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I Why do people ignore sweat dropping off as a cooling mechanism

  1. Jun 22, 2017 #1
    Hello,

    I understand how evaporation contributes significantly to heat loss through the loss of KE from the vibrating water particles on the skin; However, what about the sweat droplets that come off of a person. This water has Kinetic Energy and is now being displaced from the body(the system)? Perhaps it is a form of decreasing heat from the body but maybe not as significant as evaporation?
     
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  3. Jun 22, 2017 #2

    sophiecentaur

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    The essential requirement for 'cooling' is not to lose heat but to reduce temperature (reduce the mean internal energy of the body).
    If the water droplets are at the same temperature as the body, losing them will not affect the temperature of the body any more than cutting the body in half will affect the temperature of each half (:wink:).
    If the droplets are already cooler due to evaporation, then their cooling effect will be less due to their leaving than it they had stayed behind and reached equilibrium temperature with the body.
     
  4. Jun 23, 2017 #3
    Hello Sophiecentaur,

    I am still having a slight bit confusion. I feel I am very close to understanding this completely.


    1. during evaporation, the kinetic energy of the water molecule reaches a point in which it escapes into the atmosphere as gas. The water molecule had absorbed the heat from the body and additionally, absorbed energy from the sun. Therefore, we have the body's temperature being transferred some into the water droplet and as the water droplet leaves as gas, the logic follows that the body temperature is also lowered?

    2. If the logic from above is correct, then the sweat water that absorbed the body's heat dripping off the person will also cause the body temperature(the system) to lose heat or KE as the KE sweat droplet leaves the system?

    Maybe I am not understanding the fundamental reason how and why evaporation cools the body at the level of particles?

    I always thought losing heat was considered lowering the temperature? Heat gets exchanged and moves from high to low, and thus heat is lost and temperature is lowered.
     
  5. Jun 23, 2017 #4

    berkeman

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  6. Jun 23, 2017 #5

    russ_watters

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    Yes, but since the sweat drop that drips off is at the same temperature as the body, it doesn't take any additional thermal energy with it besides what is proportional to its mass/due to its temperature. Consider a pot of tea on the stove: when you pour some out, does what's left have a lower temperature?

    The evaporated sweat, on the other hand, takes away the heat of vaporization, which is considerable.
     
  7. Jun 23, 2017 #6
    Evaporation is caused primarily by concentration differences, rather than temperature differences. The concentration of water molecules at the interface between the liquid water and the surrounding air is higher than the concentration of water molecules in the bulk of the room air. So there is a driving force for diffusion of water molecules away from the interface into the room. This flow of mass is what we call evaporation. The diffusional flux of water vapor molecules must be replenished by liquid water molecules changing to water vapor molecules at the interface. But energy is required to bring about this phase change. This energy is supplied mainly by the liquid water itself, which cools as the water evaporates. The rate at which the water molecules diffuse from the interface to the bulk room air is enhanced by causing the air to blow past the surface. This increases the concentration gradient of the water molecules near the interface, and thus the diffusion rate. This all would happen even if the skin were perfect insulation. But, since the skin is not insulation, the cooler liquid water on the skin causes heat to flow from the hotter skin to the cooler sweat. This results in the sweat cooling less, and this helps maintain a higher sweat temperature and concentration of water vapor at the interface (and thus, a higher evaporation rate). This is how evaporation causes your skin to get cooler.
     
  8. Jun 23, 2017 #7
    Thank you sophiecentaur, russ_waters, chestermiller for all your help! I finally had the aha moment. All your comments help put together the picture.
    Physicsforums is the best!!
     
  9. Jun 24, 2017 #8

    sophiecentaur

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    I just thought of this thought experiment. Say you have a block of ice in your hand. Intuitively, will it cool you better if you throw it away or if you hang onto it? The block of ice is just an extreme version of a drop of cool sweat.
     
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