Re: [Phys-L] spontaneity and irreversibility

[Diego Saravia <dsa@unsa.edu.ar>]

> I wasn't familiar with the term "exergy" -- thanks for pointing that out.

avaibility is more used in USA,  for some people there are not the same
idea.


> I am curious about systems that have the capacity to do work.


exergy is defined in relation to environment.

you could talk of "adiabatic"  exergy if you have a system with, for
example, two temperatures.


>  Entropy is more general


For macroscopic system you have something that some people call Gouy Stodola
Theorem that show both concepts as equivalent. It states that "work
destruction"
equals entropy generation x environment temperature

MIT people, Gyftopoulos, introduce first exergy (avaibility) and then,
derive entropy.

perhaps you will like Atwood machine

You can think the machine as a power "generator"
one mass gets power from the other

If they are equals, you dont "loose" exergy, but velocity is cero. no power.
If going up mass is cero, yo get maximun aceleration and also cero power.
There was several discussions about the correct equations uf that machine.

Maximun power is with intermediate mass. you MUST "loose" exergy or
generate entropy
if you want maximun power.

In fact with a endoreversible engine maximun power is at eficiency =
1-\sqrt{TF/TC}, carnot machine do not gives you any
power. Novikov, a (russian scientist) was the first one to get that result,
Curzon Alhborn is more known in west




-- 
Diego Saravia
dsa@ututo.org
Diego.Saravia@gmail.com
NO SIEMPRE FUNCIONA->dsa@unsa.edu.ar