4-71 Air is contained in a variable-load piston-cylinder device equipped with a paddle wheel. Initially, air is at 400 kPa and 17 C. The paddle wheel is now turned by

an external electric motor until 75 kJ/kg of work has been transferred to air. During this process, heat is transferred to maintain a constant air temperature while allowing the gas volume to triple. Calculate the required amount of heat transfer, in kJ/kg.