Gases and Plasmas

1
Gases and Plasmas
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Earths Atmosphere

• Balance of
• Gravity
• Energy
• More compressed at sea level than at higher
  altitudes
• Moon has no atmosphere

3
Magdeburg Hemispheres

• Two teams of horses couldn't pull the evacuated
  hemispheres apart.

4
Weight of Air

• mass of air that would occupy a thin bamboo pole
  that extends 30 km upto the top of the
  atmosphereis about 1 kg.
• This air weighs about 10 N.

5
Weight of Air

• weight of air bearing down on a one-square-meter
  surface at sea level is about 100,000 N
• atmospheric pressure is about 105 N/m2, or about
  100 kPa.
• At sea level, 1 cubic meter of air has a mass of
  about 1 1/4 kilograms

6
Check Yourself

• About how many kilograms of air occupy a
  classroom that has a 200-m2 floor area and a
  4-m-high ceiling? (Assume a chilly 10C
  temperature.)

7
Check Yourself

• About how many kilograms of air occupy a
  classroom that has a 200-m2 floor area and a
  4-m-high ceiling? (Assume a chilly 10C
  temperature.)
• The mass of air is 1000 kg. The volume of air is
  200 m2 4 m 800 m3 each cubic meter of air
  has a mass of about 1.25 kg, so 800 m3 1.25
  kg/m3 1000 kg.

8
Check Yourself

• Why doesn't the pressure of the atmosphere break
  windows?

9
Check Yourself

• Why doesn't the pressure of the atmosphere break
  windows?
• Atmospheric pressure is exerted on both sides of
  a window, so no net force is exerted on the
  window. If for some reason the pressure is
  reduced or increased on one side only, like when
  a tornado passes by, then watch out! Reduced
  outside air pressure created by a tornado can
  cause a building to explode.

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Barometers

• Barometer instrument for measuring atmospheric
  pressure

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Mercury Barometer

• The barometer balances when the weight of
  liquid in the tube exerts the same pressure as
  the atmosphere outside
• Whatever the width of the tube, a 76-centimeter
  column of mercury weighs the same as the air that
  would fill a super-tall 30-kilometer tube of the
  same width
• mercury is literally pushed up into the tube of a
  barometer by the weight of the atmosphere.

12
Vacuum

• air pressure is repeatedly lowered by piston and
  valve action
• best vacuums attainable with mechanical pumps are
  about 1 pascal.
• Better vacuums, down to 10-8 Pa, are attainable
  with vapor diffusion or vapor jet pumps.
• Sublimation pumps can reach 10-12 Pa. Greater
  vacuums are very difficult to attain

13
Gas Molecules

• Behavior of Gas Molecules
• Kinetic Theory Applet
• Gas molecules are perpetually moving

Robert Boyle (1627-1691)
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Boyles Law
15
Check Yourself

• A piston in an airtight pump is withdrawn so that
  the volume of the air chamber is increased three
  times. What is the change in pressure?

16
Check Yourself

• A piston in an airtight pump is withdrawn so that
  the volume of the air chamber is increased three
  times. What is the change in pressure?
• the pressure in the piston chamber is reduced to
  one-third. This is the principle that underlies a
  mechanical vacuum pump.

17
Check Yourself

• A scuba diver 10.3 m deep breathes compressed
  air. If she were to hold her breath while
  returning to the surface, by how much would the
  volume of her lungs tend to increase?

18
Check Yourself

• A scuba diver 10.3 m deep breathes compressed
  air. If she were to hold her breath while
  returning to the surface, by how much would the
  volume of her lungs tend to increase?
• Atmospheric pressure can support a column of
  water 10.3 m high, so the pressure in water due
  to the weight of the water alone equals
  atmospheric pressure at a depth of 10.3 m. Taking
  the pressure of the atmosphere at the water's
  surface into account, the total pressure at this
  depth is twice atmospheric pressure.
  Unfortunately for the scuba diver, her lungs tend
  to inflate to twice their normal size if she
  holds her breath while rising to the surface. A
  first lesson in scuba diving is not to hold your
  breath when ascending. To do so can be fatal.
• (Scuba is an acronym for Self-Contained
  Underwater Breathing Apparatus.)

