Gases and Plasmas

1
Gases and Plasmas

• Dr. Don Franceschetti
• Physics 1010
• October 23, 2007

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Phases of matter

• Solid -- atoms or molecules held in place by
  bonds
• Liquids -- molecules experience enough force to
  cohere but slip past each other.
• Gas -- molecules hardly interact
• Plasmas electrical forces cannot be ignored.

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The Atmosphere

• Held in place by gravity.
• 99 below 30 km.
• Gas molecule velocities about 1600 km/hr
• Velocity proportional to ?(T/m)

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Pressure

• Pressure is force per unit area which is the
  same as P F/A
• Pressure is measured in Pascals N/m2
• The force is the impulse force of zillions of
  molecular collisions every second
• Key discovery you cant suck water to a height
  greater than 10 m. byproduct of industrial
  revolution

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Atmosphere Pressure

• 1 atmosphere 100,000 Pa, almost exactly
• weight of atmosphere above 1 sq meter 10
  metric tons
• 1 torr is the pressure exerted by a column of
  mercury 1 mm high
• 1 atmosphere 760 torr

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Gauge or absolute pressure

• Its easy to measure pressure above or below
  atmospheric (gauge pressure).
• Blood pressure and tire pressure expressed as
  gauge pressure

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A very famous experiment

• Check out the web for Otto van Guericke

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Barometer
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Boyles Law

• Doubling the pressure of a gas, doubles the
  density and halves the volume
• PV constant

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Ideal Gas Law

• pV nRT
• Avogadros Principle Equal volumes of different
  gases at the same temperature contain the same
  number of molecules.
• Temperature is a measure of the average kinetic
  energy of the gas molecules.

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Buoyancy of Air

• An object surrounded by air is buoyed up by a
  force equal to the weight of the air displaced.
• A cubic meter of air has a mass of 1.2 kg and
  weighs about 12 newtons.
• Lighter than air craft
• Hydrogen
• Helium
• Hot air

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Bernoullis Principle

• When the speed of a fluid decreases, internal
  pressure in the fluid decreases.
• Applies to laminar flow
• Explains behavior of air foils
• Curve baseballs
• Ships moving parallel

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Plasmas

• Are gases at such high temperatures that
  molecules are ionized.
• Can be manipulated by electromagnetic forces.
• Neon lights, sparks, etc

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Review

• Q 14.1 Why are gases and liquids called fluids?
• Q 14.2 What is the energy source for the motion
  of gas in the atmosphere? What prevents
  atmospheric gases from flying off into space?

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Review

• What is the mass of a cubic meter of air at room
  temperature (20 degrees C).