Ideal Gases

Random motion of gas molecules

• Gas molecules never sit still; they fly about ceaselessly in random directions at high speed
• "Random" means no preferred direction: every molecule is constantly changing direction as it bounces off other molecules and off the walls of the container
• A typical air molecule at room temperature moves at hundreds of metres per second between collisions

How collisions produce pressure

• Each time a gas molecule strikes the wall of its container, it bounces back. By Newton's third law, the wall pushes the molecule back, and the molecule pushes the wall
• Over countless collisions, the millions of tiny pushes add up to a net force on every wall, acting at right angles to the surface
• Pressure is force per unit area:

P = F / A

• Two things control how big the pressure is:
  • How often the molecules hit the wall each second (more collisions per second means more force per second)
  • How hard each collision is (faster molecules mean a harder impact)
• Anything that changes either factor (heating the gas, compressing it, adding more molecules) changes the pressure