/sci/ - Science & Math

>>10145666
Too long didn't watch. Give me a summary

>>10145710
Old man yells at KVL
It's a response to this:
https://www.youtube.com/watch?v=0TTEFF0D8SA

>>10145710
He wants people to believe KVL doesn't hold for any system involving changing flux.

>>10145666
Engineers...

>>10145666
electroboom is treating the inductor as a DC voltage source equal to -dphi/dt and using KVL while lewin is treating it as a source of flux with 0 voltage across it and using faraday

Both ways yield the exact same equation, electroboom's case yielding iR - V + Ldi/dt = 0, while lewin's case is yielding iR - V = -ldi/dt so I think this is just a case of technicalities

>>10146241
>electroboom is treating the inductor as a DC voltage source equal to -dphi/dt
clearly a retarded way to look at it. Lewin is right, Faraday's law is the most fundamental, KLR is just an EE trick

>>10146414
I agree that lewin is technically correct, KVL is just a special DC case of faraday's law, but he's getting a little too butthurt about it

>>10145666
Why does everyone keep forgetting that Lewin is a rapist?

>>10146491
1. you're a moron, it was online sexual harassment
2. nobody cares

>>10146491
you tell me

>>10146241
This. It's pathetic how many allegedly intelligent men do not understand the nature of abstraction.

>>10146517
back to >>>/pol/

>>10146241
That's one the the things people are saying that I don't get. How is the inductor being a DC voltage source? The voltage across the inductive element is perfectly defined by [math]V_L=L \frac{d I_L}{dt}[\math] in a time variant manner.

And speaking of DC systems, that is with this assumption that the electric field in a conductor is zero? Even if it is literally a superconductor that is wrong. Charge carriers can't just spontaneously shuffle themselves to ensure a zero electric field. In any electro-dynamic system you literally can and must have an electric field for there to be a changing current (accelerating charge carrier flow). This is what the London equations are.

>>10146241
That's one the the things people are saying that I don't get. How is the inductor being a DC voltage source? The voltage across the inductive element is perfectly defined by [math]V_L=L \frac{d I_L}{dt}[/math] in a time variant manner.

And speaking of DC systems, what is with this assumption that the electric field in a conductor is zero? Even if it is literally a superconductor that is wrong. Charge carriers can't just spontaneously shuffle themselves to ensure a zero electric field. In any electro-dynamic system you literally can and must have an electric field for there to be a changing current (accelerating charge carrier flow). This is what the London equations are: describing electric field in a time variant superconductor.

>>10146674
the thing I don't really get is, if the voltage across an inductor is perfectly defined as such, then why is Lewin saying it is 0 due to the assumption that it has 0 resistance?

>>10146703
Sorry, reposted as
>>10146693
to fix TeX.

I think he's trying to use the implication that there is zero electric field through the inductor due to zero resistance (which is a false) then by the electric field line integral definition of voltage there is zero voltage across the inductor. So then he seems to pull out Faraday's law to create the equivalent [math]L\frac{dI}{dt}[/math] term in the equation describing the system.

It seems this false assumption has caused Lewin and his supporters to think that KVL cannot possible be true since it fails to define the voltage across the inductor. However, there is indeed both an electric field along the current path of the ideal inductor along and an electric potential across the inductor. Furthermore, anybody using KVL isn't concerned with the first principles derivation of electric potential differences since the whole point of lumped element models and KVL is abstracted simplified circuit analysis; so they just throw in the self inductive voltage term (whether they know why it works or not).

>>10146703
> if the voltage across an inductor is perfectly defined as such
you can define velocity as momentum over mass, but that's a shitty way to define it. it may work most of the time, but when you try to think about "what's going on physically" it leads you nowhere, and then you're pretty fucked when it comes to defining the speed of light

>>10146732
>anybody using KVL isn't concerned with the first principles derivation of electric potential differences since the whole point of lumped element models and KVL is abstracted simplified circuit analysis
no, physicists have to learn this, and it's important for them to get their physical concepts correct instead of learning engineer tricks

How the fuck you can have not same voltage in same points, at the same time?

>>10146491
They don't he was unpersoned at MIT and has no standing in the physics educator community now outside of YouTube. Also this >>10146513 he asked for pictures and for grown women over the computer to touch themselves, that's just being forward, they said no and it ended.

