Ch. 1Ch. 1Sec. 2 Sec. 2

Air PressureAir Pressure

properties of airproperties of air

1.1. massmass

properties of airproperties of air

2. 2. densitydensity= =

mass/volumemass/volume

properties of airproperties of air 3. 3. pressurepressure=force =force

pushing on a surface pushing on a surface or areaor area

units of air pressureunits of air pressure==

1.1. inches of mercury inches of mercury

2.2. millibarsmillibars

barometerbarometerinstrument used instrument used

to measure to measure changes in air changes in air pressurepressure

barometerbarometer2 kinds:2 kinds:1. mercury=

glass tube open @ bottom end & partly filled w/ mercury

barometerbarometer 2. aneroid= airtight metal

chamber sensitive to changes in air pressure - smaller than mercury barometer

- doesn’t contain liquid

altitudeelevation (distance) above elevation (distance) above

sea levelsea level

air pressure ________ air pressure ________

as altitude increasesas altitude increases

decreases

higher the altitude-______ the higher the altitude-______ the air pressureair pressure

As air pressure decreases- As air pressure decreases- density of air _________density of air _________decreases

lower

The density of air

decreases as altitude increases.

Air at sea level has more gas Air at sea level has more gas molecules in each cubic meter than air molecules in each cubic meter than air

at the top of a mountain.at the top of a mountain.

21% oxygen

21% oxygen

Measuring air pressure using an Measuring air pressure using an aneroid barometeraneroid barometer

Day/Date Day/Date Air Air pressurepressure

High or High or low low pressurepressure

WeatherWeather

Fri. 3/19/10Fri. 3/19/10 1,023.5 mb1,023.5 mb

HighHigh 797900

sunnysunny

Mon. 3/22/10Mon. 3/22/10 1,024 mb1,024 mb High High 6969

Partly Partly cloudycloudy

Tue. 3/23/10Tue. 3/23/10 1,024 mb 1,024 mb High High ??

SunnySunny

Make barometerMake barometer Cover the top of the coffee can tightly with the plastic wrap, using the Cover the top of the coffee can tightly with the plastic wrap, using the

rubber band to hold it in place. (The cover should be a taut, airtight fit.)rubber band to hold it in place. (The cover should be a taut, airtight fit.)

Position the straw so that it lays across two thirds of the cover with the Position the straw so that it lays across two thirds of the cover with the remaining length of the straw suspended over air. Tape in place.remaining length of the straw suspended over air. Tape in place.

At a 90° angle, fold one short end of the index card at about one inch from At a 90° angle, fold one short end of the index card at about one inch from that end. Tape the folded end of the index card to the can behind the straw that end. Tape the folded end of the index card to the can behind the straw in such a way that allows you to make marks on the card every day.in such a way that allows you to make marks on the card every day.

Record the level of the straw onto the card.Record the level of the straw onto the card.

For the next 10 days, at the same time each day, record the level of the For the next 10 days, at the same time each day, record the level of the straw while paying close attention to how changes in the weather affect straw while paying close attention to how changes in the weather affect the straw's level. the straw's level.

Demonstrate the "Weight of Air."Demonstrate the "Weight of Air." Place one end of the thin board on a table with slightly less Place one end of the thin board on a table with slightly less

than half the board hanging off the edge. than half the board hanging off the edge. Lay a sheet of newspaper over the part of the board on the Lay a sheet of newspaper over the part of the board on the

table. table. Allow students to predict what would happen if the Allow students to predict what would happen if the

protruding end was struck as hard as possible. protruding end was struck as hard as possible. Strike the protruding board as hard as you can. The paper Strike the protruding board as hard as you can. The paper

will not move. If you hit hard enough, the stick will break. will not move. If you hit hard enough, the stick will break. This is due to the air pressure that is exerted downward. This is due to the air pressure that is exerted downward. Since the paper is flat against the board and table, no air is Since the paper is flat against the board and table, no air is beneath the paper to counteract the pressure from above.beneath the paper to counteract the pressure from above.

Calculate the surface area of the newspaper. (length by Calculate the surface area of the newspaper. (length by width) width)

Multiply the surface area by 14.5. The result will be the Multiply the surface area by 14.5. The result will be the amount of air pressure exerted on the paper. amount of air pressure exerted on the paper.

Have students predict what would happen if you pushed Have students predict what would happen if you pushed down on the board slowly. down on the board slowly.

Place your hand on the protruding piece of the board and Place your hand on the protruding piece of the board and slowly push down. slowly push down.