1Deeper means more pressure
The pressure in a liquid comes from the weight of the liquid above. The deeper you go, the more liquid there is above you, so the greater the pressure.
depth
Lesson 21.2 notes
Pressure in liquids
Liquid pressure increases with depth and pushes in every direction: dams, divers, submarines and brakes.
2Pushing in every direction
At any point in a liquid, the pressure pushes in all directions: down, up and sideways. Push the plunger and water squirts out of every hole in the ball.
3Depth, not width
In one liquid, only the depth matters. A narrow tube and a wide tank filled with water to the same depth have the same pressure at the bottom.
4Particles in a liquid
Liquid particles are packed closely together. The weight of the particles above presses down on those below, so the deeper they are, the harder they push.
5Dams
Water presses hardest on the bottom of a dam, where it is deepest, so dams are built much thicker at the bottom than at the top.
6Deep under the sea
Every 10 m of water adds about 100 kPa. At 100 m deep, the water adds about 1000 kPa: ten times the air pressure at the surface.
7Liquids can't be squashed
Liquids can't be squashed, so a push on a pipe of liquid passes along it. The same pressure on a bigger piston gives a bigger force: that's how hydraulic jacks and car brakes work.
hydraulic
Explore: Water tower
Change the depth of the water and choose a narrow or a wide tank. Watch the jets and the pressure gauge.
Common mistake: In one liquid, only the depth matters
A narrow pipe and a huge lake of water have the same pressure at the same depth: it depends on how deep you are, not on how much liquid there is. A denser liquid gives more pressure at the same depth.
Remember
- Pressure in a liquid increases with depth, because there is more liquid above pressing down.
- Liquid pressure pushes in every direction: down, up and sideways.
- In water, every 10 m of depth adds about 100 kPa. Dams are thickest at the bottom.
Going further at GCSE