>>10146767
>when you try to think about "what's going on physically" it leads you nowhere, and then you're pretty fucked when it comes to defining the speed of light
On the contrary you are actually completely wrong in that inductor elements are exactly what is used to define the speed of light (electromagnetic waves) in transmission lines.

Physics oriented derivation:
http://www.feynmanlectures.caltech.edu/II_24.html

Engineering oriented derivation:
https://www.allaboutcircuits.com/technical-articles/introduction-to-the-transmission-line/

Just look up transmission lines if you want more examples.

>...it's important for them to get their physical concepts correct instead of learning engineer tricks
You're saying KVL "doesn't work" and they need to understand the real physical concepts. Which is why it is real shitty of them to fail to understand why KVL works as a consequence of conservation of energy (who the fuck started defining it based on closed loop electric field integrals) and make a completely wrong proof by assuming an electrostatic condition in an electrodynamic system.

>>10146703
I think I'm seeing the primary flaw that's misleading Lewin and KVL oppositionaries now. The zero electric field, electrostatic assumption. Thanks.

If anybody truly thinks that electroboom is bunk in his explanation of why the probing method and position of leads are what results in the difference in measurement for flipping the leads,
then I challenge them to produce an alternative and correct model that describes the observed behaviour; preferably not making use of KVL which is supposedly in applicable to this system.

>>10146837
Ironically come to think of it, transmission lines are a case where KVL does start to fall apart due to non-negligible time taken for changes in voltage (and electric field) to propagate through the transmission line. Thus with time-variant changes in the order of time taken for changes to propagate through a circuit can result in instantaneous snapshots where KVL appears to fail.

For example, imagine turning on a light at the end of a very long wire: when the power is applied to the start of the wire, it will take a finite time for electric potential to propagate down the wire and turn the bulb on. For a brief period it seems as though there is a voltage being input but no voltage at the output across the bulb i.e. the voltage sum of the loop is zero.

This can be solved by using the inductor-capacitor lumped element transmission line or more typical is simplified to transmission line element that applies a phase shift to the complex phasor used to represent voltage.

ElectroBOOM is being an idiot

>>10146478
I don't think Lewin is used to having to argue with retards over something so simple.

>>10145713
>old man yells at KVL

https://www.youtube.com/watch?v=ototTU5NUNA
>40:00

Still a better YouTube drama than Numberphile vs Mathologer

>>10148870
is he insane

>>10145666
he was unironically BTFO by some iranian 50yo neet MSc and now he is coping hard

>>10149012
the virgin kike sex molester
The Chad Iranian Engineer

>>10149022
you would also cope hard if you taught literal hordes of engineers that a fundamental law of electronics was wrong.

This guy should have his PhD revoked

>>10146551
>go away because I know you're right

>>10149025
he never said it was wrong you stupid moron

>>10149043
>claims Kirchhoff doesn't hold in specific situations
>gets shown that all he did was a probing error all along
b..b...but he never said it was wrong

>>10149046
the probing error is bullshit cope, Lewin is right and his demo is correct, read the paper
http://www.phy.pmf.unizg.hr/~npoljak/files/clanci/guias.pdf

>>10149053
Dr Lewin is wrong and his attempt to cover it up with capacitance is a joke. Anybody that actually understands the system can see that the difference in voltmeter outputs is due to flux change in the loops produced by the measurement leads.

>>10149238
no, you're wrong. did you read the paper? show me where it's wrong

>>10149280
That scan is missing a fucking page. Page 1090 skips to 1092. From what I can see though it perfectly confirms that the difference is due to probing and arrangement of leads. This shows exactly that Dr. Lewin's analysis of the two resistor loop fails to account for change in flux through the measurement leads by failing to include appropriate transformer/coupled inductor elements thus making it an invalid lumped element model where attempting to apply KVL will naturally fail.

Dr. Lewin's experiment and Electroboom's experiment are both not equivalent to the system analysed in the paper which clearly uses a long solenoid such that there are no magnetic fields external to the solenoid. Both Lewin and Sadaghdar (Electroboom) them use short solenoids and even worse just stick the two resistor loop past the end of the solenoid where the fields diverge and go everywhere.