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Buoyancy of Air

• Archimedes' principle holds for air just as it
  does for waterAn object surrounded by air is
  buoyed up by a force equal to the weight of the
  air displaced.
• a cubic meter of air at ordinary atmospheric
  pressure and room temperature has a mass of about
  1.2 kilograms
• its weight is about 12 newtons
• any 1-cubic-meter object in air is buoyed up with
  a force of 12 newtons.

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Dirigibles Blimps

• Large dirigible airships are designed so that
  when loaded they will slowly rise in air
• their total weight is a little less than the
  weight of air displaced.

Hindenberg used hydrogen instead of helium
21
Check Yourself

• Is there a buoyant force acting on you? If there
  is, why are you not buoyed up by this force?

22
Check Yourself

• Is there a buoyant force acting on you? If there
  is, why are you not buoyed up by this force?
• There is a buoyant force acting on you, and you
  are buoyed upward by it. You don't notice it only
  because your weight is so much greater.

23
Check Yourself

• How does buoyancy change as a helium-filled
  balloon ascends?

24
Check Yourself

• How does buoyancy change as a helium-filled
  balloon ascends?
• If the balloon is free to expand as it rises, the
  increase in volume is counteracted by a decrease
  in the density of higher-altitude air.
• So, interestingly, the greater volume of
  displaced air doesn't weigh more, and buoyancy
  stays the same.
• If a balloon is not free to expand, buoyancy will
  decrease as a balloon rises because of the
  less-dense displaced air.

25
Rising Balloon

• (Left) At ground level the balloon is partially
  inflated.
• (Right) The same balloon is fully inflated at
  high altitudes where surrounding pressure is
  less.

26
Fluid Flow
Water speeds up when it flows into the narrower
pipe.

• The motion of a fluid in steady flow follows
  imaginary streamlines, represented by thin lines
• Streamlines are visible when smoke or other
  visible fluids are passed through evenly spaced
  openings, as in a wind tunnel.
• Streamlines are the smooth paths, or
  trajectories, of bits of fluid.
• One small bit of fluid follows along the same
  path as a bit of fluid in front of it.
• The lines are closer together in narrower
  regions, where the flow speed is greater.

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Bernoullis Principle
Where the speed of a fluid increases, internal
pressure in the fluid decreases.
Daniel Bernoulli 1700-1782

• Internal pressure is greater in slower moving
  water in the wide part of the pipe, as evidenced
  by the more squeezed air bubbles.
• The bubbles are bigger in the narrow part because
  internal pressure there is less.

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Bernoullis Principle

• Bernoulli's principle applies to a smooth, steady
  flow (called laminar flow) of constant-density
  fluid.
• At speeds above some critical point, however, the
  flow may become chaotic (called turbulent flow)
  and follow changing, curling paths called eddies.

29
Bernoulli Applications

1. The streamlines are the same on either side of a
   non-spinning baseball.
2. A spinning ball produces a crowding of
   streamlines. The resulting lift (red arrow)
   causes the ball to curve as shown by the blue
   arrow.

Air pressure above the roof is less than air
pressure beneath the roof.
The vertical vector represents the net upward
force (lift) that results from more air pressure
below the wing than above the wing. The
horizontal vector represents air drag.
Wing lift
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Bernoulli Applications
When you squeeze the bulb, air rushes across the
open end of a tube that sticks into the perfume.
This reduces pressure in the tube, whereupon
atmospheric pressure on the liquid below pushes
it up into the tube where it is carried away by
the stream of air.
Pressure is greater in the stationary fluid (air)
than in the moving fluid (water stream). The
atmosphere pushes the ball into the region of
reduced pressure.
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Bernoulli Applications
The troughs of the waves are partially shielded
from the wind, so air travels faster over the
crests. Pressure there is therefore lower than
down below in the troughs. The greater pressure
in the troughs pushes water into the even higher
crests.
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Plasma

• Plasma 4th phase of matter electrified gas
• Least common in everyday world
• Most prevalent in the universe stars
• Also found in gas inside operating
• fluorescent light tubes
• neon sign tubes
• vapor lights
• MagnetoHydroDynamic Power