Particles and their behaviour · Model
Gas pressure
An empty aerosol can carries a warning: do not put it on a fire, it may explode. It is empty. What is there to explode?
Start here
Nothing in it, and it still bursts.
Shake the can and it sounds empty. Press the nozzle and nothing comes out. Put it in a bonfire and within a minute it will burst hard enough to be dangerous at ten metres.
Commit to what bursts it.
The can is not empty. It is full of gas — it always was, and it always will be, because there is no such thing as a sealed can with nothing in it. Heat that gas and the particles hit the walls harder and more often, and at some point the steel gives way. Pressure is not a substance stored in the can. It is a count of collisions.
Pressure is the one idea in this unit that people think they already understand, which is what makes it dangerous. It feels like a squeezing, or a crowdedness, or a force held inside the container. It is none of those. It is particles hitting a wall, and the whole of this lesson is a machine for counting them.
The collision counter · change one thing at a time
Count the hits on the wall.
0 of 3 controls tried
Commit first. What actually causes the pressure inside a sealed container of gas?
Temperature
Size of the container
How many particles
Particle-to-particle bumps are marked in grey. They never touch the wall, so they never add to the pressure.
The grey rings are particles bumping into each other in the middle of the box. There are plenty of them, and not one contributes to the pressure — the wall never feels them. Pressure is only ever what arrives at the wall.
Smaller box, same particles, same speed — and the count is up. Nothing about the gas changed; the wall simply moved closer, so each particle gets back to it sooner.
Hot. The count jumps, and notice the reference particle at the bottom left has not changed size by a single pixel. Speed is doing all the work.
Cold. Fewer arrivals per second and each one softer. Cool a sealed can enough and the outside air will crush it, which is the same idea from the other side.
More particles, so more of them arriving. This is what you are doing to a tyre when you pump it up — not squashing the air, just putting more of it in.
Twenty-four particles at room temperature. Every orange flash is one collision with a wall, and the number above is how many happened in the last second.
Key fact
Gas pressure is caused by particles colliding with the walls of the container. More collisions, or harder ones, means more pressure.
Three predictions · then check them on the bench
Say what will happen before you do it
Each of these changes exactly one thing. Predict the effect on the wall-hit count, then go back up and try it.
The container is made smaller, and nothing else changes. What happens to the wall hits each second?
Up. The particles are travelling no faster, but the walls are closer, so each particle completes the journey between walls more often. Same speed, shorter trip, more arrivals.
Not quite — go back to the bench and try it before reading on. Change only the one thing the question changes.
The gas is heated, and the container stays exactly the same size. What happens to the wall hits each second?
Up — and for two reasons at once. Faster particles cross the box more often, and each collision carries more punch. Both push the pressure up.
Not quite — go back to the bench and try it before reading on. Change only the one thing the question changes.
Half the particles are removed, and the temperature and container stay the same. What happens to the wall hits each second?
Down, by about half. Each remaining particle behaves exactly as it did — there are simply fewer of them arriving. This is what happens when a tyre leaks.
Not quite — go back to the bench and try it before reading on. Change only the one thing the question changes.
Think again
“Heating a gas makes the particles swell up, so they fill more of the can, so the pressure goes up.”
This explanation gets the right answer for the wrong reason, which is the hardest kind of wrong idea to shift. Commit.
Heating changes the speed of the particles and nothing else. The reference particle at the bottom of the bench is drawn at the same size at every temperature setting, and that is not a simplification — it is the fact.
Here is the test that separates the two explanations. If heating made particles swell, a hot gas would be harder to squash than a cold one, because the particles would be taking up more of the room. It is not. A hot gas squashes just as easily. And if the particles swelled, the gas would eventually become a solid block of touching particles — the opposite of what heating does.
Faster particles reach the wall more often, and hit it harder when they get there. Two effects, one cause, no swelling.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What causes the pressure of a gas on the walls of its container?
Rung 2 · The one that catches people
A sealed metal can of gas is heated. The can does not change size. Which of these has NOT changed?
Rung 3 · Explain
A bicycle pump is sealed at the end and the handle is pushed halfway in. Explain, in terms of particles, why it gets harder and harder to push.
Rung 4 · Take it somewhere new
A car tyre is correctly inflated on a cold morning. After an hour of motorway driving, the pressure gauge reads noticeably higher, and no air has been added. Explain, and say what a driver should NOT conclude from it.
Key note
Pressure = collisions with the wall. Heat it and they come faster and harder. Shrink it and they come more often. Add particles and there are more of them arriving. Nothing swells, and particle-to-particle bumps do not count.
Going further
You are under pressure right now — about 100,000 pascals of it: the weight of the whole depth of the atmosphere above you, which pushes about as hard as ten metres of water would, and works out at something like a tonne pressing on your shoulders. You cannot feel it because the same pressure is pushing out from inside you, and the two balance exactly. Remove one side of that balance and it becomes obvious immediately: a sealed plastic bottle brought down a mountain crumples on its own, and a suction cup holds a shelf up using nothing but the air that is not inside it. Nothing sucks, ever. Things get pushed, by particles, from the side where there are more of them.
Before this lesson
Next in this unit
At GCSE this becomes
Where to next
Ask Mr Badmus AI
Still picturing pressure as particles pushing each other?
Lesson content © MrBadmusAI.