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Old Jun 29th 2014, 06:44 PM   #1
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Entropy as a state property

The usual "proof" entropy is a state property is like that:

"Consider a system which undergoes a reversible process from state 1 to state 2 along path A, and let cycle be completed along path B, which is also reversible. Since the cycle is reversible we can write:

∫1-2 δQ / T + ∫2-1 δQ / T = 0

Now let cycle be completed along path C, but paths B and C represent arbitrary reversible processes. So ∫2-1 δQ / T is the same for all reversible paths between states 2 and 1."

My question is, isn't the equation above already assume entropy is a state property? Only if it is a state property it can go around a cycle without changes. How can it be valid to prove entropy is state property if it is already assumed from the beginning?
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Old Sep 5th 2014, 01:15 AM   #2
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Originally Posted by kelsiu View Post
The usual "proof" entropy is a state property is like that:

"Consider a system which undergoes a reversible process from state 1 to state 2 along path A, and let cycle be completed along path B, which is also reversible. Since the cycle is reversible we can write:

∫1-2 δQ / T + ∫2-1 δQ / T = 0

Now let cycle be completed along path C, but paths B and C represent arbitrary reversible processes. So ∫2-1 δQ / T is the same for all reversible paths between states 2 and 1."

My question is, isn't the equation above already assume entropy is a state property? Only if it is a state property it can go around a cycle without changes. How can it be valid to prove entropy is state property if it is already assumed from the beginning?
mhh it is really a confusing issue oooo i will try asking people around and see what they gat to offer
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Old Sep 5th 2014, 04:37 AM   #3
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I tend to focus on fundamental physics, and I think of entropy as "sameness". You know, heat death of the universe and all that. Have a google on entropy available energy. If the energy density is the same everywhere, there's no available energy, and entropy is said to be high.

With that in mind, and noting that a "hot" particle is a fast-moving particle, I have to say I rather struggle with the way entropy is described in thermodynamics and statistical mechanics. Things like along path A have me thinking there's too much abstraction, not enough focus on the fundamentals, and too much confusion.
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Old Sep 18th 2014, 04:16 AM   #4
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nice

nice post good




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Last edited by atifali; Sep 25th 2014 at 12:54 AM.
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Old Sep 18th 2014, 05:56 AM   #5
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If anybody is interested: the entropy of the universe is said to be increasing, in that energy-density is tending to even out. Every time you do work, you reduce the amount of energy that's available to do work. You even out the energy density and increase the "sameness" as the universe heads towards heat death.

Working back from this, cosmologists usually say the entropy of the early universe was very low. It sounds reasonable doesn't it? But have a think for a minute about the very early universe. We can be fairly confident it was small, and we can be fairly confident that the oft-mooted singularity signifies some kind of failure of mathematics. So let's say there wasn't any singularity, just that the universe was small. The energy density of this very early universe would have been incredibly high. But get this: it would have been uniform. So there would have been no available energy. So the entropy would have been high too.

The entropy only became low when the universe expanded, and you then had some regions with a high energy density, and some regions with a low energy density.

As for why the universe expanded: dunno guv.
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