I like how you challenge the norm, fight for the truth even if you have to oppose the majority! I hope you're indeed correct and get to have your own claim on humanity's understanding of all things electric!
HMM... there is no common earth, thus its unfair. - i bet your anomaly disappears. if you common eath the scope, too a fixed point on the input induction bolt coil.. for both tests.
So true. It's something that bothers me in almost every discussion I (try) to have with people, the rare exceptions excluded of course. Yet it's so great to be wrong! Because now you now know better and have become a better person for it. What's not to love? :D
@MikeDonaldson-eh2ru I've watched several of Lewin videos, the guy knows his stuff but often doesn't bother to explain....must be an MIT thing, gifted students expected to figure it out on their own.
I am surprised that Dr. Lewin did not consider the mutual coupling of the coils with each other in his experiments. In his test moving the probing wires around should change his results. I could not find anything wrong with your analysis. Dr. Lewin seems to verify his hypothesis through the demonstrated experiment which appears to be flawed in how it is setup and yields an incorrect result. First step would be to correct the setup before we even discuss the issue at hand.
I think this is more an EE/Physics communication barrier. The physics view is that the EMF is not strictly a property of the circuit (and which points you choose), but of the full path enclosed by the probe wires and whichever part of the circuit forms the rest of the loop. Time-varying magnetic flux changes the situation from being a conservative potential (safely path-independent) to one where the loop integral is path-dependent. When @ElectroBOOM talks about "bad probing" or "good probing" it essentially is defined as "arranging your probes to avoid encountering the effects of flux in the path segment outside the circuit. I.e., suppressing exactly the difference Levin is talking about. When he introduces a transformer into the model, it is incorporating the probe wires into the circuit as the secondary. The controversy is about how one classifies these issues of measuring the voltage and whether they are included in the meaning of KVL or not.
he's reading a 1 turn coild with some wires that act exactly like a 1/2 turn antiparallel coil. Of course it's gonna negate half the reading, and you dont even need to flip it. Moving the wires most change the reading because he is litteraly adding 1 and substracting 0.5
I also think this is an EE/Physics communication barrier. Faraday's Law is very clear. The voltage in a closed loop is equal to the time derivative of the magnetic flux. If there is a time varying magnetic flux present, the voltage in a closed loop is not zero and Kirchhoff's Law doesn't hold.
@@ricardonunes6724 "If there is a time varying magnetic flux present, the voltage in a closed loop is not zero and Kirchhoff's Law doesn't hold." no, that is simply false. It holds true for dynamic systems just as well.
Can i just say that watching both this and your follow up video, i think the best lesson to be learned here is how to handle a disagreement like an adult. You found someone who had reached a conclusion that you disagreed with, and were still respectful of their findings and knowledge, while showing the reasoning that lead you to disagree. If more people could handle their disputes like this the world would be a happier place. Best wishes to you.
He did everything he could to be tactful and respectful and still got gruff. You can really tell no one has disagreed with Lewin in at least 30 years. He handled it so poorly.
@@pearz420 I kind of can see where prof. coming from. When people bother you with stupid questions and provide their demonstrations that doesn't make sense then after a while you just decide to stop going into any discussions. Maybe he had enough as educator. Of course he could have handled it better. I mean ElectroBoom certainly makes not only entertaining but also educational videos. And it would certainly benefit both of them to make a constructive argument.
I am really glad you did this video, not just cause I agree with you. It can be scary to challenge the findings of someone you respect but I think he would respect that challenge because we'll science.
Can anyone show me where the professor says that Kirchhoff should be changed because it is not correct, and where exactly is the experiment that he did and that he says shows this. Why are people pretending that the professor is attacking Kirchhoff. It is clear that what he is doing is showing his students that when you make an experiment where you don't take into considerations the very well known limitations of Kirchhoff, you will get the wrong results. Not probing in such a way as to make Kirchhoff work was the whole point of the experiment. It was to show that not adjusting for the effects caused by the limitations will produce bad results. I am absolutely dumfounded that people legitimately thing that the professor doesn't know how to probe a circuit, instead of thinking that he is doing so intentionally in order to show his students the dangers of not fully understanding Kirchhoff laws and their limitations.
Hi, I'm a theoretical physicist. I don't think Prof. Lewin was completely wrong, but I don't think your reasoning is wrong either. I agree with your calculations, but I think you are not applying Kirchhoff's law as is usually understood from the physicist's point of view. One may argue that Prof. Lewin is also wrong for the same matter when he says that Kirchhoff's law is sometimes wrong. It is never wrong: it's just that it does not apply on certain systems. In the end the problem, as I perceive it, is a semantic and not a physics one. What I am certain, though, is that Lewin proved himself to act rude and arrogant in that comment box. Your objection was completely legit and he had no right to call you an uneducated.
