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Re: [Phys-L] defining energy

Some energy can be located in "the field", but often it may be located in a
system. At the intro level one might appeal to the idea that gravitational
energy is located in "the g field", and get students to picture it as if the
object were attached to the earth by a gravitational "rubber band". When
the band is stretched it has the energy. In other words one has to have
both the Earth and the object to have energy and neither alone has
gravitational energy. Of course any picture has both good and bad aspects,
but lower level thinkers need a more concrete picture. Putting energy into
a system may be too abstract for many students. And appealing to energy
diagrams with potential wells is hopeless for most.

I wouldn't say that the classification of energy into "kinds" is wrong, but
that it prevents students from thinking of energy as just being transferred.
In other words it interferes with understanding of conservation. After all
if you change something to a different kind, the amount might be different.
Once conservation is understood, then whether you talk about energy
transformation or transfer is irrelevant.

John M. Clement
Houston, TX


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.




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