Pressure in Fluids (Edexcel GCSE Physics) : Revision Note

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

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

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

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:

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 decreases as the density of the molecules decreases