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The presence
of gravity provides another force on the walls of a container holding
a fluid. Molecules in the fluid have to provide a force that supports the weight
of the fluid above them. The pressure at the top of a container will, therefore,
be lower than the pressure at some depth within the fluid. The hydrostatic
pressure due to this overburden of fluid depends upon the depth below
the surface and the density of the fluid. In the diagram, the hydrostatic pressure
at the bottom of all the containers shown in (a) is hrg
(where r is the density of the
fluid which is assumed to be incompressible) and is independent of container
geometry. For the tubes shown in (b) the fluid in both arms will be at the
same height unless there is a difference in external pressure acting on the two
limbs. As shown in the right hand diagram, a higher external pressure (X)
will push down the fluid on that side. The overpressure is measured by the
height difference between the two limbs, h, and is just the hydrostatic pressure
produced by this length of liquid column, hrg. This
type of device, a manometer, provides a common pressure measuring technique.
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