Author Topic: Does Kirchhoff's Law Hold? Disagreeing with a Master  (Read 54007 times)

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Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #275 on: November 28, 2018, 02:59:13 pm »

I already provided my position here in this thread. That's why I just refer to article I agree to and do not repeat what is already said. Don't blame me if you did not read thread or do not remember what I did say or whatever.

Yeah, you said it clearly. And I quote.

Those who do not agree (to "KVL for the birds") say that Kirchhoff’s loop equations apply to Lewin’s circuit.

No they don't. Suppose that Kirchhoff didn't know about Faraday's law of induction. I show a loop of wire with two resistors and hide the solenoid under the table. I ask Kirchhoff to measure the voltages with a voltmeter. Kirchhoff wouldn't know how to explain how a loop of wire with two resistors and no voltage source has some voltage on them. Worse, he wouldn't know how to explain why the voltmeter shows diferent voltages depending on the position of the voltmeter.

Kirchhoff wouldn't know how to explain how two pieces of wire hanging out in the breeze (that today we call dipole antenna) suddenly have voltages and currents without no visible voltage source connected to them.

Although Faraday demonstrated the phenomenon of induction in 1831, he couldn't find a mathematical formulation and therefore his theories were rejected by the scientists of the time. Meanwhile in 1845 Kirchhoff came up with his law that do not account for any kind of varying field for that matter. In 1865, Maxwell could finally produce his now famous equations that gave a mathematical formulation to Faraday's law.

Kirchhoff died in october 1887. Hertz published his first paper confirming Maxwell's equations in november 1887.

So there is no paradox in Lewin's explanations. Kirchhoff died absolutely ignorant of the confirmation of Maxwell's theory so that's why his theory doesn't account for that. Period.

So who told you that the loop is a secondary of a transformer? Certainly not Kirchhoff. Because when that theory could finally be confirmed, he was DEAD. End of story.

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You better behave

Here's a dollar, kid. Go get yourself a better education.
 

Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #276 on: November 28, 2018, 04:26:22 pm »
Kirchhoff died absolutely ignorant of the confirmation of Maxwell's theory so that's why his theory doesn't account for that. Period.

You think that Kirchoff's circuit laws shall explain electromagnetism? - Better save your dollar to get better education yourself, kid:

Here's a dollar, kid. Go get yourself a better education.
 

Offline Siwastaja

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #277 on: November 28, 2018, 04:48:27 pm »
Here's a dollar, kid. Go get yourself a better education.

Don't you see how detrimental this attitude is to science and understanding?

IMHO, this is also the actual mistake Lewin is making.

Honest questioning and discussion should be valued.

Not everyone is as strong as a person as you or me or ogden. Not everyone wants to fight. Some may actually get upset about they way Lewin arrogantly attacks their "qualifications" when they just asked honest questions, especially if you are in the role of a student. They won't ask more questions, but instead, do as you teach them to do: close up your desire for understanding, just shut up and "learn". But, just reading isn't the way you learn science. You need to really understand, and for that, asking questions, yes, even questioning your professor - would be the right thing to do.

But I guess we'll disagree on this. And, I think you are genuinely a bad person for not even trying to think about this side of the coin - and the life goes on :).
« Last Edit: November 28, 2018, 04:55:12 pm by Siwastaja »
 
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Online Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #278 on: November 28, 2018, 06:14:27 pm »

So, you criticize Lewin because someone else wrote an article criticizing him? Not because you yourself master the fundamentals and is in a position to confront him? What kind of engineers do we want to be? Just a bunch of dilettantes ranting at random in forums? Let's shut up and do our homework. Thank you.

Okay i know there are a lot of opinions in this thread. But i personally was never trying to say that Dr. Lewin is a bad teacher. Id love to have him as my physics teacher when compared to all the teachers i have had. He does a great job of explaining things in a interesting way to help you understand it. I watched some of his other lectures and i enjoyed them.

His statements about KVL is the only part where i get to disagree with him. Nobody is saying Maxwell or Faraday is wrong, nobody is trying to prove them being wrong (You would have one hell of a time trying to do that). It just seams to me that Dr. Lewin has a different idea of what KVL actually is, this is what is leading him to the conclusion of KVL being just a special case of Faradays law with no magnetic field (Its not, you can't even stick an electric field into KVL).


No they don't. Suppose that Kirchhoff didn't know about Faraday's law of induction. I show a loop of wire with two resistors and hide the solenoid under the table. I ask Kirchhoff to measure the voltages with a voltmeter. Kirchhoff wouldn't know how to explain how a loop of wire with two resistors and no voltage source has some voltage on them. Worse, he wouldn't know how to explain why the voltmeter shows diferent voltages depending on the position of the voltmeter.

Kirchhoff wouldn't know how to explain how two pieces of wire hanging out in the breeze (that today we call dipole antenna) suddenly have voltages and currents without no visible voltage source connected to them.

