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Re: [Phys-l] EM, is it energy

On 12/01/2007 07:31 PM, Craig & Margaret Lucanus wrote:

This a helpful list.


One thing to keep in mind is that the discussions are
sometimes contrapuntal, due to the widely varying
needs and backgrounds of the participants. It's not
a big problem, but it bears keeping in mind.

Attributing energy to the field rather than to an object influenced by the
field is something worthwhile which I can pass on to my students. So if
there is nothing under the influence of a field, regardless of field
strength, there is no energy stored in the field. Thanks JB, this is
something I must have missed in text books, or at least it wasn't put that

There are two ways of looking at this. I'm not going to
argue for either extreme, but rather to point out the

1a) If you are hooking up a light bulb to a battery using
clip-leads, then you say Kirchhoff rules, and fields are
an unhelpful distraction.

1b) If you are hooking up some 10 GHz vFETs along a bit
of stripline, then you say the fields are all that
matter, and Kirchhoff was severely deluded.

2a) If you are playing catch with a ball, the earth is for all
practical purposes immovable, and there is a one-to-one
correspondence between the energy of the system and the
state of the ball. It makes sense to associate the energy
with the ball. Energy is a state function if you do that,
and not otherwise.

2b) If you are doing cosmology, treating the earth as immovable
doesn't work so well.

I'm not saying that (a) is better than (b) or vice versa. I can
do cosmology, but I can also play catch. Something that is a good
approximation in one situation might be a lousy approximation in
another situation.

b) A description in terms of fields is absolutely necessary in
principle. Otherwise there is no hope of being relativistically
correct i.e. Lorentz invariant.

a) There is such a thing as "too much principle". A lot of
the time in the real world, the nonrelativistic approximation
works just fine.

Bottom line: It doesn't pay to get too dogmatic about who "has"
the energy.