1. What Is Pressure?
Pressure is a measure of how much force is applied over a given area.
The equation for pressure is:
Pressure = Force ÷ Area
Pressure is measured in pascals (Pa), where 1 pascal equals 1 newton of force acting over 1 square metre (1 Pa = 1 N/m²).
This is why a sharp knife cuts more easily than a blunt one — the same force is concentrated over a much smaller area, producing much higher pressure at the blade's edge. The same principle applies to gases, but instead of one solid surface pressing down, gas pressure comes from billions of tiny, invisible collisions happening every second.
2. What Causes Gas Pressure? The Particle Model
Gases are made up of particles (atoms or molecules) that are spaced far apart and move around constantly and randomly, in straight lines, until they collide with something — either each other or the walls of their container.
Gas pressure is caused by gas particles colliding with the walls of their container. Each individual collision exerts a tiny force on the wall, but because there are enormous numbers of particles colliding constantly (trillions of times per second in even a small container), these tiny forces add up to create a measurable, steady pressure.
This is a direct result of the particle model of matter (sometimes called the kinetic theory), which describes gas particles as:
1. Far apart compared to solids and liquids.
2. Moving fast, randomly, and in straight lines between collisions.
3. Having very weak forces of attraction between them.
Anything that changes how often or how hard particles collide with the container walls will change the gas pressure. There are two main factors that do this: volume and temperature.
3. How Volume Affects Gas Pressure
If you decrease the volume of a container (while keeping the temperature and the amount of gas the same), the same number of particles are now squeezed into a smaller space.