Gas Pressure

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Gases exert pressure on the walls of a container. This is clear enough from common experiences like blowing up a balloon or filling a tire with air. On a macroscopic level, pressure is defined as: the force exerted per unit of surface area or:

Pressure is the result of gas particles colliding with surfaces. Imagine a strong hailstorm against a window. The greater the number of collisions the greater the pressure so that both gas concentration and the temperature of a gas will affect the pressure observed at a surface. Just to give you some sense of what busy bees gas molecules are even under normal conditions, consider a sample of nitrogen gas confined to a room at 20^o C and 1 atmosphere of pressure. The most probable speed of the average particle in this gas sample is 1,100 mph. This speed changes as collisions with particles and walls of the container occur. An average particle can only travel about 6.6 x 10^-8 m before running into something and it undergoes about 7.7 x 10⁹ collisions/sec.

Due to gravitational attraction to the the Earth, the gases that form the Earth's atmosphere are pulled towards its surface and exert a force on everything near its surface. The force of all of these gases creates the atmospheric pressure of about 1 atm or 14.7 pounds per square inch that we are designed to live in. At higher altitudes where there is less atmosphere the atmospheric pressure is reduced. As you move further and further away from the surface of the earth the gas pressure is reduced even more as can be seen in the diagram at left. Pressure is generally expressed in units of atmospheres (atm), Torr, or mmHg,  Units of Pressure 1 pascal (Pa) 1 N/m2 = 1 kg/m * s2 1 atmosphere (atm) 1.01325 * 105  Pa 1 atmoshphere (atm) 760 torr = 760 mmHg 1 bar 105  Pa   A good demonstration of the effects of gas pressure can be viewed in the following movie. So-called "empty" containers are in fact, filled with gases. As the movie shows,when most of those gases are removed, normal atmospheric pressure crushes the evacuated container. View Movie