A household refrigerator removes thermal energy from the air inside its insulated compartment and expels thermal energy to the kitchen through the coil on its back. Measurements show that each minute the refrigerator:
• removes 65kJ of thermal energy from the interior air,
• consumes 20kJ of electrical energy to operate the compressor.
Assume the interior air, the compressor–motor assembly, and the external coil are the only subsystems exchanging energy and that all electrical energy input appears as thermal energy.
(a) With the interior air chosen as the system, state the magnitude and sign of the thermal energy transfer Qint each minute. Classify the process for the interior air as heating or cooling and justify your answer.
(b) Choose the compressor–motor assembly as the system. Identify the sign of the 20kJ electrical energy input when it is treated as thermal energy entering or leaving that system, and classify the process as heating or cooling.
(c) Now define the entire refrigerator (interior air, compressor–motor, and external coil) as one system. Determine the magnitude and sign of the net thermal energy transfer between this system and the kitchen each minute, and explain how your result is consistent with energy conservation.
(d) Using your analyses from parts (a)–(c), verify explicitly that energy is conserved for each set of system boundaries you considered. Discuss how the different perspectives highlight the distinction between heating and cooling processes.