Air Pressure

Chapter 1, Section 2, p10-14

Properties of Air

• Air has mass– Air is made of atoms & molecules– Atoms & molecules have mass• How can you test this?

• Because air has mass, it also has other properties, including density & pressure

Density

• Density: the amount of mass in a given volume of air – Density = mass ÷ volume• Increase molecules in volume of air = increase density• Decrease molecules in volume of air = decrease density

Pressure

• Pressure: force pushing on an area or surface– Weight of atmosphere exerts force on surfaces– Air pressure: result of weight of a column of air

pushing down on an area• Column extends up through entire atmosphere• Weight on table about the same as a large school bus

– Why doesn’t your table get crushed?– Air pushes in all directions: down, side-to-side, & up

– Air pressure changes from day to day• Denser air = more pressure than less dense air

– More mass per unit volume

Measuring Air Pressure

• Barometer: instrument used to measure air pressure– Two common kinds of barometers are:• Mercury barometers• Aneroid barometers

Mercury Barometers

• Mercury barometer: consists of a glass tube open at the bottom end and partially filled with mercury– Figure 5• Space in tube is almost a vacuum (contains little air)• Open end of tube rests in a dish of mercury• Air pressure pushing on dish of mercury = weight of

column of mercury in tube– Air pressure increases – mercury rises

» More pressure pushes more mercury up the tube

Aneroid Barometers

• Aneroid barometer: instrument used to measure air pressure “without liquid”– Barometer at your home– Figure 6• Air tight metal chamber sensitive to pressure changes

– Increase pressure – chamber walls pressed in– Decrease pressure – chamber wall bulge out

• Chamber connected to a dial• Needle on dial moves as chamber walls move

Measuring Air Pressure Activity

• Active Art– PHSchool.com– Web code: (cfp-4012)

Units of Air Pressure

• Several different units for air pressure– Inches of mercury• Weather reports for general public

– Millibars• National Weather Service maps

– 1 inch of mercury = 33.87 millibars• 30” mercury x 33.87 millibars = 1,016 millibars 1 1” of mercury

Altitude and the Properties of Air

• Altitude: elevation; the distance above sea level– Sea level: average level of the surface of the

oceans– Air pressure decreases as altitude increases– Density decreases as air pressure decreases

Altitude Affects Air Pressure

• Air pressure is greater at sea-level– Has weight of entire atmosphere pushing on it• Stack of books• Bottom book feels weight of all other books

• Air near top of atmosphere has lower pressure– Less weight pushing on it• Stack of books• Second book from top has weight of one book

Altitude Also Affects Density

• Increase altitude = decrease density– Gas molecules farther apart at high altitudes– Become quickly out of breath• Sea level – 21% oxygen• Mountain top – 21% oxygen• Fewer oxygen molecules at mountain top

Homework

• Read Section 2, p.10-14• Answer Questions 1-3 p.14