Forces - 5.5.2 Atmospheric Pressure (GCSE Physics AQA)

Atmospheric Pressure

Earth’s Atmosphere

• The atmosphere surrounds the Earth. The atmosphere is a thin layer of air around the Earth. This can be compared to an orange; the fruit is the Earth and the peel is the atmosphere.
• The atmosphere is relatively thin. In comparison to the size of the Earth, the atmosphere is quite thin. Again, the analogy or an orange is appropriate. If we consider the sizes of the peel and the fruit, we can see that the peel is much thinner than the fruit.

Atmospheric Pressure

• Atmospheric pressure is caused by particle collisions. When particles in the air collide with surfaces, they create atmospheric pressure. We previously mentioned that when a force is exerted onto an area, it results in pressure. In this case, the particles in the air are exerting forces onto surfaces, causing atmospheric pressure.
• Atmospheric pressure decreases with height. As the height above the surface of the Earth increases, the atmospheric pressure decreases.

Similarly, as the height above the Earth’s surface decreases, the atmospheric pressure increases.

There are two key reasons that explain why atmospheric pressure decreases with height:

• Changes in Density – as the height above a surface increases, there are fewer air molecules per unit area. This means that the higher up we go, the lower the density of the air. In turn, this will mean fewer collisions per unit area and therefore lower atmospheric pressure.
• Changes in Weight – as the height above a surface increases, the weight of the air above the surface will decrease. This decrease in weight causes a lower atmospheric pressure.