The difference between order parameter and state parameter?

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Discussion Overview

The discussion revolves around the distinction between order parameters and state parameters within the context of thermodynamics and Landau theory, exploring their definitions, roles, and relationships in describing physical systems.

Discussion Character

  • Debate/contested

Main Points Raised

  • Some participants propose that state parameters (like pressure, temperature, and volume) are necessary to fully describe a system, while order parameters also describe system properties.
  • One participant argues that in Landau theory, the order parameter is treated as a thermodynamic variable since free energy is expanded in terms of it.
  • Another participant suggests that when applying Landau phase transition theory, the order parameter is often considered alongside temperature, implying that other thermodynamic variables are held constant.
  • It is noted that the order parameter can depend on other thermodynamic variables, prompting a participant to question whether this is a trivial observation.
  • One participant asserts that the order parameter can be introduced as an independent thermodynamic variable, similar to how gases can be described using different sets of state parameters.

Areas of Agreement / Disagreement

Participants express differing views on whether the order parameter can be considered an independent variable or if it is inherently linked to other state parameters. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants highlight the dependence of the order parameter on other thermodynamic variables, but the implications of this dependence and the conditions under which it holds are not fully explored.

Diracmai
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At first, we need some state parameters(like P/T/V) to fully describe a system. And the order parameter is also a parameter which could describe the property of the system.
However, in the Laudau theory, why it said that order parameter is an external parameter(not a independent variable)? Is there any different between order parameter and state parameters(like P/T/V etc.)?
 
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I would say no, as in Landau theory we expand the free energy into a power series in the order parameter. Hence it must be a thermodynamic variable.
 
DrDu said:
I would say no, as in Landau theory we expand the free energy into a power series in the order parameter. Hence it must be a thermodynamic variable.
Ok! Actually, I also take the some viewpoint as you. In addition, I have another question.Generally, when we use Laudau phase transition theory, we always choose T and order parameter as free energy's variables. In this condition, it that means we hold other thermodynamic variable as constant and only discuss the relation between order parameter and temperature. In fact, the order parameter can also be the function of other thermodynamic variables.
 
Diracmai said:
In fact, the order parameter can also be the function of other thermodynamic variables.
Isn't this trivial? E.g. you can describe a gas in terms of V and T or in terms of p and T, the transformation being P(T,V). So you can always introduce the order parameter as an independent thermodynamic variable.
 

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