PHYSICS 231 INTRODUCTORY PHYSICS I

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PHYSICS 231INTRODUCTORY PHYSICS I

• Lecture 18

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Chapter 12

• The Laws of Thermodynamics

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Principles of Thermodynamics

• Energy is conserved
• FIRST LAW OF THERMODYNAMICS
• Examples
• Engines (Heat -gt Mechanical Energy)
• Friction (Mechanical Energy -gt Heat)
• All processes must increase entropy
• SECOND LAW OF THERMODYNAMICS
• Entropy is measure of disorder
• Engines can not be 100 efficient

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Work done on a gas

• Adding heat Q can
• Change temperature
• Change state of matter
• Can also change ?U by doing work on the gas

Change of Internal Energy ?U
Work done on the gas
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First Law of Thermodynamics
Note (Work done by the gas) - (Work done on
the gas)
Add heat gt Increase Int. Energy Gas does work
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Example 12.1
A cylinder of radius 5 cm is kept at pressure
with a piston of mass 75 kg. a) What is the
pressure inside the cylinder? b) If the gas
expands such that the cylinder rises 12.0
cm, what work was done by the gas? c) What
amount of the work went into changing the
gravitational PE of the piston? d) Where
did the rest of the work go?
1.950x105 Pa
183.8 J
88.3 J
Compressing the outside air
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Example 12.2a
A massive copper piston traps an ideal gas as
shown to the right. The piston is allowed to
freely slide up and down and equilibrate with
the outside air. The pressure inside
thecylinder is _________ thepressure outside.
a) Greater than b) Less than c) Equal to
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Example 12.2b
A massive copper piston traps an ideal gas as
shown to the right. The piston is allowed to
freely slide up and down and equilibrate with
the outside air. The temperature inside the
cylinder is __________ the temperatureoutside.
a) Greater than b) Less than c) Equal to
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Example 12.2c
A massive copper piston traps an ideal gas as
shown to the right. The piston is allowed to
freely slide up and down and equilibrate with the
outside air. If the gas is heated by a
steady flame, and the piston rises to a new
equilibrium position, the new pressure will be
_________ than the previous pressure.
a) Greater than b) Less than c) Equal to
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Some Vocabulary
P

• Isobaric
• P constant
• Isovolumetric
• V constant
• W 0
• Isothermal
• T constant
• ?U 0 (ideal gas)
• Adiabatic
• Q 0

