Author Topic: Does Kirchhoff's Law Hold? Disagreeing with a Master  (Read 66567 times)

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Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #775 on: February 05, 2019, 12:56:50 am »
The fact that you say the field is contained does not mean that it is contained.
You have to contain all the lines of the magnetic field. Like in a toroidal transformer, or an M or EI transformer.

When you say that wires have zero resistance, I do not ague that. The same here - if I say that I talk about ideal transformer, you shall not try to argue that transformer is not ideal. It's stupid to make preconditions as an argument.
 

Offline Sredni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #776 on: February 05, 2019, 01:01:59 am »
No, you don't understand. It's not that there is a little leakage: the whole lot of field lines are missing! In the infinitely long solenoid you miss the 'return' lines. So it's impossible to contain the field.
You can force the field lines into a high magnetic permeability material, and I also produced a drawing,



but that system is a DIFFERENT system from the infinitely long solenoid.

I hope it is clear now.

EDIT: pluralSSSSSSSSSSS. What is it with me and all those SSSSSSSSS???
« Last Edit: February 05, 2019, 01:08:13 am by Sredni »
All instruments lie. Usually on the bench.
 

Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #777 on: February 05, 2019, 01:03:29 am »
Quote
  Mehi's video @6:42 do not measure 0V

Oh yes he does. I'm referring to the original video, the one Dave posted.

Well, you are welcome to show where Mehdi measure 0V. With timestamped screenshot & YT link.
 

Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #778 on: February 05, 2019, 01:16:21 am »
No, you don't understand. It's not that there is a little leakage: the whole lot of field lines are missing! In the infinitely long solenoid you miss the 'return' lines. So it's impossible to contain the field.
You can force the field lines into a high magnetic permeability material, and I also produced a drawing, but that systems is a DIFFERENT systems from the infinitely long solenoid.

I hope it is clear now.

LOL. This is hilarious. Precondition is transformer with contained fields, meaning ideal transformer., Question is: what is voltage of 1/4 turn if 1/1 turn gives 1V. Your answer is "No, you don't understand. It's not that there is a little leakage"  :palm:

 :-DD
 

Offline Sredni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #779 on: February 05, 2019, 01:19:05 am »
I produced a drawing.
Do the same: draw your quarter turn magnetically isolated transformer and I will answer your question.

All instruments lie. Usually on the bench.
 

Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #780 on: February 05, 2019, 01:24:14 am »
I produced a drawing.
Do the same: draw your quarter turn magnetically isolated transformer and I will answer your question.

Oh, you reduce our discussion to drawing. Why don't you ask me to play some music on flute so we can continue discussion?

[edit] You do not understand what is "ideal transformer", right?

Reminder that rules are easy and your request for drawing is sorry excuse:

I do not need to. Picture I did show is good enough. Inside two circuit loops there's magnetic field and it is specifically shown where's no magnetic filed, indicated by note "no magnetic field here" (leads of voltmeter "V2") - meaning leads of voltmeter "V2" are not influenced by magnetic field. Do you have problem to understand that circuit or what?

[edit] I repeat question - what will be voltage shown by voltmeter V2?
« Last Edit: February 05, 2019, 01:31:12 am by ogden »
 

Online bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #781 on: February 05, 2019, 02:18:13 am »
Quote
  Mehi's video @6:42 do not measure 0V

Oh yes he does. I'm referring to the original video, the one Dave posted.

Well, you are welcome to show where Mehdi measure 0V. With timestamped screenshot & YT link.

What is funny is that Mehdi and Mabilde give completely different explanations for the zero volts they're measuring. Mehdi thinks that the two halves of the loop have voltages that cancel each other, while Mabilde thinks that it is the probes that are canceling what he supposed to be measuring. A little earlier he says that the EMF becomes "invisible" to the voltmeter and he called the 0V that he's measuring the "MASKED EMF", typical pseudo-scientific terminology and talk.

And now you are denying what they measured by saying that the voltmeter should indicate 250mV.

You should demand them an explanation, not us.
 

Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #782 on: February 05, 2019, 06:06:47 am »
Well does anyone also notice that the solenoid used in Dr. Lewins experiment is significantly shorter than infinity. It is smaller than the height of a HP digitizing scope.

So then is his experiment a scam because he is not recreating the same circuit as on the blackboard or does it simply just not matter?
 

Offline Sredni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #783 on: February 05, 2019, 06:19:50 am »
Well does anyone also notice that the solenoid used in Dr. Lewins experiment is significantly shorter than infinity. It is smaller than the height of a HP digitizing scope.

So then is his experiment a scam because he is not recreating the same circuit as on the blackboard or does it simply just not matter?

HE IS NOT CLAIMING THAT THE CIRCUIT CAN BE LUMPED.

Quite the contrary.

In his case the only problem with the finite size of the solenoid is that there will be flux on the outside as well. And that can give rise to spurious readings from the external voltmeters.
All instruments lie. Usually on the bench.
 

Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #784 on: February 05, 2019, 06:32:49 am »
Never said he claimed anything about lumping, you are welcome to find me a quote for that.

Exactly as you say, his magnetic flux around the circuit is not the same as what he drew on the blackboard. And this was exactly your argument against using a lumped transformer. So in what way is the situation different for Dr. Lewins experimental setup? His experiment also doesn't doesn't exactly match the circuit drawing, yet acts exactly like the math says that drawing should act.

What does the field around his experimental circuit look like if you put the volmeters a significant distance away? (Such as 100 times the solenoid length)
 

Offline Simon

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #785 on: February 05, 2019, 07:29:39 am »
calm down children please.....
https://www.simonselectronics.co.uk/shop
Varied stock of test instruments and components including EEVblog gear and Wurth Elektronik Books.
Also, if you want to get ripped off: https://www.ebay.co.uk/usr/simons_electronics?_trksid=p2047675.l2559
 

Online bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #786 on: February 05, 2019, 04:26:48 pm »
Well does anyone also notice that the solenoid used in Dr. Lewins experiment is significantly shorter than infinity. It is smaller than the height of a HP digitizing scope.

So then is his experiment a scam because he is not recreating the same circuit as on the blackboard or does it simply just not matter?

He showed earlier in the same lecture how a solenoid is a fairly good approximation of what is on the board. He showed how the size and shape of the loop around the solenoid don't matter. Everyone who tried to replicate Lewin's experiment got the same result: there are no voltages in the wires, the resistors show two different voltages and they add up to a value different from zero.

Lewin explained the result using the consistent, simple and elegant Faraday's law, which contradicts Kirchhoff's law when you have varying magnetic fields in the circuit. Others tried various conflicting, convoluted, confusing and contradictory pseudo-scientific explanations without success.
« Last Edit: February 05, 2019, 04:28:27 pm by bsfeechannel »
 

Offline Sredni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #787 on: February 05, 2019, 04:33:17 pm »
Never said he claimed anything about lumping, you are welcome to find me a quote for that.

Exactly as you say, his magnetic flux around the circuit is not the same as what he drew on the blackboard. And this was exactly your argument against using a lumped transformer. So in what way is the situation different for Dr. Lewins experimental setup? His experiment also doesn't doesn't exactly match the circuit drawing, yet acts exactly like the math says that drawing should act.

EDIT: [snipped the technical part]

Quote from: Simon
calm down children please.....

Oh, shut up daddy, or I'll tell mom what you did with the nanny.
« Last Edit: February 13, 2019, 07:47:41 am by Sredni »
All instruments lie. Usually on the bench.
 

Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #788 on: February 05, 2019, 04:55:41 pm »
You can get rid of that unwanted flux because you are trying to model the ideal case of an infinitely long solenoid where that leakage flux is exactly zero. Or, if you prefer, you can get rid of the effects of that unwanted flux because it has no effect on the two resistor loop.

Yes, so its not the same thing but we treat it as being the same thing because it behaves identically. Just like a idealized transformer model behaves the same even tho the real life equivalent shows slightly different fields.

All parts of the circuit that get affected by the field are trapped inside the lumped transformer, so by definition it is correctly lumped. The only parts that are outside are the voltmeter and resistors. But as you said they have a size of zero and such can't have EMF generated inside of them so it does not matter that they are outside the transformer. No matter how strong of a field you apply to them they will act as if there is no field.

