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I recently came across a statement in a test which said "Neither reversible nor irreversible adiabatic process can cause a change in entropy of surroundings". This statement was given to be true. But I also know that there can be a change in the entropy of a system even in an adiabatic process. Then why can't the same be true for entropy of surroundings?

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    $\begingroup$ One of equivalent definitions of irreversible processes is that you cannot return the system to its initial state by returning the surrounding to its initial state. Irreversible processes increase the total entropy. Of the system or of the surrounding. Or both. // If one wants to be a nitpicker, even reversible adiabatic processes can increase entropy of surrounding, if e.g. high local static pressure breaks something. $\endgroup$
    – Poutnik
    Commented Apr 22 at 9:35

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In thermodynamics, we assume that the surroundings are outfitted with only a certain limited array of possible equipment.

  1. Ideal isothermal reservoirs (which are very large) and thus always an entropy change of $dQ/T_R$, where $T_R$ is the reservoir temperature. There can be one reservoir or a sequence of reservoirs at different temperature.

  2. Infinite volume gases at constant pressure.

  3. Frictionless pistons of negligible or finite mass

  4. Piles of pebbles that can sit on top of the piston, and can be removed one-at-a-time or all at once at specified elevations.

This will guarantee that the entropy change for the surroundings is always $dQ/T_R$, irrespective of whether the system experiences an irreversible process.

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  • $\begingroup$ Okay! I have been confused about difference between the equations for change in entropy of system and surroundings for quite a while, but this clears my doubt. Thankyou very much $\endgroup$
    – Yes
    Commented Apr 23 at 5:19

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