As a mechanical engineer and amateur electrician, I was here for type a comment like this one literally. I totally agree with you. Dr. Lewin may right about KVL is not applicable on some circuits but that doesn't make the law "wrong" because it is not a theory. It is a "Law". Even if Mehdi is not right at all, it is not ethical to call him "uneducated".
@sudan suwal Mr. Suwal, please could you explain which argument that I posted above wrong? I am really wondering it. Actually not intend to kidding. I just want to know if I misunderstood something in electrical circuits. I'll be wait for your response. Good day.
You are completely right. The professor is making the mistake that measurement of an experiment has to be the same regardless of the probes. The fact that you get different results measuring the same thing means that something is wrong with the experiment, automatically any conclusion is wrong. Testing Kirchhoff's law in the professor ‘s experiment is the first mistake. The conclusion is a ridiculous mistake. I’m a professor so I can say what ever I want that is no mistake. That is a fact.
Or facts born out of evidence? You could argue that a bunch of professors could claim something erroneous and it would still be peer review (collective delusion or even collective error in methodology). But they cannot dispute facts.
Consistently reproducible results i'd say. "What if i test your theory this way" which is exactly what mehdi is doing and coming up with a different result. Hence, theory needs adjusting.
But then it isn't really is it? Never really has been, there's probably more politics in the scientific world than in the White House. Science is dead like in around the world in 80 days, it's all about theory and what theory is popular.
What I got (reinforced) from this is that even the wire is a circuit component. Since the sense wire folds back on itself and follows about the same path back around, it induces nearly equal but opposite current from the wire that it's adjacent to, cancelling itself out. So you only read the effects of current through the opposite resistor. At least, that's what appears to be happening. He touched on that near the end when he drew in the hidden transformer.
Matija Lekovic there is no point of saying who's smarter or right, his point of view differ to that of ElectroBOOM, that's all. No matter who's correct in the end as you learn everyday, either of them will learn the truth that will change their perceptive positively 👍.
He's definitely smarter but smarter doesn't mean you know everything. He may know the math but can he make bread? Easy thing to do even a child can but I'll bet if you left him alone with all the ingredients he would have to mix them up many times before he came up with something resembling bread. Taught a lot of people to make bread and the amount of ways people find to screw it up is amazing.
Interesting to see you challenging Dr Lewin. Science drama is so much better than typical TH-cam drama. And this is science drama _on_ TH-cam! A new paradigm!
I come to this video from time to time hoping I can understand all the concepts explained here a lot better. Out of all Mehdi's videos, this one does even more hard science than Mehdi usually does. That being said, I have learned quite a bit from this great dude, and I do appreciate the fact that he shocks himself a lot just for the laughs and to enhance the learning experience.
I like how respectful this video was towards one of the greatest minds in out current time. Its not bad to disagree with someone and politely explain why. This is a great science video with awesome explanation AND a great guide to social communication. Good for you EB!
If Lewin was so smart he wouldn't have been stripped of his emeritus professorship for sexually harassing students. Arguing about incomparable achievements is pointless. So is the fetishization of hands-on knowledge. Takes all sorts to make the world go round.
I was working on this same experiment for a Book. And I couldn't find a Simple way of explaining this. You did it in just 15 minutes which is awesome. This is very well done and quite diplomatic I must say.
Not to be rude, but maybe you should wait with your book until you know what you're writing about. Mehdi is completely wrong, and what he is proposing is in direct contradiction with maxwell's third law. Kirchoff's voltage law does not hold under varying magnetic fields and has never in history been thought to. As a matter of fact, if it did hold generally we could not have engines, generators or electromagnetic waves. If you believe in the existence of those things I suggest you take another look at the theory. What mehdi did was to 100% verify the effects and then without much of a reason just dismissed them as "probing errors". They were not probing errors, they were vital parts of the experiment. If only it was this easy to disprove electromagnetism. I bet he would "disprove" gravity by showing a falling ball and saying that if only the gravitation didn't cause a probing error it would just float there.
I like how your comment says "very well done and diplomatic" almost exactly the same way that other guy's comment says, and he posted his comment a day before you... COMPLIMENT THIEF AHA
You are correct I have experienced different measurement around the loop while performing some practical in my university even my professor were stoked to see that, but I realized later that I had bad probing. Well explained Keep the videos coming and always express it good to see what other people think.
