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Cooling liquids by blowing

  • #1
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I just began reading Feynman lectures and it turns out that during evaporation, water molecules that move faster than average (higher temp) break away from the attraction of their neighbors leaving behind molecules that move slower than average (lower temp) and result in a cooler liquid. When a vapor molecule gets close to the surface of the water, its speed is increased by the attraction of molecules in the water and thus re-introduces heat to the liquid. By this logic, Feynman says that by blowing on soup (blowing away moist air filled with vapor molecules, and introducing dryer air) causes the soup to cool down faster. Can someone explain to me why blowing on the soup, or any liquid, wouldn't just increase the temperature due to increasing the speed of molecules on the surface (liquid moves with you blow on it) ? Or wouldn't it keep the surface hot and maintain continuous evaporation before a drop in temperature?
 

Answers and Replies

  • #2
DaveC426913
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1] The amount of energy inserted by blowing on the liquid to move it around is negligible.
2] Blowing on it does accelerate evaporation.
3] Evaporation allows the molecules with the most kinetic energy to escape into the atmo. They take heat with them.
4] Evaporation itself is an endothermic process; it takes heat to free a molecule and convert it to vapour. It gets the heat from the soup.
 
  • #3
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The speed of the air when you blow on the liquid adds negligible kinetic energy to the molecules, and results in negligible heating. But you are removing vapor molecules from the air near the surface of the liquid by blowing them away. So the rate of molecules being introduced from the now drier air above the surface decreases, and this results in a net increase in the rate of evaporation.

Chet
 
  • #4
Nathanael
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If I can add one more (hopefully not incorrect) statement:

As you said, blowing on the soup would increase the temperature at the surface. This increased temperature at the surface may give some atoms (which would have otherwise not evaporated) enough energy to evaporate. This means some of the kinetic energy you give the soup by blowing on it will be instantly taken away via evaporation. This should contribute to the negligibility of the energy given by blowing on it.
 
  • #5
haruspex
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blowing on the soup would increase the temperature at the surface.
Well, that gets into what does one mean by temperature. Generally it is considered to be the motion of the particles after subtracting out the bulk movement of the system. The flow of air created across the surface by blowing is a bulk movement.
It may help, instead, to convey just how negligible the extra energy is. Air molecules around us move at 400-500 m/s. Call it 450 m/s. An extra 1m/s from blowing only raises this to ##\sqrt(450^2+1)##m/s (Pythagoras), so even if turned into chaotic motion it only raises the temperature by ##280/450^2## degrees C.
 

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