[Phys-L] let's figure out how energy behaves

[John Denker <jsd@av8n.com>]

On 09/28/2015 04:24 PM, Philip Keller wrote:
> I look at the title of this thread and I can't help thinking: Let's
> NOT define energy. 

Yes, it was a pathologically ill-chosen Subject: line,
so I changed it just now.

As I have often said:
	Energy /is/ as energy /does/.

That is to say, it is far better to understand 
how energy behaves than to rote-memorize a pithy 
dictionary-style definition of what energy "is".

Words acquire meaning from how they are used,
not from some dictionary-style definition. 
Many students are shocked to hear this, but 
it's true and important.


>  Let's just define specific energy types.  We have
> these interesting quantities.  Under a particular set of
> circumstances, each can be shown to be changed by work.  So for
> example, we could call (1/2)mv^2 the "work-changed motion-related
> quantity".  Call mgh the work-changed height-related quantity. Build
> your collection. 

IMHO that is a step in the wrong direction.  The most 
salient thing about energy is the conservation law.
Focusing on specific sub-categories of energy loses
sight of the conservation law, unless you are super-
careful or super-lucky.

The fundamental conservation law recognizes only one 
kind of energy. The more you slice and dice the energy,
the more likely you are to make a mistake.

One failure mode (among many!) is that the sub-categories
are likely to overlap.  This leads to overcounting some
contributions to the energy.

On 09/28/2015 02:09 PM, Paul Nord wrote:

> > Scientists are not very consistent with the words
> > they use for speaking about energy.

Well, yes and no.

On the one hand, we have what I call the physics energy,
which AFAICT is very well defined and very well behaved.

On the other hand, there are innumerable things that some 
«scientist» here or there has called «energy» at some point.
Some of these are important and some not.  AFAICT even the
best of them seem to be poorly defined, subjective, and
context-dependent.  For example, as soon as you start talking
about «useful energy» or «available energy» you discover 
that a block of ice is more useful and more valuable in
summer than in winter.  So «usefulness» is not a function
of the state of the system.  (This stands in contrast to
the physics energy, which /is/ a function of state.)

In the introductory physics course, I recommend focusing
on the physics energy, and keeping the other notions of
«energy» on the back burner until the students have a 
good solid understanding of the physics energy /and/ 
a good solid understanding of entropy.

At the very least, you have to distinguish the physics
energy from the others.  Trying to come up with an all-
purpose definition that unifies the disparate notions
of «energy» is guaranteed to be a fiasco.