I was given a problem to identify whether the given statement is true or false.

And the statement was:

"Consider an ideal gas which is expanded from $(P_{1},V_{1},T_{1})$ to $(P_{2},V_{2},T_{2}).$ Then, the work done on the gas is maximum when it is compressed back irreversibly from $ (P_{2},V_{2}) $ to $ (P_{1},V_{1}) $ against a constant pressure $P_1$".

I concluded that the statement is false, as:

  1. It is nowhere mentioned what is greater $P_1$ or $P_2$ as there is no information on how the expansion process was done.

  2. Even if I assume that $ P_1 > P_2$, still during irreversible compression I can vary the pressure such that the pressure goes beyond $P_1$ in the middle of the process but finally returns to the required final state $ (P_{1},V_{1}) $ , which would mean that I am doing more work than what I would be doing if I keep the pressure constant.

However the answer says that the statement is true.

Where am I wrong?

  • 4
    $\begingroup$ In my judgment, this problem is woefully underspecified. $\endgroup$ Commented Feb 14, 2019 at 14:53

1 Answer 1


It is tricky wording. Usually when you compress something you want to do so as efficiently as possible. The minimum amount of work would be required if you compressed this reversibly.

If for some reason, as in the question, you want to waste your energy and overkill the work required (i.e., do things the hard way), then compress the gas IRREVERSIBLY. that will make it take the maximum amount of energy to get the desired compression.

The engineer in my cringes at this :)

  • $\begingroup$ Is the statement about constant pressure correct? $\endgroup$ Commented Feb 14, 2019 at 15:55
  • $\begingroup$ An irreversible process can be done even if the pressure is kept varying. $\endgroup$ Commented Feb 14, 2019 at 16:03
  • $\begingroup$ pressures dictate whether you are expanding or compressing. Since it says it is compressing, the outside pressure must be larger than the inside pressure. Irreversibilites, for instance friction, are present regardless of the direction or magnitude of the pressure $\endgroup$
    – B. Kelly
    Commented Feb 14, 2019 at 16:06
  • $\begingroup$ The statement about constant pressure only means that the external pressure is constant, just as the atmosphere is always 1 bar. $\endgroup$
    – B. Kelly
    Commented Feb 14, 2019 at 16:09
  • 1
    $\begingroup$ In your problem the external pressure is greater than the pressure inside the volume, hence, the volume is compressed. If it is compressed reversibly the minimum amount of work will be required because there are absolutely no losses i.e. no work is wasted. Conversely, the most amount of work is done if the process is irreversible since there will be losses and you have to both provide energy for compression and also energy for the losses. $\endgroup$
    – B. Kelly
    Commented Feb 14, 2019 at 16:13

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