Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: non-potential voltage



I hope I am not alone who appreciates Michael's effort to bring
John's formulation closer to those who do not manipulate "divs
and rots" routinely.

Sometimes say that what is in volts is a cause while what is in
amps is an effects. But I is the cause of I*R. The chicken and
the egg? Or should we say that I comes before V?

A changing current induced in the circuit of the secondary
windings of a transformer can not be distinguished from the
ac part of a current produced by a battery (for example by
modulating the value of R). Amperes seem to be more real
(less dependent on interpretations) than volts.

Ludwik Kowalski

Michael Edmiston wrote:

I had to read John Denker's description of non-potential voltage very
carefully, and I now understand it and agree with it. Thanks, John, for
this explanation.

Not being a theorist, it takes me a while to grasp the meaning of things
like "curl free" and so forth. For others out there who are like me, I
would like to try floating a different (but I think equivalent) explanation
of the problem.

(1) We typically take "potential" to mean "position dependent." Potential
energy is position-dependent energy. Electric potential difference is
[position-dependent energy] divided by [charge]... that is: [delta-U]/[q].

(2) In electrostatics, electric field lines begin on positive charges and
end on negative charges. Electric field lines have beginnings and endings.

(3) In electrodynamics, electric field lines created by changing magnetic
fields form loops... no beginning, no ending.

(4) If we move charge around the looping electric field created by a
changing magnetic field, we have had an increase or decrease in the kinetic
energy of that charge or we have had to do +/- work on that charge, even
though we ended up at the same place we started. That means work was done,
or the kinetic energy of the charge changed, even though the position did
not change.

(5) Therefore, the work-energy situation involved in moving or allowing
charge to move in a loop of magnetically induced electric field involves
work and/or energy changes even though the charge began and ended at the
same position. This is inconsistent with our position-based definition of
electric potential difference.

It's amazing to me that even though I basically have known and taught points
(1) through (4) for a considerable number of years, this current list
discussion has only now brought point (5) to my attention. I naively called
the induced emf a potential difference without it ever registering in my
mind that this is a contradiction in word usage.

However, now I am no longer sure what words to use. I cannot say I am fond
of John calling this "voltage" because I still view voltage as a slang word
for electric potential difference. I have never seen a definition of
voltage that does not tie it to potential difference. Am I missing
something here?

Until someone comes up with something better, I am inclined to use induced
emf, although I agree with those who don't like the word "force" that is
implied by emf.

Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817