Properties of Air

1
Properties of Air

• 6C

2
What is Air?

• Air is made up of
• 78 Nitrogen
• 21 Oxygen
• 1 Other gases

3
Properties of Air

• Air is a mixture of elements, compounds and
  molecules . Air has mass, volume, and therefore
  density.
• The more molecules we put in a given volume of
  air, the more mass it will have. So this air
  will be more dense.

4
Pressure

• Pressure is a force that acts over a certain
  area.
• Liquids and gases are fluids. Fluids are any
  material that is able to flow. Fluids exert
  pressure because of the motion of their
  particles.
• Pressure will always move from a high pressure to
  a low pressure area. The pressure will always
  try to equalize. You see this when you get a
  hole in your bicycle tire.

5
Inflating example

• Air inside a ball pushes against the sides. The
  more air we put in a ball, the more the molecules
  push. So the pressure increases as we add air.

6
Review

• If we increase the force, what should happen to
  the pressure?
• It should increase.
• If we increase the area over which we apply a
  force, what should happen to the pressure?
• It should decrease!

7
Air Pressure

• Air pressure is the result of the column of air
  that is above you.
• There is so much air above you that at sea level
  you have 14.7 lbs/in2 pushing on you.

8
Why are we not crushed by air pressure?

• Air pressure is equal in all directions.
• So air pushes equally in all sides of us. The
  forces are balanced!

9
Measuring Air Pressure

• We use barometers to measure air pressure.
• Mercury barometer a glass tube sealed at the
  top partially filled with mercury.
• Air pressure pushing on the mercury in the dish
  causes the mercury to rise and fall in the tube.

10
Aneroid Barometers

• Aneroid Barometer Works without liquid. Has an
  airtight metal can that is sensitive to changes
  in air pressure. This chamber is connected to a
  dial.

11
Units of Air Pressure

• TV weather stations and aviation use inches of
  mercury.
• Meteorologists (and the NWS) use millibars, an
  SI unit. A unit of pressure equal to
  one-thousandth of a bar or 100 pascals, most
  commonly used to measure atmospheric pressure.

12
Altitude and Pressure

• Altitude (or elevation) is the distance above sea
  level.
• The higher the altitude, the less air there is
  above you. So as altitude increases, air
  pressure decreases.

13
Pressure and Boiling Points

• As the air pressure decreases, it takes less
  energy to cause water to boil. There is less
  force pushing down on the water at higher
  altitudes, so it is easier for the water
  molecules to escape into the air. If you go high
  enough you could boil water at room temperature.
  It would not burn you, and would not cook your
  food!

14
Pressure analogy

• Imagine a stack of books. The bottom book feels
  all the weight of the all the books stacked above
  it, while the second one up feels slightly less
  weight. The higher in the stack, the less
  pressure one would feel.

15
Altitude and Density

• As the air pressure decreases, the density of the
  air decreases. The air particles are not
  squashed together as tightly the higher one goes.
• The air at sea level and at 6km has the same 21
  oxygen, but at 6km there are fewer molecules, so
  you take in less oxygen with each breath.

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Madgeburg Hemispheres

• Air pressure is surprisingly strong. If we have
  two hollow metal spheres full of air, the
  pressures are equal so they will easily separate.
  

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Madgeburg Hemispheres

• But if we remove the air from the hollow spheres,
  there will be much lower pressure on the inside.
  The higher pressure outside will push in on the
  spheres. If the spheres are small, with a
  diameter of 4 inches, it will take over 180
  pounds of force to pull them apart.

18
Pressure Demo

• If we attached half of the sphere to the ceiling
  and removed the air, Mr. Brown would be able to
  hang from it.
• This is also why the soda can was so easily
  crushed by the air pressure in the room.

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Layers of the Atmosphere

• Four layers
• classified by changes in temperature

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Temperature and the Troposphere

• Troposphere temperature at surface is warmed by
  the earth absorbing energy from the sun.
• The air cools by about 6.5C for every 1-km above
  the ground.

21
Temperature and the Stratosphere

• Ozone absorbs ultraviolet radiation from the sun,
  causing the temperature to increase.

22
Temperature and the Mesosphere

• This layer does not absorb energy from the sun,
  so it starts to cool again.

23
Temperature and the Thermosphere

• Solar radiation first hits this layer, so the few
  particles that are here can gain lots of energy.
  They move rapidly, so they have a very high
  temperature.
• But the air is so thin here that it takes special
  instruments to measure the temperature accurately.

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The Troposphere

• Lowest (inner) layer
• weather occurs here
• we live in it.
• tropo means turning or changing conditions
• depth varies from 9km above the poles to 16km at
  the equator
• shallowest layer, but contains most of the mass.

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The Stratosphere

• strato means layer or spreading out
• Contains the ozone layer
• Temperature increases

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The Mesosphere

• Drop in temperature marks beginning of mesosphere
• Meso means middle
• Most meteors burn up here

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The Thermosphere

• Very top layer
• Air is very, very thin, about 1/1000th as dense
  as the air at sea level
• Thermo means heat
• Extends from 80km to space
• No definite outer edge
• Very hot (1800C), but since air is so thin it
  would not feel warm at all.
• Divided into two parts, the ionosphere and the
  exosphere

28
The Ionosphere

• Energy from sun strips the electrons from the gas
  molecules creating charged particles called ions.
• Radio waves can bounce off of ions, allowing
  radio waves to travel great distances.
• The aurora borealis (Northern Lights) occur here

29
The Exosphere

• Exo means outer
• Extends for 1000s of miles
• Satellites orbit here