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Change in Entropy of surroundings for irreversible process

  1. Aug 19, 2014 #1
    1. The problem statement, all variables and given/known data
    I know that the equation ΔSsystem = Q/T is only valid for a reversible process but whenever i see problems involving a irreversible isothermal expansion of ideal gas, the ΔSsurroundings is taken as -Qirr/T. Why is that equation valid for surroundings, is it because the surroundings being a reservoir/sink, any heat exchange is done reversibly?


    2. Relevant equations

    ΔSsystem = Q/T
     
  2. jcsd
  3. Aug 19, 2014 #2
    This assumes that there are no significant temperature gradients present within the reservoir (so that the temperature of the entire reservoir is uniform) and that the reservoir temperature remains constant during the process. This can be approached in the real world by having the material in the reservoir consist of a mixture of liquid and solid at the melting point.

    Chet
     
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