Air pressure - force of air particles

Do you know what air pressure is? It's definitely not something like blood pressure. Is it? Air pressure is the force applied on you by the weight of tiny particles of air. Now you might be wondering how these invisible molecules of air can have a weight and take up space, but it is what really happens. Since there's a lot of 'empty' space between air molecules, air can be compressed to fit into a smaller volume.

When it's compressed, air is said to be 'under high pressure'. Air at sea level is what we're used to living in, and we forget we're actually feeling air pressure all the time! Weather forecasters measure air pressure with a barometer. Barometers are used to measure the current air pressure at a particular location in 'inches of mercury' or in 'milli bars' (mb).

A measurement of 29.92 inches of mercury is equivalent to 1013.25 milli bars. The Earth's atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimetre (14.7 pounds per square inch).

The force on 1,000 square centimetres (a little larger than a square foot) is about a ton! Remember that you have air inside your body too, that air balances out the pressure outside, so you stay nice and firm and not like jelly.

What happens if air pressure changes?

If you've ever been to the top of a tall mountain, you may have noticed that your ears pop and you need to breathe more often than when you're at sea level. As the number of molecules of air around you decreases, the air pressure to decreases.

This causes your ears to pop in order to balance the pressure between the outside and inside of your ear. Since you are breathing fewer molecules of oxygen, you need to breathe faster to bring the few molecules there are into your lungs to make up for the deficit.

As you climb higher, air temperature decreases. Typically, air temperatures decrease about 3.6 degrees Fahrenheit per 1,000 feet of elevation.

Air pressure experiments

While holding your hand over your ribs, take a deep breath and observe what happens to your chest. Did you feel it expand? Did you see it expand? How would you explain what happened? Your chest expands because, like blowing up a balloon, you are increasing the number of air molecules inside your lungs.

This causes your lungs to expand in order to provide space for the increased number of air molecules.

Blow up a balloon and observe what happens. Does it expand? Why does it make a noise when it's popped? When a balloon is blown up, the air pressure inside the balloon slowly becomes greater than the air pressure outside the balloon. Since the balloon is made of rubber and is expandable, it grows larger and larger.

When the balloon is popped, the air escapes instantly. The sound you hear is from the molecules of air inside the balloon coming into sudden contact with the molecules of air outside the balloon.

Get an empty plastic bottle with a screw top. Fill about a quarter of it with very hot water. Cap it tightly and let it stand for about an hour. What did you expect to happen?

The bottle will crumple in on itself. When you added the hot water, it caused the air temperature inside the bottle to rise. While the container was sealed no air could get into or out of the bottle. When the water inside the bottle was cool, the air was also cool and caused the pressure inside the bottle to decrease.

As the pressure on the inside walls of the bottle decreased, its walls collapsed. This happened since there wasn't enough air pressure inside the bottle to offset the air pressure on the outside of the bottle!

Compiled by Janani Amarasekara