Your regular, humour-filled videos with shocking situations that make you want to go Ohm, are nice and I love them, but this video was really refreshing. Seeing you explain a confusing topic and simplifying it down so those of us, not too familiar with electronics yet can understand...dude, I need more of this. I think that is a legitimate sign of intelligence.
would it be possible for you to start a lecture series about circuit analysis? i believe you are the best teacher i know. theory combined with actual applications/experimentations is the best way to learn. i haven't been bored in any of your videos. you're so good! more voltage times current to you sir!
Dr. Lewin is displaying behavior far too common in the veterened engineering academics in that he clearly believes his knowledge and opinion is higher than anyone else. It is an unfortunate side effect of hubris in this field and I've personally experienced it in many professors. Simply in the way he responded to your comments, insisting that any argument is the result of no education and only his video and lectures can educate you, all the way to that last clip you showed where he reveals that every other author and professor disagree with him and yet they're the ones that are wrong? I recall a professor refusing to allow us to use Thevenin's equivalence when analyzing BJT circuits simply because she didn't like it. Every single online tutorial, university, and textbook insists on its use over 8 KVL equations but she didn't care because her opinion with gospel. Knowledge =/= education and that's incredibly important to keep in mind. You can have all the knowledge in the world but if you don't or can't question that knowledge then you're not well educated.
" insisting that any argument is the result of no education and only his video and lectures can educate you, all the way to that last clip you showed where he reveals that every other author and professor disagree with him and yet they're the ones that are wrong?" Sadly sounds like the attitude you get from the likes of anti-vaxers and other conspiracy theorists. Even really intelligent people can fall into this trap.
I was lucky. When I changed career (I got bored with electronics as a profession as it takes 2 hours to design something then 8 months to do the damn paperwork). Several of the lecturers who were international leading experts in their fields said roughly the same thing: "This is my educated opinion, there have been many other educated opinions over the years that have been ultimately proven to be wrong. Remember the same is true of my opinion - just because I have a PhD does not mean I cannot be proven wrong at some point in the future." Basically we got marked on how many differing opinions we engaged with and how we analysed the evidence. In my dissertation I even got my supervisor questioning parts of his own PhD thesis.
Reminds me of medical academia. Most doctors today keep treating patients with medication that just makes them worse. Diabetes, for example. Countles and countless of cases where it's reverted by change in diet but they refuse to even take a look at it. Makes you wonder if theres a big pharma mafia after all. Fortunately there are more and more doctors leaving their ego at the door and raising their voice.
May I suggest an alternative test for probing this circuit: Rather than having the probe wires in the same plane as the resistor loop, instead have the wires perpendicular to the plane of the loop (parallel to the changing magnetic field). Thus, no EMF would be introduced into them until they are sufficiently far away to make the effects negligible.
The probes in walter lewin's experiment are not affected by the changing magnetic field already, because in the external loops there's not much going on, there you can safely apply kirchhoff's loop rule and that's why you can measure Vr1 and Vr2, because the same voltage is applied on the scope
Howwwwlyy Shhieeeett!! When I heard "2 different Voltages across the same 2 points", I questioned my life and all circuits that I ever made ^^ Now that I saw the great explaination it all came together for me. But I do agree with you. It does make a lot on sense when you think about it.
This and the follow-up part 2 video are my two very favorite videos of yours! Your passion is for the science itself...finding the truth...This is the same passion, Faraday, Maxwell, Feynman, and the other greats all shared...You're in good company! Thank you for this series and for the inks to the counter-arguments by Lewin...
I fully agree that the model is missing an inductor. The ability for a wire to be able to have current induced from a changing magnetic field needs to be modelled in the circuit as an inductance. Just like the lumped element model for transmission lines.
Exactly. Every wire with a length longer than zero has inductance, no matter how low its resistance is. Had these been ideal wires with 0 inductance would mean they have to also have a length of 0, this would make the diameter of this circle also 0 giving it no magnetic loop area and making it impossible for a magnetic field to induce a voltage in it, hence voltage on all nodes would also be 0V. This then matches up with the circuit of two resistors, there is no component capable of creating a voltage.
But where does the inductance go in the circuit? It is distributed everywhere in the loop, including in the resistors. Lumping it in a specific place is not an accurate representation of what's going on.
@@Steve-du6ms Each resistor can be replaced by its resistance and a series inductor (ESL). Each real life wire can be replaced by a resistor and a series inductor (ESR & ESL). These elements are joined together with circuit diagram wires, which have no properties in real life.