In the same way Dr. Lewins experimental circuit doesn't have field lines terminating at infinity, but instead right outside the coil and a lot even terminate inside the loop.

So the fact that im ignoring the fields effect on voltmeters and resistors because they can't be effected is just the same thing as Dr. Lewins experiment ignoring the extra returning field around the whole circuit. Both are different than the original drawing on the blackboard, but in both cases the minor difference has no effect on the circuit. Both are just the field closing back on itself in a different way, if not please explain why.
 

Offline Sredni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #789 on: February 05, 2019, 05:11:17 pm »
I don't know how to say it in a different way.

Picture, then

« Last Edit: February 13, 2019, 07:49:21 am by Sredni »
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Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #790 on: February 05, 2019, 05:17:48 pm »
Then explain why the results still match the experiment if its the wrong way to do it?
 

Online bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #791 on: February 05, 2019, 05:39:47 pm »
Then explain why the results still match the experiment if its the wrong way to do it?

They don't. Only the voltages across the resistors are the same. The voltages in the wires are not. In Lewin's circuit the voltages are zero. Yours have 250mV. Huge difference.
 

Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #792 on: February 05, 2019, 06:18:49 pm »
Then explain why the results still match the experiment if its the wrong way to do it?

They don't. Only the voltages across the resistors are the same. The voltages in the wires are not. In Lewin's circuit the voltages are zero. Yours have 250mV. Huge difference.

The voltages across the voltmeters also match up. So this means 4/4 components in the circuit show identical results when circuit analyzed as a lumped circuit.

To measure the 250mV across the wire section you need to connect to the section with ideal wires that ignore magnetic fields. If you do that in real life (by choosing a path with 0V EMF because we don't have these mythical ideal wires) you get the voltmeter also showing 250mV. If the wires does interact with the field then this is a different circuit and when modeled in(as extra inductors on that transformer) now can produce 0V for a loop that encloses no field. So the mesh model is behaving like the real life circuit in both cases. How does this differ from experimental results?
 

Online bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #793 on: February 05, 2019, 08:17:14 pm »
If you do that in real life (by choosing a path with 0V EMF because we don't have these mythical ideal wires) you get the voltmeter also showing 250mV.

No. You don't. Mabilde and Mehdi measured precisely 0V. And they used real life wires. Then they spent the rest of their videos (and in case of Mehdi the whole second video too) trying to cook up an excuse to justify why they couldn't measure whatever voltages different from zero.
 

Online EEVblog

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #794 on: February 05, 2019, 11:00:25 pm »
Well does anyone also notice that the solenoid used in Dr. Lewins experiment is significantly shorter than infinity. It is smaller than the height of a HP digitizing scope.
So then is his experiment a scam because he is not recreating the same circuit as on the blackboard or does it simply just not matter?

It doesn't matter to him. He's trying to make a fundamental physics point and found a way, any way, regardless if it's a good or accurate practical analogy or not, to do it. He's never really addressed this, and probably think he doesn't have to because his theory is not wrong.
« Last Edit: February 05, 2019, 11:03:55 pm by EEVblog »
 

Online SiliconWizard

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #795 on: February 05, 2019, 11:49:37 pm »
Well does anyone also notice that the solenoid used in Dr. Lewins experiment is significantly shorter than infinity. It is smaller than the height of a HP digitizing scope.
So then is his experiment a scam because he is not recreating the same circuit as on the blackboard or does it simply just not matter?

It doesn't matter to him. He's trying to make a fundamental physics point and found a way, any way, regardless if it's a good or accurate practical analogy or not, to do it. He's never really addressed this, and probably think he doesn't have to because his theory is not wrong.

I think you nailed it. Getting stuck to his (in)famous experiment and trying to criticize, explain, praise or debunk it, as many of us have done one way or another, turns out completely pointless.
You're right, it's about fundamental physics, and whereas I still think the experiment itself is flawed, and has led some of us to misinterpret his point at first, he probably couldn't care less.

