Cyclische processen en geïsoleerde systemen

A system that does not exchange heat and does no work on its surroundings is called an isolated system. The change in internal energy for such systems is zero.

In non-isolated systems, the internal energy can be constant only if the process followed is cyclic.

Suppose a thermodynamic system, like air inside the lungs, at initial pressure and volume, expanded to a new final state, returns to its initial state in a thermodynamic process. Such a process is called a cyclic process. 

In this process, the internal energy of the gas remains constant; therefore, the work done equals the net heat transfer.

In a pV diagram, such a cyclic process is represented by a closed path. Suppose the work done along the first path is W1, and work done along the second path is W2. The net work done in a cyclic process is represented by the area between these two closed paths.

It is positive if the process follows a clockwise cycle and is negative when it follows a counter-clockwise cycle.