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Re: [Phys-L] defining energy



On 10/31/2013 10:12 AM, Trivilino, Herman wrote:

You can start, as you mention, by saying that in its simplest form
work is force times distance.

It is conventional to related work to F • dx, perhaps even to
/define/ work as F • dx. That is indeed "the simplest form"
and it works fine in simple situations. However, there are
situations where that approach either fails completely or
at best conflicts with conventional concepts, notably in
situations involving friction.

Whether work is the best place to "start" is a matter of opinion,
on which reasonable persons may differ. There are surely hundreds
of people on this list who start with:
distance and force
--> work
--> energy

I am not going to argue against that, except to point out that
other approaches exist, many of which work about equally well.

For example, it is possible to start with energy on Day One
and do quite a lot with it, up to and including the celebrated
Interrupted Pendulum
http://www.av8n.com/physics/loop-de-loop.htm

while saying little if anything about force, let alone work.

This is a Big Deal when teaching people to fly airplanes. It
is both crucial and urgent that they understand about energy.
Force is way, way down on the priority list. I suppose there
must be other application domains where energy is not important,
so I don't want to over-generalize ... but the flying application
really puts a premium on energy.
http://www.av8n.com/how/htm/energy.html

stress that whenever work is done energy is being transferred. You
can also mention that the other way to transfer energy is heat,

Actually it's even more complicated than that:

________________ __________________
| | work | |
| energy in |--------------------> | energy in |
| region A | | region B |
| | heat | |
| |--------------------> | |
| | | |
| | advection | |
| |--------------------> | |
| | | |
|________________| |__________________|


YMMV, but this is one of the reasons why I prefer to think of energy
as being primary and fundamental. It is so fundamental that it
cannot be "defined" in terms of anything more fundamental. In
particular, it is misleading at best to define it -- or even
explain it -- in terms of work, or in terms of work+heat.

I find that defining work, defining heat, and (!) explaining the
difference between the two is just not worth the trouble, especially
in situations involving friction or any kind of dissipation.

There are lots of situations where trying to keep track of the work
and/or heat is very confusing, or just not worth the trouble. My
advice: Feel free to give up on work and heat. Keep track of the
energy and entropy instead.