3. Pressure
Balancing pressures P.8 of 13 
Particles in a balloon balance the atmosphere
Picture 3.6 The pressure on the inside balances the pressure on the outside.

Why do gases cause pressure?
Picture 3.6 shows a balloon filled with helium. The air is pushing in on it from all directions because of atmospheric pressure. Why doesn’t the balloon get squashed?
Staying inflated
The balloon isn’t squashed because it is filled with a gas - helium. The helium particles are moving around inside the balloon and colliding with its walls. The pressure from the helium gas pushes out and stops the balloon from collapsing.
What if we removed the atmosphere?
If aliens came to Earth and sucked away the atmosphere, then the balloon would explode. This is because the gas pressure inside the balloon would no longer be balanced by the air pressure outside. There would be nothing to stop the balloon expanding and bursting (unless it were made from very strong rubber).
The atmosphere
Picture 3.7 How would you balance atmospheric pressure if aliens removed the atmosphere. Water or mercury?

Staying alive?
If aliens removed the atmosphere, we would be in trouble. Even if you had some breathing apparatus, you’d have to think fast. Bits of your body would start bursting because of the pressure inside them (this pressure would normally balance the atmospheric pressure). You could stop this by replacing the atmosphere with, for example, 10 metres of water. This depth of water produces the same pressure as the atmosphere (see page 7).
Mad as a hatter
Mercury is much more dense than water. This means that you could get the same pressure under a smaller pool of mercury. You would only need 76 cm of mercury. However, the mercury is poisonous and would drive you mad first. In the days when hat makers used mercury, the vapour would slowly drive them mad - hence the expression "as mad as a hatter".

Click here to find out how a mercury barometer can be used to measure atmospheric pressure.

Strong canisters
The pressure inside an aerosol is bigger than atmospheric pressure. It ranges from 2 to 8 times the pressure of the atmosphere. This means that the can has to be very strong to withstand the difference in pressure between the inside and the outside.
Photo of sheet of cans
Picture 3.8a Welded tinplate cans start as a flat sheet. They are then formed and welded.
Can construction
The first aerosol was patented in 1929 in Norway. The container was made with thick brass walls to withstand the pressure inside the can - not very convenient.

Modern aerosol cans don't use such a high pressure. So they are much lighter and can be mass-produced. There are two main methods of construction.

Welded tinplate. These cans start life as a flat sheet of tinplated steel. The steel may have a protective lacquer coating to ensure the product doesn't corrode the can. A design is printed onto the steel, which is then cut and rolled into a cylinder. The two edges are welded together using an electric arc welder. The ends of the cylinder are shaped and attached to the cone (at the top) and the dome (at the bottom).

Photo of extruded can
Picture 3.8b The extruded aluminium can on the left started as a slug of aluminium (on the right). It was pressed out using a die.
Extruded aluminium. The bottom and cylindrical side of these cans are made from a slug of aluminium. It is punched into a die which presses out the can shape in one go, making a can without a seam. The can is lacquered and shaped at the top.
Question 8
The pressure under a fluid is proportional to its density. 10 m of water gives the same pressure as 76 cm of mercury.
a) How many centimetres are there in 10 metres?

b) About how much more dense is mercury than water?

c) The density of water is 1 g/cm3. What is the density of mercury (in g/cm3)?

d) Air has a density of about 0.001 g/cm3. From this, about how deep is the atmosphere of air?

e) The pressure inside a helium balloon will actually be more than the pressure outside. Explain why.
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Summary                                           Close
  • the pressure of the atmosphere pushes in on us
  • there is a pressure inside us that balances atmospheric pressure
  • the pressure in aerosol cans is up to 8 times atmospheric pressure