# properties-of-air-1

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```					Properties of Air

6C
What is Air?

78% Nitrogen
21% Oxygen
1% Other gases
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.
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.
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
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!
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.
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!
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.
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.
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.
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.
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
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.
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.
• Air pressure is
surprisingly strong.
If we have two
hollow metal spheres
full of air, the
pressures are equal
so they will easily
separate.
• 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.
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.
Layers of the Atmosphere
• Four layers
• classified by
changes in
temperature
Temperature and the Troposphere

• Troposphere:
temperature at
surface is warmed by
the earth absorbing
energy from the sun.
•   The air cools by about
6.5°C for every 1-km
above the ground.
Temperature and the
Stratosphere
• Ozone absorbs
from the sun,
causing the
temperature to
increase.
Temperature and the
Mesosphere
• This layer does not
absorb energy from
the sun, so it starts
to cool again.
Temperature and the
Thermosphere
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.
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.
The Stratosphere
• “strato” means layer
•   Contains the ozone
layer
•   Temperature
increases
The Mesosphere
• Drop in temperature
marks beginning of
mesosphere
•   “Meso” means middle
•   Most meteors burn
up here
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 (1800°C), but since air is so thin it
would not feel warm at all.
•   Divided into two parts, the ionosphere and
the exosphere
The Ionosphere
• Energy from sun strips the
electrons from the gas
molecules creating charged
particles called ions.
•   Radio waves can bounce off of
travel great distances.
•   The aurora borealis (Northern
Lights) occur here
The Exosphere
• “Exo” means outer
• Extends for 1000’s
of miles
•   Satellites orbit here

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 views: 269 posted: 8/18/2012 language: English pages: 29