Why Describe the Extensive State Using Intensive and Extensive Variables?

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In summary, the conversation discusses the use of intensive and extensive variables to describe the state of a system. The example given is a system consisting of one kind of species, helium, and the need for at least one extensive and one intensive variable to determine its internal energy. However, in general thermodynamics, two variables are sufficient to determine internal energy, and for other properties such as Gibbs free energy, two variables would not be enough.
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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.
 

FAQ: Why Describe the Extensive State Using Intensive and Extensive Variables?

What is the difference between intensive and extensive variables?

Intensive variables are properties that do not depend on the size or amount of a system, such as temperature or density. Extensive variables, on the other hand, are properties that do depend on the size or amount of a system, such as mass or volume.

How are intensive and extensive variables related?

Intensive and extensive variables are related through their mathematical relationship, where extensive variables are often expressed as the product of an intensive variable and the size or amount of a system. For example, the extensive variable energy (E) can be written as E = U × V, where U is the intensive variable internal energy and V is the volume of the system.

Can intensive variables change?

Yes, intensive variables can change. They can be affected by external factors, such as changes in pressure or temperature, which can alter their value. However, intensive variables do not change with the size or amount of the system, unlike extensive variables.

How do intensive and extensive variables affect each other?

Intensive and extensive variables can affect each other through their mathematical relationship. Changes in intensive variables can lead to changes in extensive variables, and vice versa. For example, increasing the temperature of a gas (intensive variable) can lead to an increase in its volume (extensive variable).

Why are intensive and extensive variables important in science?

Intensive and extensive variables are important in science because they allow us to describe and understand the behavior of physical systems. By studying how these variables change, scientists are able to make predictions and develop theories about the natural world.

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