• Support PF! Buy your school textbooks, materials and every day products Here!

Thermodynamic Postulates

  • Thread starter mrjohns
  • Start date
  • #1
13
0

Homework Statement



Consider relationship for a thermodynamic system:

S=A[UVN]^d , where A is a constant and d a real number.

I need to explain why d=1/3 is the only allowed value consistent with the postulates of thermodynamics.

The Attempt at a Solution



I'm having a hard time determining why this is the case from the postulates.
 

Answers and Replies

  • #2
938
9
You should start by writing down the postulates you think could be relevant.
 
  • #3
berkeman
Mentor
56,841
6,818

Homework Statement



Consider relationship for a thermodynamic system:

S=A[UVN]^d , where A is a constant and d a real number.

I need to explain why d=1/3 is the only allowed value consistent with the postulates of thermodynamics.

The Attempt at a Solution



I'm having a hard time determining why this is the case from the postulates.
Please check your PMs. You must write out the relevant equations and show your attempt at solving this problem.
 
  • #4
19,919
4,095
This is a problem involving units. The units on both sides of the equation must match.
 
  • #5
13
0
I don't have an attempt because I'm completely stumped.

The units on the left hand side are J/K, and on the right they are J^(1/3) m - which don't match.

I can't see anything in the postulates that helps either:

P1 - There exist equilibrium states characterised completely by U, V, N.
P2 - There exists a function of the macroscopic variables, the entropy, which is maximised when a constraint is removed
P3 - Entropy is additive over subsystems, and is a continuous and differentiable and increasing function of the total internal energy U
 
  • #6
vela
Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
14,580
1,198
Considering you don't know the units of A, it's pointless to match units between sides.

Can you give an example of what the third postulate means?
 
  • #7
1,082
25
While you can't match units directly because of the A term you can still make progress by requiring that the entropy be extensive.
 
  • #8
938
9
P3 - Entropy is additive over subsystems
This is important.
 
  • #9
19,919
4,095
U, V, N, and S are all extensive properties. So, if you double U, V, and N, what has to happen to S?
 
  • #10
13
0
I know that for a constant:

S(kN,KV,KU)=kS(N,V,U)

But I'm not sure how that restricts the power to a third.
 
  • #11
19,919
4,095
Substitute kU, kV, and kN, and kS into your thermodynamic relationship, and see what you get.
 

Related Threads on Thermodynamic Postulates

Replies
0
Views
3K
Replies
3
Views
768
  • Last Post
Replies
4
Views
2K
Replies
15
Views
4K
Replies
2
Views
885
Replies
4
Views
2K
Replies
0
Views
6K
  • Last Post
Replies
8
Views
2K
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
0
Views
1K
Top