Re: [Phys-L] spontaneity and irreversibility

[Matthew Heaney <matthewjheaney@gmail.com>]

On Sat, Aug 10, 2024 at 5:57 AM Diego Saravia via Phys-l <
phys-l@mail.phys-l.org> wrote:

> El jue, 8 ago 2024 a la(s) 3:58 p.m., John Denker via Phys-l (
> phys-l@mail.phys-l.org) escribió:
>
> > ================
> >
> > Defining energy in terms of "capacity to do work" is wrong
> > physics.
>
>
> Capacity to do work is usually called exegy, that could be assimilated
> (forced) to mechanical/electrical energy.
>
> The capacity to do work could be "fixed" inverting terms, saying something
> like energy change in  adiabatic system equals work done in it.
>  but you need to explain work an adiabatic previously. You can define
> adiabatic without speaking of heat defined isolated system and then
> isolation plus work.

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

Wiki page if you're interested:

https://en.wikipedia.org/wiki/Exergy

I am curious about systems that have the capacity to do work.  If we have a
suspended weight (or just a stretched rubber band), then that system can do
some work, compared to a system where the weight is already on the ground.

I have mostly been content to think of such a system as having stored
potential energy, but John got me thinking about entropy.  Entropy is more
general but I'm unsure about the utility of using entropy to describe
mechanical (macroscopic) systems (but I am open-minded about such things
and this has been a very informative thread).  Perhaps exergy is
the measure I was looking for.

There is something distinctive about a system that has the capacity to do
work versus one that has no such capacity, and I'm curious about how to
properly describe such a system.  Maybe it's stored potential energy (but
maybe it's not).  There is spontaneous action that occurs when a system is
left alone (the weight falls, the rubber band releases), and this dynamism
is interesting.



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