Although Faraday demonstrated the phenomenon of induction in 1831, he couldn't find a mathematical formulation and therefore his theories were rejected by the scientists of the time. Meanwhile in 1845 Kirchhoff came up with his law that do not account for any kind of varying field for that matter. In 1865, Maxwell could finally produce his now famous equations that gave a mathematical formulation to Faraday's law.

Kirchhoff died in october 1887. Hertz published his first paper confirming Maxwell's equations in november 1887.

So there is no paradox in Lewin's explanations. Kirchhoff died absolutely ignorant of the confirmation of Maxwell's theory so that's why his theory doesn't account for that. Period.

So who told you that the loop is a secondary of a transformer? Certainly not Kirchhoff. Because when that theory could finally be confirmed, he was DEAD. End of story.

Kirchhoff indeed did not know about Faraday or Maxwell at the time. He came up with his laws quite a few years before them. He did not understand induction and was not even trying to. He was dealing with circuits of resistors powered by batteries. Science at the time did not understand the underlying principle of how magnetism and electricity interact, but they knew very well that they do interact and experimentally determined a lot of the rules it follows. Its only when Maxwell came around with his equations that they had a concrete mathematical explanation of how the magnetic and electric worlds interact. They knew about induction and loop area and even had equations that can calculate it before that time, but those equations simply came from applying the best fitting equation onto the experimental results. Maxwells equations turned out to fit perfectly in all of those experimentally determined equations so he knew he was onto something big (Its incredibly unlikely this would happen to fit so many equations by chance).

Maxwell didn't go "Hah suck it Faraday, your math is wrong cause mine works so much better". He was instead using the work of Faraday and many other scientists to be able to come up with his equations. He surely went trough a lot of trial and error with mental models and math before he found a set of equations that fit in. Much like other scientist did before him to make other famous equations. Science builds on top of itself.

In science we can't know what is right and what is wrong. But we can use scientific methods to find the theories that seam to fit our universe the best. Every so often a new theory comes up that explains something better and so its adopted as the new best thing. Heck for a long time we didn't even know what atoms and molecules look like, we went trough increasingly better ideas of what they are. Eventually we even figured out that something as fundamental as a electron can be taken apart into even more fundamental building blocks.

Kirchhoffs has many other contributions in other fields, but in the electrical field his contribution is used for circuit analysis, much like Thevenins or Nortons theorem. These things don't deal with electrical and magnetic fields, inches, permeability etc... They ONLY deal with volts and amps flowing trough ideal components. So Maxwells equations don't really have much to do with KVL. Where Maxwells equations touch circuit analysis laws is during circuit modeling. Maxwells equations are used deep down to turn a physical coil of wire into how many Henrys an ideal inductor model should have to act like that coil. After that step circuit analysis uses the inductor model without knowing about Maxwell even tho Maxwells equations are hidden inside that inductance value.
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #279 on: November 29, 2018, 02:16:05 pm »
Don't you see how detrimental this attitude is to science and understanding?

Now we're talking.

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IMHO, this is also the actual mistake Lewin is making.

Honest questioning and discussion should be valued.

Not everyone is as strong as a person as you or me or ogden. Not everyone wants to fight. Some may actually get upset about they way Lewin arrogantly attacks their "qualifications" when they just asked honest questions, especially if you are in the role of a student. They won't ask more questions, but instead, do as you teach them to do: close up your desire for understanding, just shut up and "learn". But, just reading isn't the way you learn science. You need to really understand, and for that, asking questions, yes, even questioning your professor - would be the right thing to do.

Unfortunately people are not questioning honestly. An honest question requires an open mind to accept the answer.

Lewin, I and numerous others on this forum and perhaps on other places spent time and effort patiently explaining that a lot of phenomena are out of reach of Kirchhoff's theory. We wrote texts, drew sketches, made videos, recommended lectures, books, etc.

What is the reaction? Cool, I didn't know about this, I am going to study?

No. The reaction was variously like I am a "practical" engineer dedicated to "high level abstractions", which is just a wanky name for "I don't know the fundamentals and I don't care" and I want you to come with an explanation that conforms to my limited understanding of physics.

Sorry, but that ain't gonna happen. It's impossible to dumb Maxwell down. You'll have to make room in your head to accept a new idea, a.k.a., learn.

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But I guess we'll disagree on this. And, I think you are genuinely a bad person for not even trying to think about this side of the coin - and the life goes on :).

Forum member rfeecs published a link to a video by Cyriel Mabilde, who said he'd prove that Lewin was "wrong". My comment then was polite.

https://www.eevblog.com/forum/chat/does-kirchhoffs-law-hold-disagreeing-with-a-master/msg1987190/#msg1987190

The fact is that that demonstration is a train wreck. Some of the errors were pointed out by rfeecs himself. Why is that?

At 33:17 Mabilde says: Invoking the theory of path dependency [i.e. Faraday/Maxwell] is not the right answer for explaining these simple measurements.

