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Is the natural behavior of matter to lose energy?

  1. Feb 2, 2015 #1
    Hello everyone this is my first post here. I'm sorry if my spelling is terrible.

    I am no physicists so I would like to ask if my thinking was going in the right direction.
    When I say energy I really mean electromagnetic radiation. The earth receives it constantly from the sun but there is no constant increase of the earth's temperature. Presumably because it's radiated in to the cosmos. I also know that all matter radiates depending on it's temperature.
    Know that is leading me to think that in a closed system with no input of radiated energy all the matter would drop down to the lowest energy state?

    Also I am interested to now if that is true then what implications does it make on Newtons first law of motion (also I have no idea if that law holds in general relativity)? Is it possible that a body flying trough proposed closed system will not change it's speed as a whole but the individual atoms would drop to 0 K. Sounds logical but then again not really, just more questions.
     
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  3. Feb 2, 2015 #2

    mfb

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    If the system is closed, then no energy can escape. Over time, everything would reach the same temperature. If you have a lot of empty space in your system, this temperature can be very low, but it won't be zero. If you put an object into space far away from stars, it will reach a temperature of about 3 K, then it is in equilibrium between its own outgoing radiation and the incoming radiation.
    None. Temperature is related to unordered motion within the object, the net motion of the object as a whole has nothing to do with temperature.
     
  4. Feb 2, 2015 #3
    Thank you for replaying (and moving the thread, that was my original intention :) ).
    So I'm correct in assuming that matter will lose energy if there is no incoming radiation ?
    I get that energy can't escape a closed system I just didn't considered that some of that radiation might come back. But that is a consequence of the system being finite?
    If it was infinite original matters temperature should converge to 0 K ?
     
  5. Feb 2, 2015 #4

    mfb

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    If it has a temperature above 0 K, yes.
    Right.
    Not if you have a non-zero energy density everywhere, like we have in space today (mainly from the cosmic microwave background).
     
  6. Feb 8, 2015 #5
    Read up a little bit on the Law of Entropy ... 2nd Law of Thermodynamics. :)
     
  7. Feb 8, 2015 #6

    haruspex

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    Not so fast. You appear to be thinking of black body radiation. That's an idealised circumstance.
    In practice, emission / absorption of electromagnetic radiation requires the acceleration of a charge. Diatomic gases in the atmosphere do not readily do either at their ambient temperatures.
    Each molecule is electrically neutral, so there's no net acceleration of charge during molecular collisions.
    The atoms within the molecule are also electrically neutral, so there's no net charge acceleration in the spinning of the molecule or in the internal vibrations.
    To get these molecules involved in EMR, you need to go to ionising energy levels. These are higher than visible light energies. This explains why the atmosphere is clear and, in turn, forms part of the explanation of why our eyes are sensitive to the frequency ranges that they are. (No point in being sensitive to frequencies that don't penetrate the atmosphere.)
    Triatomic gases, such as H2O and CO2, are another matter. They have vibrational modes which do involve charge acceleration. This is why they can absorb and emit certain bands within the IR range.
    Metals have free electrons. These can easily undergo accelerations which interact with any frequency. Of course, the interaction often consists of reflection, though.
    The surface of a planet is generally such a mishmash of complex molecules that it can be largely treated as a 'black body', absorbing anything, and correspondingly emitting according to whatever thermal energy it has.
    To add a little to mfb's answer, emission and absorption are highly symmetric. The facility with which a given substance at a given temperature emits EMR of a given frequency equals that with which it would absorb the same frequency. 'Emissivity' covers both. That is why a steady state can be reached in which the bodies emit EMR in exactly the same power distribution as they absorb it from the ambient space. (But note that if they are distinctly not black bodies, the bodies could theoretically settle at different temperatures. You could have one subset only emitting and absorbing in one narrow band, with another only doing so in a separate narrow band.)
     
  8. Feb 8, 2015 #7

    mfb

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    Then you would need exactly zero emissivity over some large range - you won't get that. Not even the diatomic or even monoatomic gases have that. They still absorb and emit a tiny bit of radiation based on collisions.
     
  9. Feb 8, 2015 #8

    haruspex

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    Yes, that's true... the rate of interchange could be very low, but it won't vanish.
     
  10. Feb 10, 2015 #9
    Does entropy state that the amount of unusable energy increases naturally in the Universe? My physics book did a bad job explaining it for me.
     
    Last edited: Feb 10, 2015
  11. Feb 10, 2015 #10

    mfb

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    unsable is one letter away from both unusable and usable.
    The amount of unusable energy increases, neglecting some technical details.
     
  12. Feb 10, 2015 #11
    Right, thx.
     
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