Particle model and pressure

When gas particles collide with the walls of their container, they exert a force on the wall. The total force exerted by all the particles on a unit area of the container walls is the gas pressure. Changing the temperature of a fixed volume of gas changes the pressure it exerts — a hotter gas has faster-moving particles, which collide with the walls more often and with more force, increasing the pressure.

This happens because a larger volume means the same number of particles are more spread out, so they collide with the container walls less often per unit area — reducing the pressure. A smaller volume packs the same particles more tightly, so collisions with the walls happen more often per unit area, increasing the pressure.

• •Adding more gas particles to a fixed volume: more particles means more collisions with the container walls per second, so the pressure increases. Energy is also transferred to the gas as it is pumped in, which heats the gas.
• •Compressing a fixed amount of gas into a smaller volume: as the container wall moves inward, particles that bounce off it rebound with a greater speed than they arrived with (since they collide with a moving wall) — so the particles gain kinetic energy, meaning the gas temperature increases. The particles also now travel a shorter distance between collisions with the walls, so they collide more frequently, increasing the pressure further.