4.1.1—Origin of pressure in a gas

Gas pressure is produced by gas particles colliding with the walls of their container. Each collision transfers momentum to the wall; the combined effect of many collisions over an area gives the measured pressure.

At constant temperature, reducing the container volume packs the particles into a smaller space. Wall collisions become more frequent, so pressure increases: pressure is inversely related to volume under this condition.

At constant volume, increasing temperature raises the particles' average kinetic energy. They move faster and collide with the walls more frequently and with greater effect, so pressure increases: pressure is directly related to temperature under this condition.

State the controlled variable with every relationship: the volume–pressure relationship requires constant temperature, while the temperature–pressure relationship requires constant volume. Do not treat either proportionality as an unrestricted rule when the condition changes.