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Getting cooled by evaporation

  1. Jun 23, 2006 #1

    daniel_i_l

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    I know that the conventional explanation for this is that the water needs energy to evaporate so it conviniantly takes the heat energy from your body in order to do this. But first of all, why does the water need to evaporate in the first place? Because of the sun? Then it should be getting energy from the sun not from your body? I hope that you guys understand the question, this always seemed so obvious to me but then when I thought it over I got confused.
    Thanks!
     
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  3. Jun 23, 2006 #2

    HallsofIvy

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    The energy required to evaporate is from the water itself. Water "evaporates" when it changes from liquid into a gas. Remember that different molecules in a drop of water will have different speeds- the temperature is a function of the average speed. Even if the average molecular speed (i.e. temperature) is low enough that the water stays in liquid form, some of the molecules will have speed high enough to escape from the drop. Since it doesn't come back, that reduces the average molecular speed and so the temperature of the water.
     
  4. Jun 23, 2006 #3
    So the water itself is the thing that cools you? The water gets cooler and then so do you? I think I understand now. I always incorrectly thought the water was sucking the heat out of your body somehow.
     
  5. Jun 23, 2006 #4

    Hootenanny

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    No, the water does not get cooler. It takes energy to change the state of the water (evaporate it). This energy comes from your skin, you transfer thermal energy from your skin, to the sweat this increases the internal energy of the sweat and it evaporates. I will say again, the sweat does not get cooler.
     
  6. Jun 23, 2006 #5
    Now I am in trouble. The experts disagree. Mr. Halls of Ivy said that the most energetic water molecules would leave droplets - not by virtue of my body heat per se, but because the water vapor content of the surrounding air was lower. The energetic particles leaving the droplet would leave the cooler particles behind.
     
  7. Jun 23, 2006 #6

    Hootenanny

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    Sorry I should have made my point clearer. I meant that the water which evaporates does not get cooler. I hadn't read HOI's post, I just skipped to yours. The water that evaporates does not get cooler, but the water that it left behind does. Apologies for the confusion.
     
  8. Jun 24, 2006 #7
    Perspiration is a mechanism by which body temperature, of warm blooded animals, is maintained. When the ambient temperature is greater than 98.4F, heat transfers from ambience to the human body and thus raises the body temperature. Body pushes out sweat from sweat glands, this water absorbs heat from human body and evaporates.

    For water to evaporate from human body, the partial pressure of water vapor in the air should be less than that of saturated partial pressure of water at ambient temperature. In simple terms, RH should be lower than 100%. When RH is 100%, sweat will not evaporate and becomes visible on your body.
     
  9. Jun 24, 2006 #8

    daniel_i_l

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    So do you get cooled down because the heat gets sucked out of your body, or because the water itself cools down, I've always read that it's the later, but if it's the former then why doesn't the energy source (sun) heat up the rest of the "cold" water and make it hot to?
     
  10. Jun 24, 2006 #9

    Hootenanny

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    It is the evaporation of the sweat that cools you. If the sweat didn't evaporate it wouldn't cool your body down, that is you generally feel hotter in a humid, the air is already saturated (or close to) with water, so your sweat cannot evaporate.
     
  11. Jun 24, 2006 #10

    daniel_i_l

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    Yes but's that's my question, how does the evaporation cool you down? Thanks.
     
  12. Jun 24, 2006 #11

    russ_watters

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    The first reply (and several others) contains the answer: the evaporating sweat carries away heat, leaving your skin cooler.
     
  13. Jun 24, 2006 #12

    Hootenanny

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    I suggest that you re-read this
     
  14. Jun 26, 2006 #13
    We can check it by an example calculation. Consider a 60kg person in a 40deg.C ambience and got his body temperature raised to 40deg.C before perspiration starts (actually this is quite dynamic phenomenon and the body doesn't wait to calculate the quantity of sweat required).

    Average specific heat of human body is about 3.5 kJ/kg deg.C.

    For a 3deg.C drop of body temperature, quantity of heat to be removed is 60x3.5x(40-37) = 630 kJ. (quantity of to be evaporated is 630/2310 = 0.27 kgs.

    If this quantity of heat is to be supplied by water (itself) at 40 deg.C, the quantity of water required is 630/(4.2 x (40-37)) = 50 kgs.

    So the heat is transferred from the body to the evaporating water and you feel cool as heat is removed from the body.

    This is different from cooling phenomenon of hot water in a container, where water gets heat from rest of the water and thus it gets cooled.
     
  15. Jun 26, 2006 #14

    rcgldr

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    I thought evaporation was just very small droplets of water being asorbed by the air, not generation of steam. Maybe vapor would be a better description than gas.
     
  16. Jun 26, 2006 #15

    Hootenanny

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    Yes, technically sweat doesn't evaporate is vaporises.
     
  17. Jun 26, 2006 #16

    russ_watters

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    Huh? "Vaporize" and "evaporate" are two forms of the same word root, and the meaning of both is "to change into a vapor"

    http://dictionary.reference.com/browse/evaporate
    http://dictionary.reference.com/browse/vaporize

    And a vapor is a gas - it is not suspended particles of liquid water. If it were just droplets of still liquid water , then there'd be no energy loss due to sweating.

    http://dictionary.reference.com/browse/vapor

    "The gaseous state of a substance that is liquid or solid under ordinary conditions."
     
  18. Jun 26, 2006 #17

    rcgldr

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    So water can be steam at room temperature, which is well below boiling point?
     
  19. Jun 26, 2006 #18

    russ_watters

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  20. Jun 26, 2006 #19

    rcgldr

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    Unless I'm mis-reading the graph, this only happens at very low pressure. At one atmosphere of pressure, or about 10^5 Pa, the chart shows vapor at 373K, the normal boiling point of water. I guess vaporization is a different type of action.

    I had the impression that there wasn't much difference between mist, fog, or low level clouds (one's not composed of ice).
     
    Last edited: Jun 26, 2006
  21. Jun 26, 2006 #20

    russ_watters

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    Pressures of mixed-gases are treated separately, as described by the concept of "partial pressure" (or vapor pressure): http://en.wikipedia.org/wiki/Partial_pressure
    So, at 300K, in a sealed container, liquid water will evaporate until the vapor reaches a pressure of about 2kpa, then the air will be saturated and the water and water vapor in equilibrium.
     
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