I still think he's caused enough confusion to many - you just need to look at this endless thread - that his approach is pedagogically flawed. As I noted much earlier, his written courses are actually much clearer than the drama he tends to make with his oral lectures - at least IMO. But I know you have to keep your students attentive. Or at least "entertained"...

The good point is that this has raised a series of interesting questioning. And after all, if this was his intention, that's well done.
 

Online bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #796 on: February 06, 2019, 12:09:29 am »
Well does anyone also notice that the solenoid used in Dr. Lewins experiment is significantly shorter than infinity. It is smaller than the height of a HP digitizing scope.
So then is his experiment a scam because he is not recreating the same circuit as on the blackboard or does it simply just not matter?

It doesn't matter to him. He's trying to make a fundamental physics point and found a way, any way, regardless if it's a good or accurate practical analogy or not, to do it. He's never really addressed this, and probably think he doesn't have to because his theory is not wrong.

I beg to differ. If you pay attention to his lecture from the beginning, you'll see that he not only explains what approximations he is considering, but also he demonstrates them from a practical point of view.

So, Lewin didn't find just a careless way to make his point. He was careful, accurate and practical, precisely because he had faced objections before.
 

Online bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #797 on: February 06, 2019, 12:23:35 am »
I think you nailed it. Getting stuck to his (in)famous experiment and trying to criticize, explain, praise or debunk it, as many of us have done one way or another, turns out completely pointless.
You're right, it's about fundamental physics, and whereas I still think the experiment itself is flawed, and has led some of us to misinterpret his point at first, he probably couldn't care less.

I still think he's caused enough confusion to many - you just need to look at this endless thread - that his approach is pedagogically flawed. As I noted much earlier, his written courses are actually much clearer than the drama he tends to make with his oral lectures - at least IMO. But I know you have to keep your students attentive. Or at least "entertained"...

The good point is that this has raised a series of interesting questioning. And after all, if this was his intention, that's well done.

At first, that's what I thought. Lewin was right but messed things up when trying to explain it. However after, what?, three or four months discussing about the subject, reading and re-reading papers, books, analyzing the videos, etc., and even performing experiments in my lab, I came to conclusion that the one who nailed it was exactly Lewin.

He touched on highly sensitive taboo, or myth, that is the validity of Kirchhoff's laws. Those who bash him are exactly those who consider RF, or anything Maxwell related, black magic.

I, and others, on this thread managed to realize not only how removed from understanding the basic tenet of electronics many involved with it are, but how recalcitrant they are to even try to. And this is alarming.
 
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Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #798 on: February 06, 2019, 06:10:07 am »
Well does anyone also notice that the solenoid used in Dr. Lewins experiment is significantly shorter than infinity. It is smaller than the height of a HP digitizing scope.
So then is his experiment a scam because he is not recreating the same circuit as on the blackboard or does it simply just not matter?

It doesn't matter to him. He's trying to make a fundamental physics point and found a way, any way, regardless if it's a good or accurate practical analogy or not, to do it. He's never really addressed this, and probably think he doesn't have to because his theory is not wrong.

I was not trying to say Dr. Lewin is wrong with his experiment. My point was that his experiment is a good enough approximation of what is drawn the blackboard just like a lumped transformer is a good enough approximation of it too. The blackboard math, the experiment and a lumped transformer model all behave identically even tho the fine details of how the fields work inside are slightly different between all 3 of those.

The only spot i disagree with Dr. Lewin is the use of KVL on a circuit that has not been modeled to include the magnetic properties of wires. Once they are modeled everything works fine and gives identical results without any sort of paradox.
 

Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #799 on: February 06, 2019, 09:55:05 am »
He touched on highly sensitive taboo, or myth, that is the validity of Kirchhoff's laws. Those who bash him are exactly those who consider RF, or anything Maxwell related, black magic.

You are the one who do not consider RF black magic and see voltmeter leads as transmission lines, Dr.Lewin's experiment as loop antenna - for frequencies (< 300Hz) present in experiment. Right.
« Last Edit: February 06, 2019, 09:56:47 am by ogden »
 


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