Question

Air inside a rigid, thermally-insulated tank undergoes stirring as shown in the figure below. Which one of the following options is correct?
\includegraphics[width=0.5\linewidth]{18.png}

• A. The enthalpy of the air increases while the entropy of the air remains constant
• B. Both the enthalpy and the entropy of the air remain constant
• C. Both the enthalpy and the entropy of the air increase
• D. The enthalpy of the air decreases while the entropy of the air increases

Hint:

For thermally-insulated systems undergoing stirring, remember that entropy generally increases due to irreversibilities like friction, and enthalpy may increase due to work done on the system.

Answer 1

The Correct Option is C

In this case, air is being stirred inside a rigid, thermally-insulated tank. Since the tank is thermally insulated, no heat exchange occurs with the surroundings, meaning the process is adiabatic. The stirring causes the internal energy to change and leads to an increase in the disorder or randomness of the air molecules, which results in an increase in entropy. For a process in a thermally-insulated system, when work is done by the system (such as stirring), the system's energy increases, and the entropy generally increases as well due to irreversibilities like friction and turbulence. Enthalpy: In an adiabatic process, if the work done on the system is positive (i.e., energy is being put into the system), the enthalpy will tend to increase. Stirring the air increases the energy of the system.
Entropy: Stirring the air inside the tank increases the disorder of the system, thus increasing the entropy. Thus, the correct answer is (C).