Pressure in Fluids (Edexcel GCSE Physics): Revision Note

Exam code: 1PH0

Katie M

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Katie M

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Pressure in a Fluid

  • A fluid is either a liquid or a gas

  • When an object on the Earth's surface is immersed in a liquid, the liquid exerts a pressure upon the object

    • This pressure is in addition to the pressure already exerted by the atmosphere

  • For example, an object at sea level (on the surface of the sea) experiences a pressure of 101 kPa due to the atmosphere

  • If this object is now immersed to a depth of 10 metres underwater, it experiences an extra pressure of 100 kPa due to the water

  • This means that the object will experience a total pressure of

101 kPa + 100 kPa = 201 kPa

  • This fluid pressure arises due to both:

    • The water (liquid) pressure

    • Atmospheric (gas) pressure

Atmospheric and Water Pressure, downloadable IGCSE & GCSE Physics revision notes

When an object is immersed in a liquid, it experiences pressure due to both the liquid and the atmosphere

The Force Exerted by a Fluid

  • When an object is immersed in a fluid, the fluid will exert pressure, squeezing the object

    • This pressure is exerted evenly across the whole surface of the fluid and in all directions

    • The pressure exerted on objects in fluids creates forces against surfaces

    • These forces act at 90 degrees (at right angles or 'normal') to the surface

pressure-&-force, IGCSE & GCSE Physics revision notes

The pressure of a fluid on an object creates a force normal (at right angles) to the surface

Atmospheric Pressure

  • The Earth's atmosphere is a thin layer (relative to the size of the Earth) of air around it

    • It exerts a pressure of about 101 kPa at sea level

Earth's-atmosphere, IGCSE & GCSE Physics revision notes

The Earth's atmosphere

  • The atmosphere extends more than 100 km into space and becomes less dense with increasing altitude (height above sea level)

    • This means that the pressure becomes less too

  • Atmospheric pressure various slightly from day to day, depending on the weather, and fine clear weather is usually associated with high pressure

  • The graph below shows how the pressure varies with altitude:

Atm Pressure Altitude Graph, downloadable IGCSE & GCSE Physics revision notes

Graph of atmospheric pressure against altitude

  • Atmospheric pressure varies with height above a surface, for example, at sea level

  • This is due to air molecules colliding with a surface creating atmospheric pressure

    • These molecules create a force per area of the surface which creates the pressure

  • The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases

    • This is also known as the density of the air

  • Therefore, as height increases, there is always less air above a surface than there is at a lower height and the atmospheric pressure decreases with an increase in height

Atmospheric Pressure, downloadable IGCSE & GCSE Physics revision notes

Atmospheric pressure decreases as the density of the molecules decreases

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Katie M

Author: Katie M

Expertise: Physics Content Creator

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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