Process Variables: Pressure
"Pressure" is the ratio of a normal force to the area on which the force acts. Units are thus
or
.
Hydrostatic Pressure
Think of a column of fluid of height
h
and cross sectional area
A
. The fluid has a density rho. The pressure
P
at
the base of the column is by definition the force exerted on the base divided by the area
A
; that force is the
weight of the column plus any force acting on the top.
This is the formula for the pressure due to a column of fluid, or "hydrostatic pressure".
EXAMPLE:
What is the hydrostatic pressure exerted by the water in a 6.00 ft diameter cylindrical tank which
contains 90.0 gal?
If we look at the hydrostatic pressure formula, we see that we need the pressure at the top of the column, the
density of the fluid, and the height of the column.
The density we look up: 62.4 lbm/ft
3
for water
We aren't given the height of the column, but we do have dimensions on the tank. Since the volume is the
product of the area and the height, we should be able to back out the desired number.
Assume: tank has constant cross-section
Page 1 of 5
RMP Lecture Notes
8/18/2009
http://www.cbu.edu/~rprice/lectures/pressure.html

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