PHY 113 C Fall 2013 Lecture 24 1 PHY 113 C General Physics I 11 AM 1215 P M MWF Olin 101 Plan for Lecture 24 Review Chapters 1718 14 1922 Sound Doppler effect amp standing waves ID: 311030
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PHY 113 C Fall 2013 -- Lecture 24
1
PHY 113 C General Physics I
11 AM – 12:15
P
M MWF Olin 101
Plan for Lecture 24:
Review: Chapters 17-18, 14, 19-22
Sound; Doppler effect & standing waves
Physics of fluids; pressure, buoyant force, Bernoulli’s equation
Temperature & heat & ideal gas law
First law of thermodynamics
Cycles and their efficiencySlide2
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Comment about Exam 3:
Part I – take home portion (1 problem): available at end of class today -- 11/21/2013; must be turned in before part II
Part II – in-class portion (3 problems) --Tuesday 11/26/2013
Some special arrangements for early exams have been arranged by
prior agreement
Of course, all sections of the exam are to be taken under the guidelines of the honor codeSlide4
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iclicker
question
How are you doing on preparing your equation sheet for Exam 3?
It is completed
It is almost completed
I am in a panic because there are too many equations this timeSlide5
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Webassign
– Assignment #21
The work done by an engine equals one-fourth the energy it absorbs from a reservoir.
(
a) What is its thermal efficiency?
(b) What fraction of the energy absorbed is expelled to the cold reservoir?Slide6
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Webassign
– Assignment #21
What is the coefficient of performance of a refrigerator that operates with Carnot efficiency between temperatures -3.00°C and +27.0°C?Slide7
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Webassign
– Assignment #21
A gasoline engine has a compression ratio of 6.00 and uses a gas for which
γ
= 1.40. (a) What is the efficiency of the engine if it operates in an idealized Otto cycle?
(b) If the actual efficiency is 16.0%, what fraction of the fuel is wasted as a result of friction and energy losses by heat that could by avoided in a reversible engine? (Assume complete combustion of the air-fuel mixture.)
fraction lost=
ideal-actual=0.51-0.16=0.35Slide8
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Webassign
– Assignment #21
An idealized diesel engine operates in a cycle known as the
air-standard diesel cycle
shown in the figure below. Fuel is sprayed into the cylinder at the point of maximum compression, B. Combustion occurs during the expansion B → C, which is modeled as an isobaric process. Show that the efficiency of an engine operating in this idealized diesel cycle is given by the following expression. Slide9
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Comment on adiabatic process
(Q=0
) --Slide10
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Comment on adiabatic process
(Q=0
) -- continuedSlide11
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Comment on adiabatic process
(Q=0
) – continued
Suppose you were asked to calculate the final pressure for an expansion process where V
i/Vf=1/10 when Pi
=1 atm. and when
g
=1.3?Slide12
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Review of main ideas from Chapters:
17-18 – Sound waves
14 -- Physics of fluids
19-22 – Temperature, heat, thermodynamics Slide13
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Physics of sound waves
Sound waves are described by the wave equation
Change of average air density or pressure
position
timeSlide14
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Standing
wave
:
Standing waves
. Two sinusoidal waves, same amplitude, same f,
but opposite directionsSlide15
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Standing waves between reflecting wallsSlide16
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Doppler effectSlide17
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toward
away
Relative velocity of source toward observerSlide18
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Typical question concerning Doppler effect:
A driver travels northbound on a highway at a speed of 30.0 m/s. A police car, traveling southbound at a speed of 34.0 m/s, approaches with its siren producing sound at a frequency of 2500 Hz.
(
a) What frequency does the driver observe as the police car approaches?
(b) What frequency does the driver detect after the police car passes him?Slide19
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The physics of fluids.
Fluids include liquids (usually “incompressible) and gases (highly “compressible”).
Fluids obey Newton’s equations of motion
, but because they move within their containers, the application of Newton’s laws to fluids introduces some new forms.
Pressure: P=force/area 1 (N/m
2
) = 1 Pascal
Density:
r
=mass/volume 1 kg/m
3
= 0.001
gm
/mlSlide20
Buoyant force for fluid acting on a solid:
F
B=r
fluid
V
displaced
g
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General relationship between P and
r:
mg
A
D
ySlide21
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Bernoulli’s equation:Slide22
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Bernoulli’s equation:Slide23
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A hypodermic syringe contains a medicine with the density of water (see figure below). The barrel of the syringe has a cross-sectional area
A
= 2.40 10
-5
m
2
, and the needle has a cross-sectional area
a
= 1.00 10
-8
m
2
. In the absence of a force on the plunger, the pressure everywhere is 1.00 atm. A force of magnitude 2.65 N acts on the plunger, making medicine squirt horizontally from the needle. Determine the speed of the medicine as it leaves the needle's tip.
Webassign
questions on fluids (Assignment #17)Slide24
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Effects of temperature on materials – continued -- ideal gas “law” (thanks to Robert Boyle (1627-1691), Jacques Charles (1746-1823), and Gay-Lussac (1778-1850)
pressure in
Pascals
volume in m
3
# of moles
temperature in K
8.314 J/(
mol
K)
1 mole corresponds to 6.022 x 10
23
molecules
Notion of temperature:Slide25
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Notion of heat
Heat can be used to change temperature:
Heat
capacity: C = amount of heat which must be added
to
the “system” to raise its
temperature
by 1K (or 1
o
C).
Q
= C
D
T
Heat capacity per mass: C=mc
Heat capacity per mole (for ideal gas): C=
nC
v
C=
nC
pSlide26
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Some typical specific heats
Material
J/(kg·
o
C)
cal/(g·
o
C)
Water (15
o
C)
4186
1.00
Ice (-10
o
C)
2220
0.53
Steam (100
o
C)
2010
0.48
Wood
1700
0.41
Aluminum
900
0.22
Iron
448
0.11
Gold
129
0.03Slide27
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Heat and changes in phase of materials
Example: A plot of temperature versus Q added to
1g = 0.001 kg of ice (initially at T=-30
o
C)Slide28
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Typical question concerning heat:
Suppose
you have a well-insulated cup of hot coffee (
m=0.3kg
,
T=100
o
C) to which you
add
0.3
kg of ice (at 0
o
C). When your cup comes to equilibrium, what will be the temperature of the
coffee?Slide29
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Important equations for macroscopic and microscopic descriptions of thermodynamic properties of matterSlide30
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Question from previous exam:Slide31
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F
B
mg
TSlide32
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Question from previous exam:Slide33
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