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

[Herbert Schulz <herbs@wideopenwest.com>]

> On Jan 14, 2016, at 5:21 AM, Diego Saravia <dsa@unsa.edu.ar> wrote:
>
> Its obvious for me that any kind of energy, as internal energy is,  It 's
> not conserved. You can convert one kind of energy into another. only total
> is always conserved, if a body expands and lower its internal enegy and as
> a consecuente other rises, the gravitacional energy of the rised body
> grows. U is transformed to Potential Energy, E is conserved
> > The distinction between U and E is significant in fluid dynamics.
> > Maybe I'm missing something, but AFAICT the «internal energy» is
> > not conserved.  To see this, consider the contrast:
> > a) One parcel expands in such a way as to compress a neighboring
> >  parcel.  U is conserved in this situation.  So far so good.
> > b) One parcel expands in such a way as to /lift/ a neighboring
> >  parcel.  It seems to me that U is not conserved.

Howdy,

Energy is conserved for an ISOLATED system. If you say the `box' expands and lifts something else (i.e., does Work on another object) the `box' is certainly not an ISOLATED system---it's interacting with another object. The same is true if there is heat transfer, etc.

Good Luck,

Herb Schulz
(herbs at wideopenwest dot com)