Section 1 of 6What a Fluid Does
Pressure in a fluid arises from the motion and weight of the particles inside it.
Fluid
- A fluid is any substance that can flow, so both liquids and gases are fluids.
Particle cause
- Fluid pressure is caused by particles colliding with a surface, and each collision exerts a tiny force.
All directions
- The pressure at a point acts equally in every direction, so a submerged object is squeezed on all sides.
Normal force
- The force the fluid exerts on a surface acts at 90° to that surface, however the surface is tilted.
Additive
- An object in a liquid feels the pressure of the liquid and, in addition, the pressure of the atmosphere above it.
Section 2 of 6The Atmosphere
We live at the bottom of an ocean of air.
1
The Earth is surrounded by a layer of air that extends for more than 100 km above the surface.
2
The weight of the air above any surface pushes down on it, creating atmospheric pressure.
3
At sea level this pressure is about 101 kPa, which is roughly 100 000 N on every square metre.
4
Higher up there is less air above, and the air is less dense, so the pressure decreases with altitude.
5
Weather changes the pressure slightly from day to day; settled, fine weather is usually linked with higher pressure.
Section 3 of 6Pressure in Liquids
Pressure in Liquids
Depth
- The deeper a point is in a liquid, the more liquid lies above it, so the pressure is greater.
Total pressure
- The pressure at depth is the liquid pressure plus the atmospheric pressure on the surface.
Direction
- The liquid pressure at one depth pushes sideways and upwards as well as downwards.
Gas comparison
- A gas can be compressed, but a liquid has particles already touching, so its volume barely changes under pressure.
| Location | Pressure |
|---|---|
| Sea level in air | About 101 kPa |
| 10 m under water | About 100 kPa extra, so about 200 kPa in total |
| High mountain | Less than 101 kPa |
Section 4 of 6Examples
Diving
- A diver feels the pressure increase as they descend, which is why deep dives need special training and equipment.
Aircraft cabins
- Air is pumped into the cabin of an aircraft because the outside pressure at cruising height is much too low.
Bursting balloon
- A weather balloon expands as it rises, because the pressure of the surrounding air falls.
Section 5 of 6Worked Example
Question
- Atmospheric pressure at the surface of a lake is 101 kPa.
- A diver is 20 m below the surface.
- Find the total pressure on the diver.
- (water density = 1000 kg/m³, g = 9.8 N/kg)
- Pressure due to the water = hρg = 20 × 1000 × 9.8 = 196 000 Pa = 196 kPa.
- Total pressure = water pressure + atmospheric pressure = 196 + 101.
- Total pressure = 297 kPa.
Answer The total pressure on the diver is about 297 kPa, nearly three times atmospheric pressure.
Section 6 of 6Exam Checklist & Tips
15.2.1 – Key Points for the Exam
- Fluid A liquid or a gas, a substance that can flow.
- Atmospheric pressure The pressure caused by the weight of the air above a surface.
- Pascal (Pa) The unit of pressure, equal to one newton per square metre.
- Normal At right angles to a surface.
Common Mistakes
- Saying that fluid pressure only acts downwards; it acts in all directions.
- Forgetting to add atmospheric pressure to the pressure caused by the liquid.
- Stating that atmospheric pressure increases with height; it falls.
- Writing the force from fluid pressure as parallel to the surface instead of at right angles.