NEET 2020 Physics Question 141 Solution

The two cylinders and the connecting stopcock are described as "thermally insulated" from the external environment. A thermal insulator acts as an adiabatic wall that completely prevents the transfer of heat between the system and its surroundings: $$q = 0$$

By thermodynamic definitions: - Isothermal: System maintained at constant temperature by heat exchange with a thermal reservoir. - Adiabatic: System insulated from surroundings such that $$q = 0$$. - Isochoric: Constant volume ($$\Delta V = 0$$). - Isobaric: Constant pressure ($$\Delta P = 0$$). Because the insulated boundary ensures $$q = 0$$, the process is fundamentally categorized as an adiabatic process.

When the stopcock is suddenly opened into evacuated chamber B, the gas rushes into vacuum against zero opposing pressure ($$P_{\text{ext}} = 0$$), so work done $$w = 0$$. From the first law of thermodynamics: $$\Delta U = q + w = 0 + 0 = 0$$ For an ideal gas, $$\Delta U = n C_v \Delta T$$, which implies $$\Delta T = 0$$. However, this temperature constancy is an incidental consequence of the ideal gas state rather than an isothermal condition maintained by a heat bath.

In both NCERT physics and standard thermodynamics, sudden free expansion of an insulated system is canonically designated as an ADIABATIC process.