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In reversible process,due to infinitesimal steps,the system remains in thermal equilibrium(why?) . But in irreversible process, thermal equilibrium does not exist. What is the cause for this phenomenon? Why the former attains thermal equilibrium and the later cannot? Plz help me explaining this.

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  • $\begingroup$ First, thermodynamic and thermal equilibrium are different things. Thermodynamic equilibrium implies thermal equilibrium, but the converse is not true. So be careful with using these terms interchangeably (thermodynamic equilibrium in the title and thermal in the body). $\endgroup$ – Wildcat Aug 17 '14 at 15:38
  • $\begingroup$ Secondly, thermodynamic equilibrium is reached in an irreversible process. Yes, it is reached at the very end of the process, but it is reached. Reading the body of your question I think you meant to ask why a system is not in equilibrium during an irreversible process. But such question has a little sense since a system actually can be in equilibrium during an irreversible process. A quasistatic process (a process in which system is all the way in equilibrium) might be irreversible. $\endgroup$ – Wildcat Aug 17 '14 at 15:51
  • $\begingroup$ And finally, at the end of the day it all comes done to definitions, which you did not provide. Actually, some sources define a reversible process as a quasistatic process without entropy production. Thus, a system in a reversible process is always in equilibrium by the definition. A reversible process is nothing but a special limiting case of a quasistatic process. In an irreversible process a system might or might not be in equilibrium depending on is the process quasistatic or not. $\endgroup$ – Wildcat Aug 17 '14 at 15:55
  • $\begingroup$ Sir, my book says thermodynamic equilibrium consists of thermal,mechanical,chemical equilibrium. $\endgroup$ – user36790 Aug 17 '14 at 19:16
  • $\begingroup$ Sir,can u explain (my book says) why irreversible process cannot have equilibrium(it says, unlike reversible process,the irreversible process is done making a sudden finite change and in such a quick time,the system doesn't reach equilibrium.) $\endgroup$ – user36790 Aug 17 '14 at 19:22
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What your textbook says is that during the course of your process, the system is in thermodynamic equilibrium with its surroundings if it's reversible. Such a process is just a theoretical concept as such a process would take an infinite amount of time. Real processes (I mean irreversible processes) take place so quickly that it is impossible for it to be in thermodynamic equilibrium (i.e. in Mechanical, Chemical and Thermal equilibrium) with its surroundings at every step of the process. But the analysis of such a process becomes impossible as we cannot exactly pinpoint the non-equlibrium states that are passed through by the system between the initial and final states of the process and so, we approximate this process by an adiabatically slow one, and define a concept called a 'Quasi-static' process which is the locus of all the equilibrium states passed through by the system in the course of your process.

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