Author Topic: Does Kirchhoff's Law Hold? Disagreeing with a Master  (Read 60813 times)

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Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1025 on: April 25, 2019, 07:32:54 am »
In case you insist that there is just one wire and just one current meaning no path for return current - then I say that you don't even have circuit, thus Kirchoff's Circuit Law do not apply. There is huge difference between "do not apply" and "do not hold".

Nope. Kirchhoff says clearly that the wires can be connected in an entirely arbitrary way. So there is no requirement for them to form a circuit or to provide a path for a return current.

Exactly. Kirchhoff says "wires", thus more than single wire you desperately insist on. When you have single wire - you do not have conditions to apply KCL. That's why I looked for return current which I BTW found in the "spatially distributed capacitor", yet you managed to shift goalposts and change your mantra into talk about dotted area, not whole circuit. It is like showing one body of mass to happily conclude that Newton's law of universal gravitation do not work. It's not even unscientific. It's utterly stupid.
« Last Edit: April 25, 2019, 07:49:55 am by ogden »
 
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Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1026 on: April 25, 2019, 03:26:27 pm »
Yes the source of the error is people like Dr. Lewin applying it directly to a circuit without consideration if its applicable in those CONDITIONS.

Nope. Lewin was absolutely impeccable in the "application" of the laws. He used Kirchhoff to show that it works when you do not have varying magnetic fields.

He substituted the varying magnetic field for the battery and then showed how wrong it is to apply Kirchhoff to a circuit with varying magnetic fields. You will measure different voltages, because they are now path-dependent.

And left the problem for his students to solve. The solution is Maxwell and only Maxwell.

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Fix those conditions by properly modeling the thing as a circuit mesh,

We've already proved in this thread that Lewin's circuit can't be modeled by lumped components.

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if not then forget about Kirchhoffs circuit laws and stick to Maxwell.

The problem with your reasoning is that you think that if things are connected like a circuit, then Kirchhoff MUST hold. You think that if you find a single circuit where it can't be employed to explain its behavior then the whole theory is cactus.

This is BULLSHIT.

There are circuits for which Kirchhoff holds and others for which it doesn't. By now you should have known the difference. And the difference is that if the circuit is drenched with a varying magnetic field, KVL is out. If not, then you can merrily use your KVL and your precious LTSPICE to model it.

Lewin's circuit, if you have not paid the due attention to it yet, is just a sophisticated version of Faraday's original demonstration of induction. Lewin is just reenacting the same experiment with two fancy oscilloscopes and two resistors of relative high value. But the experiment is essentially the same.

Saying Lewin is wrong is the same as denying Faraday's law.

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Okay yes i was using more the software engineering definition of abstraction layers rather than the mathematics definition of abstraction. With the mathematics definition it is indeed the other way around.

Abstraction in software is commonly thought of from the point of view of the user, but it conforms with the general definition of abstraction in which the "abstracted" software is in fact more inclusive than the special case.

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And i fully agree, all of the levels of explaining electricity work fine as long as you use them within those limitations. Hence why i never really had any problems applying KVL to Dr. Lewins experimental circuit from his lecture. Any paradox between KVL and Maxwell with that circuit is simply down to using them wrong.

We've already showed that your "modelling" just don't apply to Lewin's circuit. It introduces gross errors. If I connect the lead of MY voltmeter (not yours with 250mV in series with the probes) from one resistor to the other where a wire should be in your "model" I will measure 0.5V, whereas in Lewin's circuit this voltage is zero, and everyone measured exactly zero volts.

There can't be any paradox between KVL and Maxwell. Lewin changed the condition for the validity of KVL and invited his students to explain the new behavior of that circuit. Those that think that Kirchhoff applies to just about any circuit are trying to find each one a different explanation. While those who understand Faraday's law could explain what is going on immediately.

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I'm pretty sure QED had nothing to do with the development of the first transistors.
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Then the first working BJT device popped up by ]accident from a bunch of people trying to build a better solid state RF mixer.

Wow! Shockley, Bardeen and Bratain won the Nobel Prize by accident! Lucky bastards!

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The usual transistors can still be explained rather well with Maxwells fields pushing charged particles around.

Isn't this what essentially QED is: electrodynamics applied to subatomic particles like the electron?

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Mostly just a case of controlling where the charge carriers are rather than making use of any quantum mechanical effect.

Strange. I had the impression that electrons received and emitted photons when they change their quantic levels of energy in the electronic band structure.

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I'm pretty sure very few forum members here work in a semiconductor fab, let alone one that works with such fine feature capabilities or work on building quantum computers. Hence why very very few engineers would have a good reason to dig deeper than Maxwell, heck for 95% of cases even Kirchhoff is close enough(As long as you know about the other 5%).

