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I am trying to gain a better understanding of thermodynamic equilibrium. Here's what (I think) I know:

If a system is in Thermal, Radiative, Chemical, and Mechanical Equilibrium, then it is in thermodynamic equilibrium. When in this state, the system can do no work as there is no available free energy to use. And when a system is in this state it is at its maximum entropy.

Please correct me if any of that is incorrect.

What I want to know is what would something look like if it was in thermodynamic equilibrium? Say a bit of matter like a rock was hypothetically in this state, would it still be a rock, but with uniform heat throughout, no net force, no net radiation and no net reaction? Or would it have changed state to become a gas? (because the entropy of gas is higher than that of a solid) Or would the rock just be heat energy? (I say this because the Heat Death of the universe suggests that everything would just be heat energy, no matter, and in thermodynamic equilibrium)

Any of your help on any of these questions would be extremely useful.

Thank you.

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  • $\begingroup$ everything would just be heat energy, no matter is a misconception - energy is a property of physical stuff, and not a form of stuff itself (here, classical stuff would mean matter and physical fields - electromagnetism and gravity - and ultimately of course just quantum fields) $\endgroup$
    – Christoph
    Commented Jan 20, 2014 at 14:21
  • $\begingroup$ Ok thank you that was very helpful. So say for instance the Earth entered thermodynamic equilibrium, would it just become a rock unable to support life or any process due to all its energy being converted into heat? $\endgroup$
    – arch
    Commented Jan 20, 2014 at 14:25
  • $\begingroup$ @Christoph: According to Einsteins mass energy relation, mass and energy are inter-convertible, then won't it suggest the possibility of everything to be heat energy with no matter. $\endgroup$
    – Sensebe
    Commented Jan 21, 2014 at 6:32
  • $\begingroup$ @VINAY: mass is another property of stuff, and when you convert mass into energy, you convert stuff into other stuff; while total energy needs to be conserved, mass doesn't; even if you get rid of matter (assuming you mean by that 'stuff with mass'), you will still need 'stuff without mass' (in particular photons) that carries the energy $\endgroup$
    – Christoph
    Commented Jan 21, 2014 at 7:18
  • $\begingroup$ @Christoph: I think the "stuff" what you are calling has got no proper significance yet. $\endgroup$
    – Sensebe
    Commented Jan 21, 2014 at 7:33

1 Answer 1

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If the system is in thermal equilibrium, EVERYTHING in the system is at exactly the same Temperature; EVERYTHING .

Otherwise it is NOT in thermal equilibrium.

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