(Carnot efficiency – conceptual reasoning)
A steam engine in a school laboratory demonstration uses boiling water at constant temperature 100∘C as the hot reservoir and the surrounding air at 27∘C as the cold reservoir.
(i) State the Kelvin–Planck statement of the Second Law of Thermodynamics.
(ii) Explain, without performing any numerical calculation, why the engine cannot convert all the heat absorbed from the boiling water directly into work.
(iii) If the demonstrator replaces the surrounding air with ice‐cold water at 0∘C, explain qualitatively how and why the maximum possible efficiency of the engine changes.
(iv) Give a reason why absolute temperature (kelvin) must be used while evaluating the theoretical efficiency of such an engine.