### Author Topic: Does Kirchhoff's Law Hold? Disagreeing with a Master  (Read 61078 times)

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#### GeorgeOfTheJungle

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #200 on: November 21, 2018, 11:21:45 pm »
Seems to me it's simple to resolve, put the meter HALFWAY and see what you read...
When you do that, what are you supposed to read? Because there's two different resistors in parallel that are in series too, with the same current going through both... I*R1? -I*R2?
If two meters that are part of super experiment circuit shows 0.1V and 0.9V accordingly, then such "halfway" meter shall read 0.4V.

It's -0.1V and 0.9V.

Quote
It is discussed/explained in this thread many times by the way

Oh, oh, ok. This, but with only one voltmeter in the center:

Thanks.
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#### ogden

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #201 on: November 22, 2018, 12:17:33 am »
It's -0.1V and 0.9V.

Yes. I omitted signs, sorry

Quote
Oh, oh, ok. This, but with only one voltmeter in the center:

Yes. You can refer to more detailed model shown in this post as well.

#### bsfeechannel

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #202 on: November 22, 2018, 12:46:07 am »
Yes, not only is KVL for the birds, but KCL is too!

Somebody finally said it.

#### Berni

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #203 on: November 22, 2018, 07:11:50 am »
Yep capacitors also break Kirchhoffs current law when you think about what electrons are actually doing.

But we are not in arguing about it because like Kirchhoffs voltage law we know it applies to circuit analysis where the capacitor is regarded as a black box with a large capacitance across the terminals. The reason we are arguing about KVL is that Dr. Lewin is trying to apply KVL to electric fields rather than circuits and is then blaming KVL for being wrong just because he is using it wrong.

There is a similar theoretical electrical circuit paradox involving two capacitors:

Just throwing circuit analysis math at it and analyzing what happens in the circuit when the two differently charged capacitors are shorted together seams to cause capacitors to violate themodynamics by violating the law of conservation of energy. Go read trough it, its pretty interesting.

So of course the reason why this happens is that this is a circuit that can't exist in our universe. Same thing as Dr. Lewins circuit with two resistors and a mysterious current forced trough it without having a circuit component doing the actual current pushing. It breaks the circuit abstraction by doing things that are not supposed to happen and as a result breaks the math used to analyze circuits (Like KVL). In this circuit with two capacitors we simply need to add the parasitic inductance and resistance of the wires and suddenly the circuit acts like it would in real life. It would oscillate back and forth trough the stray inductance while gradually reducing in amplitude as the energy (that we seamed to miss before) got turned into heat on the resistance.

I can see how Dr. Lewin would throw Maxwells equations and Kirchhoffs laws into the same basket. He is a physicist and has likely not done enough circuit analysis and modeling to see that KVL is not just Faradays law with a missing voltage.

I do have to admit he did make me look at voltage is a bit of a different light. I think should continue to do this lecture and demonstration as it is quite dramatic. BUT he should not go to a conclusion of "Kirchhoff is for the birds". Instead he should explain inside the same lecture that Kirchhoffs laws are a circuit analysis tool rather than a physics equation that describes the working of the universe. Briefly explain the importance of correct circuit modeling and understanding how the math you use works. Leave it there and continue on with whatever other physics you have to teach.

#### Sredni

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #204 on: November 22, 2018, 03:08:41 pm »
The reason we are arguing about KVL is that Dr. Lewin is trying to apply KVL to electric fields rather than circuits and is then blaming KVL for being wrong just because he is using it wrong.

He is not using it wrong. And you say so, too:

Quote
Dr. Lewins circuit with two resistors and a mysterious current forced trough it without having a circuit component doing the actual current pushing. It breaks the circuit abstraction by doing things that are not supposed to happen and as a result breaks the math used to analyze circuits (Like KVL).

Yes, KVL works for lumped circuit abstraction only. When the premises for lumped circuit analysis are missing, KVL is for the birds.

Quote
He is a physicist and has likely not done enough circuit analysis and modeling to see that KVL is not just Faradays law with a missing voltage.

No. In order to see KVL as Faraday's law with a missing voltage, lumped circuit assumptions need to be met. All fluxes should be confined inside the lumped components and you should not be allowed to run circles around them. It's as easy as that. No refinement of the theory (the concept of inductance used in the 'extended KVL' you mention is based on Faraday's law), no need to add parasitics, no nothing.

