Reversible Process and Work

$w=-P\Delta V$
and
$w=-\int_{V_{1}}^{V_{2}}PdV=-nRTln\frac{V_{2}}{V_{1}}$

Jasmine 2C
Posts: 184
Joined: Wed Sep 18, 2019 12:18 am

Reversible Process and Work

Can someone give a dumb downed explanation of how reversible processes generate maximum work?

Brian J Cheng 1I
Posts: 115
Joined: Thu Jul 11, 2019 12:15 am

Re: Reversible Process and Work

Reversible processes can be "turned" in any direction at the very slight perturbation of the system. The common one we went over in class was the slow isothermic expansion of gas in a piston chamber due to the addition of heat in gas which causes it to expand. Isothermic systems have temperature change of 0. In this system, this is possible because the heat added is used right away for the gas to do work on the piston via expansion, therefore not changing the temperature of the system.

Part 2: Pressure in this system is inversely related to Volume (if there's less volume, there's more external pressure acting on the system). If you look on Lavelle's website, you'll see a curve for this reversible system. It's a downwards "decay" curve compared to the irreversible process which has a straight line. Basically a reversible system does maximum work compared to irreversible because all the heat is used for work and not lost in raising the temperature of the system. This is shown on the curve as work (area under the curves) for the reversible one has a noticeably larger area than the irreversible one.

HOpe this helps!

Asha Agarwal 1E
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Joined: Fri Aug 30, 2019 12:16 am

Re: Reversible Process and Work

For reversible processes, the temperature remains constant while the pressure and volume change inversely to each other. In this way, the curve has pressure changing as volume changes, so the area under the curve is larger than that of an irreversible process in which external pressure stays constant. since work is equal to the area under the curve, it makes sense that reversible systems maximize work done.

DLee_1L
Posts: 103
Joined: Sat Aug 17, 2019 12:17 am

Re: Reversible Process and Work

Brian J Cheng 1I wrote:Reversible processes can be "turned" in any direction at the very slight perturbation of the system. The common one we went over in class was the slow isothermic expansion of gas in a piston chamber due to the addition of heat in gas which causes it to expand. Isothermic systems have temperature change of 0. In this system, this is possible because the heat added is used right away for the gas to do work on the piston via expansion, therefore not changing the temperature of the system.

Part 2: Pressure in this system is inversely related to Volume (if there's less volume, there's more external pressure acting on the system). If you look on Lavelle's website, you'll see a curve for this reversible system. It's a downwards "decay" curve compared to the irreversible process which has a straight line. Basically a reversible system does maximum work compared to irreversible because all the heat is used for work and not lost in raising the temperature of the system. This is shown on the curve as work (area under the curves) for the reversible one has a noticeably larger area than the irreversible one.

HOpe this helps!

Is the irreversible processes inefficient because it transform heat into work, but some of that heat in the system is released into the surroundings because the irreversible processes happen too quickly for all the heat to be transformed into work?

John Liang 2I
Posts: 102
Joined: Fri Aug 30, 2019 12:18 am

Re: Reversible Process and Work

DLee_1L wrote:
Brian J Cheng 1I wrote:Reversible processes can be "turned" in any direction at the very slight perturbation of the system. The common one we went over in class was the slow isothermic expansion of gas in a piston chamber due to the addition of heat in gas which causes it to expand. Isothermic systems have temperature change of 0. In this system, this is possible because the heat added is used right away for the gas to do work on the piston via expansion, therefore not changing the temperature of the system.

Part 2: Pressure in this system is inversely related to Volume (if there's less volume, there's more external pressure acting on the system). If you look on Lavelle's website, you'll see a curve for this reversible system. It's a downwards "decay" curve compared to the irreversible process which has a straight line. Basically a reversible system does maximum work compared to irreversible because all the heat is used for work and not lost in raising the temperature of the system. This is shown on the curve as work (area under the curves) for the reversible one has a noticeably larger area than the irreversible one.

HOpe this helps!

Is the irreversible processes inefficient because it transform heat into work, but some of that heat in the system is released into the surroundings because the irreversible processes happen too quickly for all the heat to be transformed into work?

Yes, I agree. In class I remember Lavelle talking about how in biological systems, irreversible process, though more inefficient, are useful due to their quickness and time value. While reversible processes are indeed more efficient per amount of heat/work put in, the irreversible processes can act more quickly and are useful as well. (Lavelle mentioned if you were to avoid a car crashing into you while crossing the street, you would want a quick reaction of energy, etc. to avoid it to stay alive).