Even then however, the phenomenon shown in the paper still applies to an extent and if you were to get the "true voltage" induced by the solenoid by properly isolating the coupled inductor element as Sadaghdar (Electroboom) shows when he moves the relative positions of the resistors on the loop you will not get this effect. Flipping the position of the leads will result in flipping the sign of the measured voltage.

I said Lewin is wrong not the paper you fuckwit. You read.

>>10149294
>...you will not get this effect.
I should say you will not get the effect Dr. Lewin shows of non-equal magnitude induced voltages.

But yeah I like that paper, if only it were actually complete.

>>10149053
>>10149303
Found the complete version myself:
http://materias.df.uba.ar/ft1a2016c2/files/2016/10/Romer-What-do-voltmeters-measure.pdf

>>10149294
>>10149334
the first link to the paper i posted has page 1091, pic related

i think this guy arguing against lewin isn't even clicking the links or reading anything, but just copypasta'ing retard youtuber arguments. clearly if you look at the paper, you see the voltmeters make different readings, and it's not due to an error probing

>>10149343
No, that's me. I've read it. It just didn't load for me for some reason I guess. Regardless, it seems you are not understanding what I mean when I explain why Lewin is wrong. Please address the points raised. The fact voltmeters take different reading is not the topic of the debate, that is clearly experimentally proven by both parties. The topic is that KVL fails to analyse circuits correctly.

Just bantz m8.

>>10149364
To add to that the apparent failure of KVL to describe the circuit as Dr. Lewin shows is a result of his measurement setup was has unaccounted flux coupling he fails to model *due to his probing*

>>10149001
>To Agree or Not to Agree with the Master is Not what Matters
>That Intro
At least one would not use him as the model example for sanity.

Why does he give this much of a fuck? Is it because he looked at mehdi's channel and saw the 300k view average?

>>10149999
scratch that I meant slightly under a million view average. I think lewin is being ridiculous on purpose in his responses. Like someone with autism attempting humor. Of course that's not to say that lewin is wrong, but it's certainly why he's made 3 response videos now and put on that funny hat and necklace in the recent one

>>10150013
Yeah I feel we're I'm getting trolled here. I think he's dressing up like an insane person to mock the "insane people" that "do not believe" Faraday's law. Hard to know for sure tho.

>>10149001
He's just channeling Feynman
https://www.youtube.com/watch?v=HKTSaezB4p8

Guys, the electric field is not a conservative field in this case because its curl is non-zero (due to the changing magnetic field in the coil). Therefore KVL will not hold because the voltage in a loop will not add up to 0.
Or am I just being retarded right now?
t. undergrad

>>10150200
this sounds right to me

>>10145666
Control Systems (aka Instrumentation) PE here. Improper wiring/probing is very easy and can lead to bad results.

>>10150439
Try telling that to the theoretical physics crowd.

>>10150200
>Therefore KVL will not hold because the voltage in a loop will not add up to 0.
It's more a problem with semantics, like this anon >>10146241 said. KVL will work if you model your system properly and include coupling to the external B-Field via a transformer, as is standard in the industry which Lewin praised many times in his video.

>>10150797
you could call it "just semantics" but in physics we prefer to think about "what is actually physically happening" which hopefully all of us don't think is just a matter of philosophy... so we regard this method as a sort of convenient equivalent and using it is more of a trick rather than an accurate physical picture

>>10150797
I agree with you, it's all just a silly argument anyway. The hard part is that the perfectly conducting wires in a circuit analysis model actually are of physical importance here as they add voltage. You have to model this somehow into your circuit analysis, which happens with all sorts of processes (noise for example).

amidoinitrite?

>>10150826
[math]\alpha = 4 V_emf[/math]

>>10150802
>in physics we prefer to think about "what is actually physically happening" which hopefully all of us don't think is just a matter of philosophy
The rest of the circuit was presented as a lumped element model though, which already presents a significant abstraction from "what is actually physically happening". Once properly motivated, I see no issue to include the external flux as the equivalent transformer component. Leaving the discussion at "KVL is totally wrong" is bad form on Lewin's part imo, since checking and adjusting existing physical models is what a lot of physical research is effectively about, and his understanding of KVL is not the one which is commonly used in practice nowadays.

