Partial Inductance - NOT

L D Miller (L_D_Miller@BayNetworks.COM)
Mon, 16 Jun 1997 07:43:06 -0700

That is the most Zen description of magnetism I ever heard.

Are you guys serious?

J. Eric Bracken wrote:
>
>OK, I'll give it a shot:
>
> Partial inductance is the voltage drop measured across a piece
>of conductor due to time-varying magnetic fields. You can imagine
>sticking an AC voltmeter across that piece of conductor and observing
>the drop.
>
> That wasn't so bad, was it? :-) Actually, to be precise, the
>magnitude of the voltage drop is L * omega, so you need to divide
>V by omega to get L.
>

Well, actually, your voltmeter completed the loop. But it needn't have;
magnetic fields handily bend around the ends of the conductor in any medium
that isn't superconducting. Remember the iron filings around a bar magnet?
I suppose the fact that your L * omega assumes a sine wave is trivial in
comparison.

> Usually the major source of confusion with partial inductance
>is the distinction between it and "loop inductance." Herewith,
>another definition:
>
> Loop inductance is the total voltage drop, measured around a
>closed conductor loop, due to time-varying magnetic fields. It's
>a bit harder to envision how you measure this... you need to insert
>a small gap in the loop, place a 1-Amp AC current source across it,
>and then measure the voltage across the gap.
>
> We should probably talk about the sources of the magnetic fields: if
>the source is the current in the _same_ conductor/conductor loop where
>you're making the measurement of voltage drop, you have a loop or
>partial "self-inductance." If the source is a current in a
>_different_ conductor, you have "mutual" loop or partial inductance.
>

Well, actually, inductance (only one kind) is defined in terms of effects
due to changes in current and current is in turn described by a closed-path
integral (Ampere's Law, etc). The closed path is what makes it a current;
otherwise, you are talking about static electricity. No CHANGE in current,
no inductive effect - sorry!

There is only one kind of inductance; what you are describing is different
ways to change the current (Ampere's law again).

Whether you close the loop with your instrumentation is immaterial.

> Another point of confusion in the definition of partial inductance
>is the current continuity problem. If I have an open conductor
>segment, where does the current flowing in it come from/go to? With a
>conductor loop, there's no such problem--the current just chases its
>tail in a closed circuit.
>
Only in superconductors, and even they run down eventually.

> The answer is that, for partial inductances, charges are piling up
>at both ends of the conductor... positive charges on the - terminal,
>negative charges on the + terminal. An odd situation, admittedly,
>but one that will be resolved later when you actually connect the
>conductor up with other conductors to form a circuit and enforce
>KCL. When this is done, the piling-up charges cancel one another out
>and vanish.

Great old mathematical operation, "cancelling"... What you are postulating
is magnetic monopoles. Has anyone ever found one yet? Again, currentless
charge does not interact with inductive elements, since (all) inductance is
_defined_ as resistance to changes in current flow. You know, e(t) = L *
d(i(t))/dt; inductance is the constant of proportionality relating e(t) and
i(t). No change in i, no e!

>
> If you don't hook the conductor up in a circuit (that is, you leave
>its terminals open), then its current is forced to zero and the
>charges at both ends again vanish. But you can still observe a
>voltage drop across it, due to EMF's induced by surrounding
>time-varying magnetic fields. People sometimes intensely dislike
>the thought of seeing voltage drops on a conductor carrying no current,
>but it is in fact physically reasonable. (No power is dissipated, after
>all, since p = v * i = v * 0.)
>
I would call that a capacitor.

WHERE did you get this stuff? I think you got taken in by an April 1 joke.

I don't intensely dislike these voltage drops because they are not only not
physically reasonable, they are physically impossible. Try Ohm's Law.

In fact, I'll go so far as to assert that _your_ "partial inductance" is in
fact capacitance.

> I'll cease my ramblings now. I hope this helped someone, somewhere.
>
>--Eric
>
>--
>J. Eric Bracken, Ph.D. Tel: 1.412.261.3200 x135
>Group Leader, Signal Integrity R&D Fax: 1.412.471.9427
>Ansoft Corp., Four Station Square, Suite 660 bracken@ansoft.com
>Pittsburgh, PA USA 15219-1119 http://www.ansoft.com
>