Atmospheric pressure

What you'll learn

This revision guide covers atmospheric pressure as required by the AQA GCSE Physics specification. You'll understand what causes air pressure, how it varies with altitude, and how to perform calculations involving density and column height. This topic connects to particle models and appears in both foundation and higher tier papers.

Key terms and definitions

Atmospheric pressure — the force per unit area exerted by the weight of air molecules in the Earth's atmosphere, typically 100,000 Pa at sea level

Density — mass per unit volume of a substance, measured in kg/m³ or g/cm³

Altitude — the height above sea level, measured in metres

Pressure — force per unit area, measured in pascals (Pa) or N/m²

Column of air — a vertical section of atmosphere extending from a surface upwards

Pascal (Pa) — the SI unit of pressure, equal to one newton per square metre (1 N/m²)

Atmosphere (atm) — a non-SI unit of pressure equal to approximately 101,325 Pa or 101 kPa

Barometer — an instrument used to measure atmospheric pressure

Core concepts

What causes atmospheric pressure

Air molecules have mass. Although individual air molecules are extremely light, the Earth's atmosphere extends many kilometres above the surface and contains an enormous number of molecules.

Atmospheric pressure results from the weight of all air molecules above a surface pressing down due to gravity. Air molecules constantly collide with surfaces and each other, creating a force distributed over an area.

Key points about atmospheric pressure:

• At sea level, atmospheric pressure is approximately 100,000 Pa (100 kPa or 1 atmosphere)
• This pressure acts in all directions, not just downwards
• The pressure on a 1 m² surface at sea level is equivalent to approximately 10,000 kg resting on it
• Our bodies experience this pressure constantly but we don't notice it because the pressure inside our bodies balances the external pressure

The standard atmospheric pressure value you should use in calculations is 101,000 Pa or 101 kPa unless told otherwise in the question.

How atmospheric pressure varies with altitude

As altitude increases, atmospheric pressure decreases. This relationship is fundamental to understanding weather systems, aviation, and mountaineering.

Why pressure decreases with height:

• At higher altitudes, there are fewer air molecules above you
• Less air above means less weight pressing down
• Therefore, the column of air creating the pressure is shorter and lighter

Important characteristics of this relationship:

• The decrease is not linear — pressure drops more rapidly at lower altitudes
• At approximately 5,500 m altitude, atmospheric pressure is roughly half that at sea level
• Commercial aircraft cabins are pressurised because atmospheric pressure at cruising altitude (10-12 km) is too low to breathe comfortably
• At the summit of Mount Everest (8,849 m), atmospheric pressure is about one-third of sea level pressure

Practical implications:

• Water boils at lower temperatures at high altitude because lower atmospheric pressure makes it easier for water molecules to escape the liquid
• Sealed packets of crisps expand when taken to high altitude because the internal pressure exceeds the reduced external pressure
• Mountaineers may experience altitude sickness due to lower oxygen pressure in the air

How atmospheric pressure varies with density

The density of air affects atmospheric pressure. Understanding this relationship helps explain weather patterns and pressure changes.

The relationship between density and pressure:

At a constant temperature, denser air creates higher pressure because:

• More molecules occupy the same volume
• Greater mass in the same space means more weight
• More collisions occur between air molecules and surfaces

Factors affecting air density:

• Temperature: Cold air is denser than warm air. When air cools, molecules move more slowly and occupy less space
• Humidity: Moist air is actually less dense than dry air (water molecules are lighter than nitrogen and oxygen molecules)
• Altitude: Air density decreases with height for the same reason pressure does

Weather applications:

• High pressure systems (anticyclones) typically involve descending, dense air and bring settled weather
• Low pressure systems (depressions) involve rising, less dense air and often bring unsettled weather
• Meteorologists use barometers to measure atmospheric pressure changes to predict weather patterns

Calculating pressure in liquid columns

The equation for calculating pressure in a column of liquid applies the same principle as atmospheric pressure:

p = h × ρ × g

Where:

• p = pressure in pascals (Pa)
• h = height of the column in metres (m)
• ρ (rho) = density in kilograms per cubic metre (kg/m³)
• g = gravitational field strength (approximately 10 N/kg on Earth)

This equation shows that pressure depends on:

• The height of the column (deeper = more pressure)
• The density of the fluid (denser = more pressure)
• Gravitational field strength (stronger gravity = more pressure)

Key understanding points:

• Pressure in a liquid increases with depth
• Pressure at a given depth is the same in all directions
• The shape of the container doesn't affect pressure at a particular depth
• This same principle applies to the atmosphere, though atmospheric density varies with height

Comparing atmospheric pressure to liquid pressure

While the same fundamental principles apply, there are important differences between atmospheric pressure and pressure in liquids:

Similarities:

• Both result from the weight of material above
• Both can be calculated using similar principles involving height, density and gravity
• Both increase with depth/decrease with height

Differences:

• Liquids are virtually incompressible — density stays constant with depth
• Air is compressible — density decreases significantly with altitude
• Liquid pressure changes linearly with depth (p = hρg works perfectly)
• Atmospheric pressure changes non-linearly with altitude (air density isn't constant)

This explains why the simple equation p = hρg gives accurate results for water in a swimming pool but only approximates atmospheric pressure changes.

