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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
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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
May 6, 2023 Physics 1 (Garcia) SJSU
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 )
May 6, 2023 Physics 1 (Garcia) SJSU
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.
May 6, 2023 Physics 1 (Garcia) SJSU
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.
May 6, 2023 Physics 1 (Garcia) SJSU
Check YourselfIn which case is the pressure greatest? A B
A B
May 6, 2023 Physics 1 (Garcia) SJSU
Check YourselfIn which case is the pressure greatest? A B
AB
May 6, 2023 Physics 1 (Garcia) SJSU
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.
May 6, 2023 Physics 1 (Garcia) SJSU
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)
May 6, 2023 Physics 1 (Garcia) SJSU
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)
May 6, 2023 Physics 1 (Garcia) SJSU
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.
May 6, 2023 Physics 1 (Garcia) SJSU
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
May 6, 2023 Physics 1 (Garcia) SJSU
Check YourselfDid the designers of this “water bridge” have to
account for the weight of ships or just the water?
Figure 13.20
May 6, 2023 Physics 1 (Garcia) SJSU
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.
May 6, 2023 Physics 1 (Garcia) SJSU
Archimedes’ PrincipleWeight of liquid displaced by floating or submerged
object equals the buoyant force on the object.
May 6, 2023 Physics 1 (Garcia) SJSU
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
May 6, 2023 Physics 1 (Garcia) SJSU
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?
May 6, 2023 Physics 1 (Garcia) SJSU
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
May 6, 2023 Physics 1 (Garcia) SJSU
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.
May 6, 2023 Physics 1 (Garcia) SJSU
May 6, 2023 Physics 1 (Garcia) SJSU
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
May 6, 2023 Physics 1 (Garcia) SJSU
Demo: Stop the Funnel
Water will not enter an air-tight container.
H
A
Weight
A
Weight
A
ABlock exit hole
May 6, 2023 Physics 1 (Garcia) SJSU
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)
May 6, 2023 Physics 1 (Garcia) SJSU
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?
May 6, 2023 Physics 1 (Garcia) SJSU
May 6, 2023 Physics 1 (Garcia) SJSU
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
May 6, 2023 Physics 1 (Garcia) SJSU
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
May 6, 2023 Physics 1 (Garcia) SJSU
Plasma An electrified gas containing ions and free electrons. Most of the matter in the universe is in the plasma phase.
May 6, 2023 Physics 1 (Garcia) SJSU
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
May 6, 2023 Physics 1 (Garcia) SJSU
Check YourselfWind blowing over the ocean causes waves to build due to Bernoulli’s principle.Where is the pressure lowered?
May 6, 2023 Physics 1 (Garcia) SJSU
Demo: Blow the Funnel
Blow hard through a funnel with a ping pong ball in the funnel’s bowl.
PingPongBall
BLOW
May 6, 2023 Physics 1 (Garcia) SJSU
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
•
May 6, 2023 Physics 1 (Garcia) SJSU
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.
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.