
#1
May1308, 07:58 AM

P: 21

Hello Lords of Physics,
sorry for pestering you guys again but these days a question about irreversibility is pestering me , my question is Why we say that irreversibility in process is due to finite tempreture difference ? so if there is finite temp diff. why process is not reversible? please teach me with giving some examples , thanks a lot!! 



#2
May1308, 10:35 AM

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Remember finite means nonzero as well as noninfinite.
So a process which involves no change in temperature is reversable, one which changes temperature will need extra work putting in to make it go the other way. The topic is called entropy and gets quite complicated in the details. 



#3
May1308, 03:17 PM

P: 21

Can anyone tell how isothermal reversible process takes place , means if we give heat to system , entropy will increase , and after doing work by system, does entropy will become zero again? but i heard that entropy once created can't be decreased! , so how the isothermal process is reversible? thanks!




#4
May1308, 11:08 PM

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PF Gold
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A Ques. From Irreversible Process
In an isothermal reversible process, the system gains energy Q (in the form of heat from the environment) and loses energy W (in the form of work to the environment). The energies are equal, Q = W, so conservation of energy is satisfied.
The system gains entropy Q/T by being heated and the environment loses entropy Q/T by heating the system. There is no entropy transfer associated with the work. Entropy is conserved because the process is reversible. Does this help? 



#5
May1408, 01:50 AM

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PF Gold
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It is not very clear from the OP what is exactly the question, but I guess this might help:
When heat "crosses" a finite (nonzero) temperature difference, then this is an irreversible process, in the sense that the total entropy of a thermally insulated system has augmented. The reason is that the entropy lowering dS1 =  Q / Thigh while the entropy increase dS2 = + Q / Tlow, and dS = dS1 + dS2 > 0 for the overall, insulated system. But this is not the only way to have irreversible processes. 



#6
May1408, 03:35 AM

P: 86

Could you make some reallife examples of reversible and irreversible isothermal processes/phenomena?




#7
May1408, 03:38 AM

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PF Gold
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Irreversible: cooking water with a flame. 



#8
May1408, 02:15 PM

P: 21

Actually lemme clear the question again , Actually when we add heat by hot reservoir in isothermal process , then there is net entropy decrease in entropy by doing work. but we say for reversible process that heat transfer should be done by very little temp. difference , in isothermal process temp is constant but still by giving finite heat by reservoir we breaking the law of reversibility.




#9
May1408, 02:25 PM

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#10
May1508, 03:26 AM

P: 86

So... what makes the first reversible and the second irreversible? Is it the "speed" in giving the heat? 



#11
May1508, 05:58 AM

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#12
May1508, 07:48 AM

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More generally, entropy is created (and a process becomes irreversible) whenever a flux occurs in response to a gradient in a generalized potential. Examples are:
 Flow of heat (entropy & thermal energy) in response to a temperature gradient  Flow of charge in response to a voltage gradient  Flow of matter in response to a concentration gradient. There are also cross terms; e.g., flow of charged matter. Since a flux requires a gradient, no real process can be reversible. 



#13
May1508, 08:23 AM

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#14
May1508, 08:33 AM

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Yes; I think it would be impossible to restore the electrons to their original position, in real life, with 100% efficiency. But you could come arbitrarily close, by decelerating and accelerating the electrons very slowly, capturing, storing, and dispensing their kinetic energy.




#15
May1508, 08:42 AM

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#16
May1508, 10:16 AM

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If the last Efield is 200 V/m, they are at point A, at rest. Some energy has been lost as radiation when the electrons were accelerating, correct? My undergrad training is in mechanical engineering, so it would take me a little while to make sure I'm doing the calculations correctly.




#17
May1508, 11:16 AM

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