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Is the Kelvin temperature scale actually supposed to be written as a lower-case k? I thought k stood for kilo- and K stood for Kelvins. --24.72.34.179 22:44, 14 Jun 2004 (UTC)

I think they meant that:

1. A "kelvin" is written with a lowercase "k", just like "ampere" is written with a lowercase "a", even though it's named after a person too.
2. However, the abbreviation for a kelvin is an uppercase "K", just like the abbreviation for an ampere is an uppercase "A".

200 K = 200 kelvins
200 A = 200 amperes

Perhaps the wording needs some clarification, or perhaps it's unnecessary. -- Curps 23:46, 14 Jun 2004 (UTC)

Page 31 of the United States National Institue of Standards and Technology (NIST) Special Puplication 811 (Guide for the Use of the International System of Units (SI)) states that unit names should be spelled out in full like any other English noun, with lower-case letters, except at the start of a sentence or in a title. However, if you write "degree Kelvin", Kelvin should be capitalized (since it's a type of degree, same way you'd write "Kennedy High School" and "high school") The publication actually uses celsius for the example but it's the same principle. A capital K should be used when abbreviating, since the unit is named after a person. -- Uberdog 02:14, 30 Jun 2004 (UTC)

According to the BIPM (see here), the "degree Kelvin" ceased to exist 36 years ago:

The 13th CGPM (1967-1968, Resolution 3) adopted the name kelvin (symbol K) instead of "degree Kelvin" (symbol °K) and defined the unit of thermodynamic temperature as follows (Resolution 4): ...

On the other hand, Celsius has a capital C because it hasn't been adopted by the SI. -- Heron 08:25, 30 Jun 2004 (UTC)

No. Celsius has a capital C because it's one of the quirks of the English language (these are language-specific rules; it's different in German, for example, with all nouns capitalized) that the nouns naming units after people are not capitalized, but the proper adjectives identifying particular ones of ambiguous unit names are capitalized. Thus, when kelvins were called "degrees Kelvin" they also had a capital K, and the R is capitalized in degrees Rankine.

Furthermore, you are also wrong about degrees Celsius not being adopted by the CGPM. They are indeed part of the SI (which, as a system of measurement, cannot "adopt" anything), a derived unit with a special name.Gene Nygaard 13:39, 10 Dec 2004 (UTC)

In case anyone's interested, here's a NASA styleguide, which is where I confirmed my suspicion that a space belongs between the number and the symbol for Kelvins, unlike in many places in Misplaced Pages. NASA occasionally has trouble with units, but hopefully they can spell them right. -- Tantalate 01:47, 4 Aug 2004 (UTC)

I agree. The American NIST checklist mandates a space between every number and its unit. The official SI brochure always uses a space (e.g. here), but I can't find an explicit statement on the BIPM site that a space is required. -- Heron 08:13, 4 Aug 2004 (UTC)

This topic comes up occasionally in other discussions. ISO 31-0 and the UK equivalent BS 5775 apparently recommend a space before the unit, but I have not seen the original text of either. I also find the recommendation in:

• US NIST publication 811
• US NIST checklist
• UK NPL
• IEE guide
• misc.metric-system faq
  

The scope of the references certainly includes metric units. Whether the authors would recommend the same format for non-metric units is unclear to me. It may be implicit in the IEE reference and the unseen ISO and BS references. I would be surprised if any respectable author/editor would specifically want all copy to have spaces before metric units but not before imperial units. In summary, there are respectable references that say a space should be used and none that say a space should absent. Where each character is costly (such as on a cellular phone display), I don't think people worry too much if the space is absent, but Misplaced Pages pages have plenty of room for space characters.

Bobblewik 09:17, 7 Aug 2004 (UTC)

Any reason why both 273.16 ("...one kelvin is the fraction 1/273.16 of...") and 273.15 ("°C = K − 273.15") is used? which is the right one?

