[Phys-L] fundamental definition of "conservation"

[John Denker <jsd@av8n.com>]

On 01/14/2016 06:38 AM, Herbert Schulz wrote:
> 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.

There is a difference between conservation and constancy.
 -- Constancy means X does not change.
 -- Conservation means X does not change, except insofar
  as it gets transferred across the boundary to/from an
  adjacent parcel.

A lot of grade-school science books get this wrong.

Feynman is very explicit about the importance of getting
it right, in _The Character of Physical Law_ and elsewhere.
For details on this, with explanations and examples and
diagrams, see
  https://www.av8n.com/physics/conservation-continuity.htm

Energy is conserved ... for isolated systems /and otherwise/.
Energy is conserved, strictly and locally.
Energy is conserved, period.

In the present context, when somebody writes
   dU = work + heat          (?)
and claims U is conserved, they are obviously not talking
about an isolated system.