An engineer should know the limits of their tools and theories.
KVL is lumped circuits stuff. Circuit not lumped ----> KVL for the birds.

Edit: changed highlighted part, able->allowed
« Last Edit: November 22, 2018, 03:43:45 pm by Sredni »
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#### ogden

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #205 on: November 22, 2018, 05:59:41 pm »
Dr.Lewin's demo in lecture #16 is bird trap for parrots, not KVL.

#### Berni

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #206 on: November 22, 2018, 07:30:23 pm »
He is not using it wrong. And you say so, too:

Can you show me a source that claims that KVL is allowed to be used outside of lumped cirucit schematics? And if it does use it as a simplified version of Faradays law does it ever say its valid in a non DC scenario?

Yes, KVL works for lumped circuit abstraction only. When the premises for lumped circuit analysis are missing, KVL is for the birds.

Yep exactly. Without lumped circuits KVL does not exist as it contains a summa operator () that operates on voltages across components. Without lumped circuits there are no components to put inside the equation (You can't strictly define down to the atom level a point in space where a resistor begins and where it ends). If you are operating on real life components made out of actual atoms you need the integral operator found inside Faradays law because real components have physical size. When you see an integral inside what looks like KVL, that is just a unfinished derivation from a special case of Faradays law, this is not KVL.

In the same way Faradays law does not exist in lumped circuits as the integral takes in physical dimensions that do not exist in lumped circuits, trying to apply it there would also make it appear broken, much like KVL appears broken in the real world. That would be a case of using Faradays law wrong.

No. In order to see KVL as Faraday's law with a missing voltage, lumped circuit assumptions need to be met. All fluxes should be confined inside the lumped components and you should not be allowed to run circles around them. It's as easy as that. No refinement of the theory (the concept of inductance used in the 'extended KVL' you mention is based on Faraday's law), no need to add parasitics, no nothing.

An engineer should know the limits of their tools and theories.
KVL is lumped circuits stuff. Circuit not lumped ----> KVL for the birds.

Edit: changed highlighted part, able->allowed

Can you explain why using a lumped circuit is bad?

#### GeorgeOfTheJungle

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #207 on: November 22, 2018, 08:50:41 pm »
And what can electroboom say in reply, now?
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#### Sredni

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #208 on: November 22, 2018, 09:09:44 pm »
And what can electroboom say in reply, now?

He could say he was looking for a particular set of potential differences among the infinitely many you can find inside a varying flux region. If you look at the symmetry of the problem, a uniformly variable flux directed along the z axis will generate a non conservative induced E field tangential to concentric circumferences. If you choose radial paths to reach any couple of points A and B on the circuit circumference, the contribute of the induced field will be non zero only on the arc. This will give you a nice set of potentials that end up being additive and (possibly) uniquely defined. This is essentially what Cyriel Mabilde has done.

EDIT: actually I have yet to check the E field goes around in concentric circles, but seems reasonable.

(note, that this is not equivalent to intercept equal and opposite fluxes in both the meshes the disk has been partitioned into - it's quite the opposite: intercepting different fluxes so as to give a nice set of values in a way to give VAC = VAB+VBC).

BUT, and it's a big butt-- I mean but, you can measure them only by specifying a particular class of paths. Confirming what Faraday has always said: the voltage depends on the path.

(I suppose that by offsetting the circuit with R1 and R2 with respect to the main coil you should make your probes exit the circle not from the center of the circuit but from the center of the coil).

« Last Edit: November 22, 2018, 09:24:28 pm by Sredni »
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#### Sredni

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #209 on: November 22, 2018, 09:45:28 pm »
Can you show me a source that claims that KVL is allowed to be used outside of lumped cirucit schematics? And if it does use it as a simplified version of Faradays law does it ever say its valid in a non DC scenario?

Lewin is not using KVL wrong, because he is not using it. He is using Faraday. Watch his video "Science and believing..." and follow the mesh analysis. You might have been confused by the fact that he drew what appears to be a circuit with lumped elements (and in fact, many posts ago, I said "I am not calling it schematics on purpose") but that curly arrow magnetic field representation inside the central mesh is telling it all. He is using Faraday.
And, in fact, he is getting path depending voltages.
No incongruences whatsoever.

