A Ques. From Irreversible Process

  1. 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. jcsd
  3. mgb_phys

    mgb_phys 8,952
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    Remember finite means non-zero as well as non-infinite.
    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.
     
  4. 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!
     
  5. Mapes

    Mapes 2,532
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    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?
     
  6. vanesch

    vanesch 6,236
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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 (non-zero) 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.
     
  7. Could you make some real-life examples of reversible and irreversible isothermal processes/phenomena?
     
  8. vanesch

    vanesch 6,236
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    Reversible (or almost so: there are no truely reversible processes in nature): slow melting of ice in water when the ambiant temperature is around 0 degrees, and vice versa.

    Irreversible: cooking water with a flame.
     
  9. 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.
     
  10. Mapes

    Mapes 2,532
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    This is not true, there is no entropy decrease associated with doing work. There is an energy decrease, however, when a system does work on its surroundings.
     
  11. We are assuming here that the temperature doesn't change, right (isotermal)? So we are thinking that the entire ice-melting or water-boiling process considered is at constant T, and all the input energy goes into changing the phase/state from solid to liquid or from liquid to gas. Is this right?

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

    vanesch 6,236
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    The fact that in the first case, the heat comes from a reservoir (the environment) at about the same temperature as the ice/water mixture (hence my "when the ambient temperature is about 0 degrees), while in the second case, the heat comes from a source (flame) at much higher temperature. The irreversible process is not the melting or the boiling, but the heat transfer.
     
  13. Mapes

    Mapes 2,532
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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.
     
  14. vanesch

    vanesch 6,236
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    I have a problem with this one. Voltage gradient = E-field. Now, do you consider the acceleration of a bunch of electrons in vacuum under a uniform E-field an irreversible process ?
     
  15. Mapes

    Mapes 2,532
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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 v-e-r-y slowly, capturing, storing, and dispensing their kinetic energy.
     
  16. vanesch

    vanesch 6,236
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    :confused: Imagine I have a cloud of electrons (not dense enough for the mutual repulsion to be of any effect), at point A, at rest. Now, I switch on brutally an E-field of 200 V/m in the x-direction for 10 seconds. Then I switch it off for 3 seconds. Next, I switch on an E-field of 200 V/m in the minus-x direction for 20 seconds. I switch it off for 3 seconds. I next switch on again an E-field in the +x direction for 10 seconds, and then switch it definitively off. Where are my electrons ?
     
  17. Mapes

    Mapes 2,532
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    If the last E-field 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.
     
  18. vanesch

    vanesch 6,236
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    Yes, it is correct that there is a small irreversible process going on with radiation due to acceleration. If your point was to say that every real process in nature is always a tiny bit irreversible, I cannot deny that. But I had the impression that you put "charge flow under the effect of a finite E-field" on the same footing concerning entropy production as "heat flow under a finite temperature gradient" which it isn't (in vacuum at least - it is in resistive media).
     
  19. Mapes

    Mapes 2,532
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    Agreed. The entropy increase from the gradient-driven flux of matter or energy through empty space is very much on the low end compared to that from matter or energy distributing itself in solid matter.
     
  20. Temperature and heat are not the key things here. E.g. take a gas that occupies half of a cilinder, the other half is vacuum. The boundary between the two sections is suddenly removed. Then the temperature in the initial state and the final state will be exactly the same (assuming the gas is an ideal gas). But the entropy has increased so this process is irreversible. Also no heat was transferred to the gas. The gas has also not performed any work.
     
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