Proving thermodynamic relationship

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Discussion Overview

The discussion revolves around the derivation of a thermodynamic relationship involving heat capacity at constant volume (Cv) and its connection to internal energy (U) and entropy (S). Participants explore the application of the first law of thermodynamics and the use of Maxwell relations in this context.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant states that Cv can be expressed as Cv = (∂U/∂T)v = T(∂S/∂T)v and attempts to derive this from the first law of thermodynamics.
  • Another participant questions the derivation, specifically the second term in the expression, asking for clarification on how it was obtained.
  • A participant claims to have applied the product rule to Tds, resulting in two terms, but this is challenged by another who argues that the product rule is not applicable in this context.
  • One participant suggests that the differentiation process involves dividing by dT and imposing the condition dV = 0, rather than applying the product rule.
  • A question is raised about expressing dS in terms of dT and dV, indicating a further exploration of the relationship between these variables.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the validity of the derivation or the application of the product rule, indicating that multiple competing views remain regarding the correct approach to the problem.

Contextual Notes

There are unresolved assumptions regarding the treatment of differentials and the application of mathematical rules in thermodynamic contexts. The discussion reflects varying interpretations of the first law and its implications for the derivation.

goggles31
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Cv = (∂U/∂T)v = T(∂S/∂T)v

I can prove this by using the Maxwell relations, but I have trouble deriving it from the first law.

dU = TdS - pdV
(∂U/∂T)v = T(∂S/∂T)v + ∂S(∂T/∂T)v = T(∂S/∂T)v + ∂S

Is there a problem with my derivation?
 
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goggles31 said:
dU = TdS - pdV
(∂U/∂T)v = T(∂S/∂T)v + ∂S(∂T/∂T)v = T(∂S/∂T)v + ∂S
Could you explain how you got the second term?
 
Fightfish said:
Could you explain how you got the second term?

I applied to product rule to Tds hence obtaining two terms.
 
goggles31 said:
I applied to product rule to Tds hence obtaining two terms.
You don't apply the product rule here because you are not actually differentiating per se - you are not finding ##\frac{d}{dT} (dU)##, which doesn't make sense because well, what does it mean to differentiate an infinitesimal differential quantity?

Rather what we are doing is simply "division" - the mathematicians will complain here, but what we are doing can be rigorously justified if we so wish. That is to say, we divide by ##dT## throughout and then impose the condition that ##dV = 0##.
 
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If S=S(T,V), what is dS in terms of dT and dV?
 
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