Re: Faraday induction

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

I wrote:

> > My recommended solution:  I just talk about "voltage".  It's easy.  It
> > removes any temptation to talk about "EMF".

Then at 09:16 PM 6/1/01 -0400, Geoff Nunes wrote:

> Consider the following statement that will appear to you as perfectly benign:
> "At this point in your circuit the voltage should be 0.7 V."

I don't think that is benign at all, especially in a thread about Faraday
induction.  It embodies a fundamental misconception.

It would be OK in some restricted situations (including situations where
Kirchhoff's laws apply) but it's not OK in this context.  We should be
eternally vigilant to prevent new outbreaks of this misconception.

> What does that really say?  "There should be a 0.7 V potential
> difference between this point in your circuit and some other point that
> I'm not going to tell you about, so nyah, nyah."
>
> WE know that a voltmeter has not one but two inputs, but do our
> students?  In teaching E&M to freshman, I've always re-written
> everything replacing V by Delta-V and always being careful to call it a
> potential difference instead of "voltage."

Talking about Delta-V is a step in the right direction.

But calling it a potential difference is a misconception.
What if the voltage is not a potential?

Not only do we need to worry about "other point" where the black lead of
the voltmeter is connected, we need to worry about the entire path that the
lead-wires take.

> Of course, I can't call emf a potential difference.

Agreed.... But saying something bad about potential doesn't say anything
good about emf.

Talking about the voltage-drop around a loop makes perfect sense to me.  I
just don't see any downside to this terminology.

> I am open to a new name, but in the meantime I have come up with nothing
> better than calling it "ee em eff" and refusing to say what the letters
> stand for.

Students can read.  (Some of them actually do so. :-)  They will figure out
that it means electromotive force.

Why do we need a new name?  What's wrong with voltage?

> I am sure that for 95% or more of students, it is not the _name_ that
> confuses them.

I agree with that!

There are at least three concepts in play here:
 a) Gauge invariance:  the location of the black lead matters.
 b) Non-potential voltages:  the route of the two leads matters.
 c) Dimensional analysis:  voltage is an energy per unit charge, not a
"force" (electromotive or otherwise).

For concepts (a) and (b) at least, the misconceptions can be restated using
one name or another, and be equally misconceived.

==============

To my ears, calling a voltage an "emf" sounds quaint and archaic, like
calling a capacitor a "condenser".  There are minor exceptions:
  *) When talking to auto-mechanics or airplane-mechanics, I call a
capacitor a "condenser" -- otherwise they won't know what I'm talking about.
  *) When I'm being sloppy, I use the term "back emf" as part of a
linearized model for how the electrical properties of a motor depend on its
mechanical rotation.  This is conventional, but it is just as dimensionally
unreasonable as any other use of the term emf.  A better name for this
quantity would be "induced Thevenin voltage" or some such.

===================

We are talking about serious non-nitpicky misconceptions here.  At one
point in my life I earned my living as a consultant.  One of my specialties
was "grounding and shielding".  It was ideal work for a consultant, because
I could easily solve problems that typical clients would !never! be able to
solve on their own.

I would try to explain my methods to them, but usually they just thought of
me as the witch doctor, called in to exorcise the ground loops.

I always tried not to giggle too much when I walked into the client's
premises and saw 1-inch woven copper "grounding strap" that they were using
in a vain attempt to make the so-called "ground" so-called "potential" at
point A equal to the so-called "ground" so-called "potential" at point
B.  They were appalled when I disconnected all the strapping and piled it
in a corner.

The strapping was simultaneously too little and too much:
 -- The room was full of non-potential voltages, and if you had really put
in enough copper to change that fact, the floor would have collapsed under
the weight of the copper.
 -- If you dealt with the problem properly, by controlling the geometry of
the red lead and the black lead of all measurements, then it was possible
and indeed necessary to remove the copper straps.

This is a good story to tell students, because it illustrates that physics
is not a purely academic subject.  It is possible for a physicist to earn a
living in the real world, solving real problems.