====== 8. Second law of thermodynamics ======
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===== - Heat engines =====
Heat engines work via the circulation of a working substance from a hot reservoir to a cold reservoir and the transformation of a part of the heat to work.

[{{:courses:phy101:8.3.jpg?nolink&300|Heat $Q_h$ is taken from a hot reservoir at a temperature of $T_h$, parts of it is converted to work $W$ by the heat engine, and the residual heat $Q_c$ is dumped into a cold reservoir at a lower temperature $T_c$.}}]

===== - Refrigerators =====
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===== - Second law of thermodynamics =====
[{{:courses:phy101:8.6.jpg?nolink&600|The second law of thermodynamics implies that perfect refrigerators and perfect heat engines do not exist}}]
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[{{:courses:phy101:8.7.jpg?nolink&400|Violating the refrigerator statement of second law violates the engine statement.}}]

===== - Carnot's cycle =====
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===== - Entropy =====