Pressure is defined as force per unit area. Static pressure is the pressure in or due to a non-moving fluid, such as water. A fluid does not sustain shearing forces, thus a force that compresses the object is the only force transferrable by a liquid. This fact leads to the conclusion that an object submerged in a fluid will feel only a force perpendicular to each surface.
Two kinds of pressure are commonly discussed: absolute pressure and atmospheric pressure. Atmospheric pressure is an arbitrary value based on the atmosphere of the planet, in our case, Earth. We feel a pressure exerted by the atmosphere no matter where we are or what we are doing. The average value on Earth is 1.0 x 105 Pascals or 760 mm of Hg (14.7 psi) at sea level, but it changes with altitude and weather conditions. Even though everything on the planet feels this pressure, when we use a pressure gauge, it does not register. The pressure shown by the gauge is called "gauge pressure" and it measures how much difference there is above or below atmospheric pressure. Absolute pressure is measured from absolute zero and so would always include atmospheric pressure.
French scientist Blaise Pascal was the first to recognize that pressure in a fluid varies with depth. For example, the top of a cylinder submerged in a tank of water is at the water's surface, and it has height (h), a cross sectional area, (A), and a mass, (m). The forces on the cylinder are the weight of the cylinder, the atmospheric pressure at the top, and the water pressure at the bottom. All these forces must add up to zero: PA + mg = PA.
Since we know the mass is dependent on the volume V and density, [b.rho ], of the object, this equation becomes: PA + [b.rho ]Vg = PA.
The volume of a cylinder is the height (h), multiplied by the cross sectional area, A. Substituting for volume and dividing by area, the equation becomes: P + [b.rho ]gh = P.
Thus, the pressure, P, at the bottom of the cylinder is greater than the atmospheric pressure, P, at the top by an amount equal to [b.rho ]gh. This is known as Pascal's law. It is only dependent on the height of the object (or depth in the fluid); it is not dependent on the area. Also, the forces perpendicular to the weight of the object are not included; the net force in that direction is always zero for static fluids.
Evangelista Torricelli invented a simple device to measure atmospheric pressure; the barometer. A tube with one end open is filled with liquid then turned upside down in a bowl of the same liquid. The liquid flows out of the tube until the atmospheric pressure and vacuum in the tube are sufficient to support the weight of the column. Since it is the densest liquid readily available, mercury is usually used in a barometer a water barometer would need to be several stories tall. Knowing P=0 in a vacuum, the density of the liquid, and the height of the column, the atmospheric pressure can be measured using Pascal's law.
Another simple device to measure pressure also uses Pascal's law. The manometer is a U-shaped tube with two open ends. One end is open to atmospheric pressure, while the other is open to an unknown pressure. The liquid in the tube will come to equilibrium with a height difference proportional to the pressure difference. Knowing the value for atmospheric pressure, the height, and the density of the liquid (usually water), the unknown pressure can be calculated.
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