Re: [Phys-L] Conservation of Energy vs Constant Energy

[John Denker <jsd@av8n.com>]

On 05/04/2015 07:51 AM, Paul Lulai wrote:

> I am trying to convince a group of hs physics teachers that energy is
> always conserved.

Excellent.  Please keep at it.

>  Regardless of open or closed systems.

Yes.

> E(total initial) + W + Q = E (total final)            [1]

> They accept this as the first law of thermo without question.

Well, as I see it, strict local conservation of energy
*is* the first law of thermodynamics ... nothing less,
nothing more.  Equation [1] is an approximation thereto.

In particular, equation [1] does not include advection.
As a familiar example, putting fuel in a car or putting
new batteries in a flashlight is an example of advection;
it increases the energy, but does not show up in equation
[1].

Also:  Let's keep in mind the proverb that says:
  *Learning proceeds from the known to the unknown.*

Explaining energy in terms of thermodynamics is a losing
strategy when the customers do not reliably understand
thermodynamics.  The connection between thermo and 
energy is 100% correct in terms of physics;  it's just
not expedient in terms of pedagogy.

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

> Do folks have authoritative resources I can point to?

How about this:
   Feynman
   _The Character of Physical Law_
   http://people.virginia.edu/~ecd3m/1110/Fall2014/The_Character_of_Physical_Law.pdf
   ... especially the story of Dennis and the blocks
   starting on page 69.

   Also the video.  The whole thing is excellent:
     https://www.youtube.com/watch?v=r_IfV9fkBhk
   The most directly relevant part is:
     https://www.youtube.com/watch?v=r_IfV9fkBhk#t=1355

---

Perhaps less "authoritative" but more detailed and more
pictorial, coming at it from another direction:
   https://www.av8n.com/physics/conservation-continuity.htm
or equivalently
   http://www.av8n.com/physics/conservation-continuity.htm