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Re: emf, potential, voltage

Some quick random thoughts:

Why do books sometimes refer to the emf (or back emf) of an inductor or
capacitor and not for a resister. i.e. is there something different about
the voltage that one measures across a capacitor or an inductor or battery
compared to other measurement such as one across a resistor.

One obvious statement is that the voltage differences across a capacitor and
inductor can be used drive a circuit (at least temporarily), hence the
appelation of "emf"; but the voltage difference across a resistor can not,
hence we don't call that an emf.

Joel Rauber
-----Original Message-----
From: Carl E. Mungan [mailto:mungan@USNA.EDU]


Some folks use E (big confusion possibilities with electric field!)
or "epsilon in the Symbol font" (which looks like one lower-case "c"
stacked on top of another - Is there a name for this weird symbol?
It's certainly not the actual "Greek epsilon" which looks like an
italicized version of the mathematician's "set inclusion" symbol and
usually denotes "permittivity".)

So with this in mind, I open pretty much any introductory textbook. I
look up inductors. I see equations like xi_L = -L*dI/dt. I
immediately interpret that xi as an emf. You yourself refer to the
"emf of the inductor". So isn't the voltage across an inductor an emf?

Again, I summarize as follows:

* All potential differences are voltages. Not all voltages are
potential differences because nonconservative fields may be present.

* All emf's are voltages. Not all voltages are emf's. I don't recall
anybody defining the potential drop across a resistor as an emf for
example.

Maybe John is right and the term "emf" is not very useful. But to
those who use it, are the following emf's? Many textbooks use the
symbol "xi" for all of these quantities.

* the voltage across an inductor
* the output of a transformer
* the voltage generated across the ends of a straight rod moving
perpendicularly to a uniform magnetic field (commonly and
suggestively called "motional emf")
* an ac generator
* the voltage around a hypothetical loop (no actual wire present) in
a region of changing magnetic field

Please justify your response. I am starting to suspect that the
answer goes something like the following: Inside an inductor,
battery, etc we cannot speak of a potential. But if we stay outside
of these devices and assume no leakage, then what we measure across
their terminals *is* a potential difference. I am trying to reconcile
Gene's statement:

In Kirchhoff's loop rule all
the terms are potential differences. (None are emfs.)

with the fact that I usually include emf's such as batteries and
inductors in my circuits.

--
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/