The Ocean of Air
Atmospheric pressure — the crush you never feel
What this lesson covers
Why it matters
Right now, roughly one kilogram presses on every square centimetre of you — about 15 tonnes in total. You don't feel it because you're pressurised too. Remove the balance and metal cans fold like paper.
The idea in plain words
The weight of the sky. Tap the terms.
P(atm) ≈ 1.013 × 10⁵ Pa = 76 cm of Hg
Why does a drinking straw work?
You don't suck the drink up — the sky pushes it up
- the origin: cause — the weight of the air column above you — kilometres of it
- the classics: demonstrations — crushed can, Magdeburg hemispheres (horses couldn't part them!)
- the exam list: daily consequences — drinking straws, syringes, rubber suckers, filling a dropper — all atmosphere-powered
- You expand your lungs, LOWERING pressure in the straw
- The atmosphere, pressing on the drink's surface, pushes liquid up
Predict first
Air is pumped out of a sealed tin can. The can crumples because…
A vacuum has no force to give. The atmosphere was ALWAYS pushing — pumping out the can's air just removed the counter-push.
- outside air pressure crushes it, no longer balanced from inside — correct
- the vacuum sucks the walls inward
- the metal cools and shrinks
What you do
Pump the can down stroke by stroke, watch the sky win — then answer what actually did the crushing.
Check yourself
We don't feel the atmosphere's 15-tonne squeeze because…
Balanced pressure feels like nothing — deep-sea fish are equally unimpressed by their depth.
The Magdeburg hemispheres resisted teams of horses because…
Atmospheric force over that area ≈ tonnes — the sky out-pulled the horses.
A rubber sucker sticks to smooth walls because…
On a rough wall air leaks back in and the atmosphere loses its grip — try it.
- our blood and body fluids push OUT with equal pressure — correct
- skin is too tough to notice
- the pressure only acts downward
- with the air pumped out, the atmosphere pressed the halves together — correct
- they were glued
- the vacuum inside pulled them shut
- pressing it out expels the air; the atmosphere pins it to the wall — correct
- rubber is sticky
- static electricity