Atmospheric Pressure (OCR Gateway GCSE Physics)

• 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

• Scientists identify different layers of the atmosphere according to: 
  • The height above the Earth's surface
  • The characteristics of the gases there

The Earth's atmosphere is extremely thin relative to its diameter

The Atmosphere as a Fluid

• A fluid is either a liquid or a gas
• When an object is immersed in a liquid, the liquid exerts a pressure upon the object
  • This liquid 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

• Therefore, overall pressure arises due to both:
  • The water (liquid) pressure
  • Atmospheric (gas) pressure

• These are both sources of fluid pressure

Fluid pressure due to both a gas (the atmosphere) and due to a liquid (water)

• 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