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GATE 2025 ME – Question 18

Thermodynamics · Zeroth, first and second laws, work, heat and energy changes · 1 mark · Multiple choice

Air inside a rigid, thermally-insulated tank undergoes stirring as shown in the figure below. Which one of the following options is correct?

A rigid, thermally insulated tank containing air, with a stirrer shaft entering from the top.
  1. The enthalpy of the air increases while the entropy of the air remains constant
  2. Both the enthalpy and the entropy of the air remain constant
  3. Both the enthalpy and the entropy of the air increase
  4. The enthalpy of the air decreases while the entropy of the air increases

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Show answer and explanation

Correct answer: (C) Both the enthalpy and the entropy of the air increase

Explanation

The stirrer does work on the air, and the tank is insulated and rigid, so this work raises the internal energy and the temperature of the air, and its enthalpy rises too. The work is dissipated by irreversible friction, so the entropy of the air increases as well.