chapter 14 gasses & plasmas weight of air is small but not negligible. for example, weight of...
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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
Once believed that you had to move water at a slow drop in gradient to move it.
Pressure does not depend on the amount of liquid
Volume is not the key- depth is
The force
exerted by a
fluid on a
smooth surface
is always at right
angles to the
surface
Apr 20, 2023 Physics 1 (Garcia) SJSU
Pressure in Liquids
High
Medium
Low
High
Medium
Low
Apr 20, 2023 Physics 1 (Garcia) SJSU
Pressure
Pressure is defined as
Pressure =
Metric unit of pressure is Pascal.
1 Pascal = 1 Newtons per square meter
Atmospheric pressure is about 100,000 Pascals
( Force )
( Area )
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Bed of Nails
Sample Problem
Note: Atmospheric pressure is about 100,000 Pascals so much more than pressure due to the “gold” brick.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Check Yourself
In which case is the pressure greatest? A B
A B
Apr 20, 2023 Physics 1 (Garcia) SJSU
Check Yourself
In which case is the pressure greatest? A B
AB
Crush the Rail Tanker Car
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
Apr 20, 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.
A floating object
displaces a weight of
fluid equal to its own
weight.
Ice cubes
in water?
Figure 13.15
To control how high they float in the water
Crocs swallow stones
Fish use air bladders
Figure 13.17
Apr 20, 2023 Physics 1 (Garcia) SJSU
Check Yourself
Did the designers of this “water bridge” have to account for the weight of ships or just the water?
Figure 13.20
Apr 20, 2023 Physics 1 (Garcia) SJSU
Check Yourself
?
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.
Floating & Liquid
Density
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
Figure 13.23
Surface
Tension
• adhesion- attracted to unlike
• Cohesion- attracted to like
contractive tendency of
the surface of liquids
Surface tension causes
Buoyancy
Apr 20, 2023 Physics 1 (Garcia) SJSU
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
Zeppelins
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.
AtmosphereWe live at the
bottom of an ocean of fluid—the fluid is air & “ocean” is the atmosphere.
Apr 20, 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
Apr 20, 2023 Physics 1 (Garcia) SJSU
Atmospheric Pressure
Demo: Magdeburg Hemispheres
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.
Demo: Drinking Straw
L
A AWeight
Apr 20, 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.
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.
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.
Bernoulli’s principle
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
Apr 20, 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?
• 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 forward
provided by plane’s propeller/jet
Airplane Wing
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.
Bernoulli’s principle
• How does this relate to fireplaces?
• On a windy day, waves in a lake are higher than normal. Why?
Plasma
• 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
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.