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How many are there?

  1. Aug 4, 2005 #1
    How many laws of thermodinamics are there?

    This topic came up during one of our last hours of higschool physics. Some of us sead that there are 4 and some sead that there are only 2. In different books we also found different answeres. So now I realy don't know which statement is corect. Any ideas?
  2. jcsd
  3. Aug 4, 2005 #2
    There are four laws of thermodynamics but they are numbered funny.

    The first three were discovered, then someone decided there was an implicit assumption in those 3 that required a definition of temperature, so the fourth law discovered is numbered zero.
  4. Aug 4, 2005 #3

    Claude Bile

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    0th law - If a system, C is in thermal equilibrium with two other systems, A and B, then A and B must also be in thermal equilibrium.

    1st law - Heat = Change in Internal Energy + Work done on the system. (Conservation of energy effectively).

    2nd law - Entropy always increases with time.

    3rd law - No object can ever reach a temperature of absolute zero.

    A nice discussion of laws 1-3 can be found here.

  5. Aug 5, 2005 #4
    1st Law: You can't win; you can only break even

    2nd Law: You can only break even if you get to absolute zero.

    3rd Law: You can't get to absolute zero.
  6. Aug 5, 2005 #5
    you are forgetting a law here, see the above post

  7. Aug 5, 2005 #6
    But i don't know why we can not reach the zero absolute temperature. Who can explain?
  8. Aug 7, 2005 #7

    Claude Bile

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    Essentially, because a particle at 0K violates the HUP. Google the 3rd law of thermodynamics and you should find some satisfying explanations.

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