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Re: [Phys-L] more about basic DC circuits

I think this sows how hard it is to write ANYTHING! [and yes, i just saw
the typo in that sentence, but I decided to leave it as evidence supporting
the sentence :) ]

Still, I think we know the idea he was targeting -- the idea that resistors
"use up" current. Rather than "conservation" of current, I would have
chosen "continuity" and I would emphasize that it's a rule that applies to
a given point in time. We all have our favorite ways of saying things. I
call this the "one path, one current" rule for series circuits.


On Mon, Dec 16, 2013 at 12:32 PM, John Denker <jsd@av8n.com> wrote:

On 12/16/2013 10:14 AM, Prof John Ertel wrote:
Q3: So long as we include both i(conduction) & i(displacement), our
definition of a "complete circuit" seems to demand this to be TRUE.

I wouldn't say that. Not even close. If the alleged «conservation of
current» means anything at all, it is some hitherto-unheard-of use of
the word, dramatically non-analogous to conservation of momentum.

Familiar counterexample: The current in an RC circuit. It doesn't
"leak out" into some nearby region; it just stays in place and
decreases. Momentum cannot do anything similar, because momentum
is conserved.

Another counterexample: Consider a photodetector or perhaps a Geiger-
Müller tube. There is some substance subject to an electric field.
Initially there is no current. Then a photon comes in and ionizes the
substance. Now there is a current, because the negative ion goes one
way while the positive ion goes the other way. This current did not
come in from outside; it just appeared in place, violating any reasonable
notion of conservation of current.

If you're going to use the same word with two different meanings, similar
enough to be confusing yet different enough to be non-interchangeable, it
seems like pedagogical malpractice to not carefully explain the
distinction.
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