VEGETABLE PRODUCTION in EASTERN EUROPE

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• One Way Processes
• Defining Entropy Change
• Entropy Change for Irreversible Processes
• The Second Law of Thermodynamics
• Entropy and the Performance of Engines
• Entropy and the Performance of Refrigerator
• The Efficiencies of Real Engines
• The Second Law Revisited
• A Statistical View of entropy

Content of this chapter
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24-1 One way processes
There is a property of things that happen
naturally in the world around us that is strange
beyond belief. Yet we are so used to it that we
hardly ever think about it. It is this
All naturally occurring processes proceed in
one direction only. They never, of their own
accord, proceed in the opposite direction.
Example
A cup of hot coffee left on your desk gradually
cools down. It never Gets hotter all by itself.
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It does not violate the law of conservation of
energy
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It is not the energy of the system that controls
the direction of Irreversible processes. It is
another property
Entropy S
If an irreversible process occurs in a closed
system, the entropy of that system always
increases it never decreases.
Two ways to define the change in the entropy ?S
of a system

1. A macroscopic approach heat transfer and
   the temperature
2. A microscopic approach counting the ways
   to arrange

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24-2 Defining entropy change
1. Introducing the definition of the entropy
change as
Here dQ is the increment of heat energy that is
transferred into or out of the (closed) system at
(Kelvin) temperature T, and the integral is
evaluated from the initial state i of the system
to its final state f.
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2. Entropy as a State Property
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The entropy change
The entropy change in process depends only on
properties of the initial state ( Ti and Vi ) and
of the final state ( Tf and Vf ), thus entropy is
indeed a state property!
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So we have three formulas to calculate the
entropy change
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Analysis the sample problem 24-1
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24-3 Entropy change for irreversible processes
How do we calculate the entropy change for an
irreversible process ?
Fortunately, the entropy is a state function.
?It is for that when a closed system proceeds
from an initial state i to a final state f, the
entropy change depends only on the properties of
these two states. ? It does not depend at all
on the process that connects the states or even
whether that process is reversible or
irreversible.
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• Find a reversible process that connects these
  same two states.
• Any of the many such processes will do.

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Example
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Systemstonewater
Analysis the sample problem 24-2 ?24-3 and 24-4
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24-4 The second law of thermodynamics
The second Law of thermodynamics
When changes occur within a closed system its
entropy Either increases( for irreversible
processes) or remains constant(for reversible
processes). It never decreases.
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Problem 24-3
Solution (a) work is only done along path ab,
where
(b)
(c) Both are zero for a cyclic process
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24-5 Entropy and the performance of engines
1. A Carnot Engine
Heat engine is to extracts energy from its
environment and does useful work.
At the heart of every engine is working
substance, it must operate a cycle!
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2. The Carnot Cycle
The cycle is traversed in the clockwise
direction, each process is reversible!
The net work per cycle is equal to the area
enclosed by the cycle ABCDA and shown shaded in
the figure!
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Because the engine operates in cycle, if X
represent any state property of the working
substance,we must have
T-S Carnot Cycle
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3. Efficiency of a Carnot engine
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Carnot Efficiency
No real engine can have a thermal efficiency
greater than that above, and all Carnot engines
working between the same two fixed temperature
have the same efficiency of Carnots.
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4. Search for a Perfect Engine
There is no perfect engine?!
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Analysis the cycle of engine !sample problem 24-5
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5. Different cycle has different efficiency
Diesel Cycle---two isentropic processes constant
volumeconstant pressure
adiabatic
adiabatic
For two isentropic processes
One can simplify further
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Stirling cycle
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24-6 Entropy and the performance of refrigerators
Ideal refrigerator---if the processes of the
cycle are reversible---Carnot refrigerator
The efficiency of a refrigerator
For Carnot engine
For a typical room air conditioner ,K2.5
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?Search for a Perfect Refrigerator
Consider a closed system involving
high temperature reservoir and low temperature
reservoir
According to the second law of thermo- dynamics,
the entropy of an isolated sys- tem definitely
increases.
So that this perfect refrigerator violates the
second law of thermodynamics!
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Clausius statement
No process is possible whose sole result is the
transfer of heat from a reservoir at one
temperature to another reservoir at a higher
temperature.
That is
There are no perfect refrigerators !
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Analysis the cycle of refrigerator !sample
problem 24-6
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The magic of the heat pump!---sample problem
24-7
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24-8 The second law revisited
Three equivalent statements about the second law
of thermodynamics
2. You cannot change heat energy into work with
100 efficiency. that is, there are no
perfect engines.
3. You cannot transfer heat energy from a
low-temperature reservoir to a higher
temperature reservoir without doing
work. That is, there are no perfect refrigerators.
However, they are all completely equivalent!
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24-9 A statistical view of entropy
The multiplicity of configuration is
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The probability of the balls distributed in the
box
All microstates of a system are equally probable.
But the all configurations are not equally
probable!
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The number of states
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From thermodynamic point of view
free expansion is not an isentropic process
Analysis sample problem 24-9!
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?Entropy and disorder
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Homework Exercises 4 22 23 33 Problems 5 8