What is Pressure?

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Pressure is the force per area on a surface. The SI unit for pressure is the newton (N). One Newton is the force that will increase the speed of a one kilogram object by one meter per second each second it is applied. The Earth's atmosphere exerts pressure because of the combination of the gases present in the air. The atmospheric pressure is the sum of the individual pressures of the various gases. Atmospheric pressure is measured by a barometer. The first barometer was made by Evangelista Torricelli in the early 1600s. He completed experiments that found the average atmospheric pressure at sea level and zero degrees Celsius.

Units of Pressure

UnitSymbolDefinition
PascalPaSI pressure unit, 1Pa = 1N/m2
Millimeters of MercurymmHgthe pressure that supports a 1 mm mercury comlumn in a barometer
Torrtorr1 torr = 1 mmHg, Honors Torricelli for his invention of the barometer
Atmosphereatmaverage atmospheric pressure at sea level and O degrees Celsius
  1. 1 atm = 101 325 Pa
  2. 1 atm = 760 mmHg
  3. 1 atm = 760 torr

To compare the volumes of gases, scientists have agreed that standard conditions are 0 degrees Celsius and 1 atm. These conditions are also known as standard temperature and pressure (STP).
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