Spontaneous Process and Irreversibility

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In isolated systems, the change in heat (dQ) is zero, leading to the conclusion that the difference in entropy (Sb - Sa) is always greater than or equal to zero, indicating spontaneity. However, it is clarified that while every spontaneous process is irreversible, not all irreversible processes are spontaneous. Irreversible processes often involve energy loss through mechanisms like friction and viscosity, which do not necessarily align with natural spontaneity. An example given is the act of rubbing a stone against a wall, which is irreversible but not spontaneous. Understanding these distinctions is crucial in thermodynamics.
Euphemia
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Hi,all
My problem is following this picture!
http://mykomica.org/boards/shieiuping/physics/src/1325318780058.jpg
If the system is isolated, dQ is always zero. Sb-Sa ≥0
And it satisfies the requirement of spontaneousness in isolated systems.
So ,I do wonder if irreversible process is always spontaneous!
Am I Right?
Euphemia
 
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Note: Your picture isn't coming across.

You have it backwards. Every spontaneous process is irreversible, but not every irreversible process is spontaneous.
 
A process which is irreversible is the one results in energy loss in the form of heat (dissipation) due to friction ,viscosity etc.As these are involved in processes other than natural ones- eg.,rubbing a stone againast a wall in vertically upward direction- irreversible processes need not be natural or spontaneous ones.
 
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