Second law of thermodynamics
The second law of thermodynamics states that the total entropy of an isolated system can never decrease, and is constant only when every process the system undergoes is reversible.
Essential knowledge 9.6.A.1 states it in those words, and the topic's boundary statement fences the treatment in one line: only qualitative treatment of the second law of thermodynamics is within the scope of AP Physics 2.
The first law alone would permit a cold block to warm a hot one spontaneously, since the books would still balance. The second law forbids it, and it is the reason thermodynamic processes have a direction at all.
Three consequences the exam asks about:
- Energy transfers spontaneously from hotter to cooler. The reverse conserves energy perfectly well and still never happens on its own.
- Isolated systems move toward thermodynamic equilibrium (9.6.A.3.i), and entropy is at its maximum there (9.6.A.2.iii).
- Isolated is not the same as closed. 9.6.A.3.ii is explicit: the entropy of an isolated system never decreases, but the entropy of a closed system can decrease, because energy can be transferred into or out of it. A freezer lowers the entropy of the water inside it and raises the entropy of the kitchen by more.
The word "reversible" carries weight: constancy is the equality case, and it requires every process to be reversible, an idealization no real process meets.
There is no entropy equation anywhere on the AP Physics 2 sheet, so an answer here states a direction and justifies it from the essential knowledge statements.