Silly doubt about thermodynamics: molar representation

Click For Summary
SUMMARY

The discussion centers on the entropic representation of a thermodynamic system, specifically the equation S(U,V,N) and its implications for entropy per mole. The user seeks clarification on the derivation of the term N^{1/4} in the equation S=4A^{-1/2}U^{1/4}V^{1/2}N^{1/4}+Ns_0. The confusion arises from the transformation of extensive parameters and their relationship to entropy per mole, denoted as s(u,v). The key takeaway is that the term N^{1/4} emerges from the scaling of the extensive variables U and V when expressed in terms of the number of moles.

PREREQUISITES
  • Understanding of thermodynamic principles, particularly entropy and extensive parameters.
  • Familiarity with the concept of molar quantities in thermodynamics.
  • Knowledge of mathematical functions and their homogeneity properties.
  • Basic grasp of thermodynamic equations and their representations.
NEXT STEPS
  • Study the derivation of the fundamental thermodynamic equations, focusing on entropy functions.
  • Learn about the implications of extensive versus intensive properties in thermodynamics.
  • Explore the concept of homogeneity in mathematical functions, particularly in thermodynamic contexts.
  • Investigate the relationship between molar quantities and extensive properties in thermodynamic systems.
USEFUL FOR

Students of thermodynamics, physicists, and chemists seeking to deepen their understanding of entropy and its mathematical representations in thermodynamic systems.

Telemachus
Messages
820
Reaction score
30
Well, I have a doubt about something that I've found in the book. It's really silly, but it's been bothering me for a while, so perhaps you can help me to understand this.

As you should know, the fundamental equation of a system can be represented in the entropic representation as a function S(U,V,N). And this function is a homogeneous first order function of the extensive parameters. That is, if all the extensive parameters of a system are multiplied by a constant , the entropy is multiplied by this same constant.

[tex]S(\lambda U,\lambda V, \lambda N)=\lambda S(U,V,N)[/tex]

Then one can take [tex]\lambda=1/N[/tex] to obtain [tex]S(U,V,N)=NS(U/N,V/N,1)[/tex]
U/N is the energy per mole, and is denoted by u. Similarly V/N is the volume per mole, denoted by v, and S/N is the entropy per mole.

S(U/N,V/N,1) is the entropy of a system of a single mole, denoted s(u,v).
S(U,V,N)=Ns(u,v).

I think that the previous ideas are clear to me. But then I found some difficulties with some examples.

I have that the entropy per mole of a system is:
[tex]s=4A^{-1/2}u^{1/4}v^{1/2}+s_0[/tex]
A is just a constant.
Then the entropy for all the moles in the system according to the book is:

[tex]S=4A^{-1/2}U^{1/4}V^{1/2}N^{1/4}+Ns_0[/tex]

I don't know where the N1/4 comes from. I know I'm not understanding this clearly, because as I think it, there would be no N's at all, and that's just wrong. As I'm reasoning this, I just multiply by N the left side of the equation to obtain the big S, and then in the other side I can do the same, or multiply each U and V by N. If I just multiply everything by N I would get:
[tex]S=4NA^{-1/2}u^{1/4}v^{1/2}+Ns_0[/tex]
and if I multiply U and V by N I would get:
[tex]S=4A^{-1/2}U^{1/4}V^{1/2}+s_0[/tex]

I don't know what I'm doing wrong, and I don't know how to get that N1/4 on it's place. What's so wrong with my reasoning?
 
Last edited:
Physics news on Phys.org
N u^1/4 v^1/2 = N (U/N)^1/4 (V/N)^1/2 = U^1/4 V^1/2 N^1/4, right? Sorry for not using latex, on mobile.
 
Thank you :)
 

Similar threads

Replies
1
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 7 ·
Replies
7
Views
5K
  • · Replies 1 ·
Replies
1
Views
3K
Replies
5
Views
2K
Replies
4
Views
5K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
Replies
4
Views
1K
Replies
2
Views
2K