Pressure in a Liquid (AQA GCSE Physics)

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

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

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

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