Download GASES, PLASMAS, AND STARS

GASES, PLASMAS, AND STARS
Gases
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When a liquid substance is heated...
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Gases have pressure and temperature
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Molecules gain KE → break through the liquid surface
Molecules “drift” until they collide with another molecule
Just like liquids!
Example: “Atmospheric” Pressure
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Very large due to weight of air molecules above
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1 atmosphere = about 105 N / m2 !
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Average human body has 200,000 N of force pushing in
But air is also in the lungs → force pushing outward
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Pressure Changes in Gases
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Molecule collisions → gas pressure
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Compress a gas → it “pushes” outward
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If gas expands → fewer collisions → decreased pressure
If gas is heated → more collisions → increased pressure
Expand a gas → it “pulls” inward
These can be combined to form a pump
Pulls air from one place and pushes it to another
Unequal pressures can push objects
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Example: drinking straw
Lower pressure in lungs → atmospheric pressure pushes
liquid through straw into mouth
Pressure Example: Siphon
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Device → uses pressure differences to move liquid
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Example: transfer liquid from one cup to another
With air in hose
When hose is filled (using a pump)
Atmospheric pressure on entire
surface of liquid – nothing happens
Left end → below water surface
– Pressure > P
atmosphere
Right end → open to air
– Pressure = P
atmosphere
P
submerged end
> P
open end
Uneven pressure pushes liquid!
Once siphon is started → no pump
Measuring Pressure: Barometer
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One way to measure gas pressure:
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Simple Barometer
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Measure how much weight it can lift
Fill tube with liquid...
Then flip it into a dish
No air bubbles allowed!
Why doesn't all the liquid fall into the dish?
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Empty space inside the tube is a vacuum → no pressure
Atmospheric pressure pushes on liquid in dish
Measure height of liquid in tube → can find Patm
Air Pressure and Altitude
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Air pressure exists due to weight of air above
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Effect is noticeable on mountains
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Altitude sickness / shortness of breath
Water boils at lower temperature than usual
Can measure air pressure to estimate altitude
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Higher elevation → less air above → lower air pressure
Device is called an “altimeter”
Why are airplane cabins made airtight?
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Low pressure outside → air from inside wants to escape
Buoyant Force in Air
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Pressure gets weaker with height → buoyant force
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Similar to liquids, pressure difference causes an upward force
To “float” on air:
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An object must be less dense than air!
( very light )
Common ways to do this:
1) Heat air inside a balloon
– It expands and becomes less dense
2) Use a light gas like helium or hydrogen
– Why is hydrogen a bad idea?
Energy in Fluids (Liquids and Gases)
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3 common forms of energy in fluids:
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Energy in a fluid can change forms
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Kinetic Energy → motion of individual molecules
Potential Energy → height of molecules
“Pressure Energy” → unequal pressures can exert forces
Exhaling: Higher pressure in lungs → KE of air
Hot air balloon: Higher pressure on ground → PE
Energy can also be converted into other forms
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Wind turbine: KE of air → electrical energy
Bernoulli's Principle
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Applies conservation of energy to fluids
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Important result:
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Important for understanding flowing gases and liquids
When the speed of a fluid increases...
The pressure of that fluid must decrease!
F
Low P
Air
Application: We can make a fluid exert a force
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By creating unequal fluid speeds on 2 sides of an object
Object is pushed toward high speed region
Hi P
Forces Exerted by Fluids
Using the fluid's KE
Put an object in the fluid's
path → “pushing” force
Using the fluid pressure
Can exert force by setting
up unequal speeds
Force is perpendicular to
fluid motion
Aerodynamics
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Study of moving fluids
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Determine how to shape an object to get a desired force
Plasma
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When a gas is heated to very high temperature:
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Atomic collisions become more violent
Electrons are blasted away from their nuclei
The result is a “soup” of free e– and nuclei
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Called a plasma
Particles have electric charge → very strange behavior
Applications of Plasma
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Source of light
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Source of energy
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When positively charged nuclei collide with negatively
charged electrons, light is emitted
If a plasma can be made hot enough:
Nuclear fusion begins to occur → Energy is released
Very difficult to control (about 10 million degrees)
Spacecraft propulsion
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Plasma rockets are relatively weak...
But can operate for a long time with little fuel
Stars
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Stars are giant bodies of hot plasma
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Energy comes pouring out of stars
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In the form of light, radiation, and fast-moving particles
Huge outward pressure due to escaping energy
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Nuclear fusion occurs at an incredible rate inside stars
Just enough to balance gravity's inward pressure
When stars run out of nuclear “fuel”
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Gravity dominates → star implodes in a supernova
Becomes a white dwarf, neutron star, or black hole