Atmospheric Pressure — AQA GCSE Physics (Separate)

The atmosphere is a layer of air around the Earth that exerts pressure on surfaces.

What causes atmospheric pressure?

The atmosphere is a thin layer of air (relative to the size of the Earth) that gets less dense with increasing altitude. Air molecules collide with surfaces, creating atmospheric pressure.

Pressure decreases with height

Atmospheric pressure decreases as altitude increases, for two reasons:

• As you go higher, there is less air above you, so a smaller weight of air presses down.
• The air is also less dense at higher altitudes (fewer molecules to collide).

This is why mountaineers experience lower air pressure (and less oxygen) at high altitude, and why aircraft cabins are pressurised.

Atmospheric pressure and everyday effects

The weight of the atmosphere pressing down is why, for example, a sealed can collapses if the air inside is removed — the outside atmospheric pressure is greater than the reduced inside pressure.

Exam tips

• Atmospheric pressure is caused by air molecules colliding with surfaces.
• Pressure decreases with height because there is less air (and less weight of air) above.
• The air also becomes less dense at altitude.
• Link to real examples (mountains, aircraft, collapsing cans).