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Laws of Evaporation

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  P-270

2025-11-24

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1. Evaporation increases with the temperature. Evaporation takes place even at very low temperatures. A block of ice left for a few days in a place where the temperature is below zero will lose a considerable amount by evaporation. Wet clothes hung out on a cold winter day will freeze at once, but will soon become dry.
2. Evaporation increases if the surface of the liquid is increased. Recent experiments show that evaporation is not directly proportional to the extent of the surface. Evaporation takes place more rapidly near the boundaries of a surface than at the center, and in the case of two circular surfaces the evaporation is nearly proportional to the respective circumferences.
3. Evaporation is inversely proportional to the pressure upon the liquid.
4. Evaporation decreases as the air becomes saturated.

Air is said to be saturated with moisture when it will hold no more at the given temperature. If the temperature is raised, more evaporation can take place, but if it is lowered, condensation will take place; that.is, some of the vapor will be changed back to tiny water drops or ice crystals. At any given temperature the amount of vapor necessary for saturation is always the same per cubic meter of space, no matter how much air is also in that space.

If the space into which evaporation takes place is inclosed and the air is removed, the evaporation takes place rapidly, and saturation is quickly reached. If the air is not removed, the evaporation takes place much more slowly, because the air molecules, striking upon the surface of the water, cause a pressure upon the surface and oppose the molecules that

are coming from the liquid. In the first case the final pressure is that of the saturated vapor alone. case it is that of the air plus that of the saturated vapor. The effect of atmospheric pressure upon the rate of evaporation is so great that the moisture present in the air is generally much less than that required for saturation.