Why Describe the Extensive State Using Intensive and Extensive Variables?

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SUMMARY

The discussion centers on the necessity of describing the extensive state of a system using both intensive and extensive variables, particularly in thermodynamics. It highlights that internal energy (U) is an extensive quantity, requiring three variables for its complete description, while the equation U=n(3/2)RT suggests that only two variables (number of moles n and temperature T) are sufficient. The conversation also touches on the general thermodynamic relationship U=U(S,V) and the implications for Gibbs free energy (G), which requires additional variables due to both pressure (p) and temperature (T) being intensive.

PREREQUISITES
  • Understanding of extensive and intensive properties in thermodynamics
  • Familiarity with the ideal gas law and its variables
  • Knowledge of thermodynamic equations such as U=n(3/2)RT
  • Basic concepts of Gibbs free energy and its dependence on pressure and temperature
NEXT STEPS
  • Research the definitions and examples of extensive and intensive variables in thermodynamics
  • Study the derivation and applications of the equation U=n(3/2)RT
  • Explore the relationship between Gibbs free energy (G) and its variables, particularly p and T
  • Examine the implications of using different variable combinations in thermodynamic equations
USEFUL FOR

This discussion is beneficial for students and professionals in thermodynamics, particularly those studying physical chemistry, as well as researchers focusing on energy systems and state variables.

Bert
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Hello,
In my book:
[PLAIN]http://img84.imageshack.us/img84/6264/variabelwk1.jpg[/PLAIN]

But why do the tell me that the need do discribe the extensieve state of a system by using intensive and at least one extensive variable?

Thanks a lot.
 
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Hello Bert: you are probably retired from PF by now but the issue might be interesting for others too:
we take a system consisting of 1 kind of species present (we take helium). internal energy U is an extensive quantity so we need 1+2=3 variables to know it. But U=n(3/2)RT, so here two variables seem to be sufficient, with n extensive and T intensive. In general thermodynamics takes U=U(S,V) which also has only 2 varibles, but for G=G(p,T) 2 would not be enough because both p and T are intensive. ??
Sorry for the mess.
 

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