Gases plus

1
Gases plus

• Dr. Don Franceschetti
• Physics 1010
• November 1, 2005

2
The Big Bang Revisited

• Astronomers measure large distances by looking
  for standard candles.
• Basic idea is if you know how bright a light
  source is and how bright it appears to be, you
  can figure out how far away it is.
• Hubble found vHd for speeds of galaxies running
  away from us.
• Universal gravitation says expansion should be
  slowing down.

3
The Big Bang Revisited

• Hubble Telescope lets us see standard candles
  much better, finds rate of expansion increasing
  in time.
• Possibilities
• Newton/Einstein not quite right
• A new force (fifth)
• Speed of light changes with time

4
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

5
Atmosphere Pressure

• 1 atmosphere 100,000 Pa, almost exactly
• weight of atmosphere above 1 sq meter 100
  metric tons
• 1 torr is the pressure exerted by a column of
  mercury 1 mm high
• 1 atmosphere 760 t0rr

6
A very famous experiment

• Check out the web for Otto van Guericke

7
Barometer

• On web

8
Boyles Law

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

9
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

10
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

11
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?

12
Review

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