# The difference between order parameter and state parameter?

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1. Jun 15, 2015

### Diracmai

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.)?

Last edited: Jun 15, 2015
2. Jun 18, 2015

### DrDu

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.

3. Jun 18, 2015

### Diracmai

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.

4. Jun 19, 2015

### DrDu

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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