Re: [Phys-L] spontaneity and irreversibility

[John Denker <jsd@av8n.com>]

On 8/7/24 8:23 PM, Matthew Heaney wrote:
> Suppose I have a weight suspended a few feet above ground, attached 
> to a rope over a pulley.  In principle I could use this to weight and
> pulley to do work, such as lifting something.  The system has the
> capacity to do work, so it has "free energy" and thus low entropy.
>
> What's the entropy in this case?

1) If the weight and the payload are evenly balanced, there is
 no spontaneity here. The weight might go down while the payload
 goes up, or vice versa. There is no significant creation or even
 transport of entropy.

2) If they are not quite evenly balanced, let's imagine that the
 weight goes down, bounces off a cushion, and goes back up. The
 system is a highly nonlinear oscillator. It goes up and down
 forever. The weight spends half it's time moving in the direction
 of the net force, and half moving the other way. Again entropy
 plays no role.

3) If you want it to go down and *stay* down, let's imagine that
 the weight smacks into a hydraulic shock absorber, like you have
 on your car. Entropy is produced via viscosity in the fluid in
 the shock absorber.

================

Defining energy in terms of "capacity to do work" is wrong
physics. It's also bad pedagogy, because of what we call
"ignotum per ignotius". You can't explain the unknown in
terms of the more unknown. Students don't have a usable
intuition about "work". It's more ambiguous and hard to
define than energy itself.

It's wrong physics because you can easily have a system
that has *more* energy but *less* capacity to do work.
Details can be found here:
  https://av8n.com/physics/thermo/entropy-energy.html#sec-system-subsystem


////////////////////////

Earlier:

> My favorite site about entropy:
>
> https://franklambert.net/entropysite.com/

I don't recommend that. Lambert was a crank IMHO. He crusaded
against certain thermodynamic misconceptions ... by substituting
new and different misconceptions. The following is particularly
pernicious:

> > Concisely, the second law is “Energy of all types changes from
> > being localized to becoming more spread out, dispersed in space**
> > if that energy is not constrained from doing so”.

That's just wrong. If you have two rotating rings rubbing against
each other, the energy is 100% spread out beforehand and 100% spread
out afterwards, yet huge amounts of entropy are produced. Some
discussion, with diagrams, can be found here:
  https://av8n.com/physics/thermo/entropy-energy.html#sec-spreading