Question

A sink, that is the system where heat is rejected, is essential for the conversion of heat into work. From which law does the above inference follow?
A. Zeroth
B. First
C. Second
D. Third

Answer 1

It is a memory based question. Recall the laws of thermodynamics which discusses about the change in internal energy and change in heat energy. Recall Kelvin's statement on one of the laws of thermodynamics.

Complete answer:
We know that the sink is the part of the heat engine. So, let’s discuss the working of a heat engine. In the heat engine, the working substance absorbs some heat from the source and uses this heat to convert it into work. The remaining amount of heat is rejected to the sink which is the colder part. Since the working substance repeats its path after a regular interval of time, the change in internal energy of the system is zero.
We have from the first law of thermodynamics,
$dQ = dU + W$
Since, $dU = 0$,
$dQ = W$
$\Rightarrow {Q_1} - {Q_2} = W$
Here, ${Q_1}$ is the heat given by the source and ${Q_2}$ is the heat released by the working substance to the sink.

According to Kelvin’s statement of the second law of thermodynamics, it is impossible to design a heat engine which can convert all of the heat given to it as work without rejecting the heat to the sink.
Therefore, ${Q_1} \ne W$ or ${Q_2} \ne 0$.Thus, the sink that is the colder part of the heart engine is an essential part to reject the heat.

So, the correct answer is option C.

Note: This is a memory based question. To solve such types of questions, students must be familiar with the laws of thermodynamics. In simple language, the second law of thermodynamics says that we cannot transfer heat from a cold object to a hot object without doing any work. If there is transfer of heat then and then only the system can do the work.