The Carnot engine's efficiency between source temperature T₁ (hot) and sink T₂ (cold) is:
Aη = 1 − T₂/T₁
Bη = 1 − T₁/T₂
Cη = T₂/T₁
Dη = T₁/T₂
Answer & Solution
Correct answer: A. η = 1 − T₂/T₁
1. For a Carnot cycle between absolute temperatures T₁ > T₂:
2. η = (Q₁ − Q₂) / Q₁ = 1 − Q₂ / Q₁.
3. For reversible Carnot operation, Q₂ / Q₁ = T₂ / T₁ (Kelvin scale).
4. So η = 1 − T₂ / T₁.
5. No real engine can exceed this — sets the upper limit.
_Source: NCERT Class 11 Physics Ch 11, §11.10 Carnot Engine_
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