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8/12/2019 Physics 101 Chapter 14 Gases
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Check question:
Two solid blocks of identical size, one oflead and the other wood, are put in water.Upon which is the buoyant force greater?
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Check again:
What is the buoyant force on a ten ton shipfloating in fresh water? In salt water? In alake of mercury?
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Chapter 14 - Gases
Atmospheric Pressure, Barometers,Vacuum pumps, Buoyancy of air and
Bernoullis Principle
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Gas vs. liquid
Recall, both are fluidsDifferences:
Distance between the molecules A gas expands (i.e. density variation) to fill the
space availableSecondary point: Gas is compressible!
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Atmosphere
Thickness of the atmosphere determined by: Kinetic energy of the gas molecules (temperature) Force of gravity
Fortunately Distance from sun, Size of the earth, and Presence of the correct gases are all just right for our
world.
No real edge, but 99% below 30 km . 50 % below 5.6 km !
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Fig. 14.1
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Atmospheric Pressure
We are at the bottom of an ocean of air.Butthe density of air goes down as we go
higher .One square meter (m 2) on the ground hasabout 10,000 kg of air above it.
Weight is mg, so 10,000 kg x 10 m/s 2 = 10 5 N At sea level, pressure is approx . 10 5 N/m 2
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Where (elevation) is the densityof air the highest?
_____________ ___________
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Fig. 14.5
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Atmospheric Pressure (cont.)
The exact value is 1.01 x 10 5 N/m 2,
Or 1.01 x 105 Pascals (Pa )
Or one atmosphere (1 atm.)
Or 760 mm of mercury in a barometer .
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Toricelli (mercury)Barometer
Fig. 14.7
Fill a tube with mercury andturn upside down into a bowlof mercury. Note: there is a
vacuum at the top of the tube.
76 cm or 760 mm of mercuryis pushed up the tube by air
pressure at sea level.
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2222222222222Barometer(cont.)
The 760 mm column of mercury weighs thesame as a 30 km (30,000,000 mm) columnof air of the same width.Water barometer?
Yes, if it is 13.6 times as tall: about 10.3-m .Why? Mercury is 13.6 times as dense as water .
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Why dont you noticethis weight of air?
How can we show itis there?
Fig. 14.3
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Magdeburg Spheres 1654: All the kings horses Sucked together or pushed together?
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Is the piston supportingthe weight pulled up or
pushed up?
Fig. 14.3
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Straws and Pumps
Soda is not sucked up the straw.You pull the air out, creating apartial vacuum, and atmospheric
pressure pushes soda up thestraw.
Hand pumps (vacuum type )only good to depth of about30 ft (10.3 m).
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The Real Question!
Which gives you more benefit from theatmosphere?
a) Frosty b) Milk Shake
?
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Frosty
More push for less cash!(you cant beat it)
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Altimeters and vacuum pumps
See figure 14.10. for aneriod barometer.
Vacuum of space is about 10-8
Pa.
Mechanical pumps (Fig 14.11) are capable
of achieving about 1 Pa of pressure.
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Aneroid Barometer
May be used as altimeter
Vacuum of space isabout 10 -8 Pa.
Fig. 14.10
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Mechanical Vacuum Pump
Fig. 14.11
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Boyles Law
At constant temperature, the volume of a gasis inversely proportional to pressure.
2211,,
1V P V P or k PV or
P V
p
V=
Pv
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Boyles Law: Volume 1/P or PV = k
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XXXXXPressure can be increased by:
Increasing the numberof gas molecules atconstant volume
Decreasing thevolume of the samenumber of molecules
Fig. 14.12
Fig 14.13
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Tire gauge reads gauge (above atm.)pressure, not total pressure.
p. 253 fyi
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Buoyancy of Air
Same principle of Archimedes (see Ch 13, slides21, 22)
An object in air is buoyed up by a force equalto the weight of the air it displaces.
Usually not a large force unless we are talkingabout something the size of a blimp.
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Density of air is greater at groundlevel than at higher levels .
A smaller volume of air is displacedbefore buoyant force is greaterthan the balloons weight
As the balloon rises higher, air is lessdense, so a greater volume of airmust be displaced for buoyantforce to remain greater than weightFig. 14.15
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StreamlinesImaginary lines used to trace the paths of molecules.
They get closer when the opening gets narrower.Closer lines indicate faster flow and lower pressure.
Fig. 14.18
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LiftWings and streamlines:
Angle of attack Curvature of the wing
If lift force = weight then flight is possible .
Fig. 14.21
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Curve Balls
Curve ball moves to the side of lowerpressure (higher air speed).
Fig. 14.21
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Atomizer
Fig 14.23
Increasing air speed lowers pressure above tube, so higherpressure inside reservoir pushes liquid up the tube