And why is he using Faraday and not KVL?
Because he is analyzing a circuit that is NOT LUMPED. He is INSIDE the frigging coil!!!
If you insist in using KVL, for example at the mesh with the two voltemeters, you find and impossible result.
Impossible for Kirchhoff, but not for Faraday.

As for the rest of your post, I sense confusion. I don't know how to say it. KVL works only in the lumped component assumption. Real world components like inductors, coupled coils and transformers can be modeled with lumped components AS LONG AS YOU DO NOT MESS WITH THEIR FLUXES.  In this case you can mend KVL in 'generalized KVL'.
If in your circuit you are able to mess with the flux, say goodbye to KVL and 'generalized KVL'. You need to apply Faraday, or you will find inconsistent results.

Why?

Because the voltage depends on the path.

Also, you seem to mixing togheter external and internal inductance. You might want to have a look at "Fields and Waves in Communication Electronics", by Ramo, Whinnery and Van Duzer.

Edit: IIRC, Lewin used the magnetic field representation with little crosses or dots, and not the curly arrow. Either way, that's not lumped circuits language.
« Last Edit: November 23, 2018, 02:26:00 am by Sredni »
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#### ogden

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #210 on: November 22, 2018, 10:03:26 pm »
Because he is analyzing a circuit that is NOT LUMPED. He is INSIDE the frigging coil!!!

Right. Those two resistors put inside that frigging coil does not count. Dr.Lewin can ignore internal resistance of EMF source in his equations just because Dr.Lewin is always right.

#### Vtile

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #211 on: November 22, 2018, 10:09:26 pm »
He is not using it wrong. And you say so, too:

Can you show me a source that claims that KVL is allowed to be used outside of lumped cirucit schematics? And if it does use it as a simplified version of Faradays law does it ever say its valid in a non DC scenario?

I think these two were answered in the original 'quote' of Kirchoff work some trillion posts ago.

#### EEVblog

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #212 on: November 22, 2018, 10:14:34 pm »
An engineer should know the limits of their tools and theories.
KVL is lumped circuits stuff. Circuit not lumped ----> KVL for the birds.

And that is the entire reason why Dr Lewin's experiment is flawed. He's trying to use a practical lumped circuit to show how his non-lumped "inside the inductor" Faraday thinking is right.
And that's wrong, or at best misleading, even though what's he's saying and explaining is not wrong.

#### zucca

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #213 on: November 22, 2018, 10:27:02 pm »
In real life if two voltmeters connected to the same two points are reading different values, what to do then?

0) check if was not a mistake to use two instruments to perform the same measure
1) check if one of the two is broken or need calibration (or need some new fresh battery  )
2) check if there is an induced emf to the leads going to the DVMs, by moving the leads you should see a change in the readings

Can you tell me what I forgot? Thanks!

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#### bsfeechannel

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #214 on: November 22, 2018, 11:33:21 pm »
An engineer should know the limits of their tools and theories.
KVL is lumped circuits stuff. Circuit not lumped ----> KVL for the birds.

And that is the entire reason why Dr Lewin's experiment is flawed. He's trying to use a practical lumped circuit to show how his non-lumped "inside the inductor" Faraday thinking is right.
And that's wrong, or at best misleading, even though what's he's saying and explaining is not wrong.

I think what Prof. Lewin is trying to convey is that no circuit is really lumped.

The "lumpiness" comes only after you make certain assumptions, which most of the time we do implicitly and unconsciously.

If I could translate "Kirchhoff is for the birds", I would say, "Please, put Kirchhoff circuital laws aside for a moment, while I introduce you to a more fundamental theory that not only will explain a lot of phenomena that Kirchhoff can't, but will explain Kirchhoff itself".

I think that, if he came up otherwise so politely with a soothing voice, people wouldn't pay attention to his advice and would try to derive Faraday from Kirchhoff, which is a mistake, instead of the other way around.

So I think that's why he decided to add some hyperbole to his rhetoric. Not sure if it worked though.

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#### joeqsmith

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #215 on: November 23, 2018, 01:14:35 am »
I had seen Dr Lewin's  demonstration a few years ago but he did not do a good job showing his setup and I always assumed he had made a mistake.   I later saw another MIT lecture with a slight twist to the demonstration but again, I felt a bit letdown by the lack of details surrounding his setup.   I don't have a problem with what he presents on the board, well except the miss on ohms law.  lol.   Figured I would watch them all again as it had been so long.  Rare I watch EBs channel but after about 8 minutes in, I thought he did a good job with it.

