Multiple choiceQuestion 1 · Recall
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By the end you can explain where a gas's pressure comes from, and why heating a gas or squeezing it into a smaller space raises its pressure.
A gas is an enormous number of tiny particles flying in straight lines in every direction.
They are far apart compared with their size, and they keep bouncing off whatever wall contains them — a balloon skin, a tyre wall, a piston.
Each bounce gives the wall a tiny push. Trillions of pushes every second add up to a steady, outward push we can measure.
Pressure
Pressure is the force per unit area acting at right angles to a surface:
where is pressure, is the force and is the area. The SI unit is the pascal, , and .
Stand under a tin roof in rain: one drop is nothing, but millions of drops per second make a steady roar and push on the sheet.
Gas molecules are the raindrops, the container wall is the roof, and pressure is the average push.
Where the analogy breaks: raindrops splash and stop. Gas molecules bounce off the wall and keep going — that bounce is what keeps the push going.