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Cooling without work?

  1. Feb 18, 2013 #1
    if you hold a pitcher of water under sun you sea cooling of water in it, you transfer heat from hot to cool without do work, how do you explain it according with second law of thermodynamics?
  2. jcsd
  3. Feb 18, 2013 #2


    Staff: Mentor

    No, the water gets warmer, not cooler. Especially if your pitcher is sealed to prevent evaporation.

    Yes, heat goes from the hot body to the cold body without work. That is what the 2nd law of thermo says should happen.
  4. Feb 18, 2013 #3
    if there is evaporation it cools and heat transfer from cold to hot how do u explain it?
  5. Feb 18, 2013 #4


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    Staff: Mentor

    If there is evaporation, you do not have a closed system. Evaporation makes the entropy increase.
  6. Feb 19, 2013 #5
    The molecules in the water are doing the work of vaporizing the blob of water. The molecules use their kinetic energy to do that.

    kinetic energy of molecules= heat, more heat = higher temperature
    being vaporized = latent heat, more latent heat = no change of temperature

    The heat energy of the water is turning into the latent heat energy of the water.

    The temperature of the water is going down. There's no rise of temperature of anything. There is no change of energy of anything.
  7. Feb 19, 2013 #6


    Staff: Mentor

    If there is evaporation then the air is initially too dry, which is a low-entropy state. The evaporation continues until the water vapor in the air is at the vapor pressure, which is a high-entropy state. Thus the entropy can increase even though the temperature decreases.
  8. Feb 19, 2013 #7


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    Staff: Mentor

    In addition, you transfer a lot of energy from a very hot system (the sun) to a cold system (anything on earth).
  9. Feb 19, 2013 #8
    for heat to flow from hot to cold body there should be driving force which is temp difference..

    but evaporation is case of mass transfer..in which driving force is difference between the vapor pressure and saturation pressure of water at that temp. As the difference increases evaporation increases and finally comes to an equilibrium state when two pressure become equal.

    Clausius statement is that no heat can be transferred from cold to hot until work is done.
    work is done to create the driving force( temp difference). here when air whose relative humidity is less then 100% comes,it swept away vapors with it from liquid surface thus lowering the vapor pressure and creating a driving force.

    in the fig,cylinder is insulated from sides.when vapor pressure decreases,molecules at surface of water extract energy from neighboring molecules and become vapors,thus lowering the over all energy content of liquid water(temp).

    Attached Files:

  10. Feb 20, 2013 #9
    according to clasius statement there must be work, where is work?
  11. Feb 20, 2013 #10


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    Science Advisor

    That has already been explained:
  12. Feb 20, 2013 #11


    Staff: Mentor

    This is a distortion of Clasius' statement. His statement was "Heat can never pass from a colder to a warmer body without some other change, connected therewith, occuring at the same time." See p 117 here. Evaporation is "some other change, connected therewith".
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