The Laws of Thermodynamics

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Chapter 12 The Laws of Thermodynamics
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Heat and work Thermodynamic cycle
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• Heat and work
• Work is done by the system
• Work is done on the system

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• The first law of thermodynamics
• Work and heat are path-dependent quantities
• Quantity Q W ?Eint (change of internal
  energy) is path-independent
• 1st law of thermodynamics the internal energy
  of a system increases if heat is added to the
  system or work is done on the system

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• The first law of thermodynamics
• Adiabatic process no heat transfer between the
  system and the environment
• Isochoric (constant volume) process
• Free expansion
• Cyclical process

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Chapter 12 Problem 18
Consider the cyclic process depicted in the
figure. If Q is negative for the process BC and
?Eint is negative for the process CA, what are
the signs of Q, W, and ?Eint that are associated
with each process?
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• Work done by an ideal gas at constant temperature
• Isothermal process a process at a constant
  temperature
• Work (isothermal expansion)

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• Work done by an ideal gas at constant volume and
  constant pressure
• Isochoric process a process at a constant
  volume
• Isobaric process a process at a constant
  pressure

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• Molar specific heat at constant volume
• Heat related to temperature change
• Internal energy change

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• Molar specific heat at constant pressure
• Heat related to temperature change
• Internal energy change

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Free expansion of an ideal gas
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• Time direction
• Irreversible processes processes that cannot
  be reversed by means of small changes in their
  environment

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• Configuration
• Configuration certain arrangement of objects
  in a system
• Configuration for N spheres in the box, with n
  spheres in the left half

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• Microstates
• Microstate one of the ways to prepare a
  configuration
• An example of 4 different microstates for 4
  spheres in the box, with 3 spheres in the left
  half

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• Multiplicity
• Multiplicity ( W ) a number of microstates
  available for a given configuration
• From statistical mechanics

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Multiplicity
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Multiplicity
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Multiplicity
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Multiplicity
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• Entropy
• For identical spheres all microstates are
  equally probable
• Entropy ( S ), see the tombstone
• For a free expansion of
• 100 molecules
• Entropy is growing for
• irreversible processes in
• isolated systems

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• Entropy
• Entropy, loosely defined, is a measure of
  disorder in the system
• Entropy is related to another fundamental
  concept information. Alternative definition of
  irreversible processes processes involving
  erasure of information
• Entropy cannot noticeably decrease in isolated
  systems
• Entropy has a tendency to increase in open
  systems

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• Entropy in open systems
• In open systems entropy can decrease
• Chemical reactions
• Molecular self-assembly
• Creation of information

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• Entropy in thermodynamics
• In thermodynamics, entropy for open systems is
• For isothermal process, the change in entropy
• For adiabatic process, the change in entropy

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• The second law of thermodynamics
• In closed systems, the entropy increases for
  irreversible processes and remains constant for
  reversible processes
• In real (not idealized) closed systems the
  process are always irreversible to some extent
  because of friction, turbulence, etc.
• Most real systems are open since it is difficult
  to create a perfect insulation

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• Engines
• In an ideal engine, all processes are reversible
  and no wasteful energy transfers occur due to
  friction, turbulence, etc.
• Carnot engine

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• Carnot engine (continued)
• Carnot engine on the p-V diagram
• Carnot engine on the T-S diagram

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• Engine efficiency
• Efficiency of an engine (e)
• For Carnot engine

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• Perfect engine
• Perfect engine
• For a perfect Carnot engine
• No perfect engine is possible in which a heat
  from a thermal reservoir will be completely
  converted to work

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• Gasoline engine
• Another example of an efficient engine is a
  gasoline engine

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Chapter 12 Problem 31
In one cycle, a heat engine absorbs 500 J from a
high-temperature reservoir and expels 300 J to a
low-temperature reservoir. If the efficiency of
this engine is 60 of the efficiency of a Carnot
engine, what is the ratio of the low temperature
to the high temperature in the Carnot engine?
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• Heat pumps (refrigerators)
• In an ideal refrigerator, all processes are
  reversible and no wasteful energy transfers occur
  due to friction, turbulence, etc.
• Performance of a refrigerator (K)
• For Carnot refrigerator

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• Perfect refrigerator
• Perfect refrigerator
• For a perfect Carnot refrigerator
• No perfect refrigerator is possible in which a
  heat from a thermal reservoir with a lower
  temperature will be completely transferred to a
  thermal reservoir with a higher temperature

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Questions?
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Answers to the even-numbered problems Chapter 12
Problem 36 6.06 kJ/K
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• Answers to the even-numbered problems
• Chapter 12
• Problem 56
• -4.9 10-2 J
• 16 kJ
• 16 kJ