Q121 / 150 · MHT CET 2026 (18 April Shift 1) · Physics
In thermodynamic process for free expansion, select the 'WRONG' statement out of the following options.
- AFree expansions are adiabatic expansions and there is no exchange of heat between system and environment.
- Correct answerBA free expansion can be plotted on a p-V diagram.
- CThere is no work done on the system or by the system.
- DIt is an instantaneous change and the system is not in thermodynamic equilibrium.
Solution
In a free expansion, a gas expands into a vacuum. Since the external pressure is zero, the work done by or on the system is zero ($W = 0$).
The process is typically carried out in an insulated container, meaning there is no heat exchange with the surroundings ($Q = 0$), making it an adiabatic process.
Free expansion is a rapid, highly irreversible process. During the expansion, the gas is not in thermodynamic equilibrium, and macroscopic properties like pressure and temperature are not well-defined. Because intermediate states are not equilibrium states, the process cannot be plotted as a continuous curve on a $p-V$ diagram. Only the initial and final states can be represented.
Therefore, the statement that a free expansion can be plotted on a $p-V$ diagram is incorrect.
Answer: A free expansion can be plotted on a p-V diagram.
The process is typically carried out in an insulated container, meaning there is no heat exchange with the surroundings ($Q = 0$), making it an adiabatic process.
Free expansion is a rapid, highly irreversible process. During the expansion, the gas is not in thermodynamic equilibrium, and macroscopic properties like pressure and temperature are not well-defined. Because intermediate states are not equilibrium states, the process cannot be plotted as a continuous curve on a $p-V$ diagram. Only the initial and final states can be represented.
Therefore, the statement that a free expansion can be plotted on a $p-V$ diagram is incorrect.
Answer: A free expansion can be plotted on a p-V diagram.
Hence, the correct answer is option: 'B'.