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Bernoulli's Principle of Pressure

Daniel Bernoulli* studied the flow of fluids (liquid or gas) as it reaches a region where the pipe diameter is reduced. Bernoulli stated that the velocity of a fluid, as it flows through the narrowest diameter is increased and that the pressure in that region is decreased.

A practical application of Bernoulli's Theorem is the Venturi Tube. A Venturi tube consists of a tube which narrows to a throat (reduced diameter). The airflow at the throat is increased while the pressure is decreased.

The temperature at the throat is also reduced, however this phenomenon is not related to the Bernoulli principle. The relationship between pressure and temperature is defined by Boyle's** law. Boyle's law states that the volume of a gas is inversely proportional to it pressure at a constant temperature. The expansion of fluid as it passes the throat causes a temperature decrease.
A simple way to demonstrate the Bernoulli's principle is illustrated in the figure below. The experiment clearly proves Bernoulli's principle and shows how a pressure differential generates lift.

Bernoulli's principle demonstration. (a) A person holds two curved sheets of paper The pressure between the sheets in (b) is lower than the surrounding pressure causing the sheets to get closer. (c) The same result is obtained by using a single sheet. The pressure above the sheet in (d) is lower than the pressure below, thus causing the sheet to lift.

*Daniel Bernoulli (1700-1782), a Swiss Mathematician and Physician.
**Robert Boyle (1627-1691)

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Last update May 17, 2005
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