Chapter 14 Gasses Plasmas Weight of air is small but not negligible. For example, weight of air in...

Chapter 14 Gasses & Plasmas

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).

Figure 13.5

• All vases are filled to equal depths, so the water pressure is the same at the bottom, regardless of its shape or volume

Once believed that you had to move water at a slow drop in gradient to move it.

• Now know water can flow uphill in a closed system if there is pressure behind it.

• Pipes in the ground can follow the natural slope of the land

• Garden hose full of water with one end dropped will spill water, but it will flow up hill

Pressure does not depend on the amount of liquid

Volume is not the key- depth is• Pressure is depth dependent, not volume

dependent.

The force exerted by a fluid on a smooth surface is

always at right angles to the surface

• Liquid pressure is exerted equally in all directions

• The pressure isn’t only downward

• When liquid presses against a surface, there is a net force that is perpendicular to the surface.

• While pressure does not have a specific direction, force does.

May 6, 2023 Physics 1 (Garcia) SJSU

Pressure in Liquids

Pressure in a liquid depends on depth.As with bricks, weight of what’s above

determines pressure.

High

Medium

Low

High

Medium

Low


Pressure

Pressure is defined as

Pressure =

Metric unit of pressure is Pascal.1 Pascal = 1 Newtons per square meterAtmospheric pressure is about 100,000 Pascals

( Force )( Area )


Demo: Bed of NailsOne may safely lay or sit on

a bed of nails, as long as there are enough nails so that the pressure, measured as force per nail, is small.

Weight of 150 pounds is distributed over 300 nails. Force per nail is ½ lb. Need 5 lb per nail to pierce skin.


Sample Problem

“Gold” brick is roughly:12 kilograms [Mass]120 Newtons [Weight](0.2 m)x(0.1 m) =

(0.02 m2) [Area]Pressure on surface is(120)/(0.02)

= 6000 Pascals

Note: Atmospheric pressure is about 100,000 Pascals so much more than pressure due to the “gold” brick.


Check YourselfIn which case is the pressure greatest? A B

A B


Check YourselfIn which case is the pressure greatest? A B

AB


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.


Buoyancy

Since pressure depends on depth, a submerged object has more force due to pressure below it than above it.

Net effect is to have a net upward force, which we call buoyancy.

Buoyancy

Weight

If weight exceeds buoyancy force then object sinks, otherwise it floats.

Pressure Pressure

Pressure

• If the weight of the submerged object is greater than the buoyant force= sink

• If the weight is less than buoyant= float

•When the weight is = to buoyant force = remain at same level (won’t sink or rise)


Density & Floating

By Archimede’s principle, a solid object will float if the density of the object is less than the density of the liquid.

Billiard ball (4.0 g/cm3) floating in a cup of mercury (13.6 g/cm3)


Demo: Diet or Regular?

What can you say about the density of diet cola as compared with regular cola?

Liquids like solids are difficult to compress.

Both liquids and gases can flow, so both are

called fluids.• The pressure you feel is due to the weight of

water (or air) above you• The pressure a liquid exerts depends on depth.• Also depends on density.

• Liquid pressure = weight density X depth + pressure of atm.

• Except for small changes produced by temp., the density of a liquids is practically the same at all depths.


Buoyancy & Depth

For a fully submerged object the buoyancy force does not depend on depth, even though pressure depends on depth.

1

2

3

45

6

Buoyancy

Buoyancy

Buoyancy

Only in the special case of floating does the buoyant force acting on an object equal the object’s weight.

principle of flotation:

A floating object displaces a weight of fluid equal to its own weight.

Figure 13.15

To control how high they float in the water

Crocs swallow stonesFish use air bladders

Figure 13.17

Figure 13.19


Check YourselfDid the designers of this “water bridge” have to

account for the weight of ships or just the water?

Figure 13.20


Check Yourself

?

Place block of wood in the water. Scale reading goes up, down, or stays the same?

50 N

10 N Block

10 N Water

40 N Water

A floating

object

displaces a

weight of fluid

equal to its

own weight.


Archimedes’ PrincipleWeight of liquid displaced by floating or submerged

object equals the buoyant force on the object.


Density & Floating, RevisitedWhen an object is not solid then it floats if the average density, (total mass)/(total volume), is less than the density of the liquid.

Solid iron ballVolume: 100 ccMass: 790 gDensity: 7.9 g/cc

Iron ball, 90% hollowVolume: 1000 ccMass: 790 gDensity: 0.79 g/cc


Floating & Liquid Density

The greater the density of a liquid, the greater the buoyant force on objects floating or immersed in the liquid. Floating in Great Salt Lake, Utah is

easy because the lake water is dense due to high concentration of salt.

