Imagine two infinitely large air tanks connected by a long, straight pipe. Tank A is at a higher pressure than tank B, so air is flowing from one to the other. For the sake of argument, we will say there is no heat flux or work into or out of the pipe. Now we put a perfectly insulated cylinder inside tank A that is fitted with a piston which is free to move, though there can be no heat transfer through the cylinder walls so the air inside the cylinder undergoes an adiabatic expansion/compression as the piston moves. There is air inside this cylinder also which is at the same temperature as the air in tank A, so the state of the air inside the cylinder and the state of the air inside tank A is the same. Since the piston is free to move, the pressure of the air inside the cylinder and the pressure of the air outside the cylinder is ALWAYS the same. If air pressure dropped outside the cylinder, the piston would move to allow the air inside to be at the same pressure as the air outside. The cylinder is now moved from tank A to the inlet of this pipe and allowed to travel down the pipe to tank B. As it does, the pressure in the pipe drops and the piston moves out of the cylinder so that pressure inside and outside the cylinder remains the same. When it gets to tank B, the pressures are the same. Question: Will the air inside the cylinder and the air in tank B be at the same state when the cylinder arrives at B? Note that the pressure will be the same, but that does not necessarily mean the temperature will. Why or why not? Observations: - The air inside the cylinder represents a control mass that undergoes an adiabatic expansion and the air is doing work on the piston but the pressure inside the cylinder and outside the cylinder are the same. - A mass of air flowing through the pipe can also have a control surface drawn around it, and that control mass can be made equal to the control mass of the air inside the cylinder. - The control mass of air flowing through the pipe also experiences an adiabatic expansion as it goes from tank A to tank B, but it is doing no work. - If we write the first law for the air in the cylinder, we obtain dU = W - If we write the first law for the air in the pipe, we obtain dH = 0 Both the air in the pipe and air in the cylinder undergo seemingly equivalent expansions as they progress from tank A to tank B, they are both adiabatic, and both maintain the same pressure as they expand going down the pipe. But from the perspective of the first law, the end states are different. Are they different, the same or is there an error in the observations?