GCSE Physics revision

Pressure in a fluid 1

Pressure in a fluid

AQA 4.5.5.1.1

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Explain that liquids and gases are fluids whose pressure exerts a force normal to a surface, and calculate pressure using p = F/A.

MOST 🎯🎯

Apply the specified scientific knowledge of pressure in a fluid 1 to a relevant example.

SOME 🎯🎯🎯

Analyse an unfamiliar example of pressure in a fluid 1 using the specified scientific ideas.

Revision summary

Key knowledge

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

How Gas Particles Create Pressure

Gas particles move in completely random directions inside a container, rebounding off walls when they collide.
Each collision with a wall exerts a force, and since pressure is force per unit area (KaTeX parse error: Unexpected character: ' ' at position 5: P =

̲rac{F}{A}), these collisions

create pressure.
The total pressure depends on two key factors: the number of collisions with the walls and the force involved in each collision.

Temperature and Pressure

Heating a gas transfers energy to the particles' kinetic energy stores, causing them to move faster.
Faster-moving particles collide with the container walls more frequently and with greater force.
Since both the number and force of collisions increase, pressure increases with temperature.

Concentration and Pressure

Increasing the number of particles in a fixed volume increases the concentration of the gas.
A higher concentration means more particles are available to collide with the walls, increasing the number of collisions.
More collisions result in a higher pressure inside the container.

Volume and Pressure

Decreasing the volume of a container while keeping the number of particles the same increases the concentration of particles.
With less space to travel, particles collide with the walls more frequently.
As volume decreases, pressure increases — volume and pressure have an inverse relationship.

Expandable Containers (e.g. Balloons)

In a flexible container like a balloon, the walls can expand or shrink in response to changes inside.
An increase in temperature or concentration causes the balloon to expand rather than simply increasing pressure.
In reality, both volume and pressure are likely to increase, as the balloon can only expand so much.

6. Expandable Containers (e.g. Balloons)AQA

Notes

In a flexible container like a balloon, the walls can expand or shrink in response to changes inside.
An increase in temperature or concentration causes the balloon to expand rather than simply increasing pressure.
In reality, both volume and pressure are likely to increase, as the balloon can only expand so much.

Summary of Key Relationships

Increasing temperature → particles move faster → more frequent and more forceful collisions → higher pressure.
Increasing concentration (more particles, same volume) → more collisions → higher pressure.
Decreasing volume (same number of particles) → higher concentration → more collisions
→ higher pressure.

7. Summary of Key RelationshipsAQA

Notes

Increasing temperature → particles move faster → more frequent and more forceful collisions → higher pressure.
Increasing concentration (more particles, same volume) → more collisions → higher pressure.
Decreasing volume (same number of particles) → higher concentration → more collisions
→ higher pressure.

How Gases Exert Pressure

Gas particles move freely and randomly around a container, frequently colliding with the container walls.
Each collision causes the wall to exert a reaction force on the particle, changing its direction, whilst the particle simultaneously exerts a force on the wall at right angles to it.
When all particles are considered together, their countless collisions produce an outward pressure that is spread evenly across the container walls.

8. How Gases Exert PressureAQA

Notes

Gas particles move freely and randomly around a container, frequently colliding with the container walls.
Each collision causes the wall to exert a reaction force on the particle, changing its direction, whilst the particle simultaneously exerts a force on the wall at right angles to it.
When all particles are considered together, their countless collisions produce an outward pressure that is spread evenly across the container walls.

How Liquids Exert Pressure on the Base

In a liquid, particles fall to the bottom of a container due to gravity, exerting a downward pressure on the base.
Particles higher up in the liquid are also pulled down by gravity, pushing down on the particles below and increasing the pressure on the base.
This means that liquid pressure on the base increases with the depth of the liquid above.

9. How Liquids Exert Pressure on the BaseAQA

Notes

In a liquid, particles fall to the bottom of a container due to gravity, exerting a downward pressure on the base.
Particles higher up in the liquid are also pulled down by gravity, pushing down on the particles below and increasing the pressure on the base.
This means that liquid pressure on the base increases with the depth of the liquid above.

How Liquids Exert Pressure on the Walls

Liquid particles cannot move straight down due to other particles being in the way, so they are deflected sideways into gaps.
These sideways forces cause particles near the container walls to be pushed outwards, creating an outward pressure on the walls.
Like gas pressure, liquid pressure on the walls acts at right angles to the surface of the container.

10. How Liquids Exert Pressure on the WallsAQA

Notes

Liquid particles cannot move straight down due to other particles being in the way, so they are deflected sideways into gaps.
These sideways forces cause particles near the container walls to be pushed outwards, creating an outward pressure on the walls.
Like gas pressure, liquid pressure on the walls acts at right angles to the surface of the container.

Pressure on Submerged Objects

Fluids also exert an inward pressure on any object submerged within them.
Gas particles collide with a submerged object from all directions, and the combined forces of these collisions create an inward pressure on the object.
Liquid particles press inward against a submerged object due to gravity and the pushing action of surrounding particles, resulting in an inward pressure on all sides.

11. Pressure on Submerged ObjectsAQA

Notes

Fluids also exert an inward pressure on any object submerged within them.
Gas particles collide with a submerged object from all directions, and the combined forces of these collisions create an inward pressure on the object.
Liquid particles press inward against a submerged object due to gravity and the pushing action of surrounding particles, resulting in an inward pressure on all sides.

Key Exam Points on Fluid Pressure

For your GCSE exam, you may be asked to explain the cause of pressure from either a gas or a liquid, so you must be confident with both explanations.
Gas pressure is caused by particle collisions with container walls, whilst liquid pressure is caused by the weight of particles and their sideways interactions.
In both cases, the pressure acts at right angles to the surface it is applied to.

12. Key Exam Points on Fluid PressureAQA

Notes

For your GCSE exam, you may be asked to explain the cause of pressure from either a gas or a liquid, so you must be confident with both explanations.
Gas pressure is caused by particle collisions with container walls, whilst liquid pressure is caused by the weight of particles and their sideways interactions.
In both cases, the pressure acts at right angles to the surface it is applied to.