They are both right. 273.16 is the definition of the kelvin with respect to the triple point of water. 273.15 is the offset between the Celsius scale and the kelvin. See the BIPM website. --Heron 14:18, 19 Oct 2004 (UTC)

Definition and comparison with Celsius

I just had an edit of mine reverted and I'd like to explain myself. The BIPM brochure on kelvin, which link I added to the article, states the following (emphasis is in the original):

The 13th CGPM (1967-1968, Resolution 3) adopted the name kelvin (symbol K) instead of "degree Kelvin" (symbol °K) and defined the unit of thermodynamic temperature as follows (Resolution 4):

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Because of the way temperature scales used to be defined, it remains common practice to express a thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T₀ = 273.15 K, the ice point. This temperature difference is called the Celsius temperature, symbol t, and is defined by the quantity equation

${\displaystyle t=T-T_{0}}$

I re-stated and summarized this as follows:

The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. A temperature given in kelvins, without further qualification, is measured with respect to absolute zero, where molecular motion stops. It is also common to give a temperature (a so-called Celsius temperature) relative to the reference temperature of 273.15 K, approximately the melting point of water under ordinary conditions.

I think this is a fairly faithful rendering of the BIPM's description, but I'm open to discussion on the point. Our prior (and, after the revert, our current) definition reads as follows:

The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined by two facts: zero kelvins is absolute zero (when molecular motion stops), and one kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The Celsius temperature scale is now defined in terms of the kelvin, with 0 °C corresponding to 273.15 kelvins, approximately the melting point of water under ordinary conditions.

The problem with this description is that it does not make any distinction between the kelvin unit of temperature versus the Kelvin thermodynamic temperature scale, which measures temperature relative to absolute zero. For example, take the statement that "zero kelvins is absolute zero". Consider the question: what temperature change is necessary in water at standard temperature and pressure in order to liquefy it? The answer is "zero kelvins", but this most certainly does not indicate that the liquefied water would be at absolute zero.

My version of the definition used terms and phrases introduced by the BIPM brochure:

1. a kelvin is 1/273.16 of the thermodynamic temperature of the triple point of water,
2. it is common to measure relative to 273.15 kelvins, and
3. such practice is referred to as "Celsius temperature".

It also includes some embellishments I thought were fairly benign (and were pre-existing):

• absolute zero is where (not "when") molecular motion stops, and
• 273.15 kelvins is approximately the melting point of water.

All in all, I think this is a fairly uncontrovercial definition of the kelvin, and I'd like to reinstate it, or something like it. --P3d0 03:30, Dec 30, 2004 (UTC)

No objections in almost 9 months; I have restored my paragraph with a slight modification to avoid the phrase "so-called Celcius scale". --P3d0 16:38, September 7, 2005 (UTC)

"Technical limitations"

Is the "technical limitations" note necessary? "Kelvin" is capitalized at the start of a sentence or in a title, just like cat, washing machine, or any other common noun. The page follows Misplaced Pages's capitalization convention; what's wrong with that? --tilde 00:21, September 7, 2005 (UTC)

Hmm, yeah, I guess you're right, it really isn't any different from any other noun, I just figured since it is never referred to as Kelvin anymore, but that's really true of anything -- MacAddct1984 23:08, September 7, 2005 (UTC)

Kelvins v. kelvin?

Which is it? I thought this unit was always said in the singular. That is, "0 kelvin", "10 kelvin", "a difference of 42 kelvin", "measured in kelvin", and never "kelvins". Is that right? —BenFrantzDale 22:32, 9 October 2005 (UTC)

No. Gene Nygaard 22:52, 9 October 2005 (UTC)

I think this is a habit ingrained from the "degrees kelvin" days. It's not the norm any more. --P3d0 03:13, 10 October 2005 (UTC)

Yes, that's what I would have said if I'd had more time. In the old days before the name was changed, the K was capitalized because it was an adjective, modifying the noun degrees, and that's just a quirk of English usage that we do it that way. But also, the "s" was then added to degrees, the noun part, and the adjective "Kelvin" didn't change in the plural when we had degrees Kelvin. But now the kelvins are the noun, and they take the normal plural, and are written with a lowercase k just like volts and watts and amperes and newtons and joules.

