Cyclic process

Also called Thermodynamic cycle, Closed cycle

A cyclic process takes a system around a closed path and returns it to its starting state. Because internal energy depends only on the state, the change in internal energy over a full cycle is zero, so the net heating equals minus the net work.

Take a gas around a closed loop on a PV diagram and it finishes at the point it started from. Same pressure, same volume, so the same temperature by $PV = nRT$ , so the same internal energy, because for an ideal gas $U$ is fixed by the state and not by the route. Over one complete cycle, therefore,

\Delta U = 0 \implies Q_{\text{net}} = -W_{\text{net}}

using the sheet's $\Delta U = Q + W$ with $W = -P\Delta V$ , work done on the gas.

That is the whole content of the term. Everything the gas took in by heating over the cycle left again as work, or the reverse. Neither $Q$ nor $W$ is separately zero.

The enclosed area is the net work in magnitude, from applying 9.4.B.2.ii leg by leg and cancelling the overlap. The direction round the loop sets the sign:

Loop direction	Net work on the gas	Net heating
clockwise	negative	positive, so the gas does work on its surroundings
counterclockwise	positive	negative

Where it sits in the course. AP Physics 2 names four special cases of thermal process at 9.4.B.3, isovolumetric, isothermal, isobaric and adiabatic, and a cycle is not among them. Cycles reach the course through 9.4.B.2 instead, as diagrams to be read leg by leg. The one place the course description writes about a cycle directly is an optional sample instructional activity for Topic 9.4, which has students draw a three-step cyclical process.

Working an actual cycle is the PV diagrams guide's job.

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