10_1425_web_Lec_32_MoreKineticTheory

10_1425_web_Lec_32_MoreKineticTheory - More Kinetic Theory...

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More Kinetic Theory of Gases Physics 1425 Lecture 32 Michael Fowler, UVa
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Vapor Pressure and Humidity The H 2 O molecules in liquid water strongly attract each other, holding the liquid together. But these molecules are still jiggling around, with a Maxwell speed distribution. This means a fraction of them near the surface are moving fast enough to escape , forming a vapor above the surface . In a closed container, with enough water present, an equilibrium situation is reached between escaping and returning molecules.
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Water and Vapor in Equilibrium In equilibrium in a closed container, the molecules in the vapor have the same average kinetic energy as the air molecules, so exert pressure on the walls of the container proportionate to their numbers. This is the saturated vapor pressure . It varies with temperature like e a / T , not surprising since its origin is molecules fast enough to escape. a Air + water vapor Water
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Water Vapor Pressure At room temperature, saturated vapor pressure is about 2.5% of atmospheric pressure. At 100 ° C, it equals atmospheric pressure: this means small bubbles formed in the liquid by fast moving molecules coming together are no longer crushed by the surrounding atmospheric pressure, the water boils. In mountain resorts like Aspen, water boils at a lower temperature, producing inferior tea.
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Relative Humidity and Dew Point If water is constantly boiled off in a closed room, there’s a limit to how much water vapor the air can hold: it becomes saturated . That limit depends on the temperature. Attempts to add more water result in condensation on the walls, fog formation, etc. At this point, relative humidity = 100%. Relative humidity = vapor pressure/saturated vapor pressure Dew point: temperature at which dew forms—that is, water condenses out as the air cools. (For given vapor pressure, dew point is found from tables.)
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Pressure Cooking A pressure cooker works at a gauge pressure of about one atmosphere: that is, inside the cooker, pressure is about 2 x 10 5 Pascals.
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10_1425_web_Lec_32_MoreKineticTheory - More Kinetic Theory...

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