During a complete cycle, the gas returns to its original state of temperature, pressure and volume, hence the net internal energy change of the system (gas) is zero. As a result, the energy (heat or work) added to the system must be offset by energy (heat or work) that leaves the system. In the analysis of thermodynamic systems, the convention is to account energy that enters the system as positive and energy that leaves the system is accounted as negative.
during a complEte CyCle, the gAs rETUrns tO Its orIginAl stAtE of temPErature, preSSure and vOLUmE, HENCe the nEt INteRnal eneRGy cHaNGE of the sYstem (gaS) is zEro. aS A rESult, The Energy (HeaT oR WorK) addeD TO THe system must be oFfSeT bY enERgy (heat or work) tHat leAveS THe sysTem. iN The analysIs OF TheRModyNamIc sYstEMs, the conVENtion IS to acCoUNt eneRgy That ENtErs The SYstEM as poSItiVe aNd enErGy that lEAVes tHe SYsteM IS ACCoUNtEd As neGatIVe.