AQA GCSE Physics Foundation (8463), Foundation tier · Forces › Pressure in fluids
Practise Atmospheric pressure. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The atmosphere is a thin layer of air around the Earth, and air molecules colliding with surfaces create atmospheric pressure. You need to describe a simple model of the atmosphere and explain why atmospheric pressure decreases as height increases. This is in GCSE Physics only, not in Combined Science.
Key facts
Atmosphere = thin layer of air around the Earth
Air gets less dense with increasing altitude
Atmospheric pressure is caused by air molecules colliding with surfaces
Higher altitude → less air above → smaller weight of air → lower pressure
F = p × A
Notes
A simple model of the atmosphere
The atmosphere is a thin layer of air around the Earth. It is thin compared with the size of the Earth.
The atmosphere gets less dense with increasing altitude (height above sea level).
Most of the air is close to the Earth's surface. The atmosphere has no sharp top edge: it gradually gets thinner.
What causes atmospheric pressure
Air molecules move randomly and collide with surfaces. Each collision exerts a small force on the surface.
Atmospheric pressure is the total force of these collisions on each square metre of the surface.
It acts on every surface, at right angles to it, in all directions.
At sea level, atmospheric pressure is about 100 000 Pa (100 kPa).
Why pressure decreases with height
diagram
The number of air molecules, and so the weight of air, above a surface decreases as the height of the surface increases.
So at any height there is less air above a surface than there is at a lower height.
The air is also less dense, so fewer molecules collide with each square metre of a surface each second.
So atmospheric pressure decreases as height increases.
The air gets less dense with height, and there is less air above, so the pressure falls.
How to answer each type of question
Describe a simple model of the atmosphere
2 marksGrade 4
Say it is a thin layer of air around the Earth (thin compared with the Earth's size).
Say how its density changes with altitude.
Example. Describe the Earth's atmosphere.
Show the model answerHide the model answer
A thin layer of air around the Earth, thin compared with the size of the Earth (1). It gets less dense as the altitude increases (1).
Don’t lose marks
Saying pressure decreases with height 'because there is less gravity'. The key reason is that there is less air (a smaller weight of air) above.
Describing the atmosphere as thick. It is a thin layer compared with the size of the Earth.
Saying atmospheric pressure only acts downwards. It acts on surfaces in all directions.
Saying the molecules 'push' without mentioning collisions with the surface.
More tips
Memory tricks
Less air on top = less pressure. Think of a pile of mattresses: the bottom one is squashed the most.
Sea level ≈ 100 kPa: about 100 000 N pushing on every square metre.
Pressure differences always push from high to low.
Exam technique
Use the key phrases 'air molecules collide with the surface' and 'less air (a smaller weight of air) above'.
For 3-mark explanations, give the cause of the pressure, then what changes with height, then the effect on the pressure.
Convert kPa to Pa before using F = p × A.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Describe a simple model of the atmosphereThe atmosphere is thin compared with the size of the Earth and gets less dense with increasing altitude.
Grade 5
Explain what causes atmospheric pressureAir molecules collide with a surface, and each collision exerts a force on it.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Calculate the force exerted by the atmosphere on one side of the window. Use the equation: pressure = force normal to a surface ÷ area of that surface
1.2 × 105 N
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) What causes atmospheric pressure? Tick (✓) one box.[1]
Air molecules colliding with surfaces
Clouds pressing down on the ground
The Earth’s magnetic field
The Sun heating the air
(b) How does the density of the atmosphere change as altitude (height above sea level) increases?[1]
(c) Atmospheric pressure at the top of a mountain is lower than at the bottom of the mountain. Explain why.[2]
Show the answer and mark scheme
(a)Answer: Air molecules colliding with surfaces
(b)Answer: It decreases.
it decreases
(c)Answer: There is less air (a smaller weight of air) above the summit, and the thinner air gives fewer collisions per second with a surface.
there is less air above the top of the mountain / the weight of air above is smaller
(the air is less dense so) fewer air molecules collide with a surface each second
Question 2Medium4 marks
A person can drink through a straw by sucking air out of the straw.
(a) Explain, in terms of pressure, why a person can drink through a straw.[3]
(b) Explain why a straw would not work in this way on the surface of the Moon, which has no atmosphere.[1]
Show the answer and mark scheme
(a)Answer: Sucking lowers the pressure inside the straw; atmospheric pressure on the drink’s surface is then greater, and pushes the drink up the straw.
sucking removes air from the straw (and mouth), reducing the pressure inside the straw below atmospheric pressure
atmospheric pressure, acting on the surface of the drink, is now greater than the reduced pressure inside the straw
this pressure difference pushes the drink up the straw and into the mouth
(b)Answer: There is no atmospheric pressure on the Moon to push the drink up.
there is no atmospheric pressure on the Moon to push the drink up the straw