Pressure

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  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

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