Silly doubt about thermodynamics: molar representation

In summary, the fundamental equation of a system can be represented as a function S(U,V,N), which is a homogeneous first order function of the extensive parameters. By taking a constant lambda=1/N, we can obtain S(U,V,N)=Ns(u,v), where u is the energy per mole, v is the volume per mole, and s is the entropy per mole. However, when applying this equation to an example, it can be confusing as to where the N^1/4 term comes from. The correct reasoning is to multiply N, U, and V together, resulting in S=4A^-1/2 U^1/4 V^1/2 N^1/4 + Ns0.
  • #1
Telemachus
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Well, I have a doubt about something that I've found in the book. It's really silly, but it's been bothering me for a while, so perhaps you can help me to understand this.

As you should know, the fundamental equation of a system can be represented in the entropic representation as a function S(U,V,N). And this function is a homogeneous first order function of the extensive parameters. That is, if all the extensive parameters of a system are multiplied by a constant , the entropy is multiplied by this same constant.

[tex]S(\lambda U,\lambda V, \lambda N)=\lambda S(U,V,N)[/tex]

Then one can take [tex]\lambda=1/N[/tex] to obtain [tex]S(U,V,N)=NS(U/N,V/N,1)[/tex]
U/N is the energy per mole, and is denoted by u. Similarly V/N is the volume per mole, denoted by v, and S/N is the entropy per mole.

S(U/N,V/N,1) is the entropy of a system of a single mole, denoted s(u,v).
S(U,V,N)=Ns(u,v).

I think that the previous ideas are clear to me. But then I found some difficulties with some examples.

I have that the entropy per mole of a system is:
[tex]s=4A^{-1/2}u^{1/4}v^{1/2}+s_0[/tex]
A is just a constant.
Then the entropy for all the moles in the system according to the book is:

[tex]S=4A^{-1/2}U^{1/4}V^{1/2}N^{1/4}+Ns_0[/tex]

I don't know where the N1/4 comes from. I know I'm not understanding this clearly, because as I think it, there would be no N's at all, and that's just wrong. As I'm reasoning this, I just multiply by N the left side of the equation to obtain the big S, and then in the other side I can do the same, or multiply each U and V by N. If I just multiply everything by N I would get:
[tex]S=4NA^{-1/2}u^{1/4}v^{1/2}+Ns_0[/tex]
and if I multiply U and V by N I would get:
[tex]S=4A^{-1/2}U^{1/4}V^{1/2}+s_0[/tex]

I don't know what I'm doing wrong, and I don't know how to get that N1/4 on it's place. What's so wrong with my reasoning?
 
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  • #2
N u^1/4 v^1/2 = N (U/N)^1/4 (V/N)^1/2 = U^1/4 V^1/2 N^1/4, right? Sorry for not using latex, on mobile.
 
  • #3
Thank you :)
 

FAQ: Silly doubt about thermodynamics: molar representation

What is a molar representation in thermodynamics?

A molar representation in thermodynamics refers to the representation of a system's properties in terms of the number of moles of a substance rather than its mass. This is useful because it allows for easier comparison between different substances, regardless of their molecular weight.

How is molar representation different from other representations in thermodynamics?

Molar representation is different from other representations in thermodynamics, such as mass or volume, because it takes into account the number of molecules present in a system rather than their physical properties. This allows for a more accurate understanding of the system's behavior and properties.

Why is molar representation important in thermodynamics?

Molar representation is important in thermodynamics because it allows for easier comparison and analysis of different substances. It also helps to simplify calculations and equations, making it a useful tool in understanding the behavior of complex systems.

How do you convert between molar representation and other representations in thermodynamics?

To convert between molar representation and other representations in thermodynamics, you can use the molar mass of a substance to convert between moles and mass, or the molar volume to convert between moles and volume. These conversion factors can be found in tables or calculated using the ideal gas law.

Can molar representation be applied to all substances in thermodynamics?

Yes, molar representation can be applied to all substances in thermodynamics, as it is a fundamental concept that applies to all types of matter. However, it is most commonly used for gases and ideal solutions, where the number of molecules is more easily determined compared to other states of matter.

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