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PRACTICE / Second law

An energy-balanced engine can still be impossible

A claimed cyclic engine absorbs 900 J from a 600 K reservoir and rejects 300 J to a 300 K reservoir. These are its only heat exchanges.

What is the entropy change of the two reservoirs per cycle?

Organize the givens & assumptions
  • Qh = 900 J at 600 K
  • Qc = 300 J at 300 K
  • Working substance completes a cycle

Fixed-temperature heat reservoirs; Qh and Qc are positive heat magnitudes. The working substance completes a cycle, so its net energy and entropy changes are zero; no other heat exchanges occur.

  1. A2.5 J/K2.5\ \mathrm{J/K}
  2. B0 J/K0\ \mathrm{J/K}
  3. C−0.5 J/K-0.5\ \mathrm{J/K}
  4. D0.5 J/K0.5\ \mathrm{J/K}

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