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 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 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! Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close Close