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The primitive lift illustrated in the left hand side of the following figure operates using heated water vapor ("gas") that pushes on a sealed piston. During its operation, the vapor undergoes the rectangular cyclic process shown in the right hand side of the figure.

I'm trying to describe in words how one might realize each of the four steps.

My idea is : We assume the system is isolated entirely from its surroundings such that it obeys the ideal gas law. First, heat is added through an external source to the gas while the piston is held fixed such that Temperature increases while the volume remains constant, therefore the pressure increases from $p_{1} \to p_{2}$ (A). Then, the piston is released and the gas is no longer heated. The volume will increase from $V_{1} \to V_{2}$ and the pressure will remain constant. When the piston reaches the top it is stopped and held still.

Here I'm blocked, I don't know how to explain a situation in which pressure decreases while volume is held constant .

Any help would be appreciated.

  • $\begingroup$ It would help, if the system was not closed, but there was a source of the external gas of the pressure p2, and a ventil keeping pressure p1. $\endgroup$
    – Poutnik
    Sep 13, 2020 at 7:37

1 Answer 1


To carry out the B transformation, you have to heat a lot. If you don't heat, the volume will of course increase, but the pressure will decrease. B is a process difficult to carry out, because it is not easy to heat enough a gas who is inflated to as to maintain the inner pressure. Then, at the end of B, the temperature is very high. You block the position of the piston, and you cool the system : the pressure will decrease from P2 to P1, during the process C. The last process D is also difficult to carry out. Because in D you must compress the gaz so as to decrease its volume, but the pressure must not increase. It means that the system should be cooled during D. At the end of D, the temperature will be extremely low.


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