So he cannot interpret what he is measuring because he rejects the only theory capable of explaining what is going on right in front of his nose. To add insult to injury, his denial goes on: After all, spending more time and energy in a misinterpreted demonstration, I think it is time to tackle more serious problems, problems in this world, that concern all of us, such as the climate, injustice, violence, drugs, war and refugees.

He conveniently forgot to name ignorance as a world problem. Why? Because he chose ignorance. He chose not to learn. He chose to discredit Lewin instead. Because that's the easy path.

His video is a crime against humanity.

Who is being arrogant, after all? Mabilde and all those who recalcitrantly refuse to learn, or Lewin who dedicated an entire life to teaching?

This stubborn attitude is what is getting under our skin.

So don't fool yourself. You're not doing science a favor. If you really love science do as we all do: humbly learn.
 
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Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #280 on: November 29, 2018, 03:19:57 pm »
It just seams to me that Dr. Lewin has a different idea of what KVL actually is, this is what is leading him to the conclusion of KVL being just a special case of Faradays law with no magnetic field (Its not, you can't even stick an electric field into KVL).

Kirchhoff (KVL) IS a special case of Faraday.

What does Maxwell say? That the EMF along a closed line in space is a function of how the surface integral of a magnetic field across an arbitrary surface bounded by the closed line varies in relation to time.

The magnetic field can vary in intensity and direction. The surface can vary in size, direction and/or shape (including the closed line).

When that surfce integral does not vary with time, EMF is zero. And that coincides exactly with what Kirchhoff said: that the sum of the voltages around a mesh (a closed line) is zero.

In what circumstances does the surface integral equal zero? When the magnetic field AND the surface are constant, for example, among many other cases.

So Kirchhoff ended up being a subset Faraday. I even drew it as a Venn diagram many messages ago in this thread.

« Last Edit: November 29, 2018, 03:26:38 pm by bsfeechannel »
 

Online Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #281 on: November 29, 2018, 06:39:32 pm »

Kirchhoff (KVL) IS a special case of Faraday.

What does Maxwell say? That the EMF along a closed line in space is a function of how the surface integral of a magnetic field across an arbitrary surface bounded by the closed line varies in relation to time.

The magnetic field can vary in intensity and direction. The surface can vary in size, direction and/or shape (including the closed line).

When that surfce integral does not vary with time, EMF is zero. And that coincides exactly with what Kirchhoff said: that the sum of the voltages around a mesh (a closed line) is zero.

In what circumstances does the surface integral equal zero? When the magnetic field AND the surface are constant, for example, among many other cases.

Yes this is what i find to be the issue. Faraday and Kirchhoff are put into the same basket while they describe different things.

Here is a definition of Faradays law:
"The electromotive force induced in a circuit by variation of the magnetic flux through the circuit is proportional to the negative of the time rate of change of the magnetic linkage"

Here is a definition of Kirchhoff voltage law:
"The algebraic sum of all the voltages around any closed loop in a circuit is equal to zero"

So Faradays law only talks about the induced EMF, it does not say anything about other voltages (Like voltage drops on components or batteries pushing voltage). This is very useful because we can use it to calculate the induced EMF. Kirchhoff on the other hand does talk about the voltages summing to zero. Notice that it says "all the voltages" so this implies induced EMF too as that's a voltage inside the loop. Also notice that it says "algebraic sum", this infers that the circuit mentioned afterwards is a circuit mesh model rather than a real circuit. Had it been a real circuit means that he would be using an integral of electric fields around the circuit, rather than just summing together voltages on components.

So if "all the voltages" in Kirchhoff law includes induced EMF voltage then KVL needs to know how big this voltage is. And how do we calculate that? Well we use Faradays law of course.

In a circuit mesh schematic a wire has zero length, zero resistance and zero reactance. These wires are immune to all field effects. Building the two resistor circuit with such wires in the real world results in a circuit that must have a circumference of zero, this means it also must have a loop area of zero, zero area means no induced EMF and KVL works. This however has a problem because Dr. Lewins circuit clearly has a loop area larger than zero so it makes the circuit mesh model act different(You get 0V on both resistors). We fix this by replacing out ideal wires with inductors. This plugs in Faradays law and tells it about how big the loop area is. Now the circuit model does get voltage induced in it and KVL still appears to work.

YES I KNOW that Kirchhoffs voltage law does indeed break in certain special cases, i'm not saying it always works. I know why it breaks in those special cases. But Dr. Lewins experimental circuit is not one of these special cases since i could throw together a equivalent lumped model in 5 minutes, it worked exactly like his experiment first try.

Can you explain in what way is it a special case of Faradays law, if the two laws describe two separate concepts?


So Kirchhoff ended up being a subset Faraday. I even drew it as a Venn diagram many messages ago in this thread.



That diagram i fully agree with. Kircchhoffs laws are an abstracted application of Maxwells equations and are meant to be only used on lumped circuit meshes.

