"Is Kirchoff really for the birds?"

[Muu9]

This video revisits the classic debate with some good physics and simulations in SPICE



You can skip to 42:34 to skip the physics review.

 

[Dale]

His whole "Kirchoff is for the birds" schtick is just a strawman argument. He starts by violating one of the basic assumptions of circuit theory and then tries to make a big deal about the fact that one of the laws of circuit theory doesn't work. Nobody claims that you can use circuit theory when the assumptions of circuit theory are violated, that doesn't mean that circuit theory is "for the birds". Every single theory ever developed is subject to that same silly criticism.

 

[DaveE]

Always keep in mind this first sentence of the Wikipedia entry:

"Kirchhoff's circuit laws are two equalities that deal with the current and potential difference (commonly known as voltage) in the lumped element model of electrical circuits."

The key phrase here is "lumped element model". This is the part that these videos always seem to ignore. It simply doesn't work for induced voltages unless you model that part as an inductor.

 

[kuruman]

It simply doesn't work for induced voltages ...

And the reason is that KVL is $\oint \mathbf E\cdot d\mathbf l = 0$ in disguise. Electric fields due to induced voltages are notoriously non-conservative.

Disclaimer: I have not watched the video because my laptop's speaker in inoperative at the moment.

 

[DaveE]

I think there's a broader issue on display here which is often neglected and not well taught in introductory physics.

CIRCUITS AREN'T REAL. These are always simplified approximate models that we make of the real world which allows us to use simple, powerful, analysis tools. As such, we must always keep in mind the limitations of our models. There should be no prizes for people that can point out the discrepancies between our models and the real world, we know already, we put them there.

The only mystery here is why people think it's a mystery. It's just a bad model in these cases. That's why we also have EM.

 

[Averagesupernova]

The only mystery here is why people think it's a mystery.

And if one tries to demonstrate in other ways how the system falls apart when the rules are broken it does nothing. I can think of endless demonstrations to 'blow their minds' and then try explaining that the demonstration violated the rules, etc etc...........and it does nothing.
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To me, it's about as stupid as refusing to see that five quarters and a nickel does not equal a dollar and fifty cents because we don't add the nickel to the quarter and then multiply it by 5. Obviously there are no direct parallels to that but the point is we have to follow the rules.
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I truly believe it's a refusal to see it because of who did the demonstration. I have a pretty good idea that if I or some other Joe Nobody did the demonstration and somehow managed to get the viewers that Lewin did, some smart person like @Dale who is able to see through this nonsense would explain it and everyone would agree and say: "Yeah, that guy is wrong. Dale has it right."

 

[DaveE]

I would bet that the people making these videos know the answer just as well as we do. I think they are playing up the mystery for effect. Students deserve a simple explanation, not a carnival show.

 

[Baluncore]

I would bet that the people making these videos know the answer just as well as we do. I think they are playing up the mystery for effect.

In order to profit from comments and corrections, they have knowingly and deliberately, got it wrong. Every "view to the end" pays them income. Every "like" pays them income. Every "comment" pays them income.
Do not reward troll generated slop, give them a "thumbs down" and don't link to it here.

 

[gleem]

I think the takeaway is that, for the application of KL's to be dependable, you need to understand or justify that the actual circuit is the intended one. You can draw a circuit with a particular function that should work but doesn't when constructed because unintended capacitances or inductances are introduced in the real circuit, thus changing its characteristics.

 

[Averagesupernova]

Pretty sure that the original claim that kirchoff is for the birds was done before YouTube existed. Most every YouTuber's goal is to get lots of views. But not all. As far as I know most of the folks supporting Lewin's position are commenters and folks who come on forums and social media. There have been numerous EE blogs that address this issue as well. Naturally, these bloggers do not support Lewin's position. I think the most viewed YouTube videos concerning this are done by YouTubers who try to explain why kirchoff still holds. Also notice that they all will reproduce the results of Lewin's experiment to show that the measurements in the lecture are legit as the setup was arranged. Sure, they like the income from lots of views the same as anyone else would. Nothing wrong with that.
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I read a blog that summarizes the issue that contains the following quote and I believe it is a quite accurate description of the whole situation:

I find this entire argument more a lesson in sociology than physics.

