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

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
news:aqc1051i94973qkukn6m0uokde7v5bqspg@xxxxxxxxxx
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:

The sum of the voltages around the loop will be zero. The "hidden"
batteries" belong not to Lewin, but only to Thevenin.

The "hidden' batteries belong to Thevenin, but also to Faraday. One is
used to thinking that a piece of wire connecting two resistors puts
the ends at the same potential, but of course it doesn't because of
the integral around the loop.
I don't know what you're explaining to me, but it would be real noble
if you retracted your slanderous accusation of Lewin surreptitiously
using hidden batteries to further his claim.

I already pointed out that instead of 2 resistors with two taps as
shown, we have in reality a 3-tap circuit after inserting Thevenin's
volts, so sensibilities are once again restored.

So Faraday works 100% and Kirchoff is likewise affirmed.
John Polasek
------------------------
John,
I came into this late because my computer had been down. I agree with your
final conclusion.

However, there is no need to even consider "Thevenin" .
If the resistors were non-linear, the same result would occur but
Thevenin's
theorem would be invalid.

If you consider the circuit as having a string of n (you choose n)
series
resistors- all but two being 0, then the total voltage across these two
individual resistors would be 1V and distributed the same way that they
would be distributed with the battery in circuit- even if non-linear. The
video measured voltage across the 100 ohm resistor and battery in the
first
case and the voltage across the 100 ohm resistor in the second case- with
a
lot of hype in between.

Illegal procedure: you have just assumed what needs to be proved:
namely, that the resistor are *in series* when the video plainly shows
them welded together at top and bottom, yet, inexplicably, with
different voltages.
-----------
Sorry, I should have said, "in series with your "virtual battery" or in
series with the actual battery" Single loop on the assumption of perfect
voltmeters.

While there seems no way out, we first should investigate how the
fluxrate, like a ghost, can introduce the known driving voltage. From
Maxwell we have dB/dt causing:
Curl E = -dB/dt (what a genius to think of this)
Using Stokes to deal with the curl
Int curl E = Int E.dL = 1 volt for 1 turn, showing a
continuous gradient all round
Also
1V/1K = 1 mA (= shorted current of Thevenin supply)

We can forsake the inch by inch gradient in favor of lumped constants
by invoking Thevenins theorem which says the loop represents a power
supply of 1 volt in series with 1K ohms. We can put VThev anywhere in
the loop, and we choose the bottom shorting bar. The 1 volt pushes off
rising from C to D, then falls .9 from D to B and another .1 from A to
C.

A____B
| |
| .1 | .9
C_1V_D

You are right in that *if* we can show that the resistors are in
series we don't need Thevenin. The trick is that Thevenin affords us a
'pry bar' to break the parallel circuit into a series of 3 elements.
--------
But Thevenin's theorem (as is true of most circuit theorems) applies only to
linear circuits. The simple use of KVL and KCL works well.
---

This sure brought out a lot of argumentation for a simple problem- I
wonder
if any of Lewin's student's caught on.
The students probably felt justified in not understanding and just
being as chagrined as the observing professors were said to be. (Not
very chagrined, only a 'B').

An engineer knows Thevenin's theorem; a physicist doesn't. (Check
Smythe and Panofsky who don't have a closed circuit in the whole
book). An admixture is best.
How explain without Thevenin? V = N dph/dt yes, but you need
Thevenin's insight to install V as a 'lump' which is the essential
point in the partitioning.

John Polasek

It is convenient to install V as a lump but this owes nothing to Thevenin
but a hell of a lot to Kirchoff.

There is no illegal procedure.
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.

You have 1V induced in the loop. This is a single loop (ignoring the meters
which I am treating as ideal -no use opening up a can of worms) which has a
*common* current at every part of the loop-no branching- this *is* the
basic criterion for a series circuit. Hence it is a series circuit.

In any case I can apply Kirchoff's Laws and note that the sum of the induced
voltages =the sum of the IR drops where I is common to all the elements.
Now, as you say, correctly, we can lump the induced voltages as a "virtual
battery." . Since the current is common to all parts of the circuit it
doesn't matter, for purposes of calculating the current or measuring the
voltages across the individual resistors, where the "virtual battery"is
located.
This concept owes nothing to Thevenin.

It is valid for non-linear resistors where Thevenin is, strictly speaking,
not valid (note series and parallel equivalents, loop and node equations
also depend on superposition which implies linearity). In fact, derivation
of Thevenin's theorem is a matter of using superposition- which can only
apply for linear circuits- really fairly simple. A Thevenin source assumes
constant resistance/impedance). Thevenin/Norton is very useful and I have
used these models often but they are not a fundamental basis of circuit
theory as are Kirchoff's Laws (Actually Maxwell's equations are more
fundamental andKirchoff's Laws are simply the quasistatic approximation that
is the basis of circuit theory - so that where we use circuit theory- there
should be an implicit recognition of its limits).In non-linear cases, we can
use KVL and KCL but forget all the other circuit theorems because they all
depend on linearity.

What we cannot tell, with our ideal voltmeters (except on open circuit) is
the voltage across the virtual battery in the same way that we can measure
the voltage across a physical battery (and there a Thevenin (or Norton)
source can be used to model the battery assuming it is a known ideal source
in series with a constant impedance). If we have non-ideal voltmeters-
another set of problems -lets stick with ideal meters. So, in the battery
case Levin is measuring the voltage across a resistor and a battery and in
the flux case he is measuring only the resistor voltages. He could make more
money as a con-man than as a professor- but I like his style-he is trying to
get people to think!


By the way, my background is engineering- not physics.

-- Don Kelly
dhky@xxxxxxxxxxxx
remove the x to reply
John Polasek
.

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