Entropy
Entropy is a state function that tracks how spread out a system's energy is. AP Physics 2 describes it qualitatively as the tendency of energy to disperse, or the unavailability of some of a system's energy to do work.
Essential knowledge 9.6.A.2 gives the course's description and labels it qualitative: entropy can be qualitatively described as the tendency of energy to spread or the unavailability of some of the system's energy to do work.
Neither of those is "disorder". That word appears nowhere in the AP Physics 2 Course and Exam Description, and neither does "microstate", the configuration-counting definition that carries entropy in later courses. You cannot inspect a gas and score its tidiness, so "disorder" gives you nothing to argue from. The CED's two descriptions each point at something checkable: did energy that started concentrated end up spread across more of the system, and is less of it now available to do work?
Three properties, all from the CED:
- Localized energy tends to disperse (9.6.A.2.i). That is the direction.
- Entropy is a state function (9.6.A.2.ii), so it depends only on the current state or configuration, not on how the system got there. Two routes to the same final state give the same entropy. Work and heating are not state functions, which is why the same pair of endpoints can involve different and .
- Maximum entropy occurs at thermodynamic equilibrium (9.6.A.2.iii).
Rising entropy does not compete with conservation of energy: the first law says every joule is still accounted for, and an entropy increase says less of it can be used to do work.