IB Physics HLThe Behaviour of GasesPaper 1 & 2Pressure~8 min read
Gas Pressure
We use the word “pressure” loosely in everyday life, but in physics it has one precise meaning: force spread over an area. Once that clicks, gas pressure makes complete sense — it’s just billions of tiny molecules drumming on the walls of their container. Let’s build the idea from the ground up.
📘 What you need to know
Pressure is the force applied per unit area: P = F / A
It’s measured in pascals (Pa), where 1 Pa = 1 N m−2
The force must act perpendicular (at 90°) to the surface
The same force on a smaller area gives a bigger pressure — and vice versa
Gas pressure comes from countless molecules colliding with the container walls
Each collision pushes on the wall at right angles; together, they add up to a steady pressure
What pressure really means
Pressure tells you how concentrated a force is. Push with the same force but squeeze it onto a smaller area, and the pressure shoots up. That’s why a sharp drawing pin slips into a board while your thumb (pushing just as hard, but over a much bigger area) doesn’t.
Think of a brick. Stand it on its small end and its weight presses on a tiny patch of ground — high pressure. Lay the same brick flat and that same weight is spread over a big face — low pressure. Same force, different area, different pressure.
Same brick, same weight — but standing on its end squeezes that force onto a small area (high pressure), while lying flat spreads it over a big area (low pressure).
One more thing about the force: in a fluid it always pushes at right angles (90°) to the surface it touches — straight on, never sideways. So when we talk about the area, we mean the area facing that push. Keep that in mind and you’ll always pick the right area in a calculation.
Where gas pressure comes from
Now zoom right in. A gas is a swarm of molecules zipping around at high speed. Every time one hits a wall of the container, it bounces off and gives the wall a tiny shove — a little force, at 90° to the wall. One molecule is nothing. But billions hit every second, and all those little shoves add up to a smooth, steady push we call gas pressure.
Molecules (blue) race at the wall and bounce off. Each collision pushes the wall outward (green), always at 90° to the surface. Billions of these tiny pushes every second make the steady gas pressure.
Want to feel this yourself? Close your mouth and puff air into your cheeks. That strain you feel is exactly this — air molecules pushing outwards, at right angles, on the inside of your cheeks.
Using P = F / A
Putting it into an equation:
PressureP = F / A
where P is the pressure (Pa), F is the force pushing at right angles to the surface (N), and A is the area that force is spread over (m2). Rearranged, the force is simply F = P × A.
WE 1
A trapped gas pushes on a piston of cross-sectional area 0.015 m2 with a pressure of 2.0 × 105 Pa. Calculate the force the gas exerts on the piston.
Step 1 — rearrange P = F/A for force
F = P × A
Step 2 — substituteF = (2.0×10⁵) × 0.015F = 3000 N = 3.0 kNA modest-looking pressure over a decent area still adds up to a hefty push.
WE 2
A drawing pin is pushed with a force of 15 N. Its point touches the board over an area of just 0.10 mm2. Find the pressure on the board.
Step 1 — convert the area to m²0.10 mm² = 0.10 × 10⁻⁶ = 1.0 × 10⁻⁷ m²Step 2 — use P = F/AP = 15 ÷ (1.0×10⁻⁷)P = 1.5 × 10⁸ Pa (150 MPa)A gentle 15 N push becomes a huge pressure — all because the area is so tiny. That’s why the pin sinks in.
Watch the units: 1 mm2 is not 10−3 m2 — it’s 10−6 m2 (because you square the millimetre-to-metre step). Getting this conversion wrong is the single most common slip on pressure questions.
Molecule hits wall
tiny force each time
Billions per second
add up over the area
Steady gas pressure
🛠️ Answering a pressure question
Spot what you’re given — two of pressure, force and area.
Pick the right area: the surface the force actually pushes on, at 90°.
Convert areas to m2 (cm2 → ×10−4, mm2 → ×10−6).
Use P = F/A, rearranging for whatever you need.
Quote the unit — pascals (Pa), or kPa/MPa if the number is large.
💡 Top tips
Smaller area, bigger pressure — same force, more concentrated.
Force acts at 90° to the surface, both for solids and for gas molecules on a wall.
Pick the correct area if several are quoted — the one the force presses on.
Convert to m2: mm2 → ×10−6, cm2 → ×10−4.
1 Pa = 1 N m−2 — that link makes the units easy to check.
⚠ Common mistakes
Getting the area conversion wrong — 1 mm2 = 10−6 m2, not 10−3
Using the wrong area when several are given — use the one the force pushes on
Forgetting the force must be perpendicular to the surface
Mixing up the rearrangement — F = PA, but P = F/A
Leaving off the unit, or quoting newtons where pascals are needed
Quick recap: Pressure is force per unit area, P = F/A, measured in pascals. The force always acts at 90° to the surface. Squeeze the same force onto a smaller area and the pressure rises. Gas pressure is just the sum of billions of molecules bouncing off the container walls each second.
You’ve now got the foundation for the whole gas topic: pressure is force per area, and it comes from moving molecules. Next up is Amount of Substance — the mole, the Avogadro constant, and how we count the mind-boggling number of molecules in a gas. That’s the other half we’ll need before the gas laws tie everything together.
Want this to actually click before the exam?
Book a free meeting and let’s work through the tricky bits together.
