The Second Law of Thermodynamics Chapter 6. The Second Law The second law of thermodynamics states...
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Transcript of The Second Law of Thermodynamics Chapter 6. The Second Law The second law of thermodynamics states...
The Second Law
The second law of thermodynamics states that processes occur in a certain direction, not in just any direction.
Physical processes in nature proceed toward equilibrium spontaneously:
We can reverse these processes
It requires the expenditure of workThe first law gives us no information
about the direction in which a process occurs – it only tells us that energy must balance
The second law tells us in what direction processes occur
Work Always Converts Directly and Completely to Heat, But not the ReverseWork Always Converts Directly and Completely to Heat, But not the Reverse
Work and Heat are not interchangeable forms of energy
We need a device to convert heat to work
Some Definitions
Thermodynamic CycleSystem returns to the initial conditionsThe initial state is the same as the final
state
Some Definitions
Heat Engine A heat engine is a thermodynamic system
operating in a thermodynamic cycle to which net heat is transferred and from which net work is delivered
This is the device that converts heat to work
Some Definitions
Thermal Efficiency Index of performance of a deviceWhat you want over what you have to pay
for
input required
output desiredth
Sign Conventions
When we talk about engines, we generally dispense with our sign conventions
All the energy terms are positiveWe determine the direction of flow based
on the situation, and label carefully
inoutoutnet WWW ,
For a cyclic heat engine
0,, UWQ outnetinnet
innetoutnet QW ,, outin QQ
in
outnet
Q
W ,in
outin
Q
in
out
Q
Q1
H
L
Q
Q1
Even the Most Efficient Heat Engines Reject Most Heat as Waste Heat
Even the Most Efficient Heat Engines Reject Most Heat as Waste Heat
4.0100
40thn
Steam power plants run at about 40% efficiency
A Heat-Engine Cycle Must Reject Some Heat to a Low-Temperature SinkA Heat-Engine Cycle Must Reject Some Heat to a Low-Temperature Sink
5-5
A heat-engine cycle cannot be completed without rejecting some heat to a low-temperature sink
Kelvin-Planck Statement of the Second Law
It is impossible for any device that operates on a cycle, to receive heat from a single reservoir and produce a net amount of work.
The Kelvin-Planck statement of the second law of thermodynamics states that no heat engine can produce a net amount of work while exchanging heat with a single reservoir only. In other words, the maximum possible efficiency is less than 100%.
Some Mechanical Devices accept work as input, and move heat from low to high temperaturesHeat PumpRefrigerators and Air
Conditioners
Heat always flows spontaneously from hot to cold
COP
We use coefficient of performance (COP) instead of efficiency () for heat pumps and refrigerators
COP can be greater than 1 for refrigerators
It is always greater than 1 for heat pumps
COP Refrigerators
net
LR W
QCOP But what is Wnet
equal to?
LHnet QQW Substituting in…
LH
LR QQ
QCOP
1
1
LH QQ
Since QH is always greater than QL, COP can be greater than one, but only when QH /QL is less than 2
COP Heat Pumps
net
HHP W
QCOP But what is Wnet
equal to?
LHnet QQW Substituting in…
LH
HHP QQ
QCOP
HL QQ
1
1
Since QL is always less than QH, COP is always greater than 1
Clausius Statement of the Second Law
It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.
Energy from the surroundings in the form of work or heat has to be expended to force heat to flow from a low-temperature media to a high-temperature media. Thus, the COP of a refrigerator or heat pump must be less than infinity.
Perpetual Motion MachinesAny device that violates the first or second
law is called a perpetual motion machine
If it violates the first law, it is a perpetual motion machine of the first type (PMM1)
If it violates the second law, it is a perpetual motion machine of the second type (PMM2)
Reversible ProcessesHeat pumps, refrigerators and heat engines
all work best reversiblyReversible processes don’t have any losses
such as Friction Unrestrained expansion of gases Heat transfer through a finite temperature difference Mixing of two different substances Any deviation from a quasistatic process
Internally Reversible Process
The internally reversible process is a quasiequilibrium process, which once having taken place, can be reversed and in so doing leave no change in the system. This says nothing about what happens to the surroundings around the system.
Totally or Externally Reversible Process
The externally reversible process is a quasiequilibrium process, which once having taken place, can be reversed and in so doing leaves no change in the system or surroundings.
All real processes are irreversible!!
So why should we worry about reversible processes?
Reversible processes represent the best that we can do.
Carnot Cycle
Named for French engineer Nicolas Sadi Carnot (1769-1832)
One example of a reversible cycle
Nicolas Sadi Carnot
French Physicist Developed the
theory of heat engines
http://scienceworld.wolfram.com/biography/CarnotSadi.html
Carnot Cycle
Composed of four reversible processes 2 adiabatic expansion or compression 2 reversible isothermal heat transfer
Carnot Cycle Process 1-2 Reversible isothermal heat addition at high
temperature
Process 2-3 Reversible adiabatic expansion during which the system does work and the working fluid temperature changes from TH to TL
Process 3-4 Reversible isothermal heat exchange takes place while work of compression is done on the system.
