Using Homogenuous Functions to Understand Thermodynamics

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Discussion Overview

The discussion revolves around the application of homogeneous functions in thermodynamics, particularly in relation to the internal energy function and its properties. Participants explore the conditions under which the assumption of homogeneity can be applied in thermodynamic systems.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that if a function is homogeneous of degree r, it satisfies a specific differential equation involving its partial derivatives.
  • Another participant suggests that the assumption of homogeneity can generally be applied to macroscopic amounts of a substance, implying that doubling the amount of substance also doubles the energy.
  • It is noted that exceptions to this assumption arise when dealing with very small amounts of substance, such as individual molecules, where energy does not simply double.
  • A further contribution emphasizes the need to consider the absence of long-range interactions, such as gravitational effects in astrophysical contexts, which may invalidate the homogeneity assumption.
  • A participant expresses interest in finding mathematical literature that discusses the theorem related to homogeneous functions in thermodynamics.

Areas of Agreement / Disagreement

Participants generally agree on the applicability of the homogeneity assumption for macroscopic systems but acknowledge exceptions for small quantities and specific conditions involving interactions.

Contextual Notes

The discussion does not resolve the limitations of the homogeneity assumption in various contexts, such as the effects of small sample sizes or long-range interactions.

Who May Find This Useful

Researchers and students in thermodynamics, mathematical physics, and related fields may find this discussion relevant, particularly those exploring the implications of homogeneous functions in energy calculations.

Petar Mali
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If I have homogenuous function [tex]f(x,y,z,...)[/tex] of degree [tex]r[/tex] than:
[tex]x\frac{\partial f}{\partial x}+y\frac{\partial f}{\partial y}+...=rf[/tex]

In thermodynamics:
[tex]dU=TdS-pdV+\mu dN[/tex]

If I said U is homogenuous function of degree 1 I will get

[tex]U=TS-pV+\mu N[/tex]

When can I use this assumption?
 
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You can almost always use this assumption. It amounts to the assumption that when you have some substance, and you double the amount of it, and so its volume and entropy, you also double the energy. This will be true for macroscopic amounts of a substance.

The only exceptions occur for very small amounts of a substance. If you have one molecule, and then you add one more, the energy is not just doubled. Same goes for going from two molecules to four. It will start working when there is enough of the substance that the effects of the surface are negligible and you basically have a totally homogeneous material.
 
You also have to assume that there are no long range interactions between the molecules. So e.g., in astrophysical problems where gravity is important you cannot make this assumption.
 
Thanks!
 
Hi!
You know..? This is exacly what I need for my thesis.
In which mathematic books can I find that theorem?
 

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