Lot's Lots of people haven't figured that out, of course—they probably didn't pay much attention in English class to little nuances like parts of speech—so you do often see things like 298 Kelvin or 298 kelvin. But the keepers of our standards realize what's going on, and add the normal "s" in the plural of the spelled out word. Gene Nygaard 04:38, 10 October 2005 (UTC)

I did pay attention in English (and I do appreciate your use of em dashes), but from my science and engineering studies, "kelvins" just sounds wrong. I'd be interested in seeing a reference that says that's correct if you have one. (Either way, I think P3d0 is dead on in terms of etimology.) —BenFrantzDale 20:27, 20 October 2005 (UTC)

Rule in NIST SP811 section 9.2 (kelvins not listed among exceptions)

"Plural unit names are used when they are required by the rules of English grammar. They are normally formed regularly, for example, "henries" is the plural of henry. According to Ref. , the following plurals are irregular: Singular - lux, hertz, siemens; Plural - lux, hertz, siemens. (See also Sec. 9.7.)"

Usage in BIPM SI brochure section 2.1.1.5

"A difference or interval of temperature may be expressed in kelvins or in degrees Celsius (13th CGPM, 1967-1968, Resolution 3)."

Gene Nygaard 21:23, 20 October 2005 (UTC)

It is interesting to note that in the paragraph where Gene Nygaard rails against people for not paying attention in English class the first word has been misspelt. There shouldn't be an apostrophe in "Lots". A case of the pot calling the kettle black I think. Pedro20 03:34, 9 November 2005 (UTC)

I don't spell lots with an apostrophe; thanks for calling it to my attention. It is a typo, a miscommunication between my brain and my fingers, something I find happening more often all the time. I will correct it, just as I do when the s is missing in the plural of kelvins (but it's usually only my own mistakes that I will change on a talk page). (signed later, Gene Nygaard 08:31, 9 November 2005 (UTC))

Temperature and energy

How can Kelvin be converted into electron volt, let alone with a dimensionless conversion factor as in this article? Temperature and energy are different physical properties. Kelvin is a base unit and ${\displaystyle eV=1.602*10^{-19}J=1.602*10^{-19}kg*m^{2}/s^{2}}$. --Hardy 21:46, 16 October 2005 (UTC)

That looks like its the average energy per atom, which may or may not be correct, but either way it is certainly true that temperature is not energy and any exposition of the above formula should make that clear. —BenFrantzDale 20:22, 20 October 2005 (UTC)

I agree. According to my physics book ("Gerthsen Kneser Vogel: Physik") and sources in the WWW the average kinetic energy of particles of an ideal gas is

${\displaystyle {\overline {\mathrm {E_{kin}} }}={\frac {3}{2}}\cdot k_{B}\cdot \mathrm {T} }$.

So even the constant of proportionality is incorrect. I wonder were these formulae came from originally. --Hardy 10:16, 22 October 2005 (UTC)

Shouldn't that be

${\displaystyle {\overline {\mathrm {E_{kin}} }}={\frac {f}{2}}\cdot k_{B}\cdot \mathrm {T} }$.

Where f is the number of degrees of freedom? — Sverdrup 10:52, 29 October 2005 (UTC)

That is right, but for ideal gases f is 3. So the formula applies to ideal gases and approximately to monoatomic gases as He. For gases whose molecules consist of two atoms as O₂ f is 5. For molecules with more than two atoms that are not on a straight line f is 6. Anyway imho the section "Temperature and energy" is not relevant for this article and should be removed. Otherwise we could add still more relations between temperature and energy as energy per water molecule or per iron atom. --Hardy 18:51, 30 October 2005 (UTC)

So do you guys have the rigth formula? Looks kinda silly, a disputed tag on a formula... --The Minister of War 17:01, 24 November 2005 (UTC)