GCSE Physics revision

Atmospheric pressure

Pressure and pressure differences in fluids (physics only)

AQA 4.5.5.2

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AQA student objectives

Learning pathway

ALL 🎯

:Describe a simple model of the Earth’s atmosphere and of atmospheric pressure • explain why atmospheric pressure varies with height above a surface.

MOST 🎯🎯

Apply the scientific explanation of atmospheric pressure to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving atmospheric pressure and explain the scientific reasoning.

Revision summary

Read on screen, then print for Cornell-style active revision.

When more air particles are added to a container, there are more collisions with the walls per second.
More collisions mean a greater total force on the walls, which increases the pressure inside the container.
This demonstrates why denser air (more particles in a given space) produces higher atmospheric pressure.

1. How More Particles Increase PressureAQA

When more air particles are added to a container, there are more collisions with the walls per second.
More collisions mean a greater total force on the walls, which increases the pressure inside the container.
This demonstrates why denser air (more particles in a given space) produces higher atmospheric pressure.

A balloon stays the same shape because the atmospheric pressure pushing inward equals the gas pressure from the particles inside pushing outward.
If the forces are balanced, the net force on the balloon wall is zero, so it neither expands nor contracts.
This principle shows that atmospheric pressure acts in all directions simultaneously.

2. Balanced Pressure in a BalloonAQA

A balloon stays the same shape because the atmospheric pressure pushing inward equals the gas pressure from the particles inside pushing outward.
If the forces are balanced, the net force on the balloon wall is zero, so it neither expands nor contracts.
This principle shows that atmospheric pressure acts in all directions simultaneously.

When a glass of water is covered with a card and turned upside down, the card stays in place because atmospheric pressure pushes upward on it.
The upward force from atmospheric pressure balances the downward weight of the water, preventing it from falling.
Atmospheric pressure is approximately 100,000 N/m² (100 kPa), which is large enough to support the weight of the water in the glass.
Atmospheric pressure can vary slightly, ranging from around 99 kPa on low-pressure days to over 100 kPa on high-pressure days.

3. The Card and Glass of Water DemonstrationAQA

When a glass of water is covered with a card and turned upside down, the card stays in place because atmospheric pressure pushes upward on it.
The upward force from atmospheric pressure balances the downward weight of the water, preventing it from falling.
Atmospheric pressure is approximately 100,000 N/m² (100 kPa), which is large enough to support the weight of the water in the glass.
Atmospheric pressure can vary slightly, ranging from around 99 kPa on low-pressure days to over 100 kPa on high-pressure days.

A small amount of water is boiled in a drinks can until steam fills the space above the water, replacing most of the air inside.
When the can is quickly plunged into cold water, the steam condenses rapidly, greatly reducing the number of particles inside the can.
With far fewer particles inside, the internal pressure becomes much lower than the external atmospheric pressure.
The unbalanced forces cause the can to crush inwards almost instantly, demonstrating the enormous magnitude of atmospheric pressure.

4. The Crushing Can ExperimentAQA

A small amount of water is boiled in a drinks can until steam fills the space above the water, replacing most of the air inside.
When the can is quickly plunged into cold water, the steam condenses rapidly, greatly reducing the number of particles inside the can.
With far fewer particles inside, the internal pressure becomes much lower than the external atmospheric pressure.
The unbalanced forces cause the can to crush inwards almost instantly, demonstrating the enormous magnitude of atmospheric pressure.

At higher altitudes, the density of air decreases because there are fewer air particles per unit volume.
Fewer particles means fewer collisions with surfaces, resulting in a smaller force and therefore lower pressure.
At the Earth's surface, there is a greater weight of air above pressing down, which also contributes to higher pressure at lower altitudes.
This is analogous to liquid pressure, where pressure increases with depth due to the greater weight of liquid above.

5. Why Atmospheric Pressure Decreases with HeightAQA

At higher altitudes, the density of air decreases because there are fewer air particles per unit volume.
Fewer particles means fewer collisions with surfaces, resulting in a smaller force and therefore lower pressure.
At the Earth's surface, there is a greater weight of air above pressing down, which also contributes to higher pressure at lower altitudes.
This is analogous to liquid pressure, where pressure increases with depth due to the greater weight of liquid above.

Atmospheric pressure decreases with increasing height above the Earth's surface, just as liquid pressure increases with depth.
The relationship for pressure due to a column of fluid is P = h × ρ × g, where h is height, ρ is density, and g is gravitational field strength.
A greater weight of air above a point means a greater force per unit area, and therefore greater atmospheric pressure at that point.

6. The Pressure–Height RelationshipAQA

Atmospheric pressure decreases with increasing height above the Earth's surface, just as liquid pressure increases with depth.
The relationship for pressure due to a column of fluid is P = h × ρ × g, where h is height, ρ is density, and g is gravitational field strength.
A greater weight of air above a point means a greater force per unit area, and therefore greater atmospheric pressure at that point.