ISOTHERMAL HEAT ENGINES AND THE SECOND LAW

"xxein" wrote in message
...
On Mar 25, 6:59 am, Pentcho Valev wrote:
From a heretical point of view, isothermal heat engines are much more
interesting than non-isothermal ones working between two heat reservoirs
at different temperatures. If, at the end of the isothermal cycle, the net
work done by the engine is positive, we have an unequivocal violation of
the second law of thermodynamics - this work can only be done at the
expense of heat taken from a single heat reservoir - the environment.

Curiously, conventional thermodynamics does not and cannot deal with
isothermal cycles, whether or not they violate the second law. The whole
story is too long so here I am only giving an example. A fundamental
theorem in thermodynamics - "Entropy is a state function" - requires that
the cycle in question be broken up into small Carnot cycles:

http://ronispc.chem.mcgill.ca/ronis/chem213/hnd8.pdf
"5. Entropy Changes in Arbitrary Cycles. What if we have a process which
occurs in a cycle other than the Carnot cycle, e.g., the cycle depicted in
Fig. 3. If entropy is a state function, cyclic integral of dS = 0, no
matter what the nature of the cycle. In order to see that this is true,
break up the cycle into sub-cycles, each of which is a Carnot cycle, as
shown in Fig. 3. If we apply Eq. (7) to each piece, and add the results,
we get zero for the sum."

The Carnot cycle is not isothermal so it is impossible to subdivide an
isothermal cycle into small Carnot cycles. And if this fundamental theorem
is irrelevant vis-a-vis isothermal heat engines, it would be reasonable to
suspect that the whole conventional thermodynamics is irrelevant.

Pentcho Valev


xxein: Don't throw spaghetti into saying what you want to say. Just
say it.

Show me an example of work output obtained from a closed system.
==============================================
Valev is normally quite intelligent, but for some reason he imagines
converting heat to work is a violation of the second law of thermodynamics
and perpetual motion machines are a possibility.