Section 3 Air Pressure

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Section 3 Air Pressure

• Pages 25-30
• https//www.youtube.com/watch?v1nveWCRiIlo
• https//www.youtube.com/watch?vrog8ou-ZepE

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Does air have.

• mass?
• volume?
• density?
• pressure?
• Whether it does or does not, how could we prove
  it?

3
Properties of Air

• Air has mass because it is composed of atoms and
  molecules.
• Because air has mass, it has density and
  pressure.
• The more molecules in a given volume of air, the
  greater its density.

4
Density and Pressure

• The amount of mass in a given volume of air is
  its density.
• The force pushing on an
• area or surface is pressure.

5
Air Pressure

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

6
Air Pressure

• Why doesnt air pressure
• crush your desk?
• The molecules in air push in all directions. So
  the air pushing down on the top of your desk is
  balanced by the air pushing up on the bottom.

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Measuring Air Pressure

• An instrument that is used to measure changes in
  air pressure is a barometer.
• There are two kinds of barometers
• -mercury barometers
• -aneroid barometers

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Mercury Barometers

• Mercury barometers consist of a glass tube open
  at the bottom end partially filled with mercury.

• The air pressure pushing down on the surface of
  the mercury in the dish is equal to the weight of
  the column of mercury in the tube.

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Aneroid Barometers

• Aneroid means without liquid.
• An aneroid barometer has an airtight metal
  chamber that is sensitive to changes in air
  pressure.
• As the shape of the chambers changes, the needle
  on the dial moves.

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• Why are aneroid barometers more practical than
  mercury barometers?
• They are smaller and do not contain liquid.

• Two different units are used to measure air
  pressure
• - Most weather reports use inches
• - Weather maps indicate air pressure in millibars

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Weather Reports

• Falling air pressure usually indicates that a
  storm is approaching.

• Rising air pressure usually means that the
  weather is clearing.

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Increasing Altitude

• Altitude is the distance above sea level.
• Air pressure decreases as
• altitude increases.
• As air pressure decreases, so does density.

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How Altitude Affects Air Pressure

• Which book has more weight on it, second book
  from the top or the book at the bottom?
• Air at sea level is like the bottom book.
  Sea-level air has the weight of the whole
  atmosphere pressing on it.
• Air near the top of the atmosphere is like the
  second book from the top, lower air pressure.

14
Altitude Affects Density

• Why do mountain climbers bring oxygen tanks with
  them on climbs?
• They have difficulty breathing at high altitudes
  because
• As you go up through the atmosphere, air pressure
  decreases.
• As air pressure decreases, the density of air
  decreases.
• (the air still contains 21 oxygen)

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Words Worth Knowing

• Add these words to your list on ISN 2
• density
• pressure
• air pressure
• barometer
• mercury barometer
• aneroid barometer
• altitude