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Example 14.8 from College Physics, 14.6 Convection
The average person produces heat at the rate of about 120 W when at rest. At what rate must water evaporate from the body to get rid of all this energy? (This evaporation might occur when a person is sitting in the shade and surrounding temperatures are the same as skin temperature, eliminating heat transfer by other methods.)
Work it out on paper first. Then open the solution one step at a time, and stop as soon as you can finish on your own.
Energy is needed for a phase change (). Thus, the energy loss per unit time is
We divide both sides of the equation by to find that the mass evaporated per unit time is
(1) Insert the value of the latent heat from Table 14.2, . This yields
Evaporating about 3 g/min seems reasonable. This would be about 180 g (about 7 oz) per hour. If the air is very dry, the sweat may evaporate without even being noticed. A significant amount of evaporation also takes place in the lungs and breathing passages.
How did it go?