Cyclic Processes

A cyclic process is one in which a system, after a series of changes, returns exactly to its initial state; since internal energy is a state variable, this means the net change in internal energy over one full cycle is always zero, $\Delta U_{cycle}=0$. Applying the first law over the full cycle then gives $Q_{net}=Q_{in}-Q_{out}=W$: the net work done over one cycle exactly equals the net heat absorbed. Both heat flowing into, and heat flowing out of, the system can occur at different stages within a single cycle. The textbook's key everyday example is the four-stroke petrol engine: an adiabatic compression stroke, an isochoric heat-addition stroke, an adiabatic expansion (power) stroke, and an isochoric heat-reject …