WBJEE 2025 Physics and Chemistry Papers

In an adiabatic process, there is no heat exchange, so $Q=0$.In free expansion, the gas expands into a vacuum, meaning there is no external pressure doing work, so $W=0$.From the first law of thermodynamics, we have:$∆ U=Q-W$ Substituting the values, we get:$∆ U=0-0=0$For an ideal gas, the internal energy $U$ depends only on the temperature. Since $∆ U=0$, the temperature does not change during the process. This characteristic is described as an isothermal process.Thus, the adiabatic free expansion of an ideal gas is isothermal, which corresponds to: