The efficiency of heat engines is high when the temperature difference between the hot and cold reservoirs is large. Refrigerators, on the other hand, work better when the temperature difference is small. Thinking of the mechanical refrigeration cycle shown in Fig. 20.9, explain in physical terms why it takes less work to remove heat from the working substance if the two reservoirs (the inside of the refrigerator and the outside air) are at nearly the same temperature, than if the outside air is much warmer than the interior of the refrigerator.

Fig.20.9

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(b) O Compressed fluid in condenser is at higher temperature than outside air, so fluid gives off heat |Qt| to air. Fluid partially condenses to liquid. 3 Fluid expands adiabatically. through expansion valve. Evaporator Expansion valve Condenser Evaporator Expansion valve COLD High Low pressure pressure Condenser НОT Inside of Compressor takes in refrigerant fluid, does work |W| on fluid to compress it adiabatically, and delivers fluid to condenser coil. refrigerator After expansion, fluid in evaporator is at lower temperature than inside of refrigerator. So fluid absorbs heat|Qcl, cooling contents of refrigerator. Fluid partially evaporates to vapor. Compressor Compressor Fluid returns to compressor, and another cycle begins.