Dr. Lewin's Responses

This is what makes Science great!

Fine

Good

"allow me to assume that the wire is a perfect inductor" is akin to the fable "assume a sperical cow". Assumptions don't hold. However, this seems mostly like a case of not agreeing on the terminology. What is KVL defined as to dr. Lewin vs. Mehdi? What is MKVL defined as? KLR? Define those terms first - otherwise this could be a simple example of not talking the same language.

Malthe Høj-Sunesen

@Nicholas Strahan, understood 👍 I remember when I was studying in University, we had strict instructions to not use Wikipedia. Some of our lecturers even went as far as to purposely upload false information regarding topics that they where teaching to catch students out. 🤭 this was going back 10years ago so perhaps things have changed. 😂

KVL absolutely does hold with energy container type parts. Capacitors, inductors, transformer, batteries, etc. But you can't just use an RMS voltage or current reading. You must know the complex impedance of all of the parts. ELI the ICE man. Then all instantaneous voltages around a loop sum to zero, =if= you've properly taken everything applicable into account.

Steven J Greenfield

I think it is a mistake to stoop to name calling or to be impolite.

Steven J Greenfield

Honestly i dont like seeing dr.lewin arguing with people in the comments who are trying to make him look bad , he does not deserve this and its making us all look bad. (Sorry mehdi i know your intention was good)

Yousef rawas

@ElektroBOOM he says in the comments he did watch your video in the end: <a href="https://www.youtube.com/watch?v=ototTU5NUNA&amp;lc=Ugz_DV2Jjlaatbs2V2t4AaABAg.8npeWSmyAN28npnHWz7NXn" rel="nofollow noopener" target="_blank">https://www.youtube.com/watch?v=ototTU5NUNA&amp;lc=Ugz_DV2Jjlaatbs2V2t4AaABAg.8npeWSmyAN28npnHWz7NXn</a>

dreamer

The wiki article is accurate, it was simply misquoted. The article concludes by saying KVL can be used as long as any parasitic inductance are considered. I find Wikipedia to be a pretty great source in general! It is rarely incorrect for more than a few days, especially in popular articles.

@Daniel Lang The very same (i think) wiki article that describes KVL as integral(e*dot*dl) later says that you CAN use it as long as you include the parasitic inductances that are inherent to any DC circuit which naturally makes a loop. I think the debate is simply physicist vs. engineer. The engineers are looking at the circuit as a schematic which models everything as a lumped element. In other words, the spacial dimensions are ignored entirely! Therefore the natural inductance of the circuit due to its geometry can be considered zero satisfactorily and replaced by an inductor like Mehdi said. KVL holds. The physicist prefers to view the circuit as a loop in space, which of course does not satisfy KVL as you have stated. There is no disagreement here! I'm no EE, but Electroboom's experience says that the engineer's model works well enough to fully understand the circuit, and that is why KVL is always used to describe it. Simply two different ways of modeling a system.

I trust Feynman more!

ElectroBOOM [Mehdi Sadaghdar]

Oh and by the way, I was reading Feynman, Leyton, Sands book chapter 22 on AC circuits, and it shows that as long as the magnetic fields are contained in the component itself, the sum of all voltages is zero. Dr. Lewin said otherwise in one of his videos saying no matter what, if there is magnetic fields in a circuit, the sum of voltages is not zero. Then what if the magnetic fields are not contained? The the sum of voltages won't be zero, not because KVL doesn't hold, but because leaking fields create new components in a circuit, namely the circuit loop becomes an inductor. So ignoring the new components such leakage can generate, it would seam KVL doesn't hold. It is not due to KVL inaccuracy, but due to the change in circuit model due to leakage. Basically the book says we assume all fields are contained because: "By making suitable approximations, it is possible to ignore complexities of the fields that appear inside the object".

ElectroBOOM [Mehdi Sadaghdar]

Also I don't know where the definition "KVL can only be used if Integral E ds = 0" comes from. Krichhoff only said sum of voltages should be zero. Voltage is the electromotive force (EMF) across any component, Integral of E.ds across electrical, and -dPHI/dt across the magnetic ones.

