physics 101 chapter 14 gases

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

physics 101 chapter 14 gases

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