14-May-15 Physics 1 (Garcia) SJSU Chapter 14 Gasses & Plasmas.

Apr 18, 2023 Physics 1 (Garcia) SJSU

Chapter 14 Gasses & Plasmas


Air

Most common example of a gas is air, which is mixture of nitrogen and oxygen gas

Density of water = 1 gram per cubic centimeter

Density of air = 0.001 gram per cubic centimeter = 1 kilogram per cubic meter

Water is about 1000 times denser than air

Weight of air is small but not negligible. For example, weight of air in this room is comparable to your weight (volume of room about 1000x your volume).


Atmosphere

Density of air in the atmosphere decreases with increasing altitude.

Most of atmosphere in the first 10 km (about 6 miles) of altitude.

We live at the bottom of an ocean of fluid—the fluid is air & “ocean” is the atmosphere.


Atmospheric Pressure

Atmospheric column of airBase: 1 square meterHeight: 10 kilometers (10,000 m)Volume: 10,000 cubic metersMass: 10,000 kilogramsWeight: 100,000 Newtons

(= 22,000 lb = 11 tons)Pressure: 100,000 Pascals

(= 15 lb per sq. inch)

Demo: Air Heavier than Water

Fill a glass with water to the brim then place an index card on top.

Hold the card while you carefully invert the glass over a sink.

If the seal stays tight, atmospheric pressure will support the weight of the water.



Demo: Magdeburg Hemispheres

Pair of hemispheres fit together with air-tight seal

Most of the air is pumped out from the interior.

Air pressure holds the two pieces tightly together.

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- Atmospheric pressure

- Low pressure


Check YourselfThe surface area of a Magdeburg hemisphere is

about 1/100th of a square meter. If we evacuate the air from inside a pair, how large is the force holding them together (Atmospheric pressure is 100,000 Newtons per square meter)?

What if only 90% of the air is removed?


Demo: Can Crush

Pour small amount of water into an empty soda can.

Heat water to boiling.Quickly transfer the can,

upside-down, into a pan of cold water.

Soda can is quickly crushed.


Can Crush, Analyzed

SteamL

AA

LowPressure

AtmosphericPressure

Boil the water and inside of the can fills with steam, pushing out the air.

Cool the can and steam condenses back into water, leaving almost a vacuum inside. Low pressure inside and atmospheric pressure outside.


Crush the Rail Tanker Car

The interior of the tank car was washed out & cleaned with steam. Then all the outlet valves were shut and the tank car was sealed. All the workers went home for the evening and when they returned, this is what they found. Apparently as the tank car cooled, it collapsed. The shell on these tank cars is 7/16th inch thick steel.


Demo: Drinking Straw

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With lungs, you reduce pressure in your mouth and in the straw.The higher pressure on the outside pushes the liquid up the straw.Force due to the pressure difference must match or exceed the weight.

Weight

NO “SUCKING” FORCE


Prairie Dog VacuumPrairie dogs captured by giant vacuum truck

with a padded bin.

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Force


Boyle’s Law

Density of a gas increases as it is compressed (volume decreased)

As the density of a gas increases, the pressure in the gas also increases.

Compress the gas by pushing in the syringe. Dial gauge shows increase in pressure.

SyringeGauge

Tank


Breathing & Boyle’s Law

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Demo: Stop the Funnel

Water will not enter an air-tight container.

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Weight

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Weight

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ABlock exit hole


Buoyancy in Air

Objects can float in air, just as they float in water, if the objects’ average density is less than the density of air.

Hot Air BalloonHelium-filled Blimp


ZeppelinsOn May 6, 1937, the German zeppelin Hindenburg caught fire and was destroyed in less than one minute.

Only 35 of the 97 people on board were killed.

Most deaths did not arise from the fire, but were suffered by those who leapt from the burning ship. (The lighter-than-air fire burned overhead.)

Those passengers who rode ship on its gentle descent to the ground escaped unharmed.

Hindenburg was 10 times longer than today’s blimps


Demo: Neutral Buoyancy Balloon

Tie a helium balloon to a long string and allow it to float upward until it can lift no more string.

Cut the string where it touches the floor.

Weight of balloon and string exactly balance the buoyant force.


Bernoulli’s Principle

Where the speed of a fluid increases the pressure in the fluid decreases.

This phenomenon is due to energy conservation; when fluid’s kinetic energy increases (velocity increases) its internal potential energy (pressure) decreases.

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Still Air

Win

d


Demo: Blow It Up

Hold a sheet of paper in front of your mouth and blow; the paper will rise.

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A


Check YourselfWind blowing over the ocean causes waves to build due to Bernoulli’s principle.Where is the pressure lowered?


Blow the RoofIf wind blows hard

enough the low pressure above can create a large enough force to lift the roof off.

New Orlean’s Superdome after hurricane Katrina

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Demo: Blow It Off

Bend cardboard into a U-shape. Place on table, legs down, and try to blow it off.

Side view

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AFrontview

Fast moving air in the channel between the card and the table creates a low pressure region, pressing the card downward.


Demo: Blow the Funnel

Blow hard through a funnel with a ping pong ball in the funnel’s bowl.

PingPongBall

BLOW


Airplane Wing

Pressure difference created by Bernoulli effect creates upward lift.

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Wing

LIFT FORCE


Demo: Throwing You a CurveForce on a spinning ball is perpendicular to the forward motion and

towards the sided spinning with the flow of air around the ball.

No Spin

Forward Motion

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Bernoulli

Spin


Demo: Keep It UpObjects in a moving steam of fluid are pulled to the center of the

stream because pressure is lower inside the stream than outside.

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14-May-15 Physics 1 (Garcia) SJSU Chapter 14 Gasses & Plasmas.

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Transcript of 14-May-15 Physics 1 (Garcia) SJSU Chapter 14 Gasses & Plasmas.