>>10150889
>The rest of the circuit was presented as a lumped element model though, which already presents a significant abstraction from "what is actually physically happening". Once properly motivated, I see no issue
sure, as long as you understand what approximations and assumptions are being made, then that's fine. but a guy teaching physics to undergrads has reason to harp on why maxwell's equations are more fundamental than circuit diagram representations.
>Leaving the discussion at "KVL is totally wrong" is bad form on Lewin's part imo
he didn't say it was wrong, he said it's an approximation and it's not necessary. which is true. it probably is bad form for him, since probably there are many ways to demonstrate that he's mischaracterized how the loop rule gets used in practice, you can always go to higher orders in your approximation to make it closer to the real thing. fine.

anyhow the fact remains that the two voltmeters make different readings and it's not a probing error, and a paper has been posted in this thread twice demonstrating it in detail

>>10150915
>but a guy teaching physics to undergrads has reason to harp on why maxwell's equations are more fundamental than circuit diagram representations
Any proper circuit analysis course starts by motivating the lumped element models by their derivation from Maxwell's equations, so that they form an equivalent representation of all electromagnetic phenomena in the long-wave limit. Taking an incomplete set of elements and then showing that they would lead inconsistencies with Maxwell's equations without even mentioning that there is more to it is questionable practice by a teacher imo.
>he didn't say it was wrong, he said it's an approximation and it's not necessary. which is true
His definition of KVL is the center of this whole discussion, since he defines it as in-equivalent to Faraday's law, which is not consistent with the way it is done in network theory. Had he done that step right, he would not have been able to find this inconsistency.

>anyhow the fact remains that the two voltmeters make different readings and it's not a probing error, and a paper has been posted in this thread twice demonstrating it in detail
I am not questioning those results, I was arguing against the notion that they are not included in a (truly) equivalent circuit model, which seems to be the message Lewin wants to propagate.

>>10145666
I think we can all agree on the following:

-Yes you can put the probes in a way (that you and Mudhi call 'correct probing') to measure with no net change in magnetic field through the loop - if the probe wires are aligned to bisect the soleniod B field into two equal halves that cancel out (Such as by bringing the probe wites to the centre of soleniod from intersection points 180 degrees apart).

-The point of the experiment is to show that a meter on the left reads different to when a meter is on the right, because each is looped around the B field in the opposite direction. It is not meant to be a experiment to make a voltage measurement that would be the same voltage across the two points in a lumped element model of the circuit.

-You can make a lumped element model of the circuit and measure the voltage as you get with 'correct' probing.

-KVL is violated *if* you accept Lewins defintion of KVL: that KVL is a special case of faradays law applying only when db/dt does not exist. So KVL cannot contain -db/dt since it is not an E, and thus KVL is violated any time there is a changing B.

-If you take another definition of 'KVL' to mean the sum of E's plus sum of the loop EMF's -db/dt in total sums to zero, then that 'KVL' is not violated.

-Once you apply lumped element model which adds the inductors it becomes obvious why they don't read the same value.

So all in all I think everyone agrees with everyone, and this is really just a matter of phyisists and engineers using different circuit representations (actual diagram of whats on the bench vs lumped element model), and engineering and physicists using difference definitions of KVL.

>>10150200
You're correct. ElectroBOOM is being a retard. Fucking EE's think they know about Electrodynamics when they never even took a course in Classical Electrodynamics.

HURR DURR I'll use KVL instead of Ampere's law.

>>10152574
Ampere's law doesn't hold either :-v)

>>10152574
>instead of Ampere's law.
u trolling?

>>10152574
Hi Walt, didn't know you used 4chan old man.

>>10145666
Lewin finally watched Mehdi's video and wrote a response: https://www.youtube.com/watch?v=ototTU5NUNA&lc=Ugz_DV2Jjlaatbs2V2t4AaABAg.8npeWSmyAN28npnHWz7NXn

I guess he is insane after all.

>>10153746
Open the link to go to his highlighted comment.

>>10153746
>>10153749
Wow, why is he taking such a polite remark so personal? I thought this guy was supposed to be a good professor or something.

>>10153828
He's still a good professor, his 8.01 8.02 and 8.03 lectures have helped me more than some of my professors did back when it was still on iTunes U. I still would recommend his lectures and go to them for refreshing my memory.
But he's too proud to admit he was wrong all along, and he's a little bit on the crazy side (look at his art lecture). Also now that he's been shamed and kicked off MIT because some crazy chick claimed online harassment and just like that, lost everything he worked for, it would be too much of a pain to admit his whole flamewar was wrong because of bad probing. So I guess he chooses to hold his views now that he has an internet army of indians that elevated him to some sort of semi-god.

