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Pressure
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Pressure means how much something is pushing on something else. It is expressed as force per unit area: P=F/A
In technology, pressure is often specified in multiples of atmospheric pressure.
It can also be defined as the thrust [compressive force acting perpendicularly to the surface of a body] acting per unit area.
Since pressure depends on both force and area, the same amount of force applied by a hand will make less pressure than when applied by something smaller, like a finger.
Pressure is also related to density. By the ideal gas law, for example, doubling the pressure can put twice as much gas in a bottle.
To measure the amount of pressure people use these units:
 Pounds per square inch  (psi, PSI, lb/in2, lb/sq in). This U.S. customary units is not elsewhere. Normal atmospheric pressure is 14.7 psi, which means that a column of air one square inch in area rising from the Earth's atmosphere to space weighs 14.7 pounds.
 Atmosphere  atm. Normal atmospheric pressure is called 1 atmosphere. 1 atm = 14.6956 psi = 760 torr.
 P=F/A, Torr  torr. This is called the Torr because it is based on the original Torricelli barometer design. One atmosphere of pressure will push the column of mercury (Hg) in a mercury barometer to a height of 760 millimeters. A pressure that causes the Hg column to rise 1 millimeter is called a torr. 1 atm = 760 torr = 14.7 psi.
 Bar  bar. The bar is almost the same as the atmosphere unit. One bar = 750.062 torr = 0.9869 atm = 100,000 Pa.
 Millibar  mb or mbar. There are 1,000 millibar in one bar. This unit is used by meteorologists who find it easier to refer to atmospheric pressures without using decimals. One millibar = 0.001 bar = 0.750 torr = 100 Pa.
 Pascal  Pa is the SI unit of pressure. It was named after the French physicist Blaise Pascal. 1 pascal = a force of 1 newton per square meter (1 newton = the force needed to accelerate 1 kilogram one meter per second per second = 1 kg.m/s2; this is actually quite logical for physicists and engineers, honest). 1 pascal = 10 dyne/cm2 = 0.01 mbar. 1 atm = 101,325 Pascals = 760 mm Hg = 760 torr = 14.7 psi.
 Kilopascal  kPa. The prefix "kilo" means "1,000", so one kilopascal = 1,000 Pa. Therefore, 101.325 kPa = 1 atm = 760 torr and 100 kPa = 1 bar = 750 torr.
 Megapascal  MPa. The prefix "mega" means "1,000,000", so one megapascal = 1,000 kPa = 1,000,000 Pa. Pressures in the megapascal range are common in highpressure air compressors and hydraulic systems, where high pressure liquids are used to do useful work.
 Gigapascal  GPa. The prefix "giga" means "1,000,000,000", so one gigapascal = 1,000 MPa = 1,000,000 kPa = 1,000,000,000 Pa = 9,870 atm = 10,000 bar. Pressures of several gigapascals can change graphite to diamond or make hydrogen a metallic conductor!