I also had the impression that every electronics engineer around the world learns at least some rudimentary concepts of QED as part of their regular course which is required to understand solid state electronics. But I may be wrong.
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1027 on: April 25, 2019, 03:53:51 pm »

Exactly. Kirchhoff says "wires", thus more than single wire you desperately insist on. When you have single wire - you do not have conditions to apply KCL.

Alright. I created an ogden version of KCL fail for you. I hope it now can make its way into your everything-must be-a-circuit head.



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That's why I looked for return current which I BTW found in the "spatially distributed capacitor",

Cool! Can you connect an ammeter and measure this current flowing through a... wire?
« Last Edit: April 25, 2019, 03:55:53 pm by bsfeechannel »
 

Offline ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1028 on: April 25, 2019, 04:56:19 pm »
Alright. I created an ogden version of KCL fail for you.

Brilliant. Exactly what I was looking for. Could you sign it and put high resolution file on google drive? - So I can show what stupid ingenuity looks like.
« Last Edit: May 18, 2019, 11:39:33 pm by ogden »
 
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Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1029 on: April 25, 2019, 05:48:22 pm »
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if not then forget about Kirchhoffs circuit laws and stick to Maxwell.
The problem with your reasoning is that you think that if things are connected like a circuit, then Kirchhoff MUST hold. You think that if you find a single circuit where it can't be employed to explain its behavior then the whole theory is cactus.
This is BULLSHIT.
There are circuits for which Kirchhoff holds and others for which it doesn't. By now you should have known the difference. And the difference is that if the circuit is drenched with a varying magnetic field, KVL is out. If not, then you can merrily use your KVL and your precious LTSPICE to model it.
Lewin's circuit, if you have not paid the due attention to it yet, is just a sophisticated version of Faraday's original demonstration of induction. Lewin is just reenacting the same experiment with two fancy oscilloscopes and two resistors of relative high value. But the experiment is essentially the same.
Saying Lewin is wrong is the same as denying Faraday's law.

No i said that KVL holds in every circuit mesh model. Not every physical circuit, this is an important distinction so don't carelessly throw them in the same bag.

I'm not saying Lewin is wrong. Kirchoffs circuit laws indeed don't work in such a use case. All i did was show a method of using KVL in a way that does work in that circuit, useful in cases where you would want to apply other circuit analysis tools.


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And i fully agree, all of the levels of explaining electricity work fine as long as you use them within those limitations. Hence why i never really had any problems applying KVL to Dr. Lewins experimental circuit from his lecture. Any paradox between KVL and Maxwell with that circuit is simply down to using them wrong.
We've already showed that your "modelling" just don't apply to Lewin's circuit. It introduces gross errors. If I connect the lead of MY voltmeter (not yours with 250mV in series with the probes) from one resistor to the other where a wire should be in your "model" I will measure 0.5V, whereas in Lewin's circuit this voltage is zero, and everyone measured exactly zero volts.
There can't be any paradox between KVL and Maxwell. Lewin changed the condition for the validity of KVL and invited his students to explain the new behavior of that circuit. Those that think that Kirchhoff applies to just about any circuit are trying to find each one a different explanation. While those who understand Faraday's law could explain what is going on immediately.

I did say there is no paradox in my own post so why are you arguing for the same thing.

Oh and if you do have a voltmeter that can integrate the work needed to move an electron along any chosen path id love to see a write up on its operation. Would give you a pretty good chance at a Nobel prize even.

If you are so good at it how about showing me how to correctly analyze this simple circuit:
https://www.eevblog.com/forum/chat/does-kirchhoffs-law-hold-disagreeing-with-a-master/msg2189216/#msg2189216


Feel free to use any method you want, as long as it shows what the circuit will do once given power.


...
Wow! Shockley, Bardeen and Bratain won the Nobel Prize by accident! Lucky bastards!
...
Isn't this what essentially QED is: electrodynamics applied to subatomic particles like the electron?
...
Strange. I had the impression that electrons received and emitted photons when they change their quantic levels of energy in the electronic band structure.
...
I also had the impression that every electronics engineer around the world learns at least some rudimentary concepts of QED as part of their regular course which is required to understand solid state electronics. But I may be wrong.

The inventors of the transistor certainly did amazing work in the field of semiconductors, but just saying that making a transistor was not what they ware trying to do when they made one. Its not the only major discovery that had a little bit of luck in it.

I was certainly ever shown any QED in lectures about semiconductors, tho to be honest those ware pretty dull lectures so i mostly just memorized enough stuff to pass the test, rather than show much interest. Its all mostly just electrostatic fields with a bit of electron physics thrown in. Nice to know about, but not terribly useful to know in deep detail. Much like teaching software engineers some assembler, not really practical for the majority of cases but good to know the basics.

Those electrons do certainly interact trough photons according to QED, but the overall behavior is more sensible to explain with Maxwell since it still works just fine in there.
 
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