3. Pressure
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Pressure in gases P.7 of 13 
Gas particles produce a pressure animation
Picture 3.3 Gas particles colliding produce a pressure.

Why do gases cause pressure?
The particles in a gas are all moving about randomly. As they move around, they collide with the walls of their container. Each collision produces a tiny force on the wall trying to push it outwards.

A can of gas contains many billions of particles. Each of them bumps into the walls thousands of times every second. Although we can’t feel each individual bump, they average out to produce a constant pressure on the walls.

The pressure from the atmosphere
Picture 3.4 The pressure from the atmosphere is the same as we'd get from 10 metres of water.

Experiencing the atmosphere
The atmosphere is a mixture of gases and it pushes on us with atmospheric pressure. Although atmospheric pressure changes with the weather, it is usually about 100,000 N/m2. This is equivalent to 100,000 apples piled up on a square metre (it’s easy to remember that an apple weighs about a newton).

If we stacked up all those apples, they would form a pile that is about 10 m tall. So the pressure of the atmosphere is a bit like being under a 10 metre tall pile of apples.

Or we could replace the apples with water (apples are mainly water). So the pressure of the atmosphere is the same as lying under 10 m of water.

Feel that pressure - or not
However, we don’t feel the weight of atmosphere. This is because the pressure pushes in all directions. You might think that we don't feel the pressure - but you would certainly notice if the atmosphere disappeared (see next page).

Look at the balloon in picture 3.5. It is floating in the air. The air pressure pushes it up as much as it pushes it down. This is because the air particles are moving in all directions and push on the balloon from all sides. The number of air particles pushing up on the bottom of the balloon is the same as the number pushing down on the top.

Pressure acts in all directions graphic
Picture 3.5 The atmospheric pressure pushes in all directions.

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Question 7
a) Why is it easy to remember that an apple weighs about a newton?

b) The pressure of the atmosphere is 100,000 Pa. What is the pressure 10 metres under the sea? (Hint, remember the atmosphere is still there).

c) What is the pressure 20 metres under sea?

d) How deep can a diver go if he can withstand up to 5 times atmospheric pressure?

Summary                                           Close
  • the particles in a gas are all moving
  • the gas particles collide with container walls, making a pressure
  • atmospheric pressure is 100,000 Pa
  • gas pressure pushes in all directions - not just downwards