My book says 'Maximum work can be obtained only from thermodynamically reversible processes,' but why is it so? What is the cause? Actually to me the definition of reversible process is confusing. It says that at each step during the process, equilibrium is maintained. But, let's say,we have to expand a gas in a cylinder headed by piston from pressure Pi to Pf .Suppose,the process is done with innumerable infinitesimal steps. Then, say at each step, dp is changed in the pressure. Before the step, gas and the piston were exerting same pressure Pi . Now, when the piston is raised, gas does expand to equalize the pressure,which is Pi - dp . So, at the end of the step, the gas & the piston will exert same pressure and hence will be in mechanical equilibrium. During the step,the gas and the piston was not in equilibrium;it was at the end of the step the equilibrium is achieved. So, during each step of the process the gas would be at disequilibrium . So,will it really be a reversible process? It is contradictory with the definition. But it is true that during each step, the system is at disequilibrium though infinitesimally small by . So, why does the definition tell that at each step,there is equilibrium? So, I have three questions: Why is maximum work only obtained from reversible processes? According to the definition, at each step of the reversible process,there must be equilibrium. But during each step, there is infinitesimal disequilibrium. So, why does the definition say so in-spite of the disequilibrium during each step? How do infinitesimal steps make a process reversible? I will be very grateful if anyone answer these three questions. Please help.