Pressure in a Liquid (Cambridge (CIE) IGCSE Physics): Revision Note

Liquid pressure

Factors affecting pressure in a liquid

• The pressure beneath the surface of a liquid will increase with the depth of the liquid

  • This is because the pressure in a liquid is caused by the force of the weight of the liquid above it

  • As the depth increases, there is more liquid above which causes the pressure to increase

• Pressure in a liquid also depends on the density of the liquid

  • The more dense the liquid, the greater the pressure it exerts

  • Sea water is more dense than fresh water, so the pressure exerted on a object will be greater in sea water than in fresh water at the exact same depth

Pressure in a column of water

• In a column of water, the highest pressure would be at the bottom

  • If a hole is made at the bottom of the column, the water will pour out with a large force

  • If a hole is made at the top of the column, the water will pour out with a small force

  • This is because of the difference in pressure in the column caused by the weight of the water

Pressure in a column of water increases with depth, shown by the strong and weak jet of water

• If several holes are made at the same height, the water would spurt out the same distance from each hole

  • This is because pressure at the same depth acts equally in all directions

Calculating liquid pressure

Extended tier only

• The pressure acting on an object in a liquid changes with depth

  • The deeper the object, the higher the pressure exerted upon it and vice versa

• The change in pressure with depth is given by:

$\mathrm{\Delta }p=\rho g\mathrm{\Delta }h$

• Where:

  • Δp = change in pressure, measured in pascals (Pa)

    • Where 1 Pa = 1 N/m²

  • ρ = density of the liquid, measured in kilograms per metre cubed (kg/m³)

  • g = gravitational field strength, measured in newtons per kilogram (N/kg)

  • Δh = change in height of the column, measured in metres (m)

A liquid exerts pressure on every surface of a submerged object, acting at right angles to each surface