ElectroBOOM [Mehdi Sadaghdar]

Don't be sorry, it's about science not personal preference. Unfortunately Wikipedia is written by people like us and isn't necessarily correct. I have fixed some other articles in my time. KVL only says the sum of voltages in a loop is zero, which matches Faraday's law. Yes, it was initially written for circuits with battery and resistor only, but later we realized it applies to all components, in time or frequency domain. You must sum all voltages across components with electric field and ones with magnetic field. For example, isn't L di/dt the voltage across an inductor? can you assign any other voltage across an inductor? If you can, then all our simulations, calculations and designs are out of the window! Engineers always design knowing KVL holds. This is not against Maxwell or Faraday's laws.

ElectroBOOM [Mehdi Sadaghdar]

Oh yeah, after the fact. Still didn't like my explanation

ElectroBOOM [Mehdi Sadaghdar]

I can't let him confuse the society though!

ElectroBOOM [Mehdi Sadaghdar]

@Daniel Lang, Nicky put but please don’t rely on Wikipedia for your info, it’s not admissible as a reference in any university as anyone can edit/amend the information on that site.

I am very sorry Mehdi I've learned it the same way Mr Lewin explained it. Even on Wikipedia it is explained: "KVL is based on the assumption that there is no fluctuating magnetic field linking the closed loop. This is not a safe assumption for high-frequency (short-wavelength) AC circuits. In the presence of a changing magnetic field the electric field is not a conservative vector field. Therefore, the electric field cannot be the gradient of any potential. That is to say, the line integral of the electric field around the loop is not zero, directly contradicting KVL." For my understanding KVL can only be used if Integral E ds = 0 (That's what KVL says - Sum of all Voltages must be 0 in a closed loop). But in Faraday's Law (Maxwell Equations) Closed Integral E ds = - Integral d/dt B dA. So if there is a changing magnetic flux there is a voltage non zero for KVL and therefore the possibility to measure two different voltages for two different arrangements. So maybe you should look up your informations. I hope this doesn't effect our friendship... ;-)

Daniel Lang

Mehdi HE DID WATCH THE VIDEO he replied to my comment in his video

Yousef rawas

This man is too old to be wrong, just let it go.

It's interesting to see how Dr. Lewin thinks he's been personally attacked. Medhi's video was rather respectful but this man seems to conflate his theories with his ego which is never really in the interest of science let alone a logical debate.

The guy is a bully not an educator. Just ignore him. Also..... 2,000,000 VOLTS for 2mil subs. Period. Full Stop?

Jason L Bracken

Dr Lewin is arguing like a redditor, just read the title and starts posting 1 hrs response videos.

Terence Tan

I don't have a stick in this game. I'm a software engineer who hates when EEs right code, so I'm not about to start to write electronics. That said, it's unfortunate to get a response like this. This is drawn out and undirected and exactly what I can't stand about poor educators. Good education should be terse and to the point or, in my humble opinion, it displays a lack of useful understanding. The smartest professor I ever had was also the worst professor I ever had. There is a habit of understanding things so well an so long that they cannot communicate it to anyone else. They forget what it is like to not understand something.

Paul Grodt

Mehdi, i think i got it. All these scientific guys in comments are saying that charges, moved around by magnetic field cannot be considered as voltage. Guess, we have a superconductive ring in a changing magnetic field. We have a lot of current, but no voltage at all, at any points of the ring. And in regular ring voltage is caused by current. PS: Dr. Lewin's experiment still bad probing. PPS: On the other hand, when you measure with your sense wires close to main loop, as you have showed, you ignore the half, over which you have your sense wires, so, in fact, measure only the other half. PPPS: OMG, i confused myself even more.

I’m just a regular dude with no background in physics or EE however this topic has really got me wanting to know more! Good luck ElectroBOOM I really do hope that Dr Lewin sees the light.

Very sad. The whole idea of science is that one cannot preach from authority. It is the responsibility of the enlightened to teach, not to tell. Ironically, he spends more time demeaning the so-called "religious believers" in KVL than explaining how his argument actually differs from that of Electroboom's. Perhaps it is his old age.

