The Second Law of Thermodynamics

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The Second Law of Thermodynamics
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• The second law of thermodynamics states that
  processes occur in a certain direction, not in
  just any direction.
• Physical processes in nature can proceed toward
  equilibrium spontaneously

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Water always flows downhill
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Gases always expand from high pressure to low
pressure
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Heat always flows from high temperature to low
temperature
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Can We Take Advantage of These Processes?

• Yes!! We can use them to produce work
• Or we can just let them happen and lose the
  opportunity.

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We can reverse these processes

• It requires the expenditure of work
• The 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 what direction processes
  occur

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Thermal Energy Reservoirs

• Bodies that can absorb finite amounts of heat
  without undergoing any change in temperature are
  called thermal energy reservoir or heat
  reservoir.
• A reservoir that supplied energy in the form of
  heat is called a source.
• A reservoir that absorbs energy in the form of
  heat is called a sink.

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Heat Engines

• Work can easily be converted to other form of
  energy, but converting other forms of energy to
  work is not that easy.
• Work can be converted to heat directly and
  completely, but converting heat to work requires
  the use of some special devices. These devices
  are called heat engines.

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Characteristics of Heat Engines

• They receive heat from a high temperature
  source (solar energy, oil furnace, nuclear
  reactor, etc.)
• They convert part of this heat to work (usually
  in the form of rotating shaft).
• They reject the remaining waste heat to a low
  temperature sink (the atmosphere, rivers etc).
• They operate on a cycle.

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• Working fluid Heat engines usually involve a
  fluid to and from which heat is transferred while
  undergoing cycle. This fluid is called Working
  fluid.
• Thermal Efficiency The fraction of the heat
  input that is converted to net work output is a
  measure of the performance of a heat engine and
  is called the thermal efficiency.

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• Consider a heat engine operating between a high
  temperature reservoir at temperature TH and a
  low temperature reservoir at temperature TL.
  QH be the magnitude of heat transfer between the
  heat energy and high temperature reservoir and QL
  be magnitude of heat transfer between the heat
  engine and high temperature reservoir.

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• The thermal Efficiency is always less than unity
  since both QL and QH are positive quantities.
• Thermal efficiency is a measure of how
  efficiently a heat engine converts the heat that
  it receives to work.

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REFRIGERATORS

• Refrigerators is a devices that absorb heat from
  low temperature reservoir and reject it to high
  temperature reservoir.
• Most frequently used refrigeration cycle is vapor
  compression refrigeration cycle which has for
  important component a compressor, a condenser,
  an expansion valve, and an evaporator.

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REFRIGERATORS
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REFRIGERATORS

• Coefficient of performance of refrigerator The
  efficiency of a refrigerator is expressed in
  terms of the coefficient of performance (COPR).
• The objective of a refrigerator is to remove heat
  (QL) from the refrigerated space for this it
  required work input of Wnet,in.

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Heat Pumps

• Refrigerators and heat pumps operate on the same
  cycle but differ in their objectives.
• The objective of a heat pump is to maintain a
  heated space at a high temperature.
• The measure of performance of a heat pump is also
  expressed in terms of the coefficient of
  performance COPHP.

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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.

Cold
Hot
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Clausius Statement of the Second Law

•   In order to accomplish heat transfer from cold
  to hot you need a device, like a heat pump or
  refrigerator, that consumes work.

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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.
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For a heat pump
For a Refrigerator
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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. 

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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.
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So What is the Best You Can Do?

• We know that Coefficients of Performance for
  refrigerators and heat pumps must be less than
  infinity, but how much less?
• We know that thermal efficiencies for heat
  engines must be less than 100, but how much less?

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It Depends on..

• Irreversibilities
• Reversible Process A Process tha can be reversed
  without leaving any trace on the surroundings.
• That is both the system and surrounding are
  returned to their initial states at the end of
  the reverse process.

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Irreversible Processes

• Process that are not reversible are called
  Irreversible Process.
• Factors that cause a process to be irreversible
  are called Irreversibilities. They include
• Friction
• Unrestrained expansion of gases
• Heat transfer through a finite temperature
  difference
• Mixing of two different substances
• Any deviation from a quasistatic process

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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.
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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.
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All real processes are irreversible!!
So why should we worry about reversible processes?
Reversible processes represent the best that we
can do.
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Carnot Cycle

• Named for French engineer Nicolas Sadi Carnot
  (1769-1832)
• One example of a reversible cycle
• Composed of four reversible processes
• 2 adiabatic heat transfer
• 2 reversible isothermal heat transfer

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Carnot Cycle

• Process 1-2 Reversible isothermal heat addition
  at high temperature, TH gt TL to the working fluid
  in a piston--cylinder device which does some
  boundary work.
• Process 2-3 Reversible adiabatic expansion
  during which the system does work as the working
  fluid temperature decreases from TH to TL.
• Process 3-4 The system is brought in contact
  with a heat reservoir at TL lt TH and a 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

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Process 1-2 Reversible isothermal heat addition
at high temperature, TH gt TL to the working fluid
in a piston--cylinder device which does some
boundary work.
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• Process 2-3 Reversible adiabatic expansion
  during which the system does work as the working
  fluid temperature decreases from TH to TL.

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• Process 3-4 The system is brought in contact
  with a heat reservoir at TL lt TH and a reversible
  isothermal heat exchange takes place while work
  of compression is done on the system.

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Process 4-1 A reversible adiabatic compression
process increases the working fluid temperature
from TL to TH
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Execution of the Carnot Cycle in a Closed System
Isothermal
Isothermal
Adiabatic
Adiabatic
The Carnot cycle is a reversible heat engine
The area inside the figure represents the work
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Reversed Carnot Cycle
A reversed Carnot Cycle is a refrigerator or a
heat pump
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Carnot principles

• 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.

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• These statements form the basis for establishing
  a thermodynamic temperature scale related to the
  heat transfers between a reversible device and
  the high- and low-temperature reservoirs by
  
• Therefore, 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.

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Efficiency of a Carnot Engine

• For a reversible cycle the amount of heat
  transferred is proportional to the temperature of
  the reservoir

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Efficiency of a Carnot Engine

• The thermal efficiencies of actual and reversible
  heat engines operating between the same
  temperature limits compare as follows

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COP of a Reversible Heat Pump and a Reversible
Refrigerator
Only true for the reversible case
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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

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Chapter Summary

• 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.

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Chapter Summary

• Work can be converted to heat directly, but heat
  can be converted to work only by some devices
  called heat engines.

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Chapter Summary

• 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.

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Chapter Summary

• 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.

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Chapter Summary

• 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. The effects such as friction,
  non-quasi-equilibrium expansion or compression,
  and heat transfer through a finite temperature
  difference render a process irreversible and are
  called irreversibilities.

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Chapter Summary

• The Carnot cycle is a reversible cycle that is
  composed of four reversible processes, two
  isothermal and two adiabatic.

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Chapter Summary

• 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.

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Chapter Summary

• These statements form the basis for establishing
  a thermodynamic temperature scale related to the
  heat transfers between a reversible device and
  the high- and low-temperature reservoirs by
  
• Therefore, 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.

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Chapter Summary

• 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.

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Chapter Summary

• The COPs of reversible refrigerators and heat
  pumps are given in a similar manner as
• and
• Again, these are the highest COPs a refrigerator
  or a heat pump operating between the temperature
  limits of TH and TL can have.