Find the whole article here: https://www.electronicdesign.com/te...icle/21808474/when-kirchhoffs-law-doesnt-work

 

[DaveE]

CIRCUITS AREN'T REAL. These are always simplified approximate models that we make of the real world which allows us to use simple, powerful, analysis tools. As such, we must always keep in mind the limitations of our models. There should be no prizes for people that can point out the discrepancies between our models and the real world, we know already, we put them there.

I'm reminded of a favorite quote from, maybe, the best analog EE prof ever:
"Engineering is the art of approximation" - R.D. Middlebrook

Our task is to build models that are just good enough, not perfect. Good enough to aid our understanding and answer our questions without too much effort. That's what circuit analysis is. That's what Kirchhoff's laws are for. Perfect is what the mathematicians and theoretical physicists want; they don't do circuit analysis (anymore).

 

[Muu9]

Pretty sure that the original claim that kirchoff is for the birds was done before YouTube existed. Most every YouTuber's goal is to get lots of views. But not all. As far as I know most of the folks supporting Lewin's position are commenters and folks who come on forums and social media. There have been numerous EE blogs that address this issue as well. Naturally, these bloggers do not support Lewin's position. I think the most viewed YouTube videos concerning this are done by YouTubers who try to explain why kirchoff still holds. Also notice that they all will reproduce the results of Lewin's experiment to show that the measurements in the lecture are legit as the setup was arranged. Sure, they like the income from lots of views the same as anyone else would. Nothing wrong with that.
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I read a blog that summarizes the issue that contains the following quote and I believe it is a quite accurate description of the whole situation:


Find the whole article here: https://www.electronicdesign.com/te...icle/21808474/when-kirchhoffs-law-doesnt-work

This linked paper is very interesting: https://kirkmcd.princeton.edu/examples/lewin.pdf

 

[Averagesupernova]

This linked paper is very interesting: https://kirkmcd.princeton.edu/examples/lewin.pdf

Yes. It is referenced in many places that discuss this disagreement.

 

[Averagesupernova]

I watched the video posted at the beginning of this thread and I admit I did skip through parts of it. I will likely watch it again at some point.
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I find he seems to fall into the same trap that many folks do concerning circuit analysis. Circuit analysis says that the magnetic fields must be contained within each component. In the real world this does not happen and electrical engineers everywhere know this. So this rule only applies on the paper in front of us. When we look at a schematic it is against the rules to say: "But that inductor is spewing it's field out all over the place and it affects other components!". It may very well do that in the real world, and if it doesn't matter, it is ignored on paper and everything goes on smoothly. But if it is a significant effect that an inductor spews it's magnetic field out where it is not noted on the schematic then we end up with what we have in the Lewin experiment. If an inductor couple in with other components or conductors in the circuit, it must be noted in some way.
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I do find it interesting that at some point he said we only concern ourselves with voltage and current at circuit nodes using circuit analysis with Kirchoff. I would agree with this. But this implies he would agree with my stance on the fact that Lewin fails to draw in inductors where he should.
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The creator of the video knows his physics I would say, but misses the boat the way alot of other physicists do concerning real world application and representing it on paper in the form of a schematic.

 

[DaveE]

In the real world this does not happen and electrical engineers everywhere know this.

Yep. Ask anyone that's ever done a PCB layout for a SMPS or high speed digital if the schematic tells the whole story.

YouTube creators and even physicists might do better if they could just admit that they haven't studied or worked as an EE IRL.

 

[Averagesupernova]

@Muu9 there are a handful of threads here from previous years that talk about the so-called Lewin paradox. I posted quite a lot in one of them that went a bit crazy. Alot of the controversy in it dealt with test lead positioning and whether it's possible to come up with a way to measure places on the loop while being confident that the test leads' positioning are not contributing to the reading. It branched off into different types of loops, etc. I won't clutter this thread with any of it. If you want to discuss what has already been discussed (that phrase sums up this subject quite well) start a new thread with a disclaimer: Haha. WARNING! THIS THREAD WE WILL TALK ABOUT WHAT HAS ALREADY BEEN TALKED ABOUT TO DEATH SEVERAL TIMES OVER! Or you may read these threads and have your questions answered.