IB Physics HL B.4 practice question

Question 1

Multiple choiceMedium1 mark

A real isolated system consists of parts A and B. During an irreversible change, their entropy changes are $\Delta S_A=-x$ and $\Delta S_B=+y$ where $x$ and $y$ are positive. Which relation gives the entropy change of the whole system and the resulting constraint on $x$ and $y$

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Answer: B. The changes of the two parts add: $\Delta S_{\text{whole}}=\Delta S_A+\Delta S_B=-x+y=y-x$ Since the irreversible change occurs in a real isolated system, its total entropy increases; hence $y-x>0$ so $y>x$
A: treats the decrease $-x$ as a positive contribution.
C: reverses the subtraction, giving the wrong total change and constraint.
D: counts only the increase in part B and omits part A’s entropy change.
Remember: the entropy increase applies to the isolated whole, not necessarily to each part.

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