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Re: induced emf again



"John S. Denker" wrote:

This whole thread is muddled. It repeately seems to take
general principles, apply them to a complicated special
case, and then attempt to re-state certain factoids about
the special case as if they were new general principles.

This list, and Feynman, helped me to discover that at least
two distinct phenomena are involved in the so-called Law
of Induction. Motional wire emf, as in an electric generator,
and stationary wire emf, as in a transformer, are two very
different phenomena. The formula dFLUX/dt can be used
to calculate the emf per coil in each case but that does not
mean the two phenomena are the same. Am I the only one
to become aware of this well known fact recently?

I am not trying to discover a new general principle. What
I am trying to do is to share my personal discovery with
those to whom it might also be something new, despite
many years of teaching physics. Unfortunately, most
introductory textbooks do not distinguish the two effects.

I am also trying to conceive an experiment which would
demonstrate that in one case the induced E field is
conservative while in another it is not. What is wrong
with this? Please help by criticizing the suggested
experiment, or by proposing other experiments. What
can be better for students, for example, in the second
semester of an AP course, than experiments designed
to test ideas which are new to them?
Ludwik Kowalski

Let's get a grip. We know the general principles. In
particular, we know the induced voltage on a bar moving
through a magnetic field. We also know the principle of
superposition, and how to apply it if the E field contains
source terms due to charges as well as source terms due
to magnetic induction.

There are one or two right ways to analyze the moving
bar. We know what they are.

There are infinitely many wrong ways to analyze the moving
bar. Do we need to discuss each of them separately?

In a system with N details, you can come up with N factorial
special cases where some of the details are coincidentally
related to some of the other details. But it is a waste of
time to hypothesize that there is some deep meaning or new
general principle reflected in these coincidences. Humans
have a strong predilection to find pseudo-patterns when no
real patterns exist; this is right up there with clothing
and religion on the list of "cultural universals". Part of
scientific training is to recognize and rein-in this predilection,
so as to not waste time pursuing hypotheses that have no chance
of panning out.

The White Queen believed six impossible things before breakfast.
I've believed thirty or forty. The trick is to winnow them
quickly and focus resources on the hypotheses that have a
reasonable chance of being useful.