Liquid pressure and depth
Open holes at different depths in a tall container of water and oil, and compare how hard the liquid squirts out.
Core concept
Pressure in a liquid increases with depth and with the liquid's density (p = ρgh)
Topic: Pressure
A liquid presses on everything in it and on the walls of its container. The deeper you go, the more liquid there is above, so the pressure: How much force presses on an area. A sharp knife has high pressure at its edge. All key words is greater: p = ρ × g × h, where ρ (rho) is the density: How much mass is packed into a space. Things less dense than water float. All key words of the liquid, g is about 9.8 N/kg and h is the depth. At the same depth the pressure is the same in every direction. A denser liquid gives a bigger pressure at the same depth. That is why dams are built thicker at the bottom.
What you will learn
- Show that liquid pressure increases with depth.
- Calculate liquid pressure using p = ρgh.
- Explain why dams are thicker at the base.
Before you start
- Know that pressure = force ÷ area.
Materials
- Water and cooking oil
Equipment
- Tall plastic bottle with holes at 10 cm, 20 cm, 30 cm and 40 cm, each plugged with tape
- Stand 30 cm high and a tray to catch the water
Procedure
- Open the hole 10 cm below the surface and record the pressure and where the jet lands.
- Open the holes at 20 cm, 30 cm and 40 cm, one at a time, and record each.
- Fill the container with oil and repeat for one depth.
How this simulation is simplified
- g is 9.8 N/kg and the container is kept topped up, so the depth does not change.
- Air pressure acts on both the surface and the jet, so only the liquid's own pressure is shown.
- The holes are 80 cm above the bench at the surface level (container 50 cm tall on a 30 cm stand).