GCSE · Physics · Edexcel · Spec 1PH0
Atmospheric pressure variation with height
A sealed bag of crisps swells up a mountain. An emptied can crushes itself. Nothing touched either one. Same invisible cause, and you're about to climb right through it.
Predict, then check
Air is pumped out of a can. The air outside carries on pressing on it.
A can has had the air pumped out of it, and the air outside is still pressing on it. What happens, and why?
Sealed bags and balloons
Reason it through
Why does a sealed bag puff up on the way up a mountain?
First link · your turn
As the bag is carried higher, what happens to the amount of air above it?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Climb through the air and count what's left above you.
What you need to know
- Air has weight, and its molecules hit surfaces. That makes atmospheric pressure, which acts on all objects in the atmosphere.
- Go higher and atmospheric pressure decreases, because there is less air above you pushing down.
Have a goSam insists air weighs nothing, so height can't matter. Sam climbs from the beach to a hilltop. Where is more air pushing down on Sam, and why?
On the beach, because more air is above Sam there.
Climbing leaves air below you, so there is less air above pushing down, and that is why atmospheric pressure decreases with height.
- The air gets thinner and thinner as you rise. There is no exact top, but the atmosphere is taken to reach up to about 100 km.
Have a goMarta draws the atmosphere ending at a sharp line 100 km up, like a lid on a jar. What is wrong with her drawing?
There is no exact upper boundary. The air just keeps thinning.
The atmosphere is only taken to reach up to about 100 km, where almost no air is left, so a sharp lid is the wrong picture.
- At sea level, atmospheric pressure is about 100 000 N/m², which is 100 kN/m².
- Pressure changes far more gradually with height in air than with depth in water, because air is much less dense than water.
- Atmospheric pressure acting inwards on an object is normally balanced by an equal pressure acting outwards from inside it.
Have a goA sealed bottle full of air sits on a shelf with the atmosphere pressing in on it. Why isn't it crushed?
An equal pressure from the air inside pushes outwards.
The inward atmospheric pressure is normally balanced by an equal outward pressure from inside, so the two cancel.
- Take the inside pressure away, say by pumping the air out of a can, and the unbalanced outside pressure can crush it.
- Carry a sealed bag or helium weather balloon upwards and the outside pressure falls, but the inside pressure doesn't fall as much, so it puffs up or expands.
The big picture
Air has weight, and the air above you pushes on everything beneath it. Climb higher and there is less air above, so atmospheric pressure falls. That one idea explains why a can crushes when its air is pumped out and why a sealed bag puffs up on the way up a mountain.
Key points
Worked example
Problem
Use the illustrative values to find how much atmospheric pressure falls between sea level and aircraft cruising height (about 10 km), and roughly what fraction of the sea-level pressure is left.
⚠ Watch out
Writing that air has no weight, or that it only pushes downwards. Air has weight, and atmospheric pressure acts on all objects. Pressure falls with height because there is less air above you, not because the air disappears at a sharp edge.
Memory hook
Pressure is the push of the air stacked above you. Climb, and the stack gets shorter, so the push gets smaller.
Check yourself
Cover the page. In two sentences, explain why pressure is lower on a mountain top. Then say which pressure is greater inside a balloon that has risen.
Flashcards
(11)What produces atmospheric pressure?
How does atmospheric pressure change as height increases, and why?
Where does the atmosphere end?
How does the air change as you climb?
What are the illustrative pressures at sea level and at about 9 km (Mount Everest)?
Why does pressure change much more gradually with height in air than with depth in water?
Roughly how far do you rise through air for the same pressure change that needs only about 1 m of water?
What normally balances the atmospheric pressure pushing inwards on an object?
Why can a can be crushed when the air inside it is pumped out?
Why does a sealed helium weather balloon expand as it rises?
True or false: air has no weight.
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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