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Sound waves versus equilibrium

  1. Aug 11, 2011 #1
    Postulate a parcel of air that is transmitting sound waves. Can that parcel ever be considered to be in a state of equilibrium? I maintain that it cannot. How say you?
  2. jcsd
  3. Aug 11, 2011 #2
    What is your definition of equilibrium?

    Match this against the situation and post your answer.
  4. Aug 11, 2011 #3
    Fair enough. Kinetic gas theory says a gas is in a state of equilibrium when its entropy has been maximized. I maintain that a gas transmitting sound waves is not in a state of maximum entropy. Consequently, the Ideal Gas Equation of State at Equilibrium would not apply to that parcel of air.
  5. Aug 12, 2011 #4
    Not quite.

    A system is said to be in equilibrium when it has no further tendency to change its properties of interest.

    In particular considering thermodynamic equilibrium (it is not the kinetic theory by the way) a system is isolated if the internal energy and volume are constant.

    It can be shown that in an isolated system (with constant U and V) the entropy tends to a maximum.

    So yes for an isolated system in equilibrium, the entropy is a maximum

    Is your gas an isolated system?
  6. Aug 12, 2011 #5
    Kinetic Gas Theory makes a whole range of assumptions which are not completely true. Complete elasticity of collisions is ALMOST completely true, but not quite.
  7. Jan 2, 2014 #6

    Simon Bridge

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    We would not expect a volume of air with sound waves passing through it to satisfy the OPs definition of being in equilibrium because the volume of air is not an isolated system.
    Why would anyone think it might be?

    It is not clear what the point of the 1st post is.
    If the intention is to start a conversation, as in social media, then isn't it more polite to kick things off yourself?
  8. Jan 2, 2014 #7
    I agree. Point taken. But why wait more than two years to bring this to my attention?
  9. Jan 2, 2014 #8

    Simon Bridge

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    Because I'm an idiot?

    I only just noticed and didn't check the date stamp - it happens :)
  10. Jan 2, 2014 #9
    No harm, no foul.
  11. Jan 2, 2014 #10

    Simon Bridge

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    Red Wizard needs coffee - urgently!
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