Dr. Lewin is not applying KVL to a lumped circuit hence why it does not work for him. Works perfectly fine for me when i apply KVL to my lumped equivalent version of his circuit. Is my lumped circuit model wrong in some way? (I always welcome constructive criticism)
 
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Offline radioactive

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #282 on: November 30, 2018, 01:38:36 am »
I've updated the openEMS FDTD simulation (3D field solver for Maxwell's equations).   More turns to the coil,  Added Perfect-Matched-Layer (8 levels) to the boundary conditions.  Added two more voltage probes right next to each other at points approximately 90 degrees around the loop from the resistors.  I did some experimentation with a stepped pulse instead of the nice Gaussian, but still working on that.

I really don't see how Kirchhoff's law could possibly be used to analyze something like this (and truly be successful).  Not saying that to be controversial,  I just don't see how KVL could be useful for much other than static DC analysis and this circuit is definitely not that.   

[edit] removed sim files.  I'm an idiot.  I'll post a good sim when I have it ready.
« Last Edit: December 05, 2018, 11:10:26 am by radioactive »
 

Online Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #283 on: November 30, 2018, 02:58:59 am »
Are you perhaps using a current source to power the dense coil in the middle?

The parasitic capacitance between the turns and the inductance of the coil are likely forming a parallel LC tank circuit. Parallel tank circuits are easily exited into oscillation by a current source due to the current source providing freedom to the voltage to swing as much as it wants. Similar thing happens if a voltage source step powers a series LC tank.

To test out my theory i replaced the voltage source in my LT Spice model with a current source and assigned the solenoid coil some parasitic capacitance. And yes this does appear to be the case. Try powering the inner coil with a voltage step instead.
 
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Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #284 on: November 30, 2018, 03:04:23 am »
I did some experimentation with a stepped pulse instead of the nice Gaussian, but still working on that.

You shall try to make pulse which results in waveforms demonstrated by Dr.Lewin in his SUPER DEMO. Also voltages on resistors (< 1mV) are way too small - does not match those seen in SUPER DEMO.

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I really don't see how Kirchhoff's law could possibly be used to analyze something like this (and truly be successful).

Of course! Kirchhoff's law can't be used to analyze electromagnetism :) As your results do not look like those shown by Dr.Lewin, shall I conclude that Maxwell's equations can't be used as well? [kidding] :D
 
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Offline radioactive

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #285 on: November 30, 2018, 03:59:52 am »
I'll have to work on this more later, but here is some output from an attempt at using a stepped voltage source  (vs. Gaussian current source).   I will see if there is a way to scale things on the voltages, but I think is pretty standard for excitation pulses to be 1 Volt in these sims.  For now, I have just been looking at how one might come to the conclusion that loop voltages add up to zero.  I thought maybe there was something special about this configuration.  I'm not particularly impressed by the triggered scope captures, but would be nice to see something close to that.  This output might come closer to that depending on what point in time / voltage trigger level you look at I guess.

Anyway,  I probably have it screwed up.  Let me know if somebody knows how to fix this sim up.   I have used openEMS for designing UHF microstrip planar BPF filters in the past (that work),  so I have confidence in the solver itself.

[edit]  I did have it all screwed up.  I'll post a better sim when I have it ready.
« Last Edit: December 05, 2018, 11:11:02 am by radioactive »
 

Online Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #286 on: November 30, 2018, 04:17:57 am »
Yeah the image of those scopes is a bit fuzzy in the video but i got pretty much the same results when recreating the experiment myself:
https://www.eevblog.com/forum/chat/does-kirchhoffs-law-hold-disagreeing-with-a-master/msg1987562/#msg1987562

The purple trace at the bottom is the voltage on the solenoid coil, i was using that as a stable trigger signal.

So you need to apply a similar voltage step response across your solenoid coil (Not just a pulse). Also your time scale appears to be very short in the simulation. The pulse you applied seams to last only a few picoseconds, this gives it a bandwidth of >100GHz and hence why you get funny behavior as you are mostly simulating radio waves traveling around your scene. The whole simulation only lasting what appear to be around half a nanosecond. My experiment had the pulse last 500 microseconds so about 1 000 000 times longer than your simulation time.

I don't have any experience with that EM simulation tool but it does look pretty cool.
 

Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #287 on: November 30, 2018, 05:11:08 am »
So you need to apply a similar voltage step response across your solenoid coil (Not just a pulse).

Right. Lewin uses just positive step. If possible, magnetic core shall be added as well.

I think is pretty standard for excitation pulses to be 1 Volt in these sims.

0dBV for RF PCB simulations is good default choice, but this is solenoid :) We know that EMF of experiment is 0.1V or so.
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #288 on: November 30, 2018, 07:45:17 am »
Yes this is what i find to be the issue. Faraday and Kirchhoff are put into the same basket while they describe different things.

Kirchhoff lives inside Faraday. So, if you put Faraday in a basket, Kirchhoff will be there too. The guy who realized that was Maxwell.

