**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**

Now
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 P_{ref} then we can express any other pressure as a dimensionless number P^{*} .

** P ^{*}= P / P_{ref} = 7.5/10 = 0.75**

As shown above, with P_{ref} = 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*

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