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# I hope someone in here can help me out of this problem. I know that for calculating the change in entropy of a system () we can use this formula : =δq/T Well the problem is here, when we calculating ΔS for an adiabatic process we know Q= 0 so ΔS=0 in this process, but why we just can't let Q be equal to zero in a free expansion (while we know that in free expansion W=Q=0) and get to the same result (ΔS=0) for the free expansion process?

SAGAR SINGH - IIT DELHI
879 Points
11 years ago

Dear nitin,

The formula you wrote is only valid for a reversible process. And the free expansion is not a reversible process. To calculate the difference in entropy in this case, connect the initial and final state by any reversible path, for which generally you won't have dQ = dL = 0, and use your formula on this path.

All the best.

Sagar Singh

B.Tech IIT Delhi

sagarsingh24.iitd@gmail.com