Thermodynamics

cen84959_ch03.qxd 4/1/05 12:31 PM Page 150150 | Thermodynamics(a) BeforeWaterSalt(b) AfterSaltywaterFIGURE 3–62Whenever there is a concentrationdifference of a physical quantity in amedium, nature tends to equalizethings by forcing a flow from the highto the low concentration region.P vLiquid waterTWatervaporFIGURE 3–63When open to the atmosphere, water isin phase equilibrium with the vapor inthe air if the vapor pressure is equal tothe saturation pressure of water.the temperatures are the lowest. Both fog and dew disappear (evaporate) asthe air temperature rises shortly after sunrise. You also may have noticed thatelectronic devices such as camcorders come with warnings against bringingthem into moist indoors when the devices are cold to avoid moisture condensationon the sensitive electronics of the devices.It is a common observation that whenever there is an imbalance of a commodityin a medium, nature tends to redistribute it until a “balance” or“equality” is established. This tendency is often referred to as the drivingforce, which is the mechanism behind many naturally occurring transportphenomena such as heat transfer, fluid flow, electric current, and mass transfer.If we define the amount of a commodity per unit volume as the concentrationof that commodity, we can say that the flow of a commodity isalways in the direction of decreasing concentration, that is, from the regionof high concentration to the region of low concentration (Fig. 3–62). Thecommodity simply creeps away during redistribution, and thus the flow is adiffusion process.We know from experience that a wet T-shirt hanging in an open area eventuallydries, a small amount of water left in a glass evaporates, and the aftershavein an open bottle quickly disappears. These and many other similarexamples suggest that there is a driving force between the two phases of asubstance that forces the mass to transform from one phase to another. Themagnitude of this force depends on the relative concentrations of the twophases. A wet T-shirt dries much faster in dry air than it would in humid air.In fact, it does not dry at all if the relative humidity of the environment is100 percent and thus the air is saturated. In this case, there is no transformationfrom the liquid phase to the vapor phase, and the two phases are inphase equilibrium. For liquid water that is open to the atmosphere, the criterionfor phase equilibrium can be expressed as follows: The vapor pressurein the air must be equal to the saturation pressure of water at the water temperature.That is (Fig. 3–63),Phase equilibrium criterion for water exposed to air: P v P sat @ T (3–30)Therefore, if the vapor pressure in the air is less than the saturation pressureof water at the water temperature, some liquid will evaporate. The larger thedifference between the vapor and saturation pressures, the higher the rate ofevaporation. The evaporation has a cooling effect on water, and thus reducesits temperature. This, in turn, reduces the saturation pressure of water and thusthe rate of evaporation until some kind of quasi-steady operation is reached.This explains why water is usually at a considerably lower temperature thanthe surrounding air, especially in dry climates. It also suggests that the rateof evaporation of water can be increased by increasing the water temperatureand thus the saturation pressure of water.Note that the air at the water surface is always saturated because of thedirect contact with water, and thus the vapor pressure. Therefore, the vaporpressure at the lake surface is the saturation pressure of water at the temperatureof the water at the surface. If the air is not saturated, then the vaporpressure decreases to the value in the air at some distance from the watersurface, and the difference between these two vapor pressures is the drivingforce for the evaporation of water.The natural tendency of water to evaporate in order to achieve phase equilibriumwith the water vapor in the surrounding air forms the basis for the