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Third law of thermodynamics

  1. Jul 14, 2010 #1
    The are at least two different formulation of third law of thermodynamics:
    (1) it's impossible to cool a body to absolute zero by any finite process,
    (2) as a system approaches absolute zero, all processes cease and the entropy
    of the system approaches a minimum value.
    I don't know why they are equivalnet. Can anyone explain it to me?
  2. jcsd
  3. Jul 14, 2010 #2


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    Generically, cooling something means making it stand still. You're taking the kinetic energy away from it (your table, chair, and even the snow at -40 F in my backyard in the winter all have rapidly moving molecules in them and lots of energy compared to something near 0 K).

    Another way to put it, you're ordering the system... reducing it's entropy. If the system reaches a state where it's at its minimum entropy... i.e. minimum disorder... i.e. maximum order, than you can't get it to "sit still" anymore than that. It's as ordered as it's going to get.
  4. Jul 15, 2010 #3
    My old thermodynamics lecturer summed up the three laws of thermodynamics thus
    You cant win
    You can't even break even
    You cant even leave the game
  5. Jul 15, 2010 #4


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    They are not equivalent, see Landsberg, "Thermodynamics and statistical mechanics", Dover Publ.
  6. Jul 15, 2010 #5
    which chapter?
  7. Jul 15, 2010 #6
    I like it :rofl:

    Notice the "minimum value" in (2). That's the same as "impossible to cool a body to absolute zero" in (1). If you could get something to absolute zero, then there is no "minimum value".
  8. Jul 15, 2010 #7


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    Jobrag, were you at Sussex? My lecturer there used those exact phrases. His name was John Barrow.

  9. Jul 16, 2010 #8


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    I had a lecturer who said that to. I think it's fairly common.
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