AQA GCSE Physics

Revision Notes

5.5.3 Pressure in a Liquid

Higher Tier Only

Factors Affecting Pressure in a Liquid

  • In a liquid, the pressure at a point increases with the height of the column of liquid about that point
    • If there is more liquid above that point, then the pressure is more
  • This is because the pressure in a liquid is caused by the weight of the liquid pushing against objects immersed in the liquid
    • As the liquid becomes deeper, the amount of liquid (and hence the weight) increases which causes the pressure to increase
  • This is why, for example, the pressure increases with the depth of the ocean
    • The pressure on the seabed is far higher than that on the surface of the ocean
  • The weight of the liquid also depends on its density
    • A more dense liquid has a greater weight and therefore will exert a higher pressure

Liquid Pressure, downloadable IGCSE & GCSE Physics revision notes

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

Calculating Pressure in a Liquid

  • The pressure due to a column of liquid can be calculated using the equation

p = h × ρ × g

  • Where:
    • p = pressure in pascals (Pa)
    • h = height of the column in metres (m)
    • ρ = density of the liquid in kilograms per metre cubed (kg/m3)
    • g = gravitational field strength on Earth in newtons per kilogram (N/kg)
  • The force from the pressure is exerted evenly across the whole surface of an object in a liquid, and in all directions

pressure-in-liquids, IGCSE & GCSE Physics revision notes

The force from the pressure of objects in a liquid is exerted evenly across its whole surface

  • The pressure is more accurately the difference in pressure at different depths h in a liquid, since the pressure changes with the depth

Worked Example

Calculate the depth of water in a swimming pool where a pressure of 20 kPa is exerted.

The density of water is 1000 kg/m3 and the gravitational field strength on Earth is 9.8 N/kg.


Exam Tip

This pressure equation will be given on your formula sheet, however, make sure you are comfortable with rearranging it for the variable required in the question!

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