@@@Steve-du6ms Every length of wire in the physical circuit would be replaced by a inductor in the schematic with a value equal to the amount of total loop inductance it contributes. Additionally all these inductors need to have a arrow drawn between them or a line along them to indicate they are coupled inductors that share the same magnetic field and each inductor should be given a dot at one end to indicate it going clockwise or counterclockwise to this field. When the probe connections are moved around to the left or to the right of the circuit this causes them to flip from going clockwise to counterclockwise, this flips the dot on the inductor, indicating it will create a voltage in the opposite direction hence why the probes going to the left or to the right side produce a different result on the oscilloscope. Once you include all these inductors the sum of voltages equations you should get a pretty close match to what the oscilloscope is showing. EDIT: Oh and you also need to include the inductor of the solenoid in the middle, it is also coupled to the common magnetic field and is connected to a voltage source. That is the voltage source that is powering this entire circuit.
@@berni8k The inductance needs to be added everywhere in the loop. This means that the voltage is induced everwhere in the loop, even within the physical resistors. So the model would include many, many mini-inductors, each with an induced voltage. The difficulty I have is that this cannot simply be lumped in a convenient place. So it is not obvious to me that the neat KVL circuit, with a source and lumped elements, emerges nicely from this situation.
This is a risk one encounters when delving outside of ones field of expertise. By ignoring the transformer created in his model, Lewin made a mistake that you clearly identify. Well done!
In true internet popcorn fashion, I'm trying to find out who's winning the argument here but I'm not reaching a quick answer. For example, I'm not certain Lewin ignores the transformer issue. See freepdfhosting.com/d5fc27ec92.pdf for the notes accompanying that lecture 16, check out Test 1 and Test 2.
@@clusterfork I think where this came from is that Dr. Lewin forgot to also include the transformer coupling to the wires that go to his oscilloscope. He sort of assumed that his oscilloscope is directly observing the voltage at those points. Or it could be that be knows what is wrong perfectly well and uses this as a way of finding the really bright students that figure out why this happens.
It was an excellent demonstration of the importance of considering all the details in a scientific experiment. In the demonstration, the hypothesis is raised that the consecrated Kirchhoff's Law could be nonsense, depending on the side where the instrument that measures the same induced voltage is positioned - an obviously absurd hypothesis. If the measuring instrument (oscilloscope) is to the right or left of the same circuit, the voltage reading should be the same - but in the demonstration it did not occur. Thus, the hypothesis that the said Law would be flawed was proven. The layman certainly went unnoticed that in both measurements, right and left, the circuit was not the same. The circuit, in fact, is not only what the demonstrator draws, but also the wires, cables and the internal impedance of the oscilloscope should be considered. As it is electromagnetic induction, any opening between wires will have voltage induction by the variation of the magnetic flux that surrounds them. The measuring circuit, to be the same with the instrument on the right and left, should be what was drawn by the demonstrator at 12:04. Soon after, he shows in practice that he did not follow what he drew; leaving again a new half turn wire near to the experimental loop over the magnetic field generator. It was an excellent joke of illusion. Thus, the hypothesis of failure of Kirchhoff's Law can not be confirmed.
Indeed. Science is about the continually challenging and testing ideas and theories. Peer review is a cornerstone. If you refuse to permit your claims to be challenged then you're a religion!
It doesnt necessarily have to be ego, but it can be very deeply rooted ways of understanding the principle. Especially for someone who does nothing but this for decades, you cant just step back and view it anew.
Nope. Walter Lewin is a great scientist, but great scientists are still human beings and still get caught up on personal bugbears that don't hold up to scrutiny.
I had been wondering about Dr. Lewin´s experiment since the first time I saw it. I watched it several times and had come to a similar conclusion. Since this is an air core transformer, any nearby wire is part of that transformer. Magnetic fields can have very complicated effects. Basically I´m glad you adressed this.
Kirchhoff's law holds only in cases the circuit size is much smaller than the wave length that passes through the circuit from the source. This is called a Quasistatic approximation, wiki link below. The simplest example is an antenna: An antenna broadcasts an electromagnetic field to the environment by a changing current that runs inside it. If Kirchhoff's law was true, there would be no current running through the antenna (it is cut off by KCL KVL laws). en.wikipedia.org/wiki/Quasistatic_approximation
@@noamgraham9006 no that is not true at least not fully. Also there is also the possibility of "short" antennas that are smaller than a quarter or half waverlength. but that is not what lewin is trying to show. here he is not working in HF areas
@@ElectroBOOM Good catch! I saw the next video, too. You have inspired me, after binge watching all of your videos in one week, to finally start my new channel I have been thinking about for about a year (and to do some of your builds starting with the rolled capacitor). I may not agree with all of your politics, but I still think you are a genius :)
Uh, if you get different readings, dependent an moving your scope/sense wires around, that might be the hint, that your sense wires and scope position are not just sensing wires, but part of the circuit you created.
