IB Physics HL B.4 Thermodynamics questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

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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.

Question 2

Paper 2Hard10 marks

A cyclic engine operates between two thermal reservoirs at $480\ \mathrm{K}$ and $300\ \mathrm{K}$ The engine, both reservoirs and an ideal mechanical work store form an isolated assembly. The engine returns to its initial state each cycle, and the work store’s entropy does not change.

For a reservoir at constant temperature, its entropy change is $\Delta S=Q/T$ where $Q$ is the energy received by the reservoir.

(a)

State the Clausius form and the Kelvin form of the second law of thermodynamics.

[2]

(b)

In one proposed cycle, the engine receives $1.20\ \mathrm{kJ}$ from the $480\ \mathrm{K}$ reservoir, transfers $0.50\ \mathrm{kJ}$ to the $300\ \mathrm{K}$ reservoir and delivers $0.70\ \mathrm{kJ}$ to the work store.

Assess whether this proposed cycle is consistent with energy conservation and the second law.

[4]

(c)

The engine continues to receive $1.20\ \mathrm{kJ}$ from the $480\ \mathrm{K}$ reservoir each cycle. Determine the greatest possible work delivered to the work store per cycle, consistent with the second law, and state whether the limiting cycle is reversible or irreversible.

[4]

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