AIR PRESSUREAIR PRESSURE

Earths AtmosphereEarths Atmosphere

The earth’s atmosphere is not made of pure The earth’s atmosphere is not made of pure air. It contains tiny solid and liquid particles air. It contains tiny solid and liquid particles of dust, smoke, and other particles.of dust, smoke, and other particles.

Gases that make up the earths dry air 78% Gases that make up the earths dry air 78% Nitrogen, 21% Oxygen, 1% OtherNitrogen, 21% Oxygen, 1% Other

The earths air is not dry though. It contains The earths air is not dry though. It contains water vapor water vapor

Water VaporWater Vapor

Water vaporWater vapor is water in the form of gas. is water in the form of gas. The amount of water vapor in the air varies The amount of water vapor in the air varies

greatly depending on the place and time.greatly depending on the place and time. Water vapor plays an important role in the Water vapor plays an important role in the

earth’s weather. It creates clouds and turns earth’s weather. It creates clouds and turns into precipitation as heat is removed.into precipitation as heat is removed.

Properties of airProperties of air

We often don’t think that air is made of anything; We often don’t think that air is made of anything; however, air consists of atoms and molecules.however, air consists of atoms and molecules.

These molecules have mass so air must have These molecules have mass so air must have mass.mass.

Because air has mass it also has other properties Because air has mass it also has other properties like density and pressure.like density and pressure.

The more molecules of gas in an area the greater The more molecules of gas in an area the greater the density and the fewer the molecules of gas in the density and the fewer the molecules of gas in an area the lower the density.an area the lower the density.

PressurePressure

PressurePressure is the force pushing on an area or is the force pushing on an area or surface. surface.

Dense air creates more pressure than less dense Dense air creates more pressure than less dense air.air.

Example: think about carrying a back pack. The Example: think about carrying a back pack. The more stuff you put in the back pack the harder it more stuff you put in the back pack the harder it gets to carry. The weight presses the straps into gets to carry. The weight presses the straps into your shoulders (pressure). When you take off the your shoulders (pressure). When you take off the backpack it feels like the pressure has been backpack it feels like the pressure has been removed. removed.

Air PressureAir Pressure

Air pressureAir pressure is the result of the weight of a is the result of the weight of a column of air pushing down on an area.column of air pushing down on an area.

The weight of the column of air above your The weight of the column of air above your desk right now is about the weight of a desk right now is about the weight of a school bus. So why isn’t the desk crushed?school bus. So why isn’t the desk crushed?

AnswerAnswer

The desk is not crushed because the air The desk is not crushed because the air pushes in all directions up, down, and pushes in all directions up, down, and sideways. So the air pushing down on the sideways. So the air pushing down on the air is balanced equally by the air pushing up air is balanced equally by the air pushing up on the bottom of the desk.on the bottom of the desk.

Air pressure can be increased by density of Air pressure can be increased by density of the air as discussed earlier or by altitude. the air as discussed earlier or by altitude.

Air pressure decreases as altitude Air pressure decreases as altitude increases. increases.

ExamplesExamples Think of a stack of ten books. Which book has more Think of a stack of ten books. Which book has more

weight on it a book at the top or at the bottom? The book weight on it a book at the top or at the bottom? The book at the bottom has the weight of all the other books on top at the bottom has the weight of all the other books on top of it.of it.

Likewise a man standing on top of a mountain has less Likewise a man standing on top of a mountain has less air on top of him than a man standing on the beach. Air air on top of him than a man standing on the beach. Air at sea level is like the bottom book it has the weight of all at sea level is like the bottom book it has the weight of all the atmosphere on top of it.the atmosphere on top of it.

At sea level, the air pressure is about 14.7 pounds per At sea level, the air pressure is about 14.7 pounds per square inch. As your altitude increases (for example, if square inch. As your altitude increases (for example, if you climb a mountain), the air pressure decreases. At an you climb a mountain), the air pressure decreases. At an altitude of 10,000 feet, the air pressure is 10 pound per altitude of 10,000 feet, the air pressure is 10 pound per square inch.square inch.

Answer on PaperAnswer on Paper How is atmosphere important for living things?How is atmosphere important for living things? What are the two most common gases in dry air?What are the two most common gases in dry air? What percentage of gas is nitrogen?What percentage of gas is nitrogen? What percentage of dry gas is oxygen?What percentage of dry gas is oxygen? What percentage of dry gas is not nitrogen or What percentage of dry gas is not nitrogen or

oxygen?oxygen? Name two things other than dry gas found in air.Name two things other than dry gas found in air. What three properties does air have? What gives air What three properties does air have? What gives air

these properties?these properties? Why is air hard to breath on the top of a mountain?Why is air hard to breath on the top of a mountain? What changes in air pressure would you expect as What changes in air pressure would you expect as

you walked down a mine shaft? Explain.you walked down a mine shaft? Explain.