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Here is a definition of Faradays law:
"The electromotive force induced in a circuit by variation of the magnetic flux through the circuit is proportional to the negative of the time rate of change of the magnetic linkage"

Here is a definition of Kirchhoff voltage law:
"The algebraic sum of all the voltages around any closed loop in a circuit is equal to zero"

What happens if your EMF is ZERO? Doesn't that ring you a bell? Where did you see ZERO before?

Ahh! KIRCHHOFF! He says that "The algebraic sum of all the voltages around any closed loop in a circuit is equal to ZERO"!!!!!!!

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Kirchhoff on the other hand does talk about the voltages summing to zero. Notice that it says "all the voltages" so this implies induced EMF too as that's a voltage inside the loop.

Here is where your problem lies. When Kirchhoff says all the voltages, he is not considering any kind of EMF. In Kirchhoff's wonderful world there is absolutely NO EMF whatso-fluxing-ever!

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In a circuit mesh schematic a wire has zero length, zero resistance and zero reactance. These wires are immune to all field effects. Building the two resistor circuit with such wires in the real world results in a circuit that must have a circumference of zero, this means it also must have a loop area of zero, zero area means no induced EMF and KVL works.

Philosophical question: if Kirchhoff only applies to circuits that cannot exist, why do we need his stupid theory?

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This however has a problem because Dr. Lewins circuit clearly has a loop area larger than zero so it makes the circuit mesh model act different(You get 0V on both resistors).  We fix this by replacing out ideal wires with inductors.

If, however, you have a constant magnetic flux, Kirchhoff holds. So Lewin has no problem with Kirchhoff.

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This plugs in Faradays law and tells it about how big the loop area is.

Faraday is not a plug-in, it is the chassis. It is the base upon which Kirchhoff stands. There's no Kirchhoff without Faraday.

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YES I KNOW that Kirchhoffs voltage law does indeed break in certain special cases, i'm not saying it always works.

Kirchhoff's law breaks MOST of the cases. Faraday is the rule. Kirchhoff is the exception.

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Can you explain in what way is it a special case of Faradays law, if the two laws describe two separate concepts?

We've done it several times along this thread.

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That diagram i fully agree with. Kircchhoffs laws are an abstracted application of Maxwells equations and are meant to be only used on lumped circuit meshes.

You do not only have a problem with Kirchhoff, Maxwell and Lewin. You also have a problem with Georg Cantor.

Between you and me. Forget all you know about circuits. Follow the guidelines I published in a previous message. Study calculus, vector analysis and a good book on electromagnetism. You'd be better off than struggling with theories you do not master.
 

Offline HackedFridgeMagnet

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #289 on: November 30, 2018, 09:37:34 am »
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Since the mid-20th century, it has been understood that Maxwell's equations are not exact, but a classical limit of the fundamental theory of quantum electrodynamics.

from https://en.wikipedia.org/wiki/Maxwell%27s_equations#Limitations_for_a_theory_of_electromagnetism

you might have to fix your Venn diagram.

Does that mean Maxwell "is for the birds"?
 

Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #290 on: November 30, 2018, 11:43:22 am »
When Kirchhoff says all the voltages, he is not considering any kind of EMF.

WTF are you talking about here?  :-//

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If, however, you have a constant magnetic flux, Kirchhoff holds. So Lewin has no problem with Kirchhoff.

Try to wrap discrete time measurement into your head. Levin even uses term "voltage at given time" and demonstrates experiment using oscilloscopes. Obviously "at given time" - during infinitely small time moment of observation, magnetic flux does not change. You just said that in such case Kirchhoff holds. That' it, case closed.
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #291 on: November 30, 2018, 12:06:14 pm »
Quote
Since the mid-20th century, it has been understood that Maxwell's equations are not exact, but a classical limit of the fundamental theory of quantum electrodynamics.

from https://en.wikipedia.org/wiki/Maxwell%27s_equations#Limitations_for_a_theory_of_electromagnetism

you might have to fix your Venn diagram.

If we are restricted to classical physics, which is the physics of circuit analysis, that won't be necessary. But if we consider QED, which applies to phenomena at the subatomic scale, that won't change the status of Kirchhoff as being owned by Faraday.

To really understand the limits of Maxwell, you'll have to study quantum mechanics. But do you believe that the people who are lazy enough to study Maxwell will have the cojones to study QM? I doubt it.

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Does that mean Maxwell "is for the birds"?

I don't know. Do birds study calculus, vector analyses and electromagnetism?
 

Online Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #292 on: November 30, 2018, 09:31:20 pm »
That depends on the symmetry of the induced field. Let me give you my take on that paper (who appears to be a draft considering there is at least a minor error and a (?) meaning what, exactly? Do you know if this was ever published and where is the definitive version?)
Well, it's interesting but it is nonetheless amazing that to compute the emf on an open path, the area always pops out.

