Re: Faraday "magnetic" induction without a magnetic field.



"John Polasek" <jpolasek@xxxxxxxxxx> wrote in message news:7er805t4mo7tke3r6v0qgr5jbf59uf59tu@xxxxxxxxxx
On Thu, 7 May 2009 22:51:34 -0700, " Don Kelly" <dhky@xxxxxxx> wrote:




"John Polasek" <jpolasek@xxxxxxxxxx> wrote in message
news:ggs505thqaupa88oqltlrls38079mlfh2n@xxxxxxxxxx
On Wed, 6 May 2009 21:29:59 -0700, " Don Kelly" <dhky@xxxxxxx> wrote:



"John Polasek" <jpolasek@xxxxxxxxxx> wrote in message
news:n28305l35p3adl658mc5mv3aliqsrggu73@xxxxxxxxxx
On Tue, 5 May 2009 21:43:44 -0700, " Don Kelly" <dhky@xxxxxxx> wrote:

"John Polasek" <jpolasek@xxxxxxxxxx> wrote in message
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On Tue, 5 May 2009 14:06:50 -0700 (PDT), Benj <bjacoby@xxxxxxxxxxx>
wrote:

On May 5, 9:30 am, John Polasek <jpola...@xxxxxxxxxx> wrote:

snip

By illegal procedure I meant you initially assumed a series loop where
the diagram clearly shows the two resistors connected in parallel with
each other, being the source of the astonishment and controversy. (Did
you see the video?).
Granted one VM is showing -.1V and the other +.9V, then it would be
the work of a moment to put the prods across the two shorting bars to
reveal the deficit in the equation, which should be fragments of 1
volt and its complement. I brought that up in a prior note.
-----------

I saw the video- Levin could sell the Brooklyn bridge- In this case he
proved that bull*** really can baffle brains.

I did not initially assume a series circuit but it is patently obvious that
ignoring the meters there is only a single current loop. Hence, by
definition, the circuit is (- see below**). However, there is no need to
consider the concepts of series or parallel or Thevenin- Just Kirchoff.

Lewin was loud in his disdain for Kirchoff. Using Kirchoff's law is
child's play, but only after first nailing down some phantom driving
voltage. Thevenin's voltage is an easy way that usually (not always)
forestalls a lot of oratory.
---------
I see no reason for disdain of Kirchoff in favour of Thevenin. All Thevenin really does is say that some unknown linear network with sources can be represented (as seen from outside) at a given pair of terminals, by an ideal voltage source and an impedance in series with the source (or the Norton equivalent- a current source in parallel with an impedance. This representation is true as seen from the outside but tells nothing about the internal network.
One exception- the Thevenin equivalent of an ideal source behind an impedance is the same source behind the same impedance (as in this case). A battery is not ideal and its behaviour under load is that of an ideal source behind a resistance which is the same as its Thevenin source- Thevenin is not needed to get this model. In his circuit, had an ideal source and an unrelated resistance which he considered as a separate element of the circuit. - he did not use a Thevenin model. Thevenin is derivative and the derivation of Thevenin's theorem (which is easy to do) depends on Kirchoff's Laws and superposition while Kirchoff's laws are far more fundamental than Thevenin- you can't derive KCL or KVL from Thevenin.

All I did was apply the fundamental core of circuit analysis- Kirchoff's
equations.
The sum of the voltages around a closed loop =0 KCL
The sum of currents into a junction =0 KVL
In this case- at every connection between elements, or even between
segments of the connecting wires, there is a single "in" and a single "out
lead so what goes in comes out and into the next segment/element all around
the loop and this reduces to a single current everywhere in the loop.
(assuming ideal voltmeters).
Considering KVL then we can say
The sum of the voltage rises due to the sources=the sum of the IR voltage
drops. and all elements have a common current.

I didn't see you deal with a phantom driving voltage.
-----------
What "phantom source" - If you are considering the induced voltage- it is included in the KVL equation. Lewin stated that this total induced voltage was 1V. When you went on about fragments of 1 V scattered around the circuit, the sum of these is 1V and there is absolutely no infotrmation as to how this is distributed. When you mentioned fragments I noted that the total IR drop was 1V which was equal to the total induced voltage rise. The driving voltage is not a "phantom" any more than the whole circuit is, but, as you did- noted that it can be represented by a lumped or a bunch of lumped sources. I pulled a Lewin and, effectively, as he did, assumed that this virtual battery was not between A and C or B and D - without any basis for doing so. So, yes, you may be able to measure voltages in the "shorting bars" but you may find that in doing so, they wont mean anything as the meter leads may enclose some flux which affects the reading.


That is all that is needed.
Nothing more.

**However, for your information, I checked a couple of references

I'm really, really familiar with parallel and series circuits (well,
since the '30's).
-------------
Then you should know that this is "officially" a series circuit.

You have an ideal voltage source in series with two resistors -3 elements with a single common current (Note that Lewin also assumed this when calculating the current by Ohms law.
Now, if you replaced the battery- 100 ohm resistor combination by an equivalent Norton circuit- THEN you would have a current source driving two parallel resistors.

---------------------------------

"Elements are said to be connected in series when they all carry the same
current" Johnson, Johnson, Hilburn,. "Elecric Circuit Analysis "(2nd Edition
Prentice Hall, 1992).

Fitzgerald, Higginbotham, Grabel "Basic Electrical Engineering" agrees but
is less succinct.

Wikipedia agrees as well

This indicates that this single loop circuit is "series" and not parallel.
The way he took "measurements" tended to give the impression of a parallel
circuit- (more misdirection).

That's right, he drew 2 imaginary voltmeters separated by 2 shorting
bars. (Really, you would need scope traces for this transient). There
is more to be learned from his hookup.
-------------
What transient? he is considering a steady state situation- a good DVM will do a better job than a scope for this situation. All Lewin said was that the flux was changing at a rate such as to produce a constant 1V. No L or C is assumed in the model.
--------------
Using my ABCD diagram
A B
C D
whereas he showed scope trace D-B, it would be informative to show
additionally, traces of DA and DC. That would allow more complete
analysis; for example is DA<>DB and DC<>0; or are they?
(This would be a good question for the B student for extra credit. In
fact, if there are any B students reading this, we will give them
extra credit for predicting DA and DC).
-----
I agree. But then there would be no puzzlement. He didn't want to give away the answer. We don't know and have no way of knowing the distribution of the induced voltage, except by inference, (Lewin did not say anything about this but it is implicit in his "measured" voltages) that he didn't consider that any of it was between A and C or B and D.
Unproven and unlikely assumption.


--
Don Kelly
dhky@xxxxxxxxxxxx
remove the x to reply



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