Lesson focus: Pressure in a stationary liquid increases with depthHow far down something is below a surface, such as below the surface of water..
Explain liquid pressure using depth and densityThe mass per unit volume of a material (density = mass ÷ volume), measured in g/cm³ or kg/m³., then apply it to dams and hydraulics.
Build on earlier learning
Pressure in gasesBoth topics use force per area, but liquids show a depth pattern.
You will be successful when you can…
- Describe the depth pattern.
- Explain pressure directions.
- Compare depth and density.
- Apply the pattern to structures.
Words that carry the science
- fluid
- A liquid or gas that flows.
- depth
- Vertical distance below a surface.
- density
- Mass per unit volume.
- hydrostatic pressure
- Pressure in a stationary fluid due to gravity.
- hydraulic system
- A system transmitting pressure through confined liquid.
Depth controls pressure
01Depth controls pressure
In a stationary liquid, pressure increases with depthHow far down something is below a surface, such as below the surface of water. because a deeper point supports a taller liquid column.
At equal depthHow far down something is below a surface, such as below the surface of water. in the same connected liquid, pressure is equal regardless of container shape and acts in all directions.
02Density and gravity set the increase
The change is Δp = ρgΔh. A denser liquid creates a greater pressure increase at the same depth.
This is the liquid contribution; total pressure can also include atmospheric pressure at the surface.
03Pressure differences have applications
Dams are thicker at the bottom because deeper water exerts greater pressure and force.
Hydraulics transmit pressure. A larger output piston gives larger force but moves less distance; energy is not created.
Pressure in liquids
Pressure in a liquid increases with depth, because there is more liquid pressing down from above. Water squirts out fastest, and lands furthest away, from the deepest hole.
Controls
Pressure = density × g × depth (1000 kg/m³ × 9.8 N/kg). Jet speed = √(2 × g × depth). This is the pressure from the water alone; the air pushes on top as well.
Compare two depths
Compare pressure increase at 2 m and 6 m in one lake.
- densityThe mass per unit volume of a material (density = mass ÷ volume), measured in g/cm³ or kg/m³. is the same.
- Gravitational field is the same.
- 6 m is three times 2 m.
- Δp is proportional to depth.
How to make the reasoning visible: The liquid pressure increase is three times larger at 6 m.
Compare liquid pressures
Use relative ρh because g is common.
| Point | Liquid | Depth / m | Relative density | Relative ρh |
|---|---|---|---|---|
| A | water | 1 | 1.0 | 1 |
| B | water | 3 | 1.0 | 3 |
| C | salt solution | 1 | 1.2 | 1.2 |
| D | salt solution | 3 | 1.2 | 3.6 |
Rank pressures.
Why do equal-height holes spray similarly?
Check your understanding
Answer from memory, then check against the suggested answer.
How does pressure vary with depth?
Which directions?
Name two controlling quantities.
Why is a reservoir wall thicker at its base?
Compare points 20 cm below water in differently shaped containers.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A bottle of water has two identical holes, one near the top and one near the bottom. Water from the lower hole travels further.
Explain why the water pressure is greater at the lower hole.
(2)Explain why the water from the lower hole travels further.
(3)Total for this question: 5 marks
Command word: Link depth through pressure and force to the observation.