Example 1:

Experiment to show the effect of air pressure on water. This simple action with a straw - which you have probably done many times, is a good demonstration of the basic pump theory discussed at the beginning of this pump training.

Method: Place a straw in water, cover with finger. Then lift (Fig.1) and lower (Fig.2) the straw, keeping the end under water. Observe the difference in level, and also what happens to the liquid when you remove the straw from the liquid.

The water cannot be compressed, but the air compresses and expands easily.

In Fig.1 the straw is raised, as the water level drops the air takes up more room which means its pressure is lower than air pressure acting on the surface of the water in the glass. As the straw is raised further, the air pressure gets so low that the straw can be removed from the glass without the water escaping the straw. The water can't leave the straw because air pressure from outside the straw is trying to get to the low pressure in the straw, but can't get past the water.

In Fig.2 the air is being compressed to a higher pressure than the air pressure acting on the surface of the water in the glass, so the level in the straw will be below the level of the water in the glass.

Example 2:

Air pressure Vs gravity:
Water bottle example - you get a cup of water and after pouring a small amount the flow slows and then virtually stops. If you open the top you will get full flow again.

As soon as any water is removed, the air inside the bottle is taking up more space than before, so it will reduce in pressure. The air outside the bottle is at higher pressure and tries to push its way into the bottle to equalise pressures. When the air pressure difference is the same as the height of the water trying to flow out due to gravity, the flow will stop, and only resume if air is added to the bottle as bubbles or if the top lid is opened.

Some air may bubble back up to try to equalise the air pressure inside and outside of the bottle. A fuel can has a larger hole and so more air can get into the can as bubbles and large gulps that increase the pressure of the air in the can closer to the atmospheric pressure outside the can allowing flow. Breather holes can assist in equalising the pressure inside and outside the container.

Air pressure wins against gravity in cases where water height is less than 10m (one atmosphere).

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