Process 4-1 A reversible adiabatic compression process increases the working fluid temperature from TL to TH
Process 1-2 Reversible isothermal heat addition at high temperature, TH > TL to the working fluid in a piston-cylinder device which does some boundary work.
Pre
ssu
reSpecific Volume
1
QHTH=constan
t2
Process 2-3 Reversible adiabatic expansion during which the system does work as the working fluid temperature decreases from TH to TL.
Pre
ssu
reSpecific Volume
1
QHTH=constan
t2
3
Process 3-4 The system is brought in contact with a heat reservoir at TL < TH and a reversible isothermal heat exchange takes place while work of compression is done on the system.
Pre
ssure
Specific Volume
1
QHTH=constan
t2
34 QL
TL=constan
t
Process 4-1 A reversible adiabatic compression process increases the working fluid temperature from TL to TH
Pre
ssure
Specific Volume
1
QHTH=constan
t2
34 QL
TL=constan
t
Execution of the Carnot Cycle in a Closed SystemExecution of the Carnot Cycle in a Closed System
The Carnot cycle is a reversible heat engine
The area inside the figure represents the work
IsothermalIsothermal
Adiabatic Adiabatic
Efficiency of a Carnot Engine
For a reversible cycle the amount of heat transferred is proportional to the temperature of the reservoir
H
Lrev Q
Q1
H
L
T
T1
Only true for the reversible case
COP of a Reversible Heat Pump and a Reversible Refrigerator
HLrevHP QQ
COP
1
1,
HL TT
1
1
1
1,
LH
revR QQCOP
1
1
LH TT
Only true for the reversible case
How do Reversible and Real Systems Compare?
The efficiency of a reversible heat engine, such as a Carnot engine, is always higher than a real engine
The COP of a reversible heat pump is always higher than a real heat pump
The COP of a reversible refrigerator is always higher than a real refrigerator
Carnot Principle
The thermal efficiency of all reversible heat engines operating between the same two reservoirs is the same
No heat engine is more efficient than a reversible heat engine
SummarySecond LawSummarySecond Law
•The second law of thermodynamics states that processes occur in a certain direction, not in any direction.
•A process will not occur unless it satisfies both the first and the second laws of thermodynamics. •Bodies that can absorb or reject finite amounts of heat isothermally are called thermal energy
reservoirs or heat reservoirs.
SummaryHeat EnginesSummaryHeat Engines
• Work can be converted to heat directly, but heat can be converted to work only by some devices called heat engines.
SummaryThermal EfficiencySummaryThermal Efficiency
• The thermal efficiency of a heat engine is defined as
where Wnet, is the net work output of the heat engine, QH is the amount of heat supplied to the engine, and QL is the amount of heat rejected by the engine.
SummaryRefrigerators and Heat PumpsSummaryRefrigerators and Heat Pumps
• Refrigerators and heat pumps are devices that absorb heat from low-temperature media and reject it to higher-temperature ones.
• The performance of a refrigerator or a heat pump is expressed in terms of the coefficient of performance.
SummaryKelvin-Plank StatementSummaryKelvin-Plank Statement• The Kelvin -Planck statement of the second law of thermodynamics
states that no heat engine can produce a net amount of work while exchanging heat with a single reservoir only.
SummaryClausius StatementSummaryClausius Statement
• The Clausius statement of the second law states that no device can transfer heat from a cooler body to a warmer one without leaving an effect on the surroundings.
Any device that violates the first or the second law of thermodynamics is called a perpetual-motion machine.
SummaryReversible vs. IrreversibleSummaryReversible vs. Irreversible
• A process is said to be reversible if both the system and the surroundings can be restored to their original conditions.
• Any other process is irreversible.
SummaryIrreversibilities
Effects such as
• friction • non-quasi-equilibrium expansion or
compression• heat transfer through a finite temperature
difference
render a process irreversible and are called irreversibilities.
SummaryCarnot CycleSummaryCarnot Cycle
The Carnot cycle is a reversible cycle that is composed of four reversible processes, two isothermal and two adiabatic.
SummaryCarnot PrincipleSummaryCarnot Principle
•The Carnot principles state that the thermal efficiencies of all reversible heat engines operating between the same two reservoirs are the same, and that no heat engine is more efficient than a reversible one operating between the same two reservoirs.
SummaryThermodynamic Temperature ScaleSummaryThermodynamic Temperature Scale
• Based on heat transfer between a reversible device and the high- and low-temperature reservoirs.
• The QH/QL ratio can be replaced by TH/TL for reversible devices, where TH and TL are the absolute temperatures of the high- and low-temperature reservoirs, respectively.
SummaryEfficiency of a Carnot EngineSummaryEfficiency of a Carnot Engine
• A heat engine that operates on the reversible Carnot cycle is called a Carnot heat engine. The thermal efficiency of a Carnot heat engine, as well as all other reversible heat engines, is given by …
• This is the maximum efficiency a heat engine operating between two reservoirs at temperatures TH and TL can have.