Re: Faraday induction

["John S. Denker" <jsd@MONMOUTH.COM>]

Wolfgang Rueckner wrote:

> Thank you -- I should have thought of going to Feynman in the first place.
> But now, how to explain this to an introductory physics student in an
> algebra-based course .... I guess the only answer is again, "the flux rule
> does not work in this case!"

Hmmmm.  The student might not be very happy
with that answer.  What good is a law of physics if it
works in some cases and not in other cases?  How can
a student tell when it is safe to use this rule, and when it is not?

The "exceptions" to the flux rule that Feynman discusses in
section II-17-2 are extremely artificial, and are not germane
to the present question.  Feynman's examples obviously play
fast and loose with the definition of what "the" loop is.
Under ordinary circumstances where you have an ordinary loop,
the rule is simple:
        voltage = flux dot

This is a Maxwell equation.  Always was.  Always will be.
It applies in the present case just fine.
It describes the induced voltage around a loop.

In contrast, the original question cited a "rule" about
the induced _current_ .  That does not apply in this case, for
the simple reason that there never was any such rule.

You can calculate the induced current (or lack thereof)
from the given Maxwell equation and Ohm's law.  No problem.

If (!) there is an induced current, Lenz's law is a useful
rule of thumb, a mnemonic that helps you keep the signs
straight, so you know the direction of the current.
That's all.  It doesn't say there will be a current.