Re: Faraday induction

["Carl E. Mungan" <mungan@USNA.EDU>]

John Denker wrote in part:

> The term "EMF" carries the following unfortunate baggage:
>    2) Sometimes people use the term as synonymous with voltage, while
>       other people use it to refer to the non-potential part of the
>       voltage, to the exclusion of the potential-type voltage.
>
> My recommended solution:  I just talk about "voltage".  It's easy.  It
> removes any temptation to talk about "EMF".

Recalling the last time I brought this up, there is another possibility:

2) EMF is the electromagnetic energy per unit charge supplied by
certain devices or elements via transformation from other forms such
as chemical or mechanical energy.

This statement probably still contains ambiguities but it emphasizes
the idea of an ELECTROMAGNETIC ENERGY SOURCE. In particular, at least
as of the digest of this morning, no one has mentioned an error in
the original message by Wolfgang Rueckner:

> Suppose we have a
> single loop of wire lying in the plane of the page (or monitor screen) and
> the loop is split -- that is to say, it's not a complete circuit.  Also
> imagine an increasing magnetic field into the page (monitor).
> Faraday's/Lenz's law tells us that an emf will be induced such as to
> produce a CCW current which generates a magnetic field out of the page.  If
> you envision the split in the loop being at the top of the page, then the
> end of the loop to the left of the split would be at a positive potential
> w.r.t. the other end.

You've got the wrong polarity. The end of the loop to the RIGHT of
the split is positive, because the loop is acting as a SOURCE and not
like a DISSIPATIVE element. In other words, if you attached a long
wire across the gap extending into a region where there is no
magnetic field, the current would run in such a direction that the
loop acts like a battery. Inside a battery, current runs from the
negative to the positive terminal. Or you may wish to think about the
example of the traditional motional emf circuit where a bar slides on
a U-shaped conductor in a region of CONSTANT magnetic field. What is
the polarity of the emf in the bar? But the emf doesn't change if you
remove the U-shaped conductor and just have a moving bar. This is
sort of like your loop with a huge gap except we no longer have a
time-varying magnetic field to complicate the question by introducing
the issue of how to attach the voltmeter leads.

I agree with John however that "voltage" is always correct and safe.
For example, there is no compelling reason to associate emf with ANY
voltage change across an inductor, as is typically done in most
textbooks.
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/