I think you are both saying the same thing. You are calling it bad probing, but Lewin is calling it applying Faraday's law over different paths and getting different results. The 2 of you kind of end up in the same place. You create a break in the loop and note the charge across it when the solenoid is on, at 28:11 of the second video Lewin also starts talking charges, although he doesn't actually break the wire. I think removing the resistors and leaving a gap makes it very clear. This is a link to the Romer AJP 1982 paper Lewin mentions: <a href="http://www.phy.pmf.unizg.hr/" rel="nofollow noopener" target="_blank">http://www.phy.pmf.unizg.hr/~npoljak/files/clanci/guias.pdf</a> (for some reason I have to copy the link and past it into my navigation bar) You might want to discuss the questions raised by Romer like what exactly does voltage across an inductor mean. I think in terms of physics texts Lewin and Romer are right, Kirchoff's law should be presented as a special case of Faraday's law, and it should be emphasized that voltage is the result of integrating along a path, not the difference between values at two disconnected points. Another victory for the Internet, a radical professor has made an esoteric secret of the ruling elite public.

I'm not sure if 1.5x speed is such a good idea. He starts shaking even faster…

Alexander Thomas

The last 5 mins of the video shows how he is different from you. Sadly, it shows his arrogance to a point where he didn't even watch you video. It also proves he does his job because of money, not cause he wants to ejucate people like you. So all in all l thing you are right. Even if you aren't you still win because you did it in a correct way, much more humble and conversation open.

That "Dr" is very rude in his reply. I get he is being "challenged" but come on. What's wrong with a civilized discussion? I feel he looks down upon people without the "Dr" in front of their name.

Sad story. Now there are two fan-boy factions ready to kill each other.. Dr. Lewin's other lectures are still great, just this one is somewhat questionable. But it's not easy to review own beliefs after so many years, this could be simple error in cognition + many years of arrogant comments. He will not think about it (don't matter if he is actually wrong or not). People - just stay calm, attacking Dr. Lewin can't bring any good to the case.

This video explains KVL in AC circuits. <a href="https://youtu.be/YV-MQ-WCiH4" rel="nofollow noopener" target="_blank">https://youtu.be/YV-MQ-WCiH4</a> Now, just to be anal. He did say two volt meters which you did not do;) If you did that then you have experimentally show the Dr. theory to have a flaw. The Dr. said that it a crime that text books use KVL for circuits with inductors and capacitors. Well the video above explains why is applies. RMS voltage will be wrong. Instantaneous and vector voltages work. So the text books are correct. So KVL holds true for AC circuits only when using instantaneous and vector voltages. A thought. If where the two volt meters that are connected to the loop are removed and two wires are connected perpendicular and coplanar what wold the voltage be at the free ends of those wires? If voltages are somehow different at the point on the connections then “which” voltage will be measured?

Rav

I think a little dementia has set in! At least he could have paid you the same degree of respect and courtesy you gave him... I don't care how smart or qualified people are, they are never too smart to learn~!

Richard Boyce

The problem is that the leads connecting the voltmeter in fact become part of the circuit and are hence susceptible to the induction. The effect of this induction depends on the path of the leads. There is nothing more to it than that. Simple things can be explained in a simple way.

JanM

oh another guy allready mentioned it here, sry i did not saw this before i was Typing.

What an arrogant asshole.

I have studied mechatronics, so little bit systemtheorie, and i think i can see the proffessors problem. He try to say, that the Kirchhoffs law doesn't contains the energy-container type parts of the system, just the dissipative types. So he says, that the statement that the energy of the waste-type elements and the containers is equal to the energy of the source, is not identical to the statement that the source-energy minus wastes and containers equals to zero. Kirchhoffs law also hold in thermodynamics, where ther is no inductance-type energy container. So i would say that Kircchoffs law hold for a system (5 technological systems: translational, rotational, electric, fluid, thermic). I can not more help, im waiting for other explanations.

