Question

what is the efficiency of Carnot engine when the temperature of source and sink are equal? ​

Answer 1

Such a process is an idealization because some temperature difference is always needed to drive the flow of heat between source/sink and system. In this idealization, there is no loss of ability of this system to perform work as heat flows, The cyclical process is "reversible"... 100%

Answer 2

The efficiency is zero.

• In a Carnot engine, the efficiency is given by $1-\frac{T_C}{T_H}$, where $T_C$ is the temperature of the cold reservoir and $T_H$ is the temperature of the hot reservoir.
• Hence if both the temperatures are equal, the efficiency of the Carnot engine will be 1 - 1 = 0.

The efficiency of the Carnot engine when the temperature of the source and sink are equal is zero.

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