ENGR 260 Section 6'6 6'7

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ENGR 260Section 6.6 6.7
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Clausius Statement of Second Law

• It is impossible to construct a device that
  operates in a cycle and produces no effect other
  than to transfer heat from a low temperature body
  to a higher temperature body.

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Clausius Statement

• Refrigeration must have work input!

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Perpetual Motion Machines

• PPM1 violates the first law of thermo
• PPM2 violates the second law of thermo

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Reversible and Irreversible Processes

• Reversible process a process that can be
  reversed without leaving any trace on the
  surroundings
• the system and the surrounding are returned to
  their initial state
• net work and heat exchange between process and
  surrounding is zero for the combined process

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Reversible Processes
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Reversible Processes
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Irreversibilities

• Friction
• Unrestrained expansion of a gas
• Heat transfer

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Friction
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Unrestrained expansion of a gas
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Heat Transfer
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Reversible Heat Transfer

• Does this exist?

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Heat Engine Review

• Heat engines are cyclic devices in that the
  working fluid returns to it original state at the
  end of each cycle.
• Work is done by the fluid in part of the cycle
  and on the fluid during another part of the
  cycle.
• Efficiency of a cycle is dependent on the
  processes that make up a cycle.
• Efficiency can be maximized by using reversible
  processes.

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Carnot Cycle

• Proposed by a French engineer Sadi Carnot in 1824
• Theorectical heat engine
• Comprised of four reversible processes. 2
  isothermal and 2 adiabatic
• Consider a closed system containing gas in an
  adiabatic piston-cylinder assembly.

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• Reversible Isothermal Expansion
• Reversible Adiabatic Expansion
• Reversible Isothermal Compression
• Reversible Adiabatic Compression

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Carnot P-V Diagram
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Reverse Carnot Cycle
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Carnot Principles

• The efficiency of a irreversible heat engine is
  always less than a reversible one operating
  between the same two reservoirs.
• The efficiencies of all reversible heat engines
  operating between the same two reservoirs are the
  same.