the second law of thermodynamics - web space - oit · the second law of thermodynamics . so far…...
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The Second Law of Thermodynamics
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So far…
• We have studied the second law by looking at its results
• We don’t have a thermodynamic property that can describe it
• In this chapter we will develop a mathematical definition for entropy– Entropy is a quantitative measure of disorder
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How does the second law relate to disorder?
• Disorder naturally increases, just like water flows down hill and gases expand
• Natural processes proceed spontaneously toward disorder
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Disorder
• Cleaning your room takes a lot of effort– Messing it up is much easier
• Dissolving sugar in water increases disorder– Sugar dissolves spontaneously– Sugar and water don’t separate spontaneously
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Energy
• It takes the same amount of energy to clean your room as to mess it up.
• It takes the same amount of energy to dissolve sugar as to separate it.
• We all know that these are not equivalent, reversible, processes.
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Understanding Entropy
• What is it?– It’s a measure of disorder
• We can define entropy about as well as we can define energy
• Remember – we define energy as the ability to do work
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Consider an energy balance for the entire system
( ) csysrevR
cexitin
dEWWQEEE
=+−∆=−δδδ
ccR
sysrevc
dEWQ
WWW
=−
+=
δδ
δδδ
Clausius Inequality
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If the heat engine is completely reversible then…
δ δ
δ δ
QT
QT
Q T QT
R
R
R R
=
=
This is based on Chapter 5, where we said that heat transfer rate in a reversible system is proportional to the temperature
Clausius Inequality
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δ δW T QT
dEc R c= −
That means that we can rewrite the energy balance, and solve for the net work (the work of the combined system.
∫∫ −= cRc dETQTW δ
To find the total net work we need to integrate over time
0
Clausius Inequality
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∫=TQTW Rcδ
But we know that it is impossible for a system to exchange heat with only one reservoir, and produce work!!
That means Wc can’t be positive, but it can be zero or negative!!
0≤= ∫ TQTW Rcδ
Clausius Inequality
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Since TR is always positive…
∫ ≤ 0TQδ
This is called the Clausius inequality
It is equal to 0 for the reversible case, and is less than 0 for the irreversible case
Clausius Inequality
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Can we prove that it is 0 for the reversible case?
If the the “system” is reversible, we can run it backwards.
If it produces negative work going one way, it will produce positive work going the other way
But we know you can’t exchange heat with only one reservoir, and produce work!!
Clausius Inequality
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Therefore, for a reversible system, the cyclic integral must be 0!!!
∫ =⎟⎠⎞
⎜⎝⎛ 0
revTQδ
Clausius Inequality
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Entropy
Entropy (S) is a measure of the disorder or randomness in a system, and is a state variable (like P,V,T) that does not depend on the path taken.
flowheat malinfinitesian is dQ where
S)entropy in (change TdQdS =
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Entropy
For any reversible process(e.g. Carnot cycle):
0=⎟⎠⎞
⎜⎝⎛∫
revTdQ
∫∫∫
∫∫
=−=
=+
b
aII
a
bII
b
aI
a
bII
b
aI
TdQ
TdQ
TdQ
TdQ
TdQ 0
The entropy of a system ina given state is independentof the path taken to get there,and is thus a state variable.
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Entropy
• The entropy difference between two equilibrium states a and b does not depend on how the system got from a to b.
∫∫ ==−=∆b
a
b
aab T
dQdSSSS
Entropy is a state variable (like P,V and T)
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Pre
s su r
e
Specific Volume
1
2
Reversible
Arbitrary
Consider a cycle composed of two processes
Process 1 (from state 1 to state 2) is reversible
Process 2 (from state 2 back to state 1 is arbitrary
Entropy
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Pre
s su r
e
Specific Volume
1
2
Reversible
Arbitrary
The change in entropy from state1 to 2 is
∫ ⎟⎠⎞
⎜⎝⎛2
1rev
net
TQδ
The change in entropy from state 2 to 1 is
( )21 SS −+
Since the process is a cycle, the total change in entropy is 0
0=
Entropy
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rev
net
TQSS ∫ ⎟
⎠⎞
⎜⎝⎛=−
2
112δ
So, the definition of a change in entropy becomes….
Or…
rev
net
TQdS ⎟
⎠⎞
⎜⎝⎛=δ
Pre
s su r
e
Specific Volume
1
2
Reversible
Arbitrary
Entropy
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Entropy ChangeEntropy Change
The entropy change between two specific states is the same whether the process is reversible or irreversible
Entropy is a “state function”
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Consider an adiabatic reversible process…
rev
net
TQSS ∫ ⎟
⎠⎞
⎜⎝⎛=−
2
112δ
0=QδSo… ∆S=0
A reversible, adiabatic process is called isentropic and S is constant.
leirreversib
net
TQSS ∫ ⎟
⎠⎞
⎜⎝⎛>−
2
112δ
But what if the process isn’t reversible?
Entropy ChangeEntropy Change
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Consider an isolated system
Because it is isolated, there is no heat transfer or work
∫ ⎟⎠⎞
⎜⎝⎛≥−
2
112 TQSS netδ
0
Isolated SystemsIsolated Systems
0≥∆SThe equality holds if all the processes inside the system are reversible
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Now consider the universe
• It can be considered to be an isolated system!!
0≥∆+∆=∆ gssurroundinsystemuniverse SSS
This is the second law of thermodynamics, expressed mathematically!!
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You can consider any system to be isolated, if you draw your system boundaries out far enough
0≥∆+∆=∆ gssurroundinsystemtotal SSS
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Hopefully you have a better feel for entropy now!
• How do you calculate entropy changes for real systems?– If it’s reversible adiabatic, ∆S=0– If it’s isothermal heat transfer
– If it’s not an ideal gas, just look it up on the property tables
TQ
TQSS rev
rev
net =⎟⎠⎞
⎜⎝⎛=− ∫
2
112δ
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Entropy is a property
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The entropy of a pure substance is determined from the tables, just as
for any other property
Entropy is a property
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Some Remarks about Entropy
• Entropy is a non-conserved property, and there is no such thing as the conservation of entropy principle. The entropy of the universe is continuously increasing.
• The performance of engineering systems is degraded by the presence of irreversibilities, and entropy generation is a measure of the magnitudes of the irreversibilities present during that process.
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Engineering Devices
Many engineering devices are essentially adiabatic
• They perform best when irreversibilities are eliminated
• Isentropic model serves as an idealization of a real process
• These devices work best when they are isentropic
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The assumption that a process is isentropic, gives us a connection between the inlet and outlet conditions – just like assuming constant volume, or constant pressure
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TQ
S revint,δδ =
Property Diagrams Involving Entropy
TdSQ rev =int,δ
∫=2
1int, TdSQ rev
This area has no meaning for irreversible processes
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Consider some special cases of the T-s diagram
Tem
pera
ture
Entropy
Isothermal Process
Q
1 2
Tem
pera
ture
Entropy
Isentropic Process
1
2
Q=0
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T-s Diagram for the Carnot Cycle
Tem
pera
ture
Entropy
1
Isentropic compression
QinW=Qin-Qout
Qout
2Isothermal expansion
3
Isentropic expansion
Isothermal compression
4
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Mollier Diagrams (h-s Diagrams)
For adiabatic, steady flow devices, ∆h is a measure of work, and ∆s is a measure of irreversibilities
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Mollier Diagram for Steam