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Why Thermodynamic cycles are used to obtain work for example carnot engine rather than single isothermal process which is part of cycle and can be used to obtain more work.And why isothermal process do not violate second law even when there is no friction or any irreversibility.I know they violate second law in actual world due to friction .But in theory reversible processes are possible which do not violate second law,so why it doesn't.

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An engine operating in a cycle can operate continuously i.e. for any number of cycles. If you just pull out one step in the cycle you don't have a useful engine because it can only operate once and then only for a short time.

An isothermal process is reversible by definition because temperature is not defined in an irreversible process. So an isothermal process does not increase (total) entropy. Incidentally this does not violate the second law since the second law says only that entropy cannot decrease. It does not forbid energy staying the same.

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You said second law says entropy can not decrease.Is it true?Or If I am correct second law says that for any process occurring in universe should increase it's total entropy.And Why Engine is not useful in theory after single process.I know it change state but according to isothermal process meaning temperature remains same and heat will still able to flow from hot reservoir in quasi-static manner –  Vishvajeet Patil Feb 18 at 7:33
    
Please Tell the reference for second law exact statement –  Vishvajeet Patil Feb 18 at 7:38
    
@VishvajeetPatil: see for example en.wikipedia.org/wiki/Second_law_of_thermodynamics. This states The second law of thermodynamics states that the entropy of an isolated system never decreases –  John Rennie Feb 18 at 8:13
    
I thought over second law ,for equilibrium entropy changes after attaining equilibrium are zero.But What about next question regarding engine usefulness and thanks for your quick responses. –  Vishvajeet Patil Feb 18 at 8:31
    
@VishvajeetPatil: An engine isn't useful if you can only operate it once and then only for a short time. –  John Rennie Feb 18 at 8:34

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