Measuring atmospheric pressure

Barometers measure atmospheric pressure. The most common types are:

Mercury barometer:

• A tube filled with mercury is inverted in a mercury reservoir
• Atmospheric pressure pushes down on the reservoir surface
• This supports a column of mercury in the tube
• At sea level, the mercury column is approximately 760 mm high
• As atmospheric pressure changes, the column height changes

Aneroid barometer:

• Contains a sealed metal chamber with partial vacuum
• As atmospheric pressure changes, the chamber expands or contracts
• Mechanical linkages move a pointer across a scale
• More portable than mercury barometers
• Used in altimeters for aircraft

Digital barometers:

• Use electronic pressure sensors
• Display pressure readings digitally
• Commonly found in weather stations and smartphones

Worked examples

Example 1: Calculating pressure in a water column

Question: A swimming pool has a depth of 2.5 m. Calculate the pressure on the pool floor due to the water. The density of water is 1000 kg/m³ and g = 10 N/kg. (3 marks)

Solution:

Step 1: Write down the equation p = h × ρ × g (1 mark)

Step 2: Substitute values p = 2.5 m × 1000 kg/m³ × 10 N/kg (1 mark)

Step 3: Calculate p = 25,000 Pa or 25 kPa (1 mark)

Note: This is the pressure from the water only. The total pressure on the pool floor would be this plus atmospheric pressure (approximately 101,000 Pa), giving about 126,000 Pa total.

Example 2: Comparing pressures at different altitudes

Question: Explain why atmospheric pressure at the top of Ben Nevis (1,345 m) is lower than at sea level in London. (3 marks)

Solution:

At higher altitude, there are fewer air molecules above you / shorter column of air (1 mark)

This means there is less weight of air pressing down (1 mark)

Therefore pressure is lower / fewer collisions between air molecules and surfaces (1 mark)

Examiner note: To gain full marks, you must explain the cause (fewer molecules/less air), link it to weight, and state the consequence (lower pressure). Simply stating "there's less air" without explanation earns only 1 mark.

Example 3: Application question

Question: A sealed bag of crisps is packed at sea level where atmospheric pressure is 101 kPa. The bag is then taken on an aeroplane where cabin pressure is maintained at 80 kPa. Describe and explain what happens to the bag. (4 marks)

Solution:

The bag will expand / get bigger (1 mark)

The air pressure inside the bag remains at approximately 101 kPa / the pressure at which it was sealed (1 mark)

The external pressure (80 kPa) is now less than the internal pressure (1 mark)

The higher pressure inside pushes outwards more than the lower external pressure pushes inwards, causing expansion (1 mark)

Examiner note: This question requires description (what happens) and explanation (why it happens). Use of comparative language ("higher," "lower," "more than") and clear reference to both internal and external pressure is essential for full marks.

Common mistakes and how to avoid them

• Confusing pressure with force — Pressure is force per unit area (Pa), not just force (N). Always include the area when explaining pressure. Remember: pressure = force ÷ area.

• Thinking atmospheric pressure only acts downwards — Atmospheric pressure acts in all directions equally. Air molecules collide with surfaces from all angles, not just from above.

• Believing there's no air/pressure at high altitude — Pressure decreases with altitude but never reaches zero in Earth's atmosphere. Even at 10 km altitude, pressure is about 25% of sea level pressure.

• Incorrect unit conversions — Common errors include using cm instead of m for height, or g/cm³ instead of kg/m³ for density. Always convert to standard SI units: metres (m), kilograms (kg), pascals (Pa).

• Forgetting that air is compressible — Unlike liquids, air density changes with pressure and altitude. This means atmospheric pressure doesn't decrease linearly with height.

• Adding atmospheric pressure when not required — In questions about pressure difference or pressure due to a liquid column, you may not need to include atmospheric pressure. Read the question carefully to determine what's being asked.

Exam technique for atmospheric pressure

• Command word awareness — "Explain" requires reasons or mechanisms (worth 2-3 marks); "State" or "Give" needs facts only (1 mark each); "Calculate" requires equation, substitution, and answer with units (typically 3 marks).

• Show your working in calculations — Even if your final answer is incorrect, you can earn marks for correct equation selection and substitution. Write the equation, substitute values with units, then calculate.

• Use data from the question — If a question provides values for density, g, or standard atmospheric pressure, use those values rather than remembered ones. Questions may use g = 9.8 N/kg or g = 10 N/kg.

• Link particle behavior to pressure effects — Higher-tier questions may require you to explain pressure changes using particle theory. Mention molecular collisions, number of particles, and weight of air columns for complete explanations.

Quick revision summary

Atmospheric pressure (approximately 100 kPa at sea level) results from the weight of air molecules above a surface. Pressure decreases with altitude because there are fewer air molecules and less weight above. Calculate liquid column pressure using p = hρg, where pressure depends on height, density, and gravitational field strength. Air is compressible, so atmospheric density and pressure decrease non-linearly with height. Barometers measure atmospheric pressure. Remember that pressure acts in all directions, not just downwards, and always use correct SI units in calculations.