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Why the temperature of a moving jar of gas doesn't increase

  1. Aug 11, 2016 #1
    1. The problem statement, all variables and given/known data
    Why doesn't the temperature of a moving jar of gas increase?

    2. Relevant equations
    Average KE is proportional to temperature

    3. The attempt at a solution
    This is just an extension of a problem my friend sent me, and I'm stuck on why even though average velocity is increasing, the temperature does not increase.
     
  2. jcsd
  3. Aug 11, 2016 #2

    Doc Al

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    Think about the reference frame that makes sense to use when talking about the random KE of the molecules of an object.

    And realize that the macroscopic velocity of an object is always relative to something. Relative to your chair, you are at rest. Relative to a moving train, you might be moving at 70 mph. Relative to the sun... and so on.
     
  4. Aug 11, 2016 #3
    The frame in which the average velocity is zero, so the jar.
     
  5. Aug 11, 2016 #4

    Doc Al

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    Right. (Also called the center of mass frame.)
     
  6. Aug 11, 2016 #5
    Ah thanks, makes much more sense than my friends explanation haha
     
  7. Aug 11, 2016 #6

    haruspex

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    So shouldn't the question state, just to be clear, that the thermometer is moving with the jar?
     
  8. Aug 11, 2016 #7

    Doc Al

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    Good point. :biggrin:
     
  9. Aug 12, 2016 #8
    the question makes no reference to thermometers!
     
  10. Aug 12, 2016 #9

    haruspex

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    Quite so, but how is the temperature to be measured/defined? My point is that it affects the answer.
    If the gas is heteropolar, you would be able to measure its temperature externally by infrared radiation. This would show an elevated temperature if you stand in front of it but a lower one if measured from behind, because of the Doppler effect. Measuring all around and averaging, you would still see a slightly higher temperature because of the increased total energy (KE).
     
  11. Aug 12, 2016 #10
    I am viewing temperature (of a gas) as the average translational KE of molecules which has nothing to do with how temperature is measured. It is a mathematical concept from kinetic theory. What thermometers 'measure' is a different matter.
     
  12. Aug 12, 2016 #11

    haruspex

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    The usual view is that temperature is a measure of the thermal energy. Bulk motion does not contribute to that.
    The question setter clearly agrees with that. Your view makes the question wrong, and makes it hard for observers to agree on temperature.
     
  13. Aug 12, 2016 #12
    thankyou for pointing out how wrong I have been.
     
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