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Hydrogen atom as an example of perpetual motion

  1. Sep 1, 2009 #1
    Hi All,

    Thermodynamics forbids perpetual motion based in its second law, but how should one deal with an isolated H atom, seen here as a machine? Is there a concrete perspective of its stopping? In its stationary and fundamental state may one observe dissipation?

    Thank you

  2. jcsd
  3. Sep 1, 2009 #2


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    It does not forbid perpetual motion so much as extracting work from perpetual motion.

    But in any case, a hydrogen atom is not at all like a machine. It has no moving parts. The electron is in something rather like a standing wave around the proton. You don't extract work from it; there's no dissipation; and there's no violation of the second law.

    Cheers -- sylas
  4. Sep 1, 2009 #3
    I was expecting to hear this (expected values of momentum is zero). But before continuing, are you saying that physics allows perpetual motion to exist?

    Best wishes

  5. Sep 1, 2009 #4


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    What do you mean by perpetual motion? An object moving under no forces moves perpetually in a straight line (principle of inertia). Is that what you mean? If so, such motion is allowed by physics. What is not possible according to our current physics is violations of the conservation of energy and violations of the second law of thermodynamics. When physicists say perpetual motion is impossible, this is what they are referring to.
  6. Sep 1, 2009 #5


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    Consider Newton's first law: an object in motion will stay in motion unless acted upon by an outside force.
  7. Sep 1, 2009 #6
    A said perpetual motion is not a problem while getting work from it is.

    Best ideas for you all. Zap
  8. Sep 1, 2009 #7
    In pragmatic terms, there is no such animal as an isolated hydrogen atom--ever. It this regard, it is nonphysical.
  9. Sep 2, 2009 #8


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    How is this so?

    Or more to the point, how does it alter the discussion?
    Last edited: Sep 2, 2009
  10. Sep 2, 2009 #9
    The whole universe is in perpetual motion too.
    Many parts of the universe, like the solar system, are also in perpetual motion except for some faint influence from their distant surrounding.

    Concerning the H atom, when it is in an excited state, there is clearly a perspective of getting eberny from it. This occurs spontaneously. Only the lowest level of energy is indefinitively stable. This is the fingerprint of quantum mechanics. As you know, in classical mechanics, the electron would fall on the proton emitting an infinite amount of radiation.

    We should forget about these prehistoric concepts like perpetual motion and their use to formulate the second principle.
    They just reflect the difficulty encountered by the pioneers to get the law formulated in general terms without knowing what was really going on. Today (and initiated by Gibbs) we have an understanding of all this at the microscopic level.
    Last edited: Sep 2, 2009
  11. Sep 2, 2009 #10


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    But it can only be found in its exited state after enegry is input. The enegry extracted can never equall the energy put in, so prohibition against perpetual motion (by the scientific and legal definition) remains upheld.
  12. Sep 2, 2009 #11
    A hydrogen atom is never in a pure state. It's always an evolving mixed state.
  13. Sep 2, 2009 #12


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    I don't even know what you mean by "pure state" and "mixed state" or why you think this has any bearing on the question.

    Pragmatically, you do have isolated hydrogen atoms. Your earlier use of the word "pragmatically" was turned around from normal usage, as I understand it. For all practical purposes, a hydrogen atom alone in space is isolated, and any notions of "motion" you might associate with an isolated hydrogen atom stable and unending.
  14. Sep 11, 2009 #13
    Notice: as gravitation is widely spread in universe, it is quite unlikely for a system to be an example of such case (expressed in first law). Otherwise you will have a rather artificial system.
  15. Sep 11, 2009 #14
    Hi Phrak,

    I agree, but my question is somewhat concerned with this. If you put an hidrogen atom in a calm portion of universe you may expect this atom to suffer almost no change in centuries, despite gravitation and weak EM fields acts on it all the time.
    It seems impressive to me that an atom of H preserves so well the dynamical relation between its proton and its electron.

    Best regards

  16. Sep 11, 2009 #15
    Maybe you should take my words in the sense of systems which present cyclical and self similar dynamics.

    Best Regards

  17. Sep 11, 2009 #16


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    The second law of thermodynamics? Perpetual motion was not used to formulate that. And it's just as relevant today.

    The very second sentence of Wikipedia's article on 'perpetual motion' makes it all quite clear: "the term more commonly refers to any device or system that perpetually (indefinitely) produces more energy than it consumes, resulting in a net output of energy for indefinite time."

    And for all practical purposes, 'perpetual motion' in the literal sense of what people were actually trying to build is not possible either.

    Perpetual motion in the either sense not prohibited by the second law of thermodynamics. Perpetual motion in the more common sense is prohibited by the first; conservation of energy.

    Energy is not conserved?
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