7.4 Atmospheric pressure and variation with height and suction caps!

What causes pressure in the atmosphere?

Why does atmospheric pressure vary with height?

The density of gases varies considerably with temperature and pressure.

Gases are very compressible because of the space between the particles.

You can squeeze the particles of a gas closer to together if a force is applied to them.

If you increase the temperature and the gas can expand, the density will decrease.

 Gas particle model reminder!

Air pressure is caused by the collisions between and molecules colliding with any surface.

The atmosphere is a mixture of gases (mainly ~1/5th oxygen, 4/5th nitrogen) that surrounds the surface of the Earth.

The atmosphere is relatively thin compared to the radius of the Earth but does stretch upwards for ~100 km, so there is quite a weight of air pushing down on us creating what we experience as atmospheric pressure.

However, the force acts in all directions so the internal pressure inside our body is the same as the external pressure beyond our skin.

Therefore we do not experience this pressure, we are unaware of it and we don't change in size!

One of the best demonstrations of atmospheric pressure is to pump the air out of a big steel can (empty car oil can is great).

A internal air is removed and the internal air pressure decreases, the external air pressure crushes the can inwards.

If you haven't got a suitable pump, if you boil water in the can and fill it with steam, screw the cap on and leave to cool.

As the steam condenses, the internal gas pressure decreases and the greater external air pressure crushes the can sides in with great sound effects and wicked distortions of its original shape.

You should observe that the can's sides collapse in all directions because pressure in a fluid acts in all directions.

You need to be able to explain this effect by considering the relative number of particle collisions on either side of the can walls, hence the relative total force and pressure differences.

A mention of suction caps, what do we use them for and how do they work!

Suction caps are used in many commercial and industrial applications e.g.

To fix objects to nonporous smooth vertical surfaces such as refrigerator doors and tiled walls.

To safely move large smooth objects such as panes of glass or automobile windscreens.

You can buy toy darts that will stick on a smooth dart board.

So, how do they work?

How a suction cap works

A suction cap is made of a flexible plastic or rubber materials.

In sticking the suction cap on a surface you squeeze out most of the air and on release you create a partial vacuum. This reduces the number of particle collisions on the 'internal' surface of the suction cap.

However, the external surface of the suction cap, experiences all the collisions of air in contact with the surface.

Therefore the external pressure on the outer surface is much greater than the internal pressure and the pressure difference sticks the suction cap on the smooth surface.

It must be a smooth surface, otherwise air molecules will leak out though any microscopic gap, reducing the pressure and whatever is held by the suction cap falls off.

Atmospheric pressure, causes and variation with height above the Earth's surface

The graph on the left shows in principle how the atmospheric pressure varies with height above the Earth's surface (altitude). Atmospheric pressure is also referred to as barometric pressure i.e. as measured by a barometer.

At the surface (height of zero km taken as sea level) it is normally close to an average of 101300 Pa (~101 kPa).

At the top of the world's highest mountain, Mount Everest in Nepal, the air is much thinner at ~8800 m above sea level. Here the pressure is only ~33000 Pa (~33 kPa) which is why breathing is much more difficult. Although your internal and external body pressures are equal, you take in less air-oxygen in each breath so all physical work is much harder than at sea level.

The first mountaineers to reach the summit used cylinders of oxygen, but today's super-fit climbers can manage on just thin air! The local Sherpa's come from an ethnic mountain population that have evolved in several ways to cope with the local conditions e.g. their mitochondria are more efficient at using oxygen in respiration and blood flow in small blood vessels doesn't decrease as much as happens with non-Sherpa people.

The atmospheric pressure around us is caused by the collision of air molecules on any surface AND, quantitatively, by the weight of the gas above you (note there are two contributions to atmospheric pressure).

So why does atmospheric pressure vary with height?

At very high altitudes there is little air, far few collisions, less weight of gas above and so the pressure tends towards zero Pa.

The greater the height/depth of a gas, the greater the weight of particles that gravity is pulling down to the Earth's surface, hence the increase in force per unit area the lower the level i.e. increase in pressure towards the Earth's surface - where the atmospheric pressure will be the greatest.

As you increase in height above the Earth' surface (increase in altitude) the atmospheric pressure decreases.

This is because the air is less dense and so less collisions can take place in a given volume AND there is less weight of molecules above a given altitude created by the downward force from the Earth's gravitational field.

Therefore the greatest atmospheric pressure will be the greatest at the Earth's surface.

To express and explain the trend in another way:

The increase in pressure the nearer you are to the Earth's surface, is due to the greater density - hence more collisions in the same volume AND the greater the weight of air above you - the greater force per unit area.

The weight of air above a certain height compresses the atmospheric gases below that level and compression means increase in pressure (ignoring any temperature differences) from more collisions between molecules.

Just as with liquid fluids discussed above, gases are fluids and the weight of them acting downwards creates a pressure in the same way AND acting in all directions.

At a given height above the Earth's surface, there is very little variation in the density and pressure of the atmosphere.

However, most weather systems are driven by regions of higher or lower pressure compared to the average atmospheric pressure at that height.

If you look at weather charts on the TV weather forecast you will see a 'high' (H) area with a number like 1029 by it, conversely, a 'low' (L) might have a number like 986 by it.

The average surface atmospheric pressure is ~1000 millibars (but don't worry about this unit, but 1 millibar = 0.1 kPa).

Barometers are used to measure atmospheric pressure and can indicate weather changes e.g. rise in barometric (fair weather, more sunny) or fall in barometric pressure (poorer weather e.g. rain).

Since the atmosphere gets less dense (less pressure) on breathing you take in less oxygen as you ascend to greater heights above the Earth's surface.

This is why many early mountaineers carried cylinders of oxygen to assist more efficient breathing.

Atmospheric pressure can be measured with a mercury barometer, though this is being replaced by electronic pressure transducers.

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INDEX for physics notes: pressure, forces, weight and upthrust in fluids