>>10153867
>But he's too proud to admit he was wrong all along
he's not wrong though, why does this thread keep slipping back to the brainlet side? let me quote:
>anyhow the fact remains that the two voltmeters make different readings and it's not a probing error, and a paper has been posted in this thread twice demonstrating it in detail
and
>-KVL is violated *if* you accept Lewins defintion of KVL: that KVL is a special case of faradays law applying only when db/dt does not exist. So KVL cannot contain -db/dt since it is not an E, and thus KVL is violated any time there is a changing B.

>-If you take another definition of 'KVL' to mean the sum of E's plus sum of the loop EMF's -db/dt in total sums to zero, then that 'KVL' is not violated.

>-Once you apply lumped element model which adds the inductors it becomes obvious why they don't read the same value.
which means the people claiming his demo is erroneous due to a probing error are wrong, and instead this is an argument over how you formulate a loop rule (which for sure is not relevant when talking about undergrad physics lectures)

anyhow i sort of doubt that he's such a good professor; i mean he gave very animated and inspirational, and very well-prepared plus great demos for physics 101 and physics 102, but i've never seen him teach anything beyond intro-undergrad tier. Feynman lectures are much more comprehensive, and i've had graduate-level quantum field theory profs who were actually able to explain it and answer questions well, and i find that much more impressive

>>10153942
He is wrong though that KVL fails. Lewin's analysis of a schematic circuit appears to show KVL fails but this is in fact the result of an incorrect representation of the physical system as a lumped element model. KVL in the "correct" context and definition as it is typically used (by engineers) does not fail when you properly model the system using inductors.

I agree though that, you cannot say his error is due to bad probing but I diverge from you in the reason for this. The experiment result is inherently dependent on how you setup and position the measurement leads (no one disagrees with this) but I say there is no "good" or "bad" probing because even though you can arrange the leads such that you get the result you expect you aren't so much "measuring the true value" as you are making the value what you want. The people that say is results come from "bad probing" are not wrong because the result varies from what they expect due to Lewin's probing. However, they aren't really right about there being a "correct probing" since the "correct probing" is really just you making the system do what you want.

Again I agree to the extent that there isn't really a "correct" way to probe the system, just a way that happens to give you your expected result.

Video where a guy called Cyriel Mabilde does the "correct" probings: https://www.youtube.com/watch?v=JpVoT101Azg

>>10150200
>>10150215
You are confused because Lewin gives a bullshit definition of KVL that no-one (particularly not the people i.e. mostly engineers that actually use KVL and defend it) uses. The more general, common and correct definition of KVL is: ∑ Vn=0 where Vn is the potential drop across the n'th element in a closed circuit loop (of a lumped model schematic specifically). There is then equivalence to Faraday's law by taking out inductor elements and replacing them with the physical dΦ/dt geometries. KVL shouldn’t be defined as a special case of Faraday’s law as Lewin does. It was independently developed and works without Faraday’s law as a consequence of conservation of energy.

>>10152574
and you are just a brainlet.

>>10153867
desu its hard to call the chick "crazy" after he acts like a retard in public. One can imagine the sort of stuff he may do in private. MIT arent retards, if they kicked him out was because he DID something wrong

>>10154450
>MIT arent retards
What is senior house? Oh, that's right, they've been banned by the thouht police. (they were degenerates anyway)

>>10154502
if you dont think that what he did is wrong, its your problem. But those videos with him losing his shit are sad. If it acts like a crank, and talks like a crank...

>>10154406
Faraday's law is much more fundamental than KVL shit.

>>10154654
More fundamental than conservation of energy? Because that's all KVL is a statement of.

>>10154654
And Quantum Electro Dynamics is much more fundamental than Faraday's law. I guess Lewin is wrong too?

He has apologised to Electroboom https://www.youtube.com/watch?v=d_XqrZo5_7Y

>>10154737
did you watch the video? or is english not your first language?
that's not really an apology for anything, it's a very thinly veiled "fuck you retard go back to YouTube".

>>10154737
>>10154748
>"apologised"
That better?

>>10154753
You absolute idiot