the temperatures are the lowest. Both fog and dew disappear (evaporate) as
the air temperature rises shortly after sunrise. You also may have noticed that
electronic devices such as camcorders come with warnings against bringing
them into moist indoors when the devices are cold to avoid moisture conden-
sation on the sensitive electronics of the devices.
It is a common observation that whenever there is an imbalance of a com-
modity in a medium, nature tends to redistribute it until a “balance” or
“equality” is established. This tendency is often referred to as the driving
force,which is the mechanism behind many naturally occurring transport
phenomena such as heat transfer, fluid flow, electric current, and mass trans-
fer. If we define the amount of a commodity per unit volume as the concen-
trationof that commodity, we can say that the flow of a commodity is
always in the direction of decreasing concentration, that is, from the region
of high concentration to the region of low concentration (Fig. 3–62). The
commodity simply creeps away during redistribution, and thus the flow is a
diffusion process.
We know from experience that a wet T-shirt hanging in an open area even-
tually dries, a small amount of water left in a glass evaporates, and the after-
shave in an open bottle quickly disappears. These and many other similar
examples suggest that there is a driving force between the two phases of a
substance that forces the mass to transform from one phase to another. The
magnitude of this force depends on the relative concentrations of the two
phases. 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 is
100 percent and thus the air is saturated. In this case, there is no transforma-
tion from the liquid phase to the vapor phase, and the two phases are in
phase equilibrium.For liquid water that is open to the atmosphere, the cri-
terion for phase equilibrium can be expressed as follows:The vapor pressure
in the air must be equal to the saturation pressure of water at the water tem-
perature. That is (Fig. 3–63),
Phase equilibrium criterion for water exposed to air: (3–30)Therefore, if the vapor pressure in the air is less than the saturation pressure
of water at the water temperature, some liquid will evaporate. The larger the
difference between the vapor and saturation pressures, the higher the rate of
evaporation. The evaporation has a cooling effect on water, and thus reduces
its temperature. This, in turn, reduces the saturation pressure of water and thus
the rate of evaporation until some kind of quasi-steady operation is reached.
This explains why water is usually at a considerably lower temperature than
the surrounding air, especially in dry climates. It also suggests that the rate
of evaporation of water can be increased by increasing the water temperature
and thus the saturation pressure of water.
Note that the air at the water surface is always saturated because of the
direct contact with water, and thus the vapor pressure. Therefore, the vapor
pressure at the lake surface is the saturation pressure of water at the tempera-
ture of the water at the surface. If the air is not saturated, then the vapor
pressure decreases to the value in the air at some distance from the water
surface, and the difference between these two vapor pressures is the driving
force for the evaporation of water.
The natural tendency of water to evaporate in order to achieve phase equi-
librium with the water vapor in the surrounding air forms the basis for thePvPsat @ T150 | Thermodynamics
(a) BeforeWaterSalt(b) AfterSalty
waterFIGURE 3–62
Whenever there is a concentration
difference of a physical quantity in a
medium, nature tends to equalize
things by forcing a flow from the high
to the low concentration region.
Water
P vapor
vLiquid water
TFIGURE 3–63
When open to the atmosphere, water is
in phase equilibrium with the vapor in
the air if the vapor pressure is equal to
the saturation pressure of water.