Well this paper is what i could find on the topic freely accessible on the internet. Flowing sources and searching on the topic shows more work on the topic but most of it is locked behind paywalls (The usual thing with scientific publications). I'm not a university student anymore to have access to those for free. Feel free to dig deeper into it if you want.

It makes sense to me that the geometric origin of the field would be important since that's the single point where nothing happens to the magnetic field lines as the flux changes. The integral of an area just happens to be a good way to capture the fact that a combination of two spatial dimensions affect the result. Since both Faradays law and a wire segment require it they both use an area (And besides its both magnetic induction so you expect something similar to happen). Area integrals pop up a lot in electronics just because how useful they are.

But yeah the question weather there is voltage on the open loop of wire depends on how you look at it. The EMF will balance out with the electric field of charge separation so technically the voltage is zero but there are more electrons on one end than the other. Circuit analysis makes use of this concept for making wire segments in the form of inductors.


What happens if your EMF is ZERO? Doesn't that ring you a bell? Where did you see ZERO before?

Ahh! KIRCHHOFF! He says that "The algebraic sum of all the voltages around any closed loop in a circuit is equal to ZERO"!!!!!!!

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Kirchhoff on the other hand does talk about the voltages summing to zero. Notice that it says "all the voltages" so this implies induced EMF too as that's a voltage inside the loop.

Here is where your problem lies. When Kirchhoff says all the voltages, he is not considering any kind of EMF. In Kirchhoff's wonderful world there is absolutely NO EMF whatso-fluxing-ever!

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In a circuit mesh schematic a wire has zero length, zero resistance and zero reactance. These wires are immune to all field effects. Building the two resistor circuit with such wires in the real world results in a circuit that must have a circumference of zero, this means it also must have a loop area of zero, zero area means no induced EMF and KVL works.

Philosophical question: if Kirchhoff only applies to circuits that cannot exist, why do we need his stupid theory?

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That diagram i fully agree with. Kircchhoffs laws are an abstracted application of Maxwells equations and are meant to be only used on lumped circuit meshes.

You do not only have a problem with Kirchhoff, Maxwell and Lewin. You also have a problem with Georg Cantor.

Between you and me. Forget all you know about circuits. Follow the guidelines I published in a previous message. Study calculus, vector analysis and a good book on electromagnetism. You'd be better off than struggling with theories you do not master.

Well Faradays law says that with no flux change the induced EMF voltage equals zero. There are other voltages possible in circuits that are not coming from induced EMF, batteries also produce voltage that is NOT induced. So saying that the induced EMF is zero does not automatically mean the sum of all voltages is zero as induced EMF is not the only voltage possible in a circuit

Kirchhoffs laws are a circuit analysis tool, not a law that governs how the universe works. Circuit analysis is just an practical application of physics, while physics is a practical application of mathematics.

Physics has no use for Kirchhoffs law since it doesn't deal with anything physical. Maxwells equations and the things that come from them (like for example Faradays law) are completely sufficient to explain what is going on. Okay we did discover even more fundamental quantum effects a layer deeper than Maxwell explains, but Maxwell still works perfectly on a macroscopic level so that's good enough, we can't say its wrong because of that (Just slightly abstracted that's all)

So then why do we even need Kirchhoffs law anyway if it appears to be useless in physics? Heck why do we even need circuit analysis if physics can already describe anything electrical? Lets just use physics instead!

Well.. we could. The problem is that calculating all of this for a real physical circuit involving only a few components would already result in a LOT of math. Engineers regularly deal with circuits that involve 10 to 1000 components, not just 3, this causes the math complexity to skyrocket and it simply becomes unpractical to calculate. Have a look at how hard EM simulations are on computers. They take a ton of memory and take a significant time to compute even on a modern PC.

Turns out engineers often have to calculate the behavior of circuits as part of there work. When they wonder how a RC low pass filter circuit acts at audio frequencies they simply don't care what happens with magnetic and electric fields around there circuit. To solve this problem the science of circuit analysis was created. This cherry picks the effects from physics that matter and condenses them down into a simpler form of math.

The engineer can now chose if he wants to take in account the magnetic fields or not, rather than it simply being part of the process that must be included for everything to function. Our universe breaks down if you suddenly have magnetic fields disappear, circuit mesh models keep working. If the engineer wants to ignore magnetic effects they simply leave them out and the circuit continues to work as if they are not there. However the circuit now behaves as if it is made out of these mythical ideal wires. In most cases this results in identical circuit behavior. If these effects cause a significant difference in behavior(Such as Dr. Lewins circuit) then the engineer must realize this and chose to add them. In this case this is done by adding an equivalent model of the wire in the form of inductor. The inductor model then calls upon Maxwell to only solve that single wire, this is quick to do because we have generalized easybake equations already prepared from Maxwell.

