# What causes gas pressure (in terms of kinetic theory)?

Jan 28, 2014

Gas pressure is caused by the collisions of gas particles with the walls of the container.

#### Explanation:

According to kinetic theory, molecules inside a volume (e.g. a balloon) are constantly moving around freely.

During this molecular motion, they constantly collide with each other and with the walls of the container.

In a small balloon, that would be many thousands of billions of collisions each second.

The force of impact of a single collision is too small to measure.

However, taken all together, this large number of impacts exerts a considerable force on the surface of the container.

If they hit the surface of the balloon straight on (at a 90 ° angle), they exert their maximum force.

If they hit the surface at an angle less than 90 °, they exert a smaller force.

The sum of all these forces causes the pressure, $p$, that is exerted by the gas.

The diagram above represents a balloon containing molecules of a gas (the red dots).

The yellow arrows indicate that the gas pressure, $\text{p}$, in the balloon is exerted outward against the walls of the balloon.

The larger the number of collisions per area of the container, the larger the pressure:

Pressure = $\text{Force"/"Area}$ or $p = \frac{F}{A}$.

The direction of this force is always perpendicular to the surface of the container at every point.

The video below gives a good explanation of gas pressure.