Gas pressure

The particles in a solid simply vibrate around a fixed point, the particles in a liquid roll over each other. The particles in a gas are moving very quickly in random directions. The speeds of the particles in the gas 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.

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.

Gas particles in a container with arrows showing motion.

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.

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

The pressure in the atmosphere at sea level is about 100,000 N/m2.

When a gas is heated, the particles have more kinetic energy and move faster. If the volume does not change the particles hit harder and more often causing the pressure to rise. Pressure cookers are an example of where this happens.