1. Air Pressure Chapter 1, Section 2, p10-14

2. 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

3. 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

4. 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

5. Measuring Air Pressure Barometer: instrument used to measure air pressure – Two common kinds of barometers are: Mercury barometers Aneroid barometers

6. 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

7. 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

8. Measuring Air Pressure Activity Active Art – PHSchool.com PHSchool.com – Web code: (cfp-4012)

9. 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

10. 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

11. 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

12. 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

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