center"|'''Celsius''' |style="background:#d4d4d4" align="center"|'''Fahrenheit''' |- |style="background:#d9d9d3" align="right"|Absolute zero<br> (exactly, by definition) |align="center"|0 K |align="center"|−273.15 °C |align="center"|−459.67 °F |- |style="background:#d9d9d3" align="right"|Freezing Point of water |align="center"|273.15 K |align="center"|0 °C |align="center"|32 °F |- |style="background:#d9d9d3" align="right"|Water’s triple point<br> (exactly, by definition) |align="center"|273.16 K |align="center"|0.01 °C |align="center"|32.018 °F |- |style="background:#d9d9d3" align="right"|Water’s boiling point |align="center"|373.1339 K |align="center"|[[Celsius#The melting and boiling points of water|99.9839 °C]] |align="center"|211.9710 °F |} For [[Vienna Standard Mean Ocean Water]] (VSMOW) at one [[Atmosphere (unit)|standard atmosphere]] (101.325 kPa) when calibrated solely per the two-point definition of thermodynamic temperature. Older definitions of the Celsius scale once defined the boiling point of water under one standard atmosphere as being exactly 100 °C. However, the current definition results in a boiling point that is actually 16.1 mK less. For more about the actual boiling point of water, see [[Vienna Standard Mean Ocean Water#VSMOW in temperature measurement|VSMOW in temperature measurement]]. === SI prefixes === <!-- This section is linked from [[Absolute zero]], [[Thermodynamic temperature]], as well as from other articles. Only delete or rename it with careful consideration, and please fix any resulting broken links.--> {{SI multiples|unit=kelvin|symbol=K}} == Typographical and usage conventions == ===Uppercase/lowercase, plural form usage, and written conventions=== When reference is made to the unit kelvin (either a specific temperature or a temperature interval), kelvin is always spelled with a lowercase k unless it is the first word in a sentence.<ref>BIPM: SI brochure, [http://www1.bipm.org/en/si/si_brochure/chapter5/5-2.html Section&nbsp;5.2]</ref> When reference is made to the "Kelvin ''scale''", the word "kelvin"—which is normally a noun—functions [[adjective|adjectivally]] to modify the noun "scale" and is capitalized. Until the 13th [[General Conference on Weights and Measures]] (CGPM) in 1967–1968, the unit kelvin was called a "degree", the same as with the other temperature scales at the time. It was distinguished from the other scales with either the adjective suffix "Kelvin" ("degree Kelvin") or with "absolute" ("degree absolute") and its symbol was °K. Note that the latter (degree absolute), which was the unit’s official name from 1948 until 1954, was rather ambiguous since it could also be interpreted as referring to the [[Rankine scale]]. Before the 13th CGPM, the plural forms were "degrees Kelvin" or "degrees absolute". The 13th CGPM changed the name to simply "kelvin" (symbol K).<ref>{{cite paper | author=Barry N. Taylor | title=Guide for the Use of the International System of Units (SI) | version=Special Publication 811 | format=.PDF | publisher=National Institute of Standards and Technology | url=http://physics.nist.gov/cuu/pdf/sp811.pdf | date=1995 | accessdate=2008-02-06}}</ref> The omission of "degree" indicates that it is not relative to an arbitrary reference point such as the Celsius and Fahrenheit scales, but rather an absolute unit of measure which can be manipulated algebraically (e.g. multiply by two to indicate twice the amount of heat). ===Temperatures and intervals=== Because the kelvin is an individual unit of measure, it is particularly well-suited for expressing ''temperature intervals:'' differences between temperatures or their uncertainties (e.g., “Agar exhibited a melting point hysteresis of 25 kelvins.” and “The uncertainty was 10 millikelvins.”). Of course, the kelvin is also used to express specific temperatures along its scale (e.g. “[[Gallium]] melts at 302.9146 kelvin”). One disadvantage of the kelvin is that intervals ''and'' specific temperatures on the Kelvin scale use exactly the same symbol (e.g., “Agar exhibited