Air Pressure Pushing Down on Me INTRODUCTION Was

Air Pressure “Pushing Down on Me”

INTRODUCTION • Was your backpack heavy when you took it to school this morning? • Were you happy to take it off when you got here? • Did you know even when you take it off there was still pressure on your shoulders?

PROPERTIES OF AIR • Take a look at air (well you can't really so think of a balloon). Do you think it has mass? • Remember mass means that it takes up space • It turns out that air does take up space so it has mass even though we can't see it. • Think of a balloon, the air takes up space inside an inflated balloon • Because air has mass it also has other properties such as density and pressure.

DENSITY •

PRESSURE • The atmosphere and the air around us exert pressure on the Earth's surface as well as on us. • Air pressure is the weight of a column of air pushing down on an area. • Why doesn't it crush you? • The molecules in the column are traveling every direction. They end up balancing each other out.

MEASURING AIR PRESSURE • There are different ways to measure air pressure. • One of the most common ways is to use a mercury barometer. • Another way is to use a aneroid barometer.

MERCURY BAROMETERS • A mercury barometer is a glass tube that is open at one end and partially filled with mercury. • The space above the mercury creates a vacuum • The open end of the tube rests in container of mercury. • The air pressure pushes on the exposed mercury causing the mercury in the tube to rise. • You can take measurements off of how high the mercury is in the tube.

ANEROID BAROMETERS • If you have a barometer at home it is probably an aneroid barometer. • Aneroid barometers do not use liquid to measure air pressure. • Instead they use a metal chamber that is sensitive to pressure changes. • When the chamber's walls are pushed in a series of dials and gears moves a needle. • The chamber's walls will relax if the pressure's low and push in if the pressure is high.

UNITS OF AIR PRESSURE • Barometers usually measure air pressure in units of pressure. • Weather forecasters usually use the term millibars.

ALTITUDE AND PROPERTIES OF AIR • Altitude is the elevation or distance above sea level. • Sea level is the average level of the surface of the oceans. • Air pressure decreases as altitude increases. • Air pressure decreases then so does the density.

HOW ALTITUDE AFFECTS AIR PRESSURE • Stack some books on your desk. • Which book has the most weight on it? The top? The middle? The bottom? • The answer is the bottom and the same goes for the air pressure. • The air at sea level as the most pressure because it has more weight upon it from the rest of the atmosphere. • This also means that it has the most density compared to the rest of the atmosphere.

HOW ALTITUDE AFFECTS DENSITY • As you climb into higher altitudes it gets harder to breath • This is due to the fact that air because less dense the higher you go. • Atmosphere still contains the same amount of oxygen, but the molecules are much farther apart. • With each breath you are taking in less oxygen than you would at sea level. • This means you could become short of breath very quickly at high altitudes.

REVIEW QUESTIONS • How does increasing the density of a gas affect its pressure? • What two instruments are used to measure air pressure? • Why is it so hard to breathe at the top of a mountain? • What is altitude? • Critical Thinking: What changes in air pressure would you expect if you carried a barometer down a cave?