1 mol of monoatomic ideal gas (temperature T1) is inside a cylinder with a moving piston (all are isolated). The initial external pressure on the piston is P1. at some point the external pressure is changed to (2/3)P1, the gas undergoes (irreversible) adiabatic expansion and reaches equilbrium state at temperature T2.
write an expression of the entropy change of the system using R,T1,T2.
The Attempt at a Solution
Well, I know that in order to calculate entropy change from state A to state B, I need to construct a reversible path from A to B, and find the heat in this path. A reversible adiabatic expansion from A (P1,T1,V1) to B( (2/3)P1,T2,V2) would mean that there is no heat transfer. why then the entropy change isn't 0?