Physics 103: Lecture 24 Thermodynamics, part 2

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Physics 103 Lecture 24Thermodynamics, part 2

• Laws of thermodynamics
• First DU Q W
• Second Efficiency lt 100
• Engines and refrigerators
• Entropy and disorder
• Short lecture today. 15 minutes for evaluations
  of Prof. Herndon at the end. Please take an
  evaluation sheet when you come in.

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Applications of the First Law Adiabatic Process

• Adiabatic process
• Energy transferred by heat is zero
• The work done is equal to the change in the
  internal energy of the system
• One way to accomplish a process with no heat
  exchange is to have it happen very quickly.
  Another way is to keep the system isolated.
• In an adiabatic expansion, the work done is
  negative and the internal energy decreases
  Adiabatic compression, pos work
• Important in the Heat engine - Carnot cycle
• PV? C, ?CP/CV
• ? adiabatic index
• Molar Specific heats
• CP heat transfer
• CV change in internal energy

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Heat Engine.

• Based on cyclical process, DU 0
• Its initial and final internal energies are the
  same
• Therefore, Qnet Weng
• The work done by the engine equals the net energy
  absorbed by the engine
• The work is equal to the area enclosed by the
  curve of the PV diagram

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Carnot Cycle
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Q DU - W
Q DU - W
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Heat Engine Efficiency vs. Refrigerator
Coefficient Of Performance
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Heat Engine Carnot Cycle
No real engine operating between two energy
reservoirs can be more efficient than a Carnot
engine operating between the same two
temperatues. - Sadi Carnot
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Heat Pumps Refrigerators
Adiabatic expansion
Heat engines operated backward Work done to
transfer heat from cold to hot reservoir Reverse
path in the PV diagram compared to Carnot
cycle Carnot cycle is a reversible process
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Real Engines vs Carnot Engines

• All real engines are less efficient than the
  Carnot engine
• Real engines are irreversible because of friction
• Real engines are irreversible because they
  complete cycles in short amounts of time

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Entropy

• A state variable related to the Second Law of
  Thermodynamics, the entropy
• The change in entropy, DS, between two
  equilibrium states is given by the energy, Qr,
  transferred along the reversible path divided by
  the absolute temperature, T, of the system in
  this interval
• When energy is absorbed, Q is positive and
  entropy increases
• When energy is expelled, Q is negative and
  entropy decreases
• Can also be thought as an increase in disorder

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More About Entropy

• Note, the equation defines the change in entropy
• The entropy of the Universe increases in all
  natural processes
• This is another way of expressing the Second Law
  of Thermodynamics
• There are processes in which the entropy of a
  system decreases
• If the entropy of one system, A, decreases it
  will be accompanied by the increase of entropy of
  another system, B.
• The change in entropy in system B will be greater
  than that of system A.

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Second Law of Thermodynamics

• Heat cannot be transferred spontaneously from
  cold to hot
• No engine operating between two reservoirs is
  more efficient than a Carnot engine
• Systemenviroment Qc/Tc?QH/TH
• Qc/QH ? Tc/TH (equal for Carnot)
• Eff 1 - Qc/QH ? 1 - Tc/TH (equal for Carnot)
• No disordered state cannot spontaneously
  transform into an ordered state
• The entropy change (Q/T) of the
  systemenvironment ? 0
• Entropy change 0 for Carnot

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Extra
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Question

• Heat is applied to ice-water mixture and a
  portion of the ice melts. Which of the following
  quantities increase?
• The mixtures temperature and internal energy.
• The mixtures temperature and entropy.
• The mixtures entropy and internal energy.
• The mixtures temperature, entropy and internal
  energy.
• None of the above three quantities increase

Temperature remains constant during the melting -
heat goes into increasing energy for ice
molecules to free themselves from neighbors and
flow as water. Since there is heat flow into the
mixture, its entropy also increases.
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Adiabatic Compression

• During adiabatic compression of an ideal gas, the
  entropy of the gas
• Decreases.
• Stays constant.
• Increases.

No heat is transferred in or out of the system
during an adiabatic process - therefore, entropy
remains constant.
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Entropy Question

• Suppose your roommate is Mr. Clean and tidies
  up your messy room after a big party. What
  happens to the entropy of the room (assume that
  room and its contents are isolated from rest of
  the universe)?
• Decreases
• Stays the same
• Increases

The books may be in order after the cleanup, but
the work done by your roommate generates heat,
resulting in increase of the rooms entropy.
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Perpetual Motion Machines

• A perpetual motion machine would operate
  continuously without input of energy and without
  any net increase in entropy
• Perpetual motion machines of the first type would
  violate the First Law, giving out more energy
  than was put into the machine
• Perpetual motion machines of the second type
  would violate the Second Law, possibly by no
  exhaust
• Perpetual motion machines will never be invented

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Heat Engine
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Carnot Cycle