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PHYSICS · Stage 8 · Chapter 6: Forces, Motion and Pressure

Physics 6.16 Pressure in liquids — Depth, Density and Hydraulic Systems

5 min read 6 parts Lesson 16 of 19

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Physics 6.16 Pressure in liquids — Depth, Density and Hydraulic Systems — Alhussan International Academy — Physics (Stages 7–9) notes: Explain liquid pressure using particles, why it increases with depth and density, and how hydraulic systems work.Liquid pressure — It is due to f…

  • All directions
  • Demonstration
  • Incompressible
  • Syringe of water
  • Consequences
  • Pressure gauge

Part 1 of 6How liquid pressure arises

  • Liquid pressure — It is due to forces between the liquid particles and the surfaces of a container.
    • It is different from the pressure of one solid on top of another.
  • Particles — Liquid particles are very close together and free to move, and they collide with each other and with the walls.
  • All directions — The pressure in a liquid acts in all directions, so however you move your head under water at the same depth the pressure feels the same.
  • Demonstration — Water squeezed from a bag with holes comes out at 90° to the surface.
    • The curved path of the jet is due to gravity.
  • Upthrust — Liquid pressure produces upthrust, the force that keeps things afloat.

Part 2 of 6Liquids are incompressible

  • Incompressible — The particles are very close together, so a liquid cannot be squashed into a smaller volume.
  • Syringe of water — With the outlet blocked, pushing the plunger does nothing; the force increases the forces between particles instead.

Part 3 of 6Depth and density

  • Depth — Liquid pressure at a point depends on the weight of liquid above it.
    • There is twice as much water above a point at the bottom of a lake as half way down, so the pressure there is double.
    • A container with holes at different heights shows water leaving faster from the deeper hole.
  • Density — A denser liquid has more particles in each cubic metre, so the pressure at the same depth is greater.
  • Equation — Pressure = depth × density × g, so doubling the depth doubles the pressure.
  • Consequences — Dam walls are thicker at the bottom, and deep-sea divers use diving vessels or submarines with very strong walls.

Part 4 of 6Measuring pressure

  • Pressure gauge — It measures the pressure in a liquid or gas.
  • Bourdon gauge — It contains a coiled metal tube.
    • The tube straightens as the pressure rises.
    • This moves a needle to show a reading on a scale.
    • Gauges have a maximum pressure they can measure.

Part 5 of 6Hydraulic systems

  • PrinciplePressure applied to a liquid in a closed system is passed on equally to every part of the liquid.
  • Small pistonA small force on a small piston produces a pressure in the liquid.
  • Large pistonThe same pressure acts on a large piston and, since F = P × A, produces a much larger force.
  • UsesCar brakes, hydraulic lifts and diggers all use this principle.

Part 6 of 6Investigating liquid pressure

  1. Fill a tall plastic bottle with three holes at different heights with water.
  2. Measure how far the jet from each hole travels sideways with a ruler.
  3. Keep the hole size and liquid the same and keep the water level topped up.
  4. Repeat for reliability; the lowest hole gives the longest jet because the pressure is greatest.
  5. To test density, change the liquid at the same depth and compare.

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