What is Pressure?

Pressure is defined as force per unit surface area. Where does this force originate from in a container filled with gas? Gas consists of a large number of particles (atoms, molecules) which are permanently in motion. When they collide with a surface, their recoil produces a force. Pressure is the sum of these forces produced by particles on unit surface area. The unit, in which the pressure is measured, is the Pascal (Pa = N/m2, Newton per square meter). Another valid unit of pressure is the bar (1 bar = 105 Pa). The standard atmospheric pressure (dry air at sea level) is 1.013 bar. This value comes from the older, and no longer used unit the Torr (760 Torr = 760 mm mercury column).