Gas pressure and the particle model
Physics · Key Stage 3 · The School
Pressure from countless tiny collisions
Gas particles move constantly, in random directions, at high speed. Every time a particle collides with the container wall, it exerts a tiny force on that wall for a fraction of a second. There are so many particles, colliding so often, that these countless tiny forces add up to a steady, measurable PRESSURE — not one big push, but billions of small ones every second.
Squeezing a gas into less space
Push the same number of gas particles into a smaller container, and they collide with the walls far more often, because there is less distance between each collision and the next. More collisions per second means higher pressure. This is why a bicycle pump gets harder to push as the air inside is squeezed into a smaller volume.
Heating a gas
Heating a gas gives its particles more energy, so they move faster. Faster particles hit the walls both more often AND with more force each time. Both effects increase pressure. This is why a sealed can heated over a flame can genuinely explode — the trapped gas inside cannot expand, so rising pressure has nowhere to go.
A sealed syringe holds a fixed number of air particles. Push the plunger in, halving the space inside. The same number of particles now has half as much room to move in, so each one collides with the walls roughly twice as often as before. Roughly double the collisions per second means roughly double the pressure — which is exactly why the plunger pushes back harder the further in you press it. Nothing about the particles themselves changed. Only how often they were hitting the walls did.