I really don't have much to add.  I repeated the test using some coax.  Braid terminated at one side, core at the other.   Then made both sections symmetrical.  My mind wasn't blown.  It behaves as I would expect.  As someone else said, take into account all the loops.

There are a few things to note.  I didn't take a lot of care to try and get good numbers as I didn't think that was important.   Where both Dr Lewin and EB use a mechanical switch,  I am driving the coil and you can see both halves of the cycle.

***************

I finished Romer's paper.  Had I been aware of it at the time I had first seen Dr Lewin's demonstration, I would have understood what he was  trying to show.   I am now watching Dr Lewin's latest videos where he is going over Romer's paper.   Still he misses the opportunity to discuss the various ways the circuit could be arranged as Romer's paper covers.   I feel as if I an being expertly trolled by an 80 year old!
« Last Edit: November 23, 2018, 02:54:05 am by joeqsmith »

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#### Sredni

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #216 on: November 23, 2018, 02:22:48 am »
An engineer should know the limits of their tools and theories.
KVL is lumped circuits stuff. Circuit not lumped ----> KVL for the birds.

And that is the entire reason why Dr Lewin's experiment is flawed. He's trying to use a practical lumped circuit to show how his non-lumped "inside the inductor" Faraday thinking is right.
And that's wrong, or at best misleading, even though what's he's saying and explaining is not wrong.

I'm not sure I'm understanding this.
Where did Lewin used a "practical lumped circuit"?

In the demo at the end of the famous lecture 16
he used a physical system constituted by two resistors connected in a loop of metal wire. The resistors themselves are inside the loop, or as engineers like to call it, the coil. That alone should have alerted the student and made him think: "Oh, wait a minute... I have always seen coils made of wire only, and then the rest of the circuit attached to its terminals. This is something different".
Then he used two oscilloscopes as 'voltmeters', still in the physical world to show that they gave different readings despite being connected to the very same two points.
That should be mind blowing for a second year university student used to probe circuits that do not mess with the flux. Now, the student should have started to use the matter between the ears and ask himself "how is this possible?" and should have started questioning the assumptions made.
And that is the purpose of the demo. Stimulate thinking. Lewin did many similar thought-provoking demos in his lectures. He never gave an explanation on purpose, not to spoon-feed the students.
That's good teaching, I see no flaws here. (but that's me) [note1]

In the explanation and subsequent videos
He explained why the instruments in the real world gave that apparently impossible result. Because that circuit, for the fact of having resistors inside the coil and the instruments linking opposite flux depending on how they encircled the coil (you can't do otherwise when they are part of the coil itself EDIT: unless you want to cross the flux, which is bad as Venkman would tell you) could not be considered a lumped circuit where KVL works.
And in fact he solved it using Faraday. If you look at his 'schematic' there are symbols representing the magnetic field in the middle of the main mesh representing the metallic loop with the resistors. There is no such thing in lumped circuit theory: you draw that wiggly symbol that represents an inductance or the secondary of a transformer and you are sure the magic flux-voodoo happens inside and do not draw the crosses or dots that represents the direction of the magnetic field.
EDIT: Also, the equations he penned are Faraday's, and yes they obviously look like 'generalized KVL' but the source of emf is no longer localized in a lumped element (if you do that, you end up with single valued potentials). (end EDIT)
There is, therefore, no 'practical lumped circuit'. Neither in the real world, nor on the blackboard (or whiteboard).

If the student does not understand it, how should it be Lewin's fault?
But even if we agree Lewin should have spoon-fed his students (which are MIT students, so I guess he had somewhat higher than average expectations), the 'he did not explain it well enough' excuse could work for about 10-15 minutes. After that, well, it's no longer Lewin's fault.
And this thing has been going on for much longer than that.

