Two cylinders A and B of equal capacity are connected to each other via a stop cock. A contains an ideal gas at standard temperature and pressure. B is completely evacuated. The entire system is thermally insulated. The stop cock is suddenly opened. The process is :
Answer: (B) adiabatic
The system is thermally insulated, so no heat is exchanged: $Q = 0$. The process is adiabatic (a free expansion).
The gas expands into vacuum, so it does no work ($W = 0$). Then $\Delta U = 0$, and for an ideal gas the final temperature equals the initial temperature. But the process is sudden and irreversible, so it is not a quasi-static isothermal process. The volume doubles (not isochoric) and the pressure halves (not isobaric).
Solution by Sreeraj P, M.Sc Physics