Pascal’s Principle• When force applied to a

confined fluid, an increase in pressure is transmitted equally to all parts of the fluid

A liquid completely filling a bottle exerts pressure in all directions

Pascal’s Principle

Figure 13.22 • The additional pressure is exerted against every square centimeter of the larger piston. Since there is 50 times the area, 50 times as much force is exerted on the larger piston. Thus, the larger piston will support a 500-kg load—fifty times the load on the smaller piston!

the piston on the right

has 50 times the area

of the piston on the left

(let’s say that the left

piston has a cross-

sectional area of 100

square centimeters

and that the right

piston has a cross-

sectional area of 5000

square centimeters).

Hydraulic devise•Multiplies a force by applying

the force to a small surface area.

•The increase in pressure is then transmitted to another part of a confined fluid, which pushes on a larger surface area

Unnumbered Figure 13.9

Figure 13.23

Figure 13.24

Surface

Tension

• When the bent wire is lowered into the water

and then raised, the spring will stretch

because of surface tension.

• adhesion- attracted to unlike

• Cohesion- attracted to like

contractive tendency of

the surface of liquids

Figure 13.27

• These molecular attractions thus tend to pull the molecule from the surface into the liquid, and this tendency minimizes the surface area.

Surface tension is caused by molecular

attractions.

Beneath the surface, each molecule is

attracted in every direction by neighboring

molecules, resulting in no tendency to be

pulled in any specific direction.

A molecule on the surface of a liquid,

however, is pulled only by neighbors on

each side and downward from below;

there is no pull upward.

The water surface sags like a piece of

plastic wrap, which allows certain insects,

such as water striders, to run across the

surface of a pond.

Surface tension causes

• a thin film of water to be drawn up

over the inner and outer surfaces

of the tube (Figure 13.31a).It

causes this film to contract The

film on the outer surface contracts

enough to make a rounded edge.

• The film on the inner surface

contracts more and raises water

with it until the adhesive force is

balanced by the weight of the

water lifted

BuoyancyThe pressure at the bottom of a submerged object is greater than the pressure at the top. The result is a net force in the upward direction.

What is this upward force called?


Buoyancy in AirObjects can float in air, just as they float in

water, if the objects’ average density is less than the density of air.

Hot Air Balloon Helium-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

Atmosphere• The thickness is determined by• kinetic energy From sun) -tends to spread the molecules

apart; tend to fly away: if disappeared- molecules moved too slowly our “atmosphere” would be a liquid or solid layer

• gravity, which tends to hold molecules near the Earth. shut off= dissipate and disappear.

• height of the atmosphere• gets thinner and thins out to emptiness in interplanetary

space.



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.

Unnumbered Figure 13.2


Demo: Stop the Funnel

Water will not enter an air-tight container.

H

A

Weight

A

Weight

A

ABlock exit hole


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: 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.

A A

AA

L

A

L

- Atmospheric pressure

- Low pressure

Barometer Any device that measures

atmospheric pressure.• A simple mercury barometer: The

vertical height of the mercury column remains constant even when the tube is tilted, unless the top of the tube is less than 76 centimeters above the level in the dish—in which case the mercury completely fills the tube.

• 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.

• A water barometer would be —13.6 times as long, or 10.3 meters high—too tall to be practical.

• You may recognize this number as the density of mercury relative to that of water

• Strictly speaking, these two do not suck the soda up the straws. They instead reduce pressure in the straws and allow the weight of the atmosphere to press the liquid up into the straws. Could they drink a soda this way on the Moon?



Demo: Drinking Straw

L

A A

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.

L

A

Force

There is a 10.3-meter limit on the height that water can be lifted with vacuum pumps.

At sea level, however strong your lungs may be, or whatever device you use to make a vacuum in the straw, the water cannot be pushed up by the atmosphere higher than 10.3 m.

• “hard vacuum- orbiting in space- satellites can reach 10−13 Pa.

• A vacuum pump simply provides a region of lower pressure into which fast-moving gas molecules randomly move.

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.

P1V1 = P2V2

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

SyringeGauge

Tank

Boyle’s Law

• A flat tire registers zero pressure on the gauge, but a pressure of about one atmosphere exists there. Gauges read “gauge” pressure—pressure greater than atmospheric pressure.

Double volume & •½ pressure•Dec speed of partaicles

•Dec. temp

Decrease volumeDensity and pressure are increased

P1V1 = P2V2

applies to ideal gases.

Boyle’s Law

• Boyle’s law applies to ideal gases. An ideal gas is one in which the disturbing effects of the forces between molecules and the finite size of the individual molecules can be neglected. Air and other gases under normal pressures approach ideal-gas conditions.

• the product of pressure and volume for a given mass of gas is a constant as long as the temperature doesn’t change

• pressure and volume are inversely proportional;

Archimedes’ principle

• holds for air just as it does for water: An object surrounded by air is buoyed up by a force equal to the weight of the air displaced.

• any object less dense than air will rise in air

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

• Rising motion of balloons are caused by unbalanced forces.

• The air beneath exerts more force than the air above.

• When it reaches an altitude where the forces are equal, it stops rising.

• Unlike water, the atmosphere has no definable surface. There is no “top.” Furthermore, unlike water, the atmosphere becomes less dense with altitude


Plasma An electrified gas containing ions and free electrons. Most of the matter in the universe is in the plasma phase.


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.

How high will a balloon rise?At ground level the balloon is partially

inflated, but at high altitues it is full when the surrounding pressure is less.

• (1) A balloon will rise only so long as it displaces a weight of air greater than its own weight. Air becomes less dense with altitude, so, when the weight of displaced air equals the total weight of the balloon, upward acceleration of the balloon ceases.

• (2) When the buoyant force on the balloon equals its weight, the balloon will cease to rise.

• (3) When the average density of the balloon (including its load) equals the density of the surrounding air, the balloon will cease rising

Bernoulli’s Principle- fluid dynamics

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.

A

L

Still Air

Win

d

Bernoulli’s principle

May 6, 2023

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

L

A

Even a small pressure difference over a large roof area can produce a large upward “lifting” force


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


Demo: Blow the Funnel

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

PingPongBall

BLOW


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.

L

AL

A

A

A

L

L

AA

•Airfoil - shape of wing• Lift- upward force which

results when the pressure on upper surface is less

•Drag- opposing force to forward motion

• Thrust - force which pushes the plane forwardprovided by plane’s propeller/jet

•


Airplane Wing

Pressure difference created by Bernoulli effect creates upward lift.

L

A

Wing

LIFT FORCE

Curving may be increased by threads or fuzz, which help to drag a thin layer of air with the ball and to produce further

crowding of streamlines on one side.

• a) The streamlines are the same on either side of a nonspinning baseball.

• (b) A spinning ball produces a crowding of streamlines. The resulting “lift” (red arrow) causes the ball to curve, as shown by the blue arrow.

Bernoulli’s principle plays an important role for animals living in underground burrows.

• Entrances to their burrows are usually mound shaped, producing variations in wind speed across different entrances. This provides necessary pressure differences of air to enable circulation in the burrow

• How does this relate to fireplaces?

• On a windy day, waves in a lake are higher than normal. Why?

Plasma

• An electrified gas containing ions and free electrons.

• Most of the matter in the universe is in the plasma phase.

• The Sun and other stars are largely plasma

• Also fluorescent & neon lights, plasma TV, auroras

• a plasma (different from a gas)

• readily conducts electric current,

• it absorbs certain kinds of radiation that pass unhindered through a gas,

• it can be shaped, molded, and moved by electric and magnetic fields.

Plasma PowerMHD power, the magnetohydrodynamic interaction

between a plasma and a magnetic field. • Low-pollution MHD power is

in operation at a few places in the world already.

• operate at high temperatures without moving parts

• Fusion power may not only make electrical energy abundant, but it may also provide the energy and means to recycle and even synthesize elements

• • In submarines, low speed MHD generators using liquid metals would be nearly silent, eliminating a source of tell-tale mechanism noise.

• In spacecraft and unattended locations, low-speed metallic MHD generators have been proposed as highly reliable generators, linked to solar, nuclear or isotopic heat sources.

http://en.wikipedia.org/wiki/Moving_parts

Pascal’s Principle

•Explains hydraulic systems•Pressure is exerted equally throughout a closed container•Hydraulic systems multiply force (over a greater distance)

Bernoulli’s Principle

Pressure exerted on a moving steam of

fluid is less than the pressure of the

surrounding fluid•Explains Flight

Archimedes’ Principle

(Sinks) Buoyant force on an object is equal to the

weight of the fluid displaced

If an object floats- the volume of displaced water = volume of the portion of

the object that is submerged.

Pressure

•P = force/areaThe force of all the collisionsForce-Measured in Newtons, N Area- m2

•Pascal- SI unit of pressure = pascal, Pa, 1 N/m2

Fluid pressure in the Environment

•The atm. Pushes downward due to gravity causing atm pressure= 100 000 Pa

•As you move up & down in the atm & in water, the pressure changes

•Pressure is greatest the farther down you go

•Altitude is the most important factor in surface air pressure.

Chapter 14 Gasses Plasmas Weight of air is small but not negligible. For example, weight of air in...

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