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What is the meaning of degree of temperature?

by dotancohen
Tags: degree, meaning, temperature
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dotancohen
#1
Feb7-10, 02:14 PM
P: 106
I see that there are four temperature scales, two of which start at absolute zero and who's values are referred to as "Kelvin" and "Rankine", and two others which do not start at absolute zero and who's values are referred to as "degrees Celsius" and "degrees Fahrenheit". What is the exact meaning of the word degree in this sense? Why is it used for two of the scales but not the other two? Is the reason because C and F do not start at absolute zero?

Also, is the degree here the same degree as in angles? We say "the temperature is 20 degrees Celsius", but not "the length is 5 degrees meter". Furthermore, we say that "the angle is 45 degrees" without giving a unit (as if degree is the unit).

Please enlighten me! Thanks!
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cepheid
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Feb7-10, 02:40 PM
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Hi,

1. As far as I know, it was just an arbitrary historical decision that the full names of the units on the Celsius and Fahrenheit scales are "degrees C" and "degrees F" respectively. For the other two temperature scales, this wasn't done, and the names of the units are just "kelvins" and "rankines" respectively. Again, as far as I know, this is a matter of convention.

2. The degrees used to measure angles have nothing to do with the degrees used to measure temperature. You are right that when referring to angles, the word "degree" is used with no further modifier, because the word "degree" IS the name of the unit. By definition, when you divide a full rotation into 360 parts, a degree is the angle spanned by one of those parts. In contrast, on the temperature scale, a "degree" is the temperature "step" you get when you divide the temperature difference between the freezing and boiling points of water into 100 equal parts. We called these degrees "degrees Celsius" after their inventor, in order to distinguish them from the units on another temperature scale, which also happened to be called "degrees," but which are defined in a totally different way.
cepheid
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Feb7-10, 02:53 PM
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Hmm, actually, according to the Wikipedia article, the unit started out being typset with a degree symbol and referred to as degree, and this was later changed. (Unfortunately, there are no citations, so you should seek corroboration if you really want to know whether this is true). So, I was right about it being a historical decision about what the convention/standard should be, but the decision turned out to be a little bit more deliberate than my first post implied (i.e. it was a conscious decision by some standards committee). Not only that, but your supposition that the absence of degrees was used to distinguish this as being an absolute temperature scale was correct after all (according to this). Good call!

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. 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 form was "degrees absolute". The 13th CGPM changed the name to simply "kelvin" (symbol K).[7] The omission of "degree" indicates that it is not relative to an arbitrary reference point like the Celsius and Fahrenheit scales, but rather an absolute unit of measure which can be manipulated algebraically (e.g., multiplied by two to indicate twice the amount of "mean energy" available among elementary degrees of freedom of the system).

dotancohen
#4
Feb7-10, 02:57 PM
P: 106
What is the meaning of degree of temperature?

> As far as I know, it was just an arbitrary historical
> decision that the full names of the units on the
> Celsius and Fahrenheit scales are "degrees C" and
> "degrees F" respectively.

Actually, according to wikipedia, "Kelvin" was once called "degree Kelvin. The word "degree" was dropped.


> The degrees used to measure angles have nothing
> to do with the degrees used to measure temperature.

Yes, I know that the units are completely different, however, I am asking about the word "degree" itself. For instance, an intake valve on a diesel engine has nothing to do with a valve on an artificial heart. However, both regulate the flow of fluid. Likewise, could "degree" in both senses mean "a distance from an arbitrary starting point" as opposed to a well-defined starting point (absolute zero is well defined, but the direction used for 0 degrees in an angle is arbitrary).
cepheid
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Feb7-10, 03:22 PM
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Quote Quote by dotancohen View Post
Actually, according to wikipedia, "Kelvin" was once called "degree Kelvin. The word "degree" was dropped.
Yeah, I figured that out. Did you not read my second post?

Quote Quote by dotancohen View Post
Yes, I know that the units are completely different, however, I am asking about the word "degree" itself. For instance, an intake valve on a diesel engine has nothing to do with a valve on an artificial heart. However, both regulate the flow of fluid. Likewise, could "degree" in both senses mean "a distance from an arbitrary starting point" as opposed to a well-defined starting point (absolute zero is well defined, but the direction used for 0 degrees in an angle is arbitrary).
It wasn't clear at all from your original post that this is what you actually meant to ask. In any case, I don't know the answer to your question. If there is some general sense in which all units named with the word "degree" are related to each other (such has being measured from an arbitrary reference point), it's not something I've come across in the course of my education.
mgb_phys
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Feb7-10, 04:01 PM
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Yes, I know that the units are completely different, however, I am asking about the word "degree" itself.
Using the word degree is a historical anachronism, it's because the temperature scale was divided between two fixed points - degree just meant a division.
It's like the wording of 'latent heat of vapourisation' doesn't mean heat in the modern definition.

Changing everyday usage of C and F would be difficult and confusing while changing scientist's usage of Kelvin is easier.
dotancohen
#7
Feb19-10, 09:23 AM
P: 106
> Yeah, I figured that out. Did you not read my second post?

No, I didn't! I must have come right at the critical moment between posts.

> It wasn't clear at all from your original post that this is what you
> actually meant to ask.

Sorry, I really didn't know how to word that. Part of developing concrete ideas is to describe them thoroughly, and I should have invested more time in doing that.


> It's like the wording of 'latent heat of vapourisation' doesn't mean
> heat in the modern definition.

I see, thanks. I learned Thermodynamics in Hebrew, not in English, and that distinction was never really clear. Then again, not much in Thermo is!


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