Talk:Specific heat ratio/Draft

Wikipedia has an article on the same subject

This article was completely re-written from scratch but it probably still has some Wikipedia content in it. I was not a major contributor to the Wikipedia article. Milton Beychok 17:01, 3 July 2008 (CDT)

Okay. I don't know what that does to its eligibility for New Draft of the Week. I'll look for the rules.... (Or, better yet, someone who knows off the bat can answer.)Aleta Curry 18:54, 4 July 2008 (CDT)

Naming this article

I had a difficult time deciding whether to name this article Specific heat ratio or Heat capacity ratio. I finally used Google's book search function and Google's scholar search function on both names. The results were:

• Book search: 7,070 books contained "specific heat ratio" and 3,820 books contained "heat capacity ratio".

• Scholar search: 2,140,000 publications by scholars contained "specific heat ratio" and 595,000 publications contained "heat capacity ratio"

Based on those results, I named the article Specific heat ratio. Milton Beychok 17:01, 3 July 2008 (CDT)

Comments/questions

Milt,

although you have a link to "specific heat", I think it would be helpful as a reminder to start with definition, like: "The specific heat is the amount of heat per amount of substance required to raise the temperature by one degree kelvin". It could go as an introductory sentence or as footnote. PS: when reading further I saw that you end the article by it. Maybe you should move the final section up to the beginning?

Paul, this article was intended to be about the specific heat ratio and it was assumed that CZ would eventually have a separate article about specific heats. That is why the link to specific heat is in the first sentence. I don't like to bite off more than I can chew. For that reason, I am reluctant to include a definition of specific heat in the introductory section. What if I simply moved that later section "Definitions of specific heat and heat capacity" up to be the first subsection just after the "Table of Contents" ? Milton Beychok 19:58, 9 April 2009 (UTC)

I would also remind the reader of the general thermodynamic relations

$C_{V}=\left({\frac {\partial U}{\partial T}}\right)_{V}$

and

$C_{p}=\left({\frac {\partial H}{\partial T}}\right)_{p}$

because otherwise your comments about the ideal gas seem to appear out of the blue.

I agree with this comment but am not quite sure where best to put those two thermodynamic equations. What if I added them in at the very beginning of the "Ideal gas relations" section to lead into my discussion of H = C_pT and U = C_vT ? Milton Beychok 19:58, 9 April 2009 (UTC)

Why do you state that a diatomic molecule has 5 degrees of freedom at room temperature? Do you want to exclude vibrational excitations? But for atoms you excluded implicitly electronic excitations. PS: Reading one sentence further I found the answer. You are repeating yourself here on the degrees of freedom of a diatomic, this could be a few words shorter.

Paul, as you found be reading the next sentence, I explained why the vibrational degree of freedom was excluded. I don't understand what you mean by "this could be a few words shorter". Please give me the exact wording you would use to make it shorter ... so that I understand what you feel is needed. Milton Beychok 19:58, 9 April 2009 (UTC)

Cheers, --Paul Wormer 17:50, 9 April 2009 (UTC)

Here it comes (a couple of words less):

---

Thus for a monatomic gas, with three degrees of freedom:

$k={\frac {5}{3}}=1.67$

and for a diatomic gas, with five degrees of freedom (three translational and two rotational degrees of freedom, the vibrational degree of freedom is not involved except at high temperatures):

$k={\frac {7}{5}}=1.4$.

Earth's atmospheric air is primarily made up of diatomic gases with a composition of ~78% nitrogen (N₂) and ~21% oxygen (O₂). At 20 °C and an absolute pressure of 101.325 kPa, the atmospheric air can be considered to be an ideal diatomic gas. This results in the same value of k, namely 7/5.

---

--Paul Wormer 20:23, 9 April 2009 (UTC)

With regard to the definition of "specific heat": the article is about the ratio, I see that, but when the reader has not readily available what a specific heat is, (s)he is lost in the very first sentence. With regard to the thermo relations: maybe in a footnote? --Paul Wormer 20:34, 9 April 2009 (UTC)

More dialogue

Paul, I think that I have now implemented your suggestions:

• I added a footnote in the first sentence defining "specific heat". That is in addition to the internal link to a CZ article not yet written.

• I added the two thermodynamic relations into the beginning of the subsection on ideal gas relations.

• I shortened the wording about diatomic gases, as you suggested.

Regards, Milton Beychok 22:40, 9 April 2009 (UTC)

$k$ or $\kappa$ ?

I would prefer to use $\kappa$ thoughout, so as to avoid confusion with $k_{B}$, particularly in those relations that involve $R$. --Daniel Mietchen 09:50, 15 April 2009 (UTC)

Temperature dependence

The sentence "Some correlations exist to provide values of k as a function of the temperature." could probably use a pointer to a suitable reference. I didn't find a good one after a brief search. --Daniel Mietchen 09:58, 15 April 2009 (UTC)