Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
**Assertion A:** Efficiency of a reversible heat engine will be highest at $-273^\circ\text{C}$ temperature of cold reservoir.
**Reason R:** The efficiency of Carnot's engine depends not only on temperature of cold reservoir but it depends on the temperature of hot reservoir too and is given as $\eta=\left(1-\dfrac{T_2}{T_1}\right)$.
In the light of the above statements, choose the correct answer from the options given below:
Answer: (D) Both A and R are true and R is the correct explanation of A
From $\eta=1-\dfrac{T_2}{T_1}$, the efficiency is largest (ideally $1$) when the sink is at $T_2=0\ \text{K}=-273^\circ\text{C}$. R is true and the formula in R directly explains A.
Solution by Sreeraj P, M.Sc Physics