Thrust And Pressure
TYPES OF PRESSURE
Atmospheric pressure (Patm):
Atmospheric pressure is the pressure exerted by environmental air. (or) The force exerted by atmosphere per unit area on the surface of earth is called as Atmospheric pressure.
1. The magnitude of average atmospheric pressure on the surface of earth is 76 cm of Hg.
3. The instrument used for measuring of pressure is called Barometer. It was constructed by Torricelli in 1643.
4. Other kinds of barometers are i) Aneroid barometer ii) Fortin’s barometer.
5. Atmospheric pressure decreases with the increase of height (or) altitude.
Gauge Pressure (Pgauge):
The pressure that is measured with respect to atmospheric pressure is called gauge pressure.
Gauge Pressure can be positive (+ve) or Negative (-ve).
Vacuum pressure (PVacuum) or pressures less than atmospheric are known as negative (-ve) gauge pressure.
Absolute pressure (Pabs):
It is the pressure that is measured with respect to the zero pressure line.
Absolute pressure is always positive.
Note: The excess pressure above atmospheric pressure is usually called gauge pressure and the total pressure is called absolute pressure. Thus,
Gauge pressure = absolute pressure – atmospheric pressure