- #1
gajagaja
- 3
- 0
Hello all!
I have a hard time about this one:
Think of an adiabatic process (can be either reversible or irreversible - doesn't matter). dQ = 0. Assume only, mechanical work is done on/by the system. So by 1st law of thermodynamics, dU =dW.
Now, everything is clear to me when I am dealing with an adiabatic process in a closed system, i.e, there is both: (a) no mass transfer (dM = 0), and (b) no heat transfer (dQ = 0).
But, let us say, I imagine an adiabatic process in an open system (where only mechanical work is done), i.e, dQ = 0, but dM NOT EQUAL to 0.
In other words, I am making dM to not be zero, by either doing some mechanical work or letting the system do some mechanical work (using the dW term).
But now, the second law of thermdynamics tells us that, mechanical work, is never possible without some amount of it getting lost as heat (Am I right about this understanding of the second law - or is this wrong??)
Therefore, part of dW is also used to generate a non-zero dQ term. But I started off with the assumption that the process was adiabatic, so I can not have a non-zero dQ term.
So now, I contradict myself, if I assume that, mass transfer also happens in an adiabatic process. But mass transfer happens in all open systems, so overall am I right to say that, adiabatic processes are not possible in open systems? Or have I reached this contradiction, only due to a poor understanding of the second law??
I would thank you all for your help and useful discussion on this!
Regards,
Gajagaja
I have a hard time about this one:
Think of an adiabatic process (can be either reversible or irreversible - doesn't matter). dQ = 0. Assume only, mechanical work is done on/by the system. So by 1st law of thermodynamics, dU =dW.
Now, everything is clear to me when I am dealing with an adiabatic process in a closed system, i.e, there is both: (a) no mass transfer (dM = 0), and (b) no heat transfer (dQ = 0).
But, let us say, I imagine an adiabatic process in an open system (where only mechanical work is done), i.e, dQ = 0, but dM NOT EQUAL to 0.
In other words, I am making dM to not be zero, by either doing some mechanical work or letting the system do some mechanical work (using the dW term).
But now, the second law of thermdynamics tells us that, mechanical work, is never possible without some amount of it getting lost as heat (Am I right about this understanding of the second law - or is this wrong??)
Therefore, part of dW is also used to generate a non-zero dQ term. But I started off with the assumption that the process was adiabatic, so I can not have a non-zero dQ term.
So now, I contradict myself, if I assume that, mass transfer also happens in an adiabatic process. But mass transfer happens in all open systems, so overall am I right to say that, adiabatic processes are not possible in open systems? Or have I reached this contradiction, only due to a poor understanding of the second law??
I would thank you all for your help and useful discussion on this!
Regards,
Gajagaja