Kinetic Theory

In the kinetic theory of gases, the concept is of a large number of randomly moving molecules, which bounce off each other and the walls of the vessel containing the gas. The concepts of pressure and temperature follow naturally from this model. Pressure is due to the bouncing of the molecules off the walls, and temperature is a measure of their average kinetic energy.

• Molecules are in continuous random motion.
• They are point particles in that their volume is much smaller than the volume of the gas.
• No forces except during collisions.
• Collisions with each other and the walls are elastic.
• The force on the wall is due to collisions, as the figure on the right shows: molecules reverse their momentum component perpendicular to the wall during the collision with the wall, whereas the component along the wall remains the same. This reversal of the momentum is an impulse, and from momentum conservation, the wall will receive the opposite impulse.
• The average force on the wall is then the sum of all impulses received by the molecules from the wall during a given time interval, divided by that time interval:
  
  

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