Which of the following processes (a) isothermal, (b) isobaric, work done will be maximum?

Question

Sir i am facing very difficultly in this following question. It will be so kind of you if you can explain this question to me:

This question is about the thermodynamics from the applications of 1st law of thermodynamics the question is (in which of the following processes, workdone will be maximum

• (a) isobaric

• (b) isothermal

Explanation about the difficulty: In my college my physics sir told me that the maximum work done is obtained from iso thermal process with the reasons that it is a reversible case in which all the heat applied is converted to workdone and the graph obtained from this is like a curve making shape of trapezium,however many of the other physist recommend that according to the graph obtained from iso baric process which is a straight line so the maximum work done is obtained from the iso baric process.... But i wonder that in carnot engine you know that he adopted that engine as the most efficient engine so in that he considered isothermal process why not iso baric

So sir it will be so kind of you to answer me this question. It may come in my exam of HCC1 also. I goggle it but got multiple answers and now i don't know which one to follow...

Answer 1

Your physics teacher is referring to the maximum thermal efficiency in performing work over a complete cycle where the maximum thermal efficiency means the largest ratio of net work out divided by the gross heat added. The most efficient being a Carnot cycle consisting of two reversible isothermal and two reversible adiabatic processes.

The "other physicists" appear to be simply comparing an isothermal expansion process to an isobaric expansion process. For the same initial and final volumes, more work is done in the isobaric process since, as you already know, work is the area under a horizontal rather than sloping line. But you need to understand the work done by a process is a one off thing and thus is of little value. What matters is the work that can be done repeatedly by the system over multiple cycles.

For that you would need to compare a cycle with the two reversible isobaric processes plus two reversible adiabatic processes (which is a reversible Brayton cycle), with a Carnot cycle operating between the same two temperature extremes.

See the figures below for this comparison using temperature-entropy graphs. The areas contained in the graph represent the net work done for each cycle. You can draw your own conclusions,

Hope this helps.