Re: [Phys-L] ?conservation of _internal_ energy

[Herbert Schulz <herbs@wideopenwest.com>]

> On Jan 14, 2016, at 1:14 PM, John Denker <jsd@av8n.com> wrote:
>
> In the context of:
>  https://www.av8n.com/physics/thermo/state-func.html#sec-internal-energy
>
> On 01/14/2016 08:23 AM, Herbert Schulz wrote:
> > Yes, imagine a gas instead of the spring. As the gas expands it
> > pushes the piston and does work. 
>
> I say it does /not/ do work.  No work is done by (or on) the blue
> system. There is no F·dx: At the place where there is a nonzero
> force, there is no displacement. At places where there is a
> nonzero displacement, there is no force.
>
> > The Temperature of the gas will go
> > down and the Internal energy of the gas will go down. So yes, the
> > Internal energy is not constant. What is your argument that the
> > Internal energy SHOULD be constant?
>
> I'm not arguing that it is constant or "SHOULD" be constant.
> I'm arguing quite the opposite.
>
> I suggest the discussion is worth having because this stuff is
> evidently quite non-obvious.  The wrong ideas are widely believed.
> I cite Wikipedia, which is a cornucopia of things that are widely
> believed without actually being true.
>    https://en.wikipedia.org/w/index.php?title=Internal_energy&oldid=697674365
>
> > > If the containing walls pass neither matter nor energy, the system
> > > is said to be isolated. Then its internal energy cannot change.
>
> Furthermore, even if we replaced that by a similar-sounding
> true statement, it would still be highly misleading, for
> reasons discussed in the vicinity of
>  https://www.av8n.com/physics/thermo/state-func.html#tab-internal-energy
>
> Just because some variable is constant in an isolated system 
> does not mean it is conserved.

Howdy,

Somewhere along the line you have to define `The System'. 

If you define `The System' as both boxes then the Internal Energy of that System is Constant (i.e., no work, etc., is done by `The System' on the rest of the Universe). How that Internal Energy is distributed may have changed from Spring Potential Energy to Kinetic Energy of the sub-parts of The System but the total Internal Energy of THAT System is the same.

If your System is just the `blue' box then it (i.e., its internal spring) has certainly done work on the `red' box (i.e., something outside The System as defined by you) and the Internal Energy of your System has gone down.

Good Luck,

Herb Schulz
(herbs at wideopenwest dot com)