AQA GCSE Physics

Revision Notes

5.5.2 Atmospheric Pressure

The Atmosphere

  • 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 100 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

Atmospheric 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

Exam Tip

Explaining why atmospheric pressure decreases with increasing altitude is a common exam question – make sure you can summarise these points!

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