ABSTRACT
When a state of the systems returns to the original state after changes of a state, the series of the changes is called a “cycle.” After one cycle, the internal energy of the closed system returns to the initial value. Thus, when the net heat added to the system is Q and the net work done by the system is W, during one cycle respectively, Equation 21.1 becomes Q = W. Here, because the heat and also the work may sometimes go out and sometimes come in during one cycle, the word “net” is used.