(There is a silver lining, though: many people have been learning about Faraday better than ever)

(Note0: when I said 'inside the coil' I meant the loop with the two resistors. As for the primary coil - the one generating the field - it could have been substituted by a falling magnet, or a nuclear explosion...)
[Note1: I found this comment by Lewin in one of his videos
Quote
>>>asks me to find a mistake on my own>>
that is by far the best way to teach Trust me I have been teaching Physics for 58 years. If present a student on a silver plate what (s)he did wrong they will quickly forget. But if they have to put in the effort to find their mistake (after I have sent them my lectures which address their topic) they will never forget.﻿
Edit: typos (some of them) and added part on equations on board.
« Last Edit: November 23, 2018, 04:11:46 am by Sredni »
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#### @rt

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #217 on: November 23, 2018, 06:15:09 am »
So nobody posted the proper pounding yet?

“Sincere Apology” nice one. Sorry you’re wrong!

#### Berni

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #218 on: November 23, 2018, 07:19:24 am »
Lewin is not using KVL wrong, because he is not using it. He is using Faraday. Watch his video "Science and believing..." and follow the mesh analysis. You might have been confused by the fact that he drew what appears to be a circuit with lumped elements (and in fact, many posts ago, I said "I am not calling it schematics on purpose") but that curly arrow magnetic field representation inside the central mesh is telling it all. He is using Faraday.
And, in fact, he is getting path depending voltages.
No incongruences whatsoever.

And why is he using Faraday and not KVL?
Because he is analyzing a circuit that is NOT LUMPED. He is INSIDE the frigging coil!!!
If you insist in using KVL, for example at the mesh with the two voltemeters, you find and impossible result.
Impossible for Kirchhoff, but not for Faraday.

Well at some point in the video he does add up all the voltages according to KVL and points out you don't get zero. This is his reasoning for saying "its for the birds". So one of the flowing cases is happening:
a) Assuming a circuit is lumped: He is claiming the circuit drawn is an lumped equivalent circuit of the real circuit so KVL should work inside it but it does not.
b) KVL in non lumped circuit: He is wrongfully applying KVL to a non lumped circuit so it does not work because its not meant to work
c) Inaccurate lumped circuit: His lumped lumped circuit model is describing a physical circuit consisting of zero length wires that can't exist in real life

Mesh analysis is a big part of the circuit analysis toolkit and is used on lumped circuits. This is another reason why i get the feeling he is mixing lumped and real circuits as if they are the same thing. As soon as you draw a resistor symbol you are creating a lumped model of a resistor. This resistor has a physical size of zero, acts perfectly according to Ohms law and has exactly 2 strictly defined connection terminals. A real resistor has a physical size larger than zero, stops following Ohms law when taken outside of operating limits and has a volume of material representing the connection terminals rather than a infinitely small point in space.

Lumped circuits are not ignoring Maxwell. They are fully embracing it and using Maxwell in a way that hides it away under abstraction in order to make math easier because 3d space integrals are hard. Its a math shortcut.

So rather than saying KVL is wrong he should have said ether "KVL can't be applied here because this is not a lumped cirucit" or "Here is a lumped equivalent circuit that is required to apply KVL"

As for the rest of your post, I sense confusion. I don't know how to say it. KVL works only in the lumped component assumption. Real world components like inductors, coupled coils and transformers can be modeled with lumped components AS LONG AS YOU DO NOT MESS WITH THEIR FLUXES.  In this case you can mend KVL in 'generalized KVL'.
If in your circuit you are able to mess with the flux, say goodbye to KVL and 'generalized KVL'. You need to apply Faraday, or you will find inconsistent results.

Yes that is the main point of using a lumped circuit. You don't have to mess with the fluxes. All the flux interaction are calculated using Maxwell into a inductance and coupling coefficient number. Giving you inductors that act like the real one by simply multiplying its values with the correct things rather than having to calculate flux in 3d space every time. All of the flux interactions are modeled in that single coupling coefficient.

The only case where i can see lumped models of wires being a bad idea is when the wires are physically moving during the circuits operation. This would make you need to recalculate the inductance and coupling coefficients as well as the induced EMF according to Maxwell for every timestep of the calculation. Since you are calculating Maxwells equations in 3D space every time it makes sense to just use Maxwells equations directly, because the lumped circuit equivalent of that wire is no longer a math shortcut if you can only reuse the values once. So if you are calculating what happens inside a electric motor its better use Maxwell directly.

Additionally all off the shelf passive electronic components are specified in Ohms, Farads or Henrys. The manufacturer has calculated Maxwell for you (Tho more likely measured with an instrument) and created a equivalent model of the component. You can no longer stick this component in a non lumped circuit. You would need to disassemble the inductor and accurately measure the path the wire takes inside and then use that in your non lumped circuit model.