So KVL only interacts with Faradays law when circuit mesh modeling deems it necessary. Its all just part of a elaborate mathematical shortcut that we call "circuit analysis". Engineers use it to calculate stuff faster and you apply it outside of that there is no grantee it will work. It might still work in special cases outside of circuit meshes but that's just a special case, not intended use.
« Last Edit: November 30, 2018, 09:35:20 pm by Berni »
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #293 on: November 30, 2018, 10:57:00 pm »
Physics has no use for Kirchhoffs law since it doesn't deal with anything physical.

Kirchhoff, one of the most important physicists of the 19th century, must be rolling over in his grave.

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The problem is that calculating all of this for a real physical circuit involving only a few components would already result in a LOT of math.

You don't get it. You have an impressive equipment on your bench all of it making extensive use of Maxwell and yet you have no clue about the theory used to design it.

In Kirchhoff's world you wouldn't have coaxial cables, ground planes, canned circuits, EMI certification, impedance matching, delay lines, coupling, decoupling, transformers, inductors, motors, generators, radiation, etc., etc., etc.

Kirchhoff is from a time when the only source of electrical energy widely available were batteries. There were no changing fields. No transients. No kind of interference. Only well-behaved DC circuits. Today, even toys using batteries have SMPSs with inductors and transformers, these specifically making use of Faraday's law.

So you can't escape Maxwell. It's everywhere these days. When we say Kirchhoff, we are in reality saying Maxwell, or Faraday in case of induction, under certain VERY special conditions. But it is still Maxwell.

Don't fool yourself or you'll end up like Cyriel Mabilde: an advocate of ignorance.

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So KVL only interacts with Faradays law when circuit mesh modeling deems it necessary.

So you're saying that on your planet the laws of Nature obey the desires of the engineer? I'm moving there right now.
« Last Edit: November 30, 2018, 11:14:36 pm by bsfeechannel »
 
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Online Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #294 on: December 01, 2018, 01:07:33 am »
Physics has no use for Kirchhoffs law since it doesn't deal with anything physical.

Kirchhoff, one of the most important physicists of the 19th century, must be rolling over in his grave.

Sorry if i worded that a bit too broadly. I did not mean to include any of Kirchhoffs other laws! Those are indeed very important for physics.

I meant to say that only for Kirchhoffs circuit laws. Maxwells equations can already describe the behavior found in Kirchhoffs cirucit laws, so physics doesn't really need KVL and KCL. In that way i guess i could say that Dr. Lewin could say that KVL is "for the birds"

But for circuit analysis really does need KVL and KCL to work.


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The problem is that calculating all of this for a real physical circuit involving only a few components would already result in a LOT of math.

You don't get it. You have an impressive equipment on your bench all of it making extensive use of Maxwell and yet you have no clue about the theory used to design it.

In Kirchhoff's world you wouldn't have coaxial cables, ground planes, canned circuits, EMI certification, impedance matching, delay lines, coupling, decoupling, transformers, inductors, motors, generators, radiation, etc., etc., etc.

Kirchhoff is from a time when the only source of electrical energy widely available were batteries. There were no changing fields. No transients. No kind of interference. Only well-behaved DC circuits. Today, even toys using batteries have SMPSs with inductors and transformers, these specifically making use of Faraday's law.

So you can't escape Maxwell. It's everywhere these days. When we say Kirchhoff, we are in reality saying Maxwell, or Faraday in case of induction, under certain VERY special conditions. But it is still Maxwell.

Don't fool yourself or you'll end up like Cyriel Mabilde: an advocate of ignorance.


I was never trying to say that Maxwell is useless. I was going among the lines that its rare that you absolutely need to get down and dirty with electric and magnetic fields. Yes Kirchhoff is not going to do much in helping you understand how a coax cable works, but for 99% of use cases i can just think of a coax cable as a delay line with some attenuation and characteristic impedance, wack that into the equivalent circuit model and done.

Why doesn't everyone write software in a hex editor? Its the most fundamental way of doing it after all. Its just simply more practical to work on a higher level with a compiler. Yes you are missing out on some fine details with all these high level language abstractions, but in 99% of cases it doesn't matter and it does get the job done significantly faster.

Not saying one should ignore the underlying physics. One should indeed understand how it works underneath for the sake of the big picture. I was making an example of why circuit analysis is useful in order to justify why one might want to use KVL rather than the more the accurate alternatives (the answer is deadlines). This is my argument for why KVL is not "for the birds"

Im not trying to get away from Maxwell. His equations do an amazing job in explaining how electricity works. Just showing where KVL fits in to it all.

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So KVL only interacts with Faradays law when circuit mesh modeling deems it necessary.

So you're saying that on your planet the laws of Nature obey the desires of the engineer? I'm moving there right now.

Nah sadly the same stubborn laws of nature apply to the planet i live on. But in the world of mesh circuits they do indeed obey the desires of the engineer, so far no airline is offering one way trips into that world so i guess we are out of luck. But at least the bent laws help things calculate faster in my real world, the trick is bending them just right so that it acts almost the same as in the real world while only taking 1% of the math to get there. Bending these laws just this right way is the responsibility of the engineer.