a melting point hysteresis of 25 K,” and “The triple point of hydrogen is 13.8033 K”). ===Formatting and typestyle for the K symbol=== The kelvin symbol is always a [[Roman type|roman]], [[Italic type|non-italic]] capital K. In the SI naming convention, all symbols named after a person are capitalized; in the case of the kelvin, capitalizing also distinguishes the symbol from the SI prefix “kilo”, which has the lowercase k as its symbol. The admonition against italicizing the symbol K applies to all SI unit symbols; only symbols for variables and constants (e.g. ''P'' = pressure, and ''c'' = 299,792,458 m/s) are italicized in scientific and engineering papers. As with most other SI unit symbols (angle symbols, e.g. 45° 3&prime; 4&Prime;, are the exception) there is a space between the numeric value and the kelvin symbol (e.g. “99.987 K”).<ref name=nist>{{cite web | title=SI Unit rules and style conventions | url=http://physics.nist.gov/cuu/Units/checklist.html | publisher=National Institute of Standards and Technology | date=September 2004 | accessdate=2008-02-06}}</ref><ref name=sib533>{{cite web | title=Rules and style conventions for expressing values of quantities | work=SI Brochure, 8th edition | pages=Section 5.3.3 | url=http://www.bipm.org/en/si/si_brochure/chapter5/5-3-2.html#5-3-3 | publisher=Bureau International des Poids et Mesures | date=1967 | accessdate=2008-02-06}}</ref> ===The special Unicode kelvin character=== [[Unicode]] provides a [[Unicode compatibility characters|compatibility character]] for the kelvin at U+212A (decimal 8490), for compatibility with CJK encodings that provide such a character (as such, in most fonts the width is the same as for [[fullwidth]] characters). Below in maroon text is the kelvin character followed immediately by a simple uppercase K: :<span class="Unicode" style="color:#721F01;font-size:bigger">&#x212a; K</span> When viewed on computers that properly support Unicode, the above line may be similar to the line below (size may vary): :[[Image:Kelvin symbol plus K.svg|23px]] The canonical decomposition of this character is U+004B (uppercase K), so some browsers may simply display a "K" in its place due to [[Unicode normalization]]. ===Mixed use of Kelvin and Celsius scales in technical articles=== In science and in engineering, the Celsius scale and the kelvin are often used simultaneously in the same article (e.g. “…its measured value was 0.01023 °C with an uncertainty of 70 µK…”). This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing Celsius temperatures and the magnitude of the degree Celsius is exactly equal to that of the kelvin.<ref name=sib222>{{cite web | title=Units with special names and symbols; units that incorporate special names and symbols | work=SI Brochure, 8th edition | pages=Section 2.2.2, Table 3 | url=http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html | publisher=Bureau International des Poids et Mesures | date=2006 | accessdate=2008-02-06}}</ref> Notwithstanding the official endorsement provided by Resolution 3 of the 13th CGPM, states “a temperature interval may also be expressed in degrees Celsius,” the practice of simultaneously using both “°C” and “K” remains widespread throughout the scientific world as the use of [[SI prefix#List of SI prefixes|SI prefixed]] forms of the degree Celsius (such as “µ°C” or “microdegrees Celsius”) to express a temperature interval has not been well-adopted.<ref name=res313>{{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 | date=1967 | accessdate=2008-02-06}}</ref> == History of the Kelvin scale == Below are some historic milestones in the development of the Kelvin scale and its unit increment, the kelvin. For more on the history of thermodynamic temperature, see ''Thermodynamic temperature: [[Thermodynamic temperature#History of thermodynamic temperature|History of thermodynamic temperature]].'' * '''1848:''' [[William Thomson, 1st Baron Kelvin|Lord Kelvin (William Thomson)]], wrote in his paper, ''On an Absolute Thermometric Scale,'' of the need for a scale whereby “infinite cold” (absolute zero) was the scale’s null point, and which used the degree Celsius for its unit increment. Thomson calculated that absolute zero was equivalent to −273 °C on the air thermometers of the time.