Nobody is stopping you from only analyzing circuits with non lumped models, but don't expect anyone else to do so. Most people enjoy calculating the result in a few lines of math rather than a few pages (And still get the same result).

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#### ogden

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #219 on: November 23, 2018, 07:38:54 am »
There is, therefore, no 'practical lumped circuit'. Neither in the real world, nor on the blackboard (or whiteboard).

So wrong. Indeed there is practical lumped circuit (for coil loop with resistors) - consisting of as many fragments of the coil as needed for purpose and two resistors. Actually kind of lumped approach is used in Romer's paper stating that 1/2 of the coil have 1/2 of the EMF induced. Dr.Lewin forgot about Faradays law in lecture about Faradays law It is obvious that he had "ups, I fracked up" because in the apology "To Agree or Not to Agree with the Master" video circuit he did not use superconductive wire anymore- to further smear things up and make his error(s) less obvious for those (parrots and cultists) who learn physics just by memorizing it.
« Last Edit: November 23, 2018, 01:20:12 pm by ogden »

#### sectokia

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #220 on: November 23, 2018, 08:07:38 am »
Summary:
1. Title is "my sincere apologies"
2. 90% of video is arguing against the nonexistent strawman who is apparently suggesting that Farady's law is incorrect, and Kirchoffs is always correct. Take great glee in squashing this strawman by repeating the same thing he has stated like a broken record for the past week.
3. State that anyone with a masters in electronics who suggests that the different readings are only* due to placement of the probing leads is an idiot.
4. Clarifies that the "sincere apology" is for being too blunt in laying out his 100% correct argument against the fairy tale strawman while completely brushing off the actual point of contention.

*but let's forget that I was the one to apply real oscilloscpes and their readings to a real test circuit. If we applied these real oscilloscopes to my theoretical model, we would apparently learn that [start broken record]

Super shorter summary:
"I can see the sailboat, and you can't."

Next up:
"Newton's laws are just a special case of relativity. It's a crime that we call these things laws, when it is really Newton's Loopholes."

You have to remember that Electroboom is making a really really bad claim here. Electroboom is claiming that induction in the probe wires is what causes the result.

This is easy to prove incorrect experimentally: You can use shielded grounded coxial cable for the meter probes - the volt meters still show different voltage.

They show different voltages because there is different currents in the left and right loop because of the induction in the centre loop. Lewin explains why this is with actual math and has done detailed videos of 30min+ length about this multiple times.

Mehdi makes motherhood statements, uses napkin diagrams, and refers to analogies about voltage, fields and inductions.

At the end Mehdi shows a circuit which he claims is the 'correct' way. What he actually shows is a circuit of two meter probe loops that DO have induction. So he actually changes the circuit from 3 to two loops, both of which now have induction, and he basically claims that this is the correct way to not have induction. He is totally backwards!

If you look at Mehdis answer, I bet he pulled it from somewhere else, because his solution at the end doesn't follow at all from what he had been blabbing about in the minutes leading up to it, and this is evidence by the fact he says he isn't 100% sure, makes no mention of the magnetic field nor even the position of the probes relative to the soleniod. His solution involves re-arranging circuit so there is induction in the meter probe loops, something that he said was causing the incorrect readings. Hence the only reasonable explination is: he is wrong. It it his is 'probing' that is wrong.

Sorry but Mehdi got owned here.... if he is going to do a video like that he should brush up first. As Lewin said, this is embarrassing for him.
« Last Edit: November 23, 2018, 08:29:16 am by sectokia »

#### ogden

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #221 on: November 23, 2018, 12:33:19 pm »
You have to remember that Electroboom is making a really really bad claim here. Electroboom is claiming that induction in the probe wires is what causes the result.

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Sorry but Mehdi got owned here.... if he is going to do a video like that he should brush up first. As Lewin said, this is embarrassing for him.

You just embarrassed yourself - because induced EMF in the probe wires is actual cause of "surprising" result. Dr.Lewin explains it in the video "Believing and Science are Very Different".

