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Q. Two ideal Carnot engines operate in cascade (all heat given up by one engine is used by the other engine to produce work) between temperatures, $T_{1}$ and $T_{2}$ . The temperature of the hot reservoir of the first engine is $T_{1}$ and the temperature of the cold reservoir of the second engine is $T_{2}$ . $T$ is temperature of the sink of first engine which is also the source for the second engine. How is $T$ related to $T_{1}$ and $T_{2}$ , if both the engines perform equal amount of work?

NTA AbhyasNTA Abhyas 2022

Solution:

$Q_{1}:$ Heat input to $I^{s t}$ engine
$Q:$ Heat rejected from $I^{s t}$ engine
$Q_{2}:$ Heat rejected from $II^{n d}$ engine
Work done by $I^{s t}$ engine $=$ work done by $II^{n d}$ engine.
$Q_{1}-Q=Q-Q_{2}$
$2Q=Q_{1}+Q_{2}$
$2=\frac{T_{1}}{T}+\frac{T_{2}}{T}$
$\text{T}=\frac{\text{T}_{1} + \text{T}_{2}}{2}$