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  • Thermo geek needed

    It's been 25 years since I've had formal schooling, which did include thermodynamics.

    The 2nd Law of Thermodynamics basically states

    It is an expression of the fact that over time, differences in temperature, pressure, and chemical potential decrease in an isolated non-gravitational physical system, leading eventually to a state of thermodynamic equilibrium.
    Here's what got me thinking on this... Imagine a solar system's worth of gas and dust (including the higher elements beyond iron) is moved to an isolated region of space, between galaxies. It can be considered closed; the amount of radiation bombarding it is negligible relative to what will happen.

    Soon, the H and He coalesce, and we have fusion; a star is born. The remaining gas and dust form planets. In a few billion years, we have a civilization of grey aliens who come and visit us. We have gone from uniform gas and dust, into a higher form.

    Note the bolded "non-gravitational" in the quoted definition. This to me is a giant cop-out. All matter has gravity. One may as well state that the 2nd law of thermodynamics applies ONLY to radiation in all its forms.

    What gives?

  • #2
    Disclaimer: I aced Thermodynamics 1 (got A+) but flunked thermodynamics 2 twice (and because of that I changed my major! LOL)

    Here it goes: As it settles across space and motion becomes negligible, temperature, pressure, and chemical potential decrease to the point where it attains equilibrium. However it does not last long for remember the Newton's Third Law so over time, gravitational forces build-up to the point where it becomes a gravitational system.

    What you described in the above example is in the theoretical sense where no gravity exists but we are in reality so gravity exists so thermodynamics law 3 will only work for a time until Newtons laws come into play.

    There it is. Aiight I am outta this thread.

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    • #3
      Not a thermo geek by any means, but I did just finish a course in thermodynamics. However, it dealt mostly with practical engineering applications, not esoteric thought experiments like yours. Ill put my oar in anyhow.

      I am not entirely certain what is meant by the term "non-gravitational system" in the quote. I have a feeling it does not mean "without gravity". I suspect it is meant to exclude a scenario like yours where work is being done on the coalescing gas as a result of gravity. In such a scenario you would expect differences in temperature, pressure and chemical potential to increase.

      If you take the view of the 2nd law that all real physical processes result in an increase of entropy, than your hypothetical gas cloud coalescing is not a violation of the 2nd law. Any localized reduction in entropy is more than compensated for by the increase in temperature, radiation etc.

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      • #4
        Thanks for the replies... I guess this is one of those things where you just have to trust the egg heads. Bit still, it makes little sense to me.

        I think a basic understanding of it is important... it helps explain the formation of our own solar system and the evolution of life in that the Earth is NOT a closed system, being bombarded by solar energy. But that got me thinking about how our own solar system formed from a homogeneous mass of gas and dust and apparently become more ordered; less entropy.

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        • #5
          You might find it helpful to brush up on the concept of a thermodynamic system. To thermo eggheads, a closed system is one which exchanges no matter with its surroundings. However, energy, in the form of heat and work are free to cross the boundary between the closed system and its surroundings. A closed system can undergo a reduction in entropy without violating the second law. This is because any entropy loss within the closed system is always more than compensated for by an increase of entropy in the surroundings.
          I'll give you an example which you can then extend to your primordial gas cloud. Say you grab a can of warm soda from you car and stick it in the fridge. In this case the can and its contents are the closed system. The boundary of the system begins where can meets air. As the can cools, it goes from a higher entropy state to a lower one. (Warm liquids have higher entropy than cold ones) Energy crosses the boundary in the form of heat. Any entropy removed from the system is transferred to the surroundings. Moreover, the increase of entropy in the surroundings is always higher than the loss within the closed system. In this case, you can even work out how much new entropy is generated by the heat transfer fairly simply.
          Likewise, you can define a closed system which includes your primordial gas nebula. Your gas cloud is free to cool and congeal into a nice, tidy solar system. But it can only do so by transferring energy and hence, entropy across the boundary. In the process of forming your ordered system, energy is lost into the surroundings. Again, the entropy transferred to, and generated in the surroundings, more than compensates for the reduction of entropy in the solar system.

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          • #6
            Originally posted by Chogy View Post
            Thanks for the replies... I guess this is one of those things where you just have to trust the egg heads. Bit still, it makes little sense to me.

            I think a basic understanding of it is important... it helps explain the formation of our own solar system and the evolution of life in that the Earth is NOT a closed system, being bombarded by solar energy. But that got me thinking about how our own solar system formed from a homogeneous mass of gas and dust and apparently become more ordered; less entropy.
            Could one not make the argument that the Earth and its atmosphere are a closed system in that our magnetic field keeps our atmosphere from being swept away by solar winds?

            Also, I'm in no way an expert on any of this, but I used to make steam for a living to make ships move through the water, and I find it curious that nowhere in this discussion has enthalpy been mentioned. My point being that energy changes form all the time within closed systems and even crosses the barriers of that system in the form of either heat or work. How much different then a closed system in the cosmos?
            Last edited by desertswo; 08 Sep 13,, 01:08.

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            • #7
              Very tantalizing topic, but guicho ruined everything by making it sort of easy to understand. I was closing in on it here.
              To be Truly ignorant, Man requires an Education - Plato

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              • #8
                Fast forward it to several billion years, then Hydrogen will burn out in the star and planets will fall on it. You have nothing but cold body in the center with maximum entropy. Everything else in the middle between dust cloud at beginning and cold body in the end is nothing but transfer of potential energy of gravaty in to radiation energy which will leave the system.
                Last edited by NUS; 08 Sep 13,, 13:40.
                Winter is coming.

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