#### sectokia

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #222 on: November 23, 2018, 01:25:04 pm »
Fine... well I will do a full dissection of the all the mistakes Mehdi makes:

At 5:47 Mehdi claims that in a twisted wire there is induced current inbetween each twist that cancels out the one in the next twist. This is false. The cables cross over for each twist, they do not connect, so there is no induction loop in the 'circle' of each twist. The reason twisting cables stops induction is because as you twist the cable, you are flipping the area of the loop over. So the magnetic field is now effectively passing though the next twist upside down. So the magnetic field through the first twist is in one direction, and the magnetic field through the next twist is in the other direction - when you consider the plain the loop made up by the wire run. So by twisting you can make the net field through the overall loop zero - therefore no induction.

At 8:18 Mehdi measures the current 'across both resistors'. Mehdi then concludes that voltage R1 plus voltage R2 is the same as voltage across the loop. In adding the volt meter across R1 and R2, Mehdi has changed the topology of the circuit. It now has 4 loops instead of 3.  Most importantly, the meter is now in the inductive loop, not the resistors. Mehdi is oblivious that he has done this. In this case the voltage around the loop is the voltage shown on the meter. By adding the voltage meter here there is no longer any induction in the smaller loop containing the resistors, as that smaller loop does not enclose any of the magnetic field.

At 9:10 he admits that if the meter is put on the out side, re-forming the original induction loop with the resistors in it, that the voltage again goes to zero. He then asks who is correct.  Obviously it is Lewin who is correct, as Mehdi is oblivious the he changed the topology of the loops when he measured what he is calling the 'loop voltage'. With the circuit back in its proper arrangement, he measures as per Lewin. Ironically it is Mehdi who is doing 'incorrect probing' here.

At 9:27 his inability to think of the inductive loop is shown further at 9:27, when he claims to be 'measuring across the gap'. Of course he is just completing a loop with his meter.... there is no gap. Once again he is obvious to his own incorrect probing....

At 9:42 he then runs the meter the other way, so there is no loop around the magnetic field, and he reads zero. He later attributes this to 'cancelling out', but there is nothing to cancel out, as he has formed no loop around the magnetic field.

At 9:49 he claims if he makes the sense lines 'a little bit shorter' than the loop, but without crossing (or encircling) the magnetic field in soleniod, and says "I will have voltage jumps". This shouldn't happen of course because he doesn't have any loop around the magnetic field inside the soleniod. The reason he reads a bizzare wave form here is because there is a weak magnetic OUTSIDE the soleniod (imaging it come out the top stretching to infinity as it spills down and comes back in the bottom of the soleniod). The thing he is measuring at this point is actually the magnetic field 'falling' through his loop in the opposite direction as the magnetic field through the solneiod. This is why his wave form is opposite to when he just measures around the soleniod. He incorrectly attributes this to the meter probes no longer 'canceling out' properly. You can see plainly that he has made a fork shape loop, and he puts his loop in the worst possible position: over the end of the soleniod while not enlcosing the magnetic field inside the soleniod. Lewin explains how you need at least a 30cm soleniod with your loop half way down, the point of this is to ensure the magnetic field outside the soleniod is for practical purposes zero. Mehdi has a complete failure of experiment setup here. You can replicate it yourself, if you have any fork shape loop around the soleniod (but not enclosing it) you read zero. Start moving it up to the end of the soleniod and you will start getting induction in the OPPOSITE direction compared to when you enclose the soleniod.

At 10:00 to 10:27 Mehdi makes the extraordinary claim that a non-closed loop will have half V induction, cancelled out by half V the other way. He never produces a single measurement to show this is true. If his claim was correct, he could measure the Half V by putting meter across left and right sides of the 'semi circle' he shows. This is of course complete garbage. There is no loop formed around the magnetic field, so there is no induction.

At 11:18 Mehdi claims "the same voltage of the loop wire are induced on the sense line". The sense lines are on the left and right side loops, which enclose no magnetic field, so there is no induction involving them. The induction loop is just the inner loop with the resistors. He is 100% wrong about induction 'in' the sense lines.

