Explain, using the second law of thermodynamics and the concept of entropy, why a refrigerator requires a continuous external energy input in order to operate.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A refrigerator moves heat from a cold interior to a warmer surroundings, a non‑spontaneous process. According to the second law, heat cannot flow spontaneously from cold to hot; doing so would decrease the total entropy of the universe. To prevent this, the refrigerator supplies external work, which increases the entropy of the surroundings (the hot reservoir) by an amount that compensates for the entropy decrease in the cold interior. Thus the overall entropy change of the universe is zero or positive, satisfying the second law.
Examiner tips
- Use the phrase "heat flows naturally from hot to cold" to show understanding of spontaneous direction.
- Explain that the refrigerator’s work input raises the entropy of the surroundings.
- Show that the net entropy change is zero or positive, linking to the second law.
Common mistakes
- Claiming the refrigerator works without external energy; ignoring the need for work input.
- Saying the refrigerator increases entropy of the cold interior rather than the surroundings.
- Using the wrong direction for spontaneous heat flow (cold to hot).
Mark scheme (4 marks)
- Heat flows naturally from hot to cold / energy will not spontaneously transfer from a cold reservoir to a hot reservoir
- A refrigerator transfers thermal energy from a cold interior to a warmer surroundings, which is a non-spontaneous process
- Such a process would decrease the total entropy of the universe, violating the second law
- External work input increases the entropy of the surroundings (hot reservoir) by enough to ensure the total entropy of the universe does not decrease / overall entropy change is zero or positive
Key terms in this question
second law of thermodynamics · entropy
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Thermodynamics (HL only) questions
- Explain why the Carnot cycle is described as a reversible cycle and discuss what…
- A heat engine operates between a hot reservoir at temperature T_H and a cold res…
- Explain why the entropy of an ideal gas increases when it undergoes a free expan…
- Explain why no real heat engine operating between two fixed temperature reservoi…