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Summary

4.1 Reversible and Irreversible Processes

  • A reversible process is one in which both the system and its environment can return to exactly the states they were in by following the reverse path.
  • An irreversible process is one in which the system and its environment cannot return together to exactly the states that they were in.
  • The irreversibility of any natural process results from the second law of thermodynamics.

4.2 Heat Engines

  • The work done by a heat engine is the difference between the heat absorbed from the hot reservoir and the heat discharged to the cold reservoir, that is, W=Qh−Qc.
  • The ratio of the work done by the engine and the heat absorbed from the hot reservoir provides the efficiency of the engine, that is, e=W/Qh=1−Qc/Qh.

4.3 Refrigerators and Heat Pumps

  • A refrigerator or a heat pump is a heat engine run in reverse.
  • The focus of a refrigerator is on removing heat from the cold reservoir with a coefficient of performance KR.
  • The focus of a heat pump is on dumping heat to the hot reservoir with a coefficient of performance KP.

4.4 Statements of the Second Law of Thermodynamics

  • The Kelvin statement of the second law of thermodynamics: It is impossible to convert the heat from a single source into work without any other effect.
  • The Kelvin statement and Clausius statement of the second law of thermodynamics are equivalent.

4.5 The Carnot Cycle

  • The Carnot cycle is the most efficient engine for a reversible cycle designed between two reservoirs.
  • The Carnot principle is another way of stating the second law of thermodynamics.

4.6 Entropy

  • The change in entropy for a reversible process at constant temperature is equal to the heat divided by the temperature. The entropy change of a system under a reversible process is given by ΔS=∫ABdQ/T.
  • A system’s change in entropy between two states is independent of the reversible thermodynamic path taken by the system when it makes a transition between the states.

4.7 Entropy on a Microscopic Scale

  • Entropy can be related to how disordered a system is—the more it is disordered, the higher is its entropy. In any irreversible process, the universe becomes more disordered.
  • According to the third law of thermodynamics, absolute zero temperature is unreachable.
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