Transcript
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Chapter 20Entropy and
the 2
nd
Law ofThermodynamics
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Things to learn
We define the concept of entropy and
understand the 2nd law ofthermodynamics (Entropy never
decreases)
We will understand engines andrefrigerators as devices
interconverting heat and work.
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20-2 Irreversible Processes
and EntropyIrreversible process
Inverse process is improbable !
Fig 20-1 free expansion
[entropy is increasing]
Entropy : multiplicity of a system
Reversible process
Inverse process happens[entropy is invariant]
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20-3 Change in
Entropy
Irreversible
process
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Reversible
process
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Entropy
calculation
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SP 20-1
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Water is heated on a stove. Find the entropy
changes of the water as its temperature rises.
CP 1
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CP 2
Find the entropychanges of the
two processes of an
ideal gas.
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SP 20-2
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Entropy as a state function
Entropy change is independent of the path.
Once we know i and f states,
we know the Entropy change.
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SP 20-1 Once again
free expansion
Gas was the only system (closed).Entropy of the closed system has been
increased! irreversible process
Corresponding Reversible process
Gas + Reservoir was the closed system
Entropy of the closed system has been
unchanged reversible processReservoir gave the same dQ to gas at the same T
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20-4 The 2nd Law of
Thermodynamics
The entropy of a closed system never decreases
Reversible process : constant
Irreversible process : increasing
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20-5 The entropy in the real world
Heat Engine
Heat (QH) is supplied fromthe hotter reservoir.
(gasoline explosion)
Through the expansion of gas,
work W is performed.
(rotate the crank)
Partial heat (QL) is lost to the
colder reservoir.
(outside the engine)
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The Carnot Engine
= ideal engine (reversible).
= the most efficient engine.
No energy loss due to
friction and turbulence.
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The Carnot Engine
(Work)
Work = Heat in Heat out
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The Carnot Engine
(Entropy)
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The Carnot Engine (Efficiency)
Perfect engine
impossible
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SP 20-4
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20-6 Refrigerator
Refrigerator is cooling
a cold reservoir cooler.
Input : Work (compressor)
Output : QL(heat lost by the cold reservoir)
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20-7 The efficiencies of real
engines No real engine has
efficiency greater than
C. If X > C, couple X to
Carnot refrigerator:
Violate the 2nd law of
thermodynamics
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20-8 A statistical view of
entropy
Macroscopic definition
dS=dQ/T
Microscopic definition
S=k ln W
k=Boltzmann constant W=multiplicity
W=1 for each microscopic configuration
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Multiplicity calculation
S=k ln W
Count the number of ways todistribute balls in the two
partitions
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Multiplicity calculation
As n ,the peak value of the W becomes
sharper and sharper at
n1=n2=n/2Entropy (log multiplicity)
becomes maximum at this point
For a system with a large number ofparticles (order of mol), system tends to
make a transition into a state with more
multiplicity, which has a larger entropy.
Second law of thermodynamics
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