Air Pressure

What is air pressure?

• The weight of the air that is above a horizontal area.

• The pressure at point X is the weightof the column of air above the horizontalsurface of unit area. Image: http://www.weather.gov.hk/education/edu01met/wxobs/pressure/pres-fig1e.jpg

The amount of air pressure on earthAir that exists at different heights has different amounts of air pressure. Like water pressure, the deeper the sea, thegreater the pressure because theweight of the water has increased. Hence, the higher the place is, the lower the air pressure. Conversely, the lower the place is, the higher the air pressure. blue dots show the molecules of the air Image: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter1/graphics/column.free.gif

How to prove that there is air pressure ? Three hundred years ago, in a small town in Germany -

Magdeburg, the mayor Magdeburger Halbkugeln wanted to prove the power of air/ atmospheric pressure. Hence, he made two hollow copper hemispheres which have a diameter of thirty-seven cm and can be sealed together tightly. One of the hemispheres had a hole that allowed the air to be pumped out. He closed the two hemispheres tightly , pumping out the air before attaching strings from both hemispheres to horses. The horses then ran in opposite directions to try to pull the hemispheres apart. It took a total of sixteen horses to pull the two hemispheres apart! However, the hemispheres can be easily pulled apart by one man if the air is not being pumped out.

Why ?

http://www.bud.org.tw/Hu/images/essay34-2.gif http://i.ytimg.com/vi/NZCq5J0g0G8/0.jpg

When there is no air in the two hemispheres, the air pressure outside is greater than the air pressure inside the hemispheres. Hence, the hemispheres are pushed together tightly together by great force.

Air pressure in daily life When we want to drink something using a straw, we must suck out the air inside the straw first. Then, the air pressure outside the straw which is higher than the internal part of the straw will push the liquid upwards through the straw.

• Suction cups which are commonly used to hang objects in households use the theory of air pressure. Water is dabbed onto the suction cup and it is then placed on a smooth surface. When the air is squeezed out, the air pressure internally is lower than the air

pressure externally. Therefore, the air pressure externally keeps the suction cup in place.

http://www.christmaslightsetc.com/images/productdetail/Mini_Suction_Cup.jpg

Why we don’t feel air pressure?• Everyone lives under air pressure, but why we don’t actually feel it?

• The air pressure inside our body balances out the air pressure of the surroundings.

• We will only feel air pressure when we encounter an imbalanced air pressure between our body and the surrounding.

• Changes in air pressure can be felt when you go on a plane and the air pressure is reduced a little bit in the cabin, or when you drive in the mountains. You can feel a popping sensation in your ears.

• Actually we don't feel air pressure directly. We are under the same pressure all the time.

Air pressure experiments

Beer can destroyed by air pressure

Explanation When the beer can is heated, the water inside

evaporates into water vapour. The water vapour occupies approximately 1000 times more space than it does as a liquid. Hence, when the beer can is placed in cool water immediately after heating, the water vapour turns back to liquid very quickly. This creates a partial vacuum in the beer can, causing the air pressure inside the beer can to decrease. Thus, the higher external air pressure crushes the beer can.

Air pressure experiments II

Air pressure - physics experiment

Explanation The heat of the candle flame causes the air

inside the bottle to expand. When the oxygen inside the candle jar is insufficient to sustain the candle flame, the candle flame expires. The expanded air then cools down and contracts, thus creating a partial vacuum. This lowers the air pressure inside the candle jar. Hence, the higher external pressure forces water up into the bottle until the internal and external pressure are equal.

The End