Particle motion

The particles in a gas move very quickly in random directions. Collisions frequently happen between:

  • particles
  • particles and the wall of the container

The effect of collisions with the container is to produce a net force acting on the container walls. This force acts at right angles to the container walls, which is detected as gas pressure. This pressure can be measured using a pressure gauge.

Gas particles in a container with arrows showing motion.

For example, the collisions caused by a gas trapped inside a balloon cause forces to act outwards in all directions, giving the balloon its shape.

The pressure caused by a gas can be calculated using the equation:

pressure = \frac{force}{area}

This is when:

  • pressure is measured in newtons per metre squared (N/m²)
  • force is measured in newtons (N)
  • area is measured in metres squared (m²)

Another unit of pressure is the pascal (Pa):

  • 1 Pa = 1 N/m2
  • 1 kPa = 1,000 Pa

A closed system is a system where no substances can enter or leave, such as a balloon filled with air and tied off, or a piston. A gas in a closed system can be:

  • compressed by reducing its volume, which increases its pressure
  • expanded by increasing its volume, which decreases its pressure.
A piston closes the end of a cylinder with some gas molecules inside. The pressure applied to the piston is doubled and the volume inside the cylinder halfs.