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Question:

The coefficient of performance of a refrigerator is 5. If the temperature inside freezer is -60oC, the temperature of the surroundings to which it rejects heat is :

31oC

41oC

11oC

21oC

Solution:

coefficient of performance :COP=Q1/W=Q1/(Q1-Q2)=T2/(T1-T2) where T1 is temp of heat bath and T2 is temp of cold bath.
→5=T2/(T1-(-60))
→6T2=5T1
→6×(-60+273)=5T1
→T1=31oC