Vapor Pressure II
As we noted previously, the osmosis process of molecular movement through a membrane is directly related to vapor pressure difference. This agrees with other models for fluid flow. Flow is directly related to pressure difference and flow proceeds from a high-pressure area to a low-pressure area.
But, what is pressure? We introduce some concepts with simple math.
Pressure is defined as a force per unit area. P = Force / Area = F / A
When we multiply any equation by one (1), the equation is unchanged. i.e.
1 = Length / Length = L / L
If we apply this to our pressure equation,we find that pressure can also be expressed as an energy per unit volume.
P = F / A = F/A * L/L =(F*L) / (A*L) = Energy / Volume
This makes sense. At the same pressure, a larger tank contains more energy.
If we rearrange the equation we get.
Energy = Pressure * Volume
we introduce one other concept. Sometimes, it is interesting to use
dimensionless values for variables such as pressure. First, we select a
reference pressure such as atmospheric pressure or vapor
pressure at a specified reference temperature call this Pref then we can express any other pressure as a dimensionless number P* .
P*= P / Pref = 7.5/10 = 0.75
As shown above, with Pref = 10, and a measured pressure of 7.5, we can express our dimensionless pressure as P*= 0.75 . Dimensionless values are very useful for communication with others that use a different unit system than you work with. i.e. metric system users can readily communicate with someone using the English system of units.
P=Pressure; A= Area; L = Length; Volume = Area * Length; Energy = work = F * L = F * distance