Re: [Phys-L] defining energy

[Bruce Sherwood <Bruce_Sherwood@ncsu.edu>]

I had in mind E_grav and E_elec etc. as "different kinds", but I'm
comfortable not using the word "kind". As you say, the important thing is
to be as explicit as possible about where the energy has gone.

Do you mean "location" as a position in space? In that case, what is the
"location" of gravitational or electric potential energy, at the intro
level?

It is true that thermal energy is indeed just kinetic energy and potential
energy (writ small). Yet one can with a thermometer (and knowledge of heat
capacity) measure change in something which we might usefully call thermal
energy, as distinct from the kinetic energy and potential energy at the
subnuclear and subnucleon level that we typically just lump into the rest
mass. A rotating wheel has slightly more rest mass than a stationary wheel,
but it is usually convenient to call this extra energy "rotational kinetic
energy" and calculate it from 0.5I*omega^2. There is more than one way to
carve up the energy of a system into various useful categories, to simplify
discussion and calculation, but the multiplicity of possible descriptions
doesn't mean that these categories are wrong.

Bruce


On Tue, Oct 29, 2013 at 8:21 PM, John Clement <clement@hal-pc.org> wrote:

> The idea of "many kinds" of energy tends to confuse students.  The concept
> works much better if you get them to think of where you put the energy
> rather than it being a different kind.  Along with this the notation should
> always be E_location for the various places you put energy.  E_g, E_e,...
> Once they have the concept of energy as just being moved from one place to
> another, using conservation of energy is much more automatic.  But texts
> still use KE rather than E_k, or PE rather than E_g.  And is thermal energy
> really different?  It is just the kinetic energy of the molecules and/or
> the
> potential energy when you stretch the bonds during a phase change.
>
> Students at the high end of formal operational or who are at the
> theoretical
> level should not have much difficulty, but since the majority of students
> in
> an intro class are transitional with some thinking at the concrete
> operational level, visualizing where energy goes is a vital process.  Make
> the concept coherent rather than splintered into different types of energy.
>
> John M. Clement
> Houston, TX
>
>
> > This is perhaps related to the fact that there is only one
> > kind of impulse but many kinds of energy inputs, and only one
> > kind of momentum but many kinds of energy.