Timeline for Is the work done by gas necessarily positive if the cycle is travelled clockwise in the $P-V$ plane (and viceversa)?
Current License: CC BY-SA 3.0
7 events
| when toggle format | what | by | license | comment | |
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| Aug 17, 2017 at 1:28 | vote | accept | Sørën | ||
| Jul 11, 2016 at 17:46 | comment | added | valerio | @march You are right, for some reason I had a non quasi-static process in mind when I wrote the post. Nevertheless, as you remarked, there are difficulties when drawing an irreversible adiabatic process in the PV plane. | |
| Jul 11, 2016 at 16:51 | comment | added | march | Note that it is perfectly fine to draw an irreversible process in the system's $p$-$V$ plane as long as the system undergoes only quasi-static (or quasi-equilibrium) processes, in which case the irreversibility is usually a consequence of heat transfer between system and environment while they are at different temperatures. On the other hand, I'm pretty sure that if the process is irreversible and adiabatic, it must also be non-quasi-static, which means that that irreversible adiabatic process should not be drawn in the $p$-$V$ plane. | |
| Jul 11, 2016 at 14:39 | comment | added | valerio | @Sørën The problem is that I suspect that the cycle you described is not possible, even if I haven't been able to come up with an explanation of why it should be impossible. Maybe we should run an experiment and see if it is possible to close the CAB cycle with an irreversible adiabatic process...My message anyway is that you have to be careful when using the PV plane: irreversible processes cannot be represented there, because curves in the PV plane are by definition made of equilibrium states. | |
| Jul 11, 2016 at 11:02 | comment | added | docscience | @valerio92 other than convention, the choice of direction is arbitrary. What matters is how you define directionality relative to the direction of energy flow, and maintaining consistency in your definition | |
| Jul 11, 2016 at 7:41 | comment | added | Sørën | Thanks for the reply and the example you made! I'm still a bit confused on what is wrong in what I asked. Is it the fact that irreversible processes cannot be represented, and so in a case like the one I proposed I cannot talk about "clockwise" or "anticlockwise" direction? (Although it seems to me that I can define a direction of travelling the cycle even if only one of the curves is not represented, for istance in my example - without B->C - the cycle would still be anticlockwise with no ambiguity) Or is it the fact that this "rule" ("clockwise" or "anticlockwise") does not hold always? | |
| Jul 10, 2016 at 23:31 | history | answered | valerio | CC BY-SA 3.0 |