At 12:02 the proof of Mehdi being wrong is made final when he shows a circuit claiming to show "properly" how to measure the voltage across the loop. Once again this just changes the topology of the circuit. By bringing the probes through the middle he is now creating two induction loops, the left loop and the right loop, each enclosing half the magnetic field. Only this time HIS METER PROBES ARE PART OF THE INDUCTION LOOP. So he has gone from the original Lewin circuit that had no magnetic field through the left and right loops that contain the meter probes, to a circuit with only two loops, both of which form an inductive loop by using the meter probes. Yet he claims it is Lewin who incorrectly probed

He overall main mistake is that for some reason he thinks voltage must exist across the wire portion of the induction loop. This is because he is used to thinking of circuit elements in conservative fields, which have a voltage drop across them. He out right says it himself when he claims Lewin 'forgot' the 'transformer'. He has no idea at all when it comes to non conservative fields, not even the most basic requirement of a closed loop around the magnetic field being required for induction.... Throughout his video he is completely obviously when he creates new induction loops and changes the topology of the circuit.

In all seriousness... has anyone checked this guy even is actually qualified? Is it just a show? His videos are entertaining but his video on this is absurd junk science. What is scary is the amount of people on here, and youtube, and reddit, who are siding with Mehdi...

« Last Edit: November 23, 2018, 02:28:06 pm by sectokia »

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#### Sredni

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #223 on: November 23, 2018, 02:35:24 pm »
b) KVL in non lumped circuit: He is wrongfully applying KVL to a non lumped circuit so it does not work because its not meant to work

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Mesh analysis is a big part of the circuit analysis toolkit and is used on lumped circuits. This is another reason why i get the feeling he is mixing lumped and real circuits as if they are the same thing.

I called it mesh analysis to make the sentence short. And in fact he is analyzing the meshes, applying Faraday. Where's the big difference with lumped circuit analysis? That the emf is not localized. This is best seen when you bring the voltmeter inside the loop and you probe the points on a diameter. Now, if you use Faraday and the emf is not localized, the emf contribute appears in BOTH sub-meshes, proportional to the area encircled by the sub-mesh. If you localize it as if it were a lumped circuit, the emf contribute appears only in one sub-mesh, no matter the area.

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As soon as you draw a resistor symbol you are creating a lumped model of a resistor.

And where is it written that that lumpiness is contagious?
Come on, what should he have done? Invent a new symbology? I've never seen that in any of the EM books I've read.
You should have enough mental flexibility to understand what is going on.

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So rather than saying KVL is wrong he should have said ether "KVL can't be applied here because this is not a lumped cirucit" or "Here is a lumped equivalent circuit that is required to apply KVL"
Good if you want your students to think in a compartmentalized way. Like technicians. But if you are forming scientists, or even engineers, you should teach them to question the limits of the tools (practical and theoretic) they use.

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Yes that is the main point of using a lumped circuit. You don't have to mess with the fluxes.
[...]
The only case where i can see lumped models of wires being a bad idea is when the wires are physically moving during the circuits operation.

Lewin's circuit IS A CASE WHERE A LUMPED MODEL IS INAPPROPRIATE.
How can this be so hard to grasp?

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Nobody is stopping you from only analyzing circuits with non lumped models, but don't expect anyone else to do so. Most people enjoy calculating the result in a few lines of math rather than a few pages (And still get the same result).

Seriously? After all these umptillion pages of discussion you still have not realized that Lewin's circuit cannot be analyzed with lumped circuit theory?
Nobody is saying that you have analyze all circuits with 'non lumped models' only. You just have to do that only when it is necessary. Such is the case of Lewin's circuit.

Oh, for the love of... Physics! :-)
All instruments lie. Usually on the bench.

#### Sredni

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##### Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #224 on: November 23, 2018, 02:43:54 pm »
You just embarrassed yourself - because induced EMF in the probe wires is actual cause of "surprising" result. Dr.Lewin explains it in the video "Believing and Science are Very Different".

ogden, I promise I will not interact with you again on this matter (I did it before when you refuse to watch the video, but then you watched it so I turned back on my steps, but now no conditions).

I will leave you with this question.

When the voltmeter is outside the loop, for example measuring 0.9 V, does it matter the area its probes are enclosing in excess of the loop area? I mean, assuming the field is all confined inside the two resistors loop, does it even matter to twist the probes? Will you read, for example, a different voltage when that excess area enclosed by the probes changes from nearly zero (twisted probes) to say, half the area of the loop, equal to the area of the loop, twice the area of the loop, ten times the area of the loop?

That's all.
Now, believe what you wish.

Edit: typos, syntax, grammar, oh Lord! Somebody get me a dictionary and a basic English course!
« Last Edit: November 23, 2018, 02:48:48 pm by Sredni »
All instruments lie. Usually on the bench.

Smf