<ref name=lordkelvin>{{cite journal | last=Thomson | first=William | coauthors= | title=On an Absolute Thermometric Scale | journal=Philosophical Magazine | month=October | year=1848 | url=http://zapatopi.net/kelvin/papers/on_an_absolute_thermometric_scale.html | accessdate=2008-02-06}}</ref> This absolute scale is known today as the Kelvin thermodynamic temperature scale. It’s noteworthy that Thomson’s value of “−273” was actually derived from 0.00366, which was the accepted expansion coefficient of gas per degree Celsius relative to the ice point. The inverse of −0.00366 expressed to five significant digits is −273.22 °C which is remarkably close to the true value of −273.15 °C. * '''1954:''' Resolution 3 of the 10th CGPM gave the Kelvin scale its modern definition by designating the triple point of water as its second defining point and assigned its temperature to exactly “273.16 degrees Kelvin.”<ref name=res310>{{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/ | date=1954 | accessdate=2008-02-06}}</ref> * '''1967/1968:''' Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature “kelvin”, symbol K, replacing “degree absolute”, symbol °K.<ref name=res313/> Further, feeling it useful to more explicitly define the magnitude of the unit increment, the 13th CGPM also held in Resolution 4 that “The kelvin, unit of thermodynamic temperature, is equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.”<ref name=res413>{{cite web | title=Resolution 4: Definition of the SI unit of thermodynamic temperature (kelvin) | work=Resolutions of the 13th CGPM | url=http://www.bipm.fr/en/CGPM/db/13/4/ | publisher=Bureau International des Poids et Mesures | date=1967 | accessdate=2008-02-06}}</ref> * '''2005:''' [[International Committee for Weights and Measures|The Comité International des Poids et Mesures]] (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 thermodynamic temperature scale would refer to water having an isotopic composition specified as ''[[Vienna Standard Mean Ocean Water|VSMOW]]''.<ref name=sib2115/> == See also == <div style="-moz-column-count:2; column-count:2;"> * [[Comparison of temperature scales]] * [[Celsius]] * [[Fahrenheit]] * [[Heat]] * [[International Temperature Scale of 1990|ITS-90]] * [[Rankine scale]] * [[Thermodynamic temperature]] * [[Triple point]] </div> == References == {{reflist|2}} == External links == {{wiktionary}} * {{cite paper | author=Bureau International des Poids et Mesures | title=The International System of Units (SI) Brochure | version=8th Edition | format=.PDF | publisher=International Committee for Weights and Measures | url=http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf | date=2006 | accessdate=2008-02-06}} {{TemperatureScales}} [[Category:Scottish inventions]] [[Category:SI base units]] [[Category:Units of temperature]] [[af:Kelvin]] [[als:Kelvin]] [[ar:كلفن]] [[ast:Kelvin]] [[bn:কেলভিন]] [[bs:Kelvin]] [[br:Kelvin]] [[bg:Келвин]] [[ca:Kelvin]] [[cs:Kelvin]] [[da:Kelvin]] [[de:Kelvin]] [[et:Kelvin]] [[el:Κλίμακα Kelvin]] [[es:Kelvin]] [[eo:Kelvino]] [[eu:Kelvin (unitatea)]] [[fa:کلوین]] [[fr:Kelvin]] [[ga:Ceilvin]] [[gl:Kelvin]] [[ko:켈빈]] [[hi:कैल्विन]] [[hr:Kelvin]] [[id:Kelvin]] [[is:Kelvin]] [[it:Kelvin]] [[he:יחידות מידה לטמפרטורה]] [[ka:კელვინი]] [[sw:Kelvini]] [[lv:Kelvins (mērvienība)]] [[lt:Kelvinas]] [[ln:Kelvin]] [[jbo:kelvo]] [[hu:Kelvin]] [[mk:Келвин]] [[ms:Kelvin]] [[nl:Kelvin (eenheid)]] [[ne:केल्भिन]] [[ja:ケルビン]] [[no:Kelvin]] [[nn:Kelvin]] [[oc:Kelvin]] [[uz:Kelvin]] [[nds:Kelvin]] [[pl:Kelwin]] [[pt:Kelvin]] [[ro:Kelvin]] [[ru:Кельвин]] [[simple:Kelvin]] [[sk:Kelvin]] [[sl:Kelvin]] [[sr:Келвин (јединица)]] [[sh:Kelvin]] [[fi:Kelvin]] [[sv:Kelvin]] [[ta:கெல்வின்]] [[th:เคลวิน]] [[vi:Kelvin]] [[tr:Kelvin (birim)]] [[uk:Кельвін (одиниця)]] [[vec:Kelvin]] [[zh:热力学温标]] [[zh-yue:開氏]]