P
P
P
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Example 12.3a
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, isothermally compressing the gas to
half its original volume (b) Pb is _______ Pa
a) Greater than b) Less than c) Equal to
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Example 12.3b
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, isothermally compressing the gas to
half its original volume (b) Tb is ________ Ta
a) Greater than b) Less than c) Equal to
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Example 12.3c
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, isothermally compressing the gas to
half its original volume (b) Wab is ________ 0
a) Greater than b) Less than c) Equal to
Vocabulary Wab is work done by gas between a and
b
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Example 12.3d
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, isothermally compressing the gas to
half its original volume (b) Ub is ________ Ua
a) Greater than b) Less than c) Equal to
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Example 12.3e
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, isothermally compressing the gas to
half its original volume (b) Qab is ________ 0
a) Greater than b) Less than c) Equal to
Vocabulary Qab is heat added to gas between a
and b
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Example 12.4a
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, adiabatically compressing the gas to
half its original volume (b) Pb is _______ Pa
a) Greater than b) Less than c) Equal to
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Example 12.4b
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, adiabatically compressing the gas to
half its original volume (b) Wab is ______ 0
a) Greater than b) Less than c) Equal to
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Example 12.4c
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, adiabatically compressing the gas to
half its original volume (b) Qab is _______ 0
a) Greater than b) Less than c) Equal to
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Example 12.4d
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, adiabatically compressing the gas to
half its original volume (b) Ub is _______ Ua
a) Greater than b) Less than c) Equal to
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Example 12.4e
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) Weight is slowly added to the
piston, adiabatically compressing the gas to
half its original volume (b) Tb is _______ Ta
a) Greater than b) Less than c) Equal to
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Example 12.5a
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) The gas is cooled, isobarically
compressing the gas to half its original volume
(b) Pb is _______ Pa
a) Greater than b) Less than c) Equal to
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Example 12.5b
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) The gas is cooled, isobarically
compressing the gas to half its original volume
(b) Wab is _______ 0
a) Greater than b) Less than c) Equal to
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Example 12.5c
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) The gas is cooled, isobarically
compressing the gas to half its original volume
(b) Tb is _______ Ta
a) Greater than b) Less than c) Equal to
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Example 12.5d
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) The gas is cooled, isobarically
compressing the gas to half its original volume
(b) Ub is _______ Ua
a) Greater than b) Less than c) Equal to
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Example 12.5e
Outside Air Room T, Atm. P
A massive piston traps an amount of Helium gas
as shown. The piston freely slides up and down.
The system initially equilibrates at room
temperature (a) The gas is cooled, isobarically
compressing the gas to half its original volume
(b) Qab is _______ 0
a) Greater than b) Less than c) Equal to
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P-V Diagrams
P
Path moves to right Wby the gas Area under
curve
P
V
Path moves to left Wby the gas - Area under
curve
V
(Won the gas - Wby the gas)
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Work from closed cycles
Consider cycle A -gt B -gt A
(work done by gas)
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Work from closed cycles
Consider cycle A -gt B -gt A
WA-gtB-gtA Area
(work done by gas)
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Work from closed cycles
Reverse the cycle, make it counter clockwise
(work done by gas)
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Work from closed cycles
Reverse the cycle, make it counter clockwise
WA-gtB-gtA - Area
(work done by gas)
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Internal Energy in closed cycles
in closed cycles
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Example 12.6
a) What amount of work is performed by the gas in
the cycle IAFI? b) How much heat was inserted
into the gas in the cycle IAFI? c) What amount
of work is performed by the gas in the cycle IBFI?
W3.04x105 J
Q 3.04x105 J
V (m3)
W -3.04x105 J
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Example 12.7
Consider a monotonic ideal gas. a) What work was
done by the gas from A to B? b) What heat was
added to the gas between A and B? c) What work
was done by the gas from B to C? d) What heat
was added to the gas beween B and C? e) What work
was done by the gas from C to A? f) What heat
was added to the gas from C to A?
P (kPa)
A
75
20,000 J
50
20,000
B
25
-10,000 J
C
V (m3)
-25,000 J
0.2
0.4
0.6
0
15,000 J
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Example Continued
Take solutions from last problem and find a)
Net work done by gas in the cycle b) Amount of
heat added to gas
WAB WBC WCA 10,000 J QAB QBC QCA
10,000 J
This does NOT mean that the engine is 100
efficient!
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Example 12.8a
C
P
Consider an ideal gas undergoing the trajectory
through the PV diagram. In going from A to B to
C, the work done BY the gas is _______ 0.
B
A
V

1. gt
2. lt

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Example 12.8b
C
P
In going from A to B to C, the change of the
internal energy of the gas is _______ 0.
B
A
V

1. gt
2. lt

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Example 12.8c
C
D
P
In going from A to B to C, the amount of heat
added to the gas is _______ 0.
B
A
V

1. gt
2. lt

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Example 12.8d
C
D
P
In going from A to B to C to D to A, the work
done BY the gas is _______ 0.
B
A
V

1. gt
2. lt

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Example 12.8e
C
D
P
In going from A to B to C to D to A, the change
of the internal energy of the gas is _______ 0.
B
A
V

1. gt
2. lt

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Example 12.8f
C
D
P
In going from A to B to C to D to A, the heat
added to the gas is _______ 0.
B
A
V

1. gt
2. lt