Hey Mehdi Dr. Lewins respond to you, and he said he thinks you were insulting him and attacked him. I think you were realy nice and kindly. Also very respectful. I do not understand him. Its under your Comment under his Video, but not Adressed to you,with an @. This is his Post : From: Lectures by Walter Lewin. They will make you ♥ Physics. @Vikas Poddar My demos at MIT were all in a very controlled environment. remember: The Earth is NOT Flat and Faraday's law is NOT wrong. ElectroBOOM's long video is dedicated to discredit my lectures. It is very insulting. His 2 major attacks were related to KVL and my famous demo in Lect #16 of 8.02 in which I show that the 2 Voltmeters attached to the same 2 points in my circuit read very different values (as they should according to Faraday's Law) and that the polarities are reversed. He shows that he does not understand Faraday's Law. Kirchhoff's loop rule (KVL) is older than Faraday's Law. But he now introduces a "modern version" of KVL which is really Faraday's Law; this is a joke. Watch my video 5+3-8=0 <a href="https://www.youtube.com/watch?v=f3-SfzepuPs&amp;index=9&amp;list=PLyQSN7X0ro22zanLOcvkaSY-IZqheFYM5." rel="nofollow noopener" target="_blank">https://www.youtube.com/watch?v=f3-SfzepuPs&amp;index=9&amp;list=PLyQSN7X0ro22zanLOcvkaSY-IZqheFYM5.</a> In my 2 latest videos ("To Agree or to NOT to Agree . . .) I set the record straight and I called a spade a spade; he has a masters degree in EE. His video is therefore an embarrassment.

I'm talking to another MIT professor to understand this. Most answers I get are vague from everyone. Stay tuned!

ElectroBOOM [Mehdi Sadaghdar]

I thought in science everything is a theory until it can be observed. A much better response would have been to identify where in your you did something wrong. I loved one of the comments - how does the voltmeter know on which side it is. A war will drive up views for both of you :) Maybe post a video asking any other physics professor to contact you to explain it. Maybe visit the local university?

Rav

What!? He answered without even watching your video? In that case I won't watch him... You'll do a recap in your next video anyway I guess.

Dukefazon

My comment seems to have disappeared from here. Well, it seems Dr. Lewin has now watched Mehdi's video. He states that in the comment section of <a href="https://youtu.be/ototTU5NUNA" rel="nofollow noopener" target="_blank">https://youtu.be/ototTU5NUNA</a>. It seems he was not impressed: "HUMBLE?????? ha ha ha ElectroBOOM's long video is dedicated to discredit my lectures. It is very insulting. His 2 major attacks were related to KVL and my famous demo in Lect #16 of 8.02 in which I show that the 2 Voltmeters attached to the same 2 points in my circuit read very different values (as they should according to Faraday's Law) and that the polarities are reversed. He shows that he does not understand Faraday's Law. Kirchhoff's loop rule (KVL) is older than Faraday's Law. But he now introduces a "modern version" of KVL which is really Faraday's Law; this is a joke. Watch my video 5+3-8=0 <a href="https://www.youtube.com/watch?v=f3-SfzepuPs&amp;index=9&amp;list=PLyQSN7X0ro22zanLOcvkaSY-IZqheFYM5." rel="nofollow noopener" target="_blank">https://www.youtube.com/watch?v=f3-SfzepuPs&amp;index=9&amp;list=PLyQSN7X0ro22zanLOcvkaSY-IZqheFYM5.</a> In my 2 latest videos ("To Agree or to NOT to Agree . . .) I set the record straight and I called a spade a spade; he has a masters degree in EE. His video is therefore an embarrassment."

Please, more videos where shit is explained. Not that conversations/drama with a professor with a ego problem.

he put out almost an hour and a half of responses without watching what he was responding too

Main problem: Dr. Lewin thinks KVL is ∮E·dl = 0 for some reason.

Kind of sad that he goes through the trouble of making a response, but can't even be bothered to watch the video he's responding to?? Right or not, thats just a bad behaviour.

Mehdi - Funny, your video explaining your reasoning is like 12 minutes or something, his are 26 + 46 minutes. On that basis alone (as I will not waste an hour of my life even to discover the cure for cancer) - YOU WIN.

As a commentor recently noted, it seems like we (you and Dr. Lewin) all agree on the concepts, but Lewin is arguing for the sake of argument. It is pretty clear that KVL holds as long as you include any inductance as a passive inductor in the circuit. In your next video, I don't think you need to call him a nice guy anymore! He is undeserving of that distinction.

I remember when i created lot of drama in my physics class, asking teacher about centrifugal force. She replied that there is no such force. I insisted, and she went hysterical, but all i needed to hear from her: "this force exists in ROTATING reference frame." Sometimes i think they are just reading the book without any understanding of things.

As much as I love the science drama, it's sad to see Lewin misrepresent the arguments and being so dismissive. Even with the demonstration that you can prove easily was not measured correctly is dismissed, he just refers to the author of a paper like an authority.