A refrigerator is characterized by a number called the coefficient of performanc

Question

A refrigerator is characterized by a number called the coefficient of performance (COP), which is defined as the heat removed from the cool place (inside the fridge) divided by the work required to remove that heat: COP = Q_L/W. (a) If you are buying a new fridge is it better to buy one with a higher COP or a lower COP? higher lower it doesn't matter Let's say that a fridge operates between temperatures of 40 degree C and 32.0 degree C. A particular fridge has a COP of 5.50. (b) If 480 of work is done by the compressor in the fridge to extract heat from inside the fridge, how much heat is removed from inside the fridge? (c) How much heat is dumped by the fridge into the surroundings, in this case? (d) What is the maximum possible COP that would be achieved by an ideal fridge operating between the two temperatures specified above?

Explanation / Answer

a) Always remember. Coefficient of performance of refrigerator or efficiency of engine, both qualitatively are defined as (Desired effect)/(Input needed to achieve it).

Thus as we can see clearly, we need higher desired effect with lesser input and hence high COP.

b) Since COP = Q removed /W in; If W =480 and COP = 5.5, we have Q = 5.5*480 = 2640 J

Heat dumped to the surrounding would be simply Work (Which, in effect is converted to heat) + Heat removed from refrigerator = 480+2640 = 3120 J

Maximum possible COP is achieved (hypothetically) by refrigerator working on reverse carnot cycle and is given by COP max = T lower/(T higher - T lower) temperatures taken in kelvin

Hence, COP = 277/(305-277) = 9.89

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