Too much confusion in my head about these concepts... Is every reversible process quasi static? If not, what are some examples? If process is irreversible then it doesn't need to be non quasi-static, I understand that. (eg. free expansion of the gas) Can irreversible process be quasi static? If so what are some examples? When is the formula for calculating entropy change applicable (dS=dQ/T)? Is it for reversible or it can also be for irreversible quasi-static (if they exists)?