Applying KVL without modeling the effects of wires to Dr Lewins experimental circuit is an example of bending the "circuit mesh world" laws too far, creating a circuit that behaves significantly different compared to real life where all the laws apply all the time.

 

Offline zucca

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #295 on: December 01, 2018, 01:33:16 am »
If I believe that in two same points there will be the same voltage ALWAYS (I cut the part with the two identical multimeters) regardless the "path", does it implies I do not undesrtand Mr. Maxwell?

I don't get it. Sorry I am an idiot.
« Last Edit: December 01, 2018, 01:37:47 am by zucca »
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Offline rfeecs

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #296 on: December 01, 2018, 05:43:05 am »
Looking at a circuit theory textbook, "Basic Circuit Theory" by Desoer and Kuh, they devote the first 10 pages of a 900 page book to "Lumped circuits and Kirchoff's laws".  They state:

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KVL applies to any lumped electric circuit; it does not matter whether the circuit elements are linear, nonlinear, active, passive, time-varying, time-invariant, etc.  In other words, KVL is independent of the nature of the elements.

You can't leave out the word lumped in that statement!  They also point out that the entire book only applies to lumped circuits.

Yes, you can make a lumped circuit model for just about anything.  You can even model a circuit with an antenna by replacing the antenna with a lumped element that represents it's terminal impedance.

But for this example, why bother?  For a circular magnetic flux region, you can easily calculate the electric field.  It goes around in circles:

If you insert the loop of wire with resistors into the electric field loop, you will get a current.  Also, the charges in the wire will arrange themselves to cancel the electric field in the wire.  (this also would happen to an unconnected segment of wire in the electric field).  So the wire has no (tangential) electric field inside it or at it's surface, but it does have a charge distribution.

Some of these papers are just defining the voltage across a section of the wire as the electric scalar potential.  This is saying: take the electric field caused by the charge distribution only and integrate it along the path of the wire.  Call this the "voltage".  Here's another paper by Kirk McDonald where he is doing this:
http://www.hep.princeton.edu/~mcdonald/examples/volt.pdf

So they are making up an imaginary voltage that would be created by the charge distribution in the wire.  But actually there is no field there, because it is cancelled by the induced electric field.  So you can't measure that "voltage" unless you could freeze the charge distribution and place it outside the field.  Or if you could form a probing loop that is unaffected by the field, then you could measure the effect of that charge distribution because it would cause a field in your meter.  I suppose this is what Cyriel Mabilde is trying to do.
« Last Edit: December 01, 2018, 08:27:32 am by rfeecs »
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #297 on: December 01, 2018, 11:38:27 am »
so physics doesn't really need KVL and KCL.

And I thought KVL and KCL were descriptions of physical phenomena. My life is a lie.

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Why doesn't everyone write software in a hex editor? Its the most fundamental way of doing it after all. Its just simply more practical to work on a higher level with a compiler. Yes you are missing out on some fine details with all these high level language abstractions, but in 99% of cases it doesn't matter and it does get the job done significantly faster.

Abstraction and neglect of the fundamentals are orthogonal things.

Abstraction means to take away from the designer tasks and routines that are necessary but will not have any kind of influence on the result. So, if you have a menial task to do, you hand it down to a computer, or another person, or company, or the compiler, etc., so that you can concentrate on the specifics of your project. However this doesn't remove from the designer the responsibility to have full mastery of the fundamental concepts of his field.

If abstraction were the same as neglect of the fundamentals, any computer science or software engineering course will be just a class on some stupid "high level" language and nothing more.

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I was making an example of why circuit analysis is useful in order to justify why one might want to use KVL rather than the more the accurate alternatives (the answer is deadlines).

So. You have, say, a radar to design. You have a tight deadline. You turn to your colleague and say "Bugger that! Let's use Kirchhoff and go home."

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But at least the bent laws help things calculate faster in my real world, the trick is bending them just right so that it acts almost the same as in the real world while only taking 1% of the math to get there.

The math required for the work with Kirchhoff requires a decade or so of learning. You do not learn Kirchhoff in the first grade, do you? If the math required for Kirchhoff were so simple, you wouldn't have spice programs out there to solve them for you.

The  basic math required to work with Maxwell requires less than 100 hours of study. And if the equations get really complex, you also have proper software to deal with them.

So, never try anymore to hide your lack of knowledge behind excuses like that. Convince yourself and others of the need to be ready to learn something new every day.
« Last Edit: December 01, 2018, 11:48:10 am by bsfeechannel »
 
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Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #298 on: December 01, 2018, 01:25:51 pm »
So, never try anymore to hide your lack of knowledge behind excuses like that.

You are troll. Shame on you and respect to Berni.
 
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Offline radioactive

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #299 on: December 01, 2018, 02:48:30 pm »
[edit]
shouldn't have posted.
« Last Edit: December 01, 2018, 02:59:40 pm by radioactive »
 


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