Yeah, I don't get how Walter Lewin didn't see this. It's something you learn really early on in physics, and something that definitely shouldn't make you think that something like Kirchhoffs Law is broken.
4:48 "I have a coil or solenoid"... Me: This is going to explode. 4:57 "The resistor limit the current to 10-12 Amps"... Me: This is going to explode. 5:30 "Now I'll measure across these two points..." Me: This time for sure.
I remember having a similar conversation with one of my instructors as well. He simply said, high tolerance applications, use Kirchhoff's Law. For low tolerance applications, use Faraday's Law. I doubt I would have caught this. This clears up a lot for me.
From Maxwells equations, the electric field E = minus grad V minus derivative of magnetic vector potential A. Obviously, mathematically, the sum of grad V around a loop is zero (~V2-V1+V1-V2). The question is, does the voltmeter measure grad V or E or something else in portions where A matters. It clearly doesn’t measure grad V, because the result depends on how you position its leads. The magnetic field affects the leads of the voltmeter and induces an additional E, thus current, inside them, which depends on how you position its leads in this magnetic field. The additional E is given by the rate of change of the total magnetic flux (magnetic field times area) through the closed loop formed by the meter leads connected to some circuit element. So, even if your leads follow the wires of the circuit, when you flip their position perfectly, grad V changes sign, while the additional E doesn’t flip sign. So, to eliminate it, the loop of your meter leads have to be parallel to the magnetic field (zero flux).
Oh, I worked in that field - years ago. The KVL is derived from "E = -grad( phi )" and the corresponding integration theorem. If there is an time varying magnetic field, the true electric field is "E = - grad(phi) - (dA/dt)" . I.e. the KVL holds in the electro-quasistatic approximation assumption, that dA/dt is approximately 0. The KVL is false otherwise.
Kirchoff's Voltage Law always works under one of two possible conditions. The first condition is that you wait long enough for the system that you are measuring voltage from to stabilize, or alternatively that it is a lumped parameter system (this would assume that the wires connecting components are negligible). The concepts explaining why this is so are summarized under the topic of transmission lines. With the help of the telegrapher equations, derived from Maxwell's equations considering a source-free space and the transmission lines that carry the signal, one can analyse the response of a signal in a circuit looking at it as a wave that travels at a fraction of the speed of light. This fast moving signal does not behave according to Kirchoff's voltage law until it has reflected and super-positioned onto itself sufficiently many times to stabilize to the expected Kirchoff's voltage. In other words, Kirchoff's voltage law is not wrong, it's only wrong to apply it to non-stabilized circuits or with non-zero length lines (which would not be a lumped parameter model) between components. With regards to the measuring of two different voltages between the same two points I completely agree with ElectroBoom. Not even at the fast speeds of waves traveling on transmission lines, given no reflections, will you be able to measure two different voltages at the same points. This has less to do with KVL and more to do with simple equa-potential of a point. The potential difference between two points in a medium, from Maxwell's equations, can be defined as the integral of the Electric field between the two points along some chosen simple path. Now, in the case where the B-field is changing, you can simply treat the total E-field as the sum of the regular E-field and the induced E-field (which is the partial time derivative of the magnetic vector potential) and voila, you have a net E-field from which a specific potential difference can be calculated. Having one equation and one unknown implies, from simple mathematics, that it can definitely be solved.
Dear Mehdi Mercury, I am (was) an electrical freshmen. After watching all your videos over the weekend, I decided to switch to Business & Management because I can no longer solder or plug in something to the outlet without imagining sparks.
Medhi, I love your channel. I'm sorry to say both of you are arguing past each other... I'm a physicist who plays an EE at work so let me see if I can offer my 2 cents... The secondary of the transformer you refer to is an effective lumped element representation of the mutual inductance between the solenoid and the loop with the resistors. There is a secondary for the sense wire loop as well. However, the lumped element approach isn't general. The mutual inductance has to be calculated for a particular geometry, not a circuit, using you guessed it... Faraday's law. So you can only model the secondary if you have already solved the emf for the system. Lumped element then can be generalized for the flux profile so you can change the waveform, which makes it a powerful approach for circuit analysis, and makes the EE masters happy because they can just treat it like a circuit element and do normal AC analysis. See the appendix of Clayton Paul's book Intro to EMC for a fully worked example of this exact treatment. Now give me a damn oscope please.
I love this about a person of science. Challenge even the most established idea. Your humble nature shows, and is really appreciated. Good vid, as you demonstrate how the exact positioning of sensory wires makes a