11.3 Atmospheric Pressure

A free GCSE Physics lesson for AQA (8463) from OBA School: why the air exerts a pressure, how it changes with altitude, and forces from pressure differences. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain atmospheric pressure in terms of air molecules colliding with surfaces
• Explain why atmospheric pressure and the density of the air decrease with increasing altitude
• Calculate the force on a flat surface due to a pressure difference: F = pressure difference × area
• Explain everyday effects such as suction caps, drinking straws and crushed bottles
• (Higher tier) use p = h ρ g for a simple model atmosphere and comment on the model

Key words

• Atmosphere: The thin layer of air around the Earth. It stretches more than 100 km up but most of the air is in the lowest 10 km.
• Atmospheric pressure: The pressure exerted by the air, about 100 kPa (100 000 Pa) at sea level.
• Altitude: Height above sea level.
• Air density: Mass of air per cubic metre. About 1.2 kg/m³ at sea level; it falls as altitude increases.
• Pressure difference: The difference between the pressures on the two sides of a surface. It causes a resultant force.
• Collision: A molecule hitting a surface and bouncing off, exerting a tiny force on it.

What causes atmospheric pressure?

Air is made of molecules moving fast in random directions. Every second, an enormous number of them collide with each square centimetre of every surface. Each collision exerts a tiny force. Added together, these forces cause atmospheric pressure, which acts at right angles to surfaces and in all directions.

At sea level the atmospheric pressure is about 100 kPa. That means a force of about 100 000 N – the weight of roughly 10 tonnes – on every square metre. We do not notice it because the pressure inside our bodies pushes outwards equally.

Pressure falls as altitude increases

The air at any height is pushed down by the weight of all the air above it. The higher you go:

• the less air there is above you, so there is less weight of air pressing down;
• so the air is less squashed: it is less dense, with fewer molecules per cubic metre;
• so there are fewer collisions per second with each square metre of a surface;
• so the atmospheric pressure is lower.

At the top of the highest mountains the pressure is only about a third of its sea-level value, and each breath contains much less oxygen. Airliners cruise at about 11–12 km, where it is below 25 kPa, so the cabin is pumped up (pressurised) to about 75–80 kPa: a little below sea-level pressure, but safe and comfortable. Atmospheric pressure also changes slightly from day to day with the weather.

Exam tip: Pressure does not fall in a straight line. The curve gets less steep with height because the air is least dense high up, so each extra kilometre removes less air from above.

Forces from pressure differences

When the pressures on the two sides of a flat surface are different, there is a resultant force towards the low-pressure side:

force = pressure difference × area

F = (p₁ − p₂) × A F in N, pressures in Pa, A in m²

Worked example: opening a vacuum-sealed jar

The air inside a jar is at 30 kPa. Atmospheric pressure outside is 101 kPa. The lid has an area of 0.0050 m².
pressure difference = 101 − 30 = 71 kPa = 71 000 Pa
F = 71 000 × 0.0050 = 355 N

Answer: about 360 N pushes the lid on – which is why it is hard to open

• Suction cap: it is not “sucked” on. Atmospheric pressure pushes on the outside, and there is little or no air inside to push back.
• Drinking straw: you remove air from the straw, lowering the pressure inside. The greater air pressure on the surface of the drink pushes the liquid up the straw.
• Crushed bottle: a plastic bottle sealed at high altitude contains low-pressure air. Brought down to sea level, the greater pressure outside squashes it.

Exam tip: Use the difference in pressure, not just the larger pressure, and convert kPa to Pa before multiplying by the area.

A simple model atmosphere (Higher tier)

If the air had the same density at every height, the atmosphere would behave like a column of liquid and p = h ρ g would give the pressure at the bottom. We can use this to estimate a height or an average density.

Worked example: how tall would a uniform atmosphere be?

Sea-level pressure = 101 kPa, density of air at sea level = 1.2 kg/m³, g = 9.8 N/kg
h = p ÷ (ρ g) = 101 000 ÷ (1.2 × 9.8) = 8588 m

Answer: about 8.6 km

This model is too simple: the real atmosphere stretches more than 100 km up, because the air gets less and less dense with height instead of ending suddenly. The model is still useful for estimating the average density between two heights:

Worked example: average density between sea level and 2.0 km

From the graph: 101 kPa at 0 km and about 79 kPa at 2.0 km, so the pressure difference = 22 kPa
ρ = Δp ÷ (h g) = 22 000 ÷ (2000 × 9.8) = 1.12 kg/m³

Answer: about 1.1 kg/m³

Common mistakes

• Saying suction caps or straws “suck”. It is the atmosphere outside that pushes.
• Thinking air pressure only pushes downwards. It acts in all directions, at right angles to every surface.
• Using the full outside pressure instead of the pressure difference when calculating a force.
• Forgetting to convert kPa to Pa (× 1000) before calculating a force.
• Saying the air at high altitude has “no pressure”. It is lower, not zero, and density falls gradually.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the approximate value of atmospheric pressure at sea level? (Choose from: 100 kPa, 100 Pa, 10 kPa, 1000 kPa.)

Answer: 100 kPa

Working: About 100 kPa (100 000 Pa). That is a force of about 100 000 N on each square metre.

Question 2. Why does atmospheric pressure decrease as altitude increases? (Choose from: There is less air above, so the air is less dense and fewer molecules hit each square metre per second, The air is colder, so the molecules stop moving, There is no gravity at high altitude to hold the air down, The air molecules are smaller at high altitude.)

Answer: There is less air above, so the air is less dense and fewer molecules hit each square metre per second

Working: Higher up there is less weight of air above, so the air is less dense. Fewer molecules per m³ means fewer collisions per second with each square metre, so the pressure is lower.

Question 3. True or false? Atmospheric pressure only acts downwards on surfaces.

Answer: False

Working: False. Air molecules move randomly in all directions, so atmospheric pressure acts at right angles to every surface: sideways and upwards as well as downwards.

Related lessons

• Previous lesson: 11.2 Pressure in a Liquid at Rest
• Next lesson: 11.4 Upthrust and Flotation
• In this chapter: 11.1 Pressure and Surfaces

Continue in the full course

This lesson is section 11.3 (AGCPH1103) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.