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For a refrigerator absorbing heat QL from a cold region and rejecting heat QH to a hot region, the coefficient of performance is written as

  • A.\(\frac{Q_{L}}{Q_{H} - Q_{L}}\)
  • B.\(\frac{Q_{H}}{Q_{H} - Q_{L}}\)
  • C.\(\frac{Q_{H} - Q_{L}}{Q_{L}}\)
  • D.\(\frac{Q_{L}}{Q_{H}}\)

Answer: A

The coefficient of performance is \(\frac{Q_{L}}{Q_{H} - Q_{L}}\).

A refrigerator's useful effect is the heat removed from the cold space, \(Q_{L}\), and the cost is the net work input, which by the first law is \(W = Q_{H} - Q_{L}\). So \(COP_{R} = Q_{L}/(Q_{H} - Q_{L})\).

\(Q_{H}/(Q_{H} - Q_{L})\) is the COP of a heat pump, whose useful effect is the heat delivered. \((Q_{H} - Q_{L})/Q_{L}\) inverts the refrigerator COP, and \(Q_{L}/Q_{H}\) does not involve the work at all.

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