Particle motion

The particles in a gas are moving very quickly in random directions. The speeds of the particles vary but, on average, they move quicker than they do in liquids and solids.

This means that it does not take long for a gas to spread out to fill its entire container. The smell of an air freshener can spread all around a room very quickly.

Gas pressure

Gas particles in a container with arrows showing motion.

Since the particles in a gas are moving fast and randomly, collisions occur frequently. These collisions may be between two particles, between a particle and the wall of the container, or between a particle and something else in the container.

The force acting on the container due to these collisions is at right angles to the container.

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

A person standing in a room full of air is constantly being hit by the particles of the gases that make up the air.

curriculum-key-fact
The pressure in the atmosphere (atmospheric pressure) at sea level is about 100,000 N/m2.

The pressure from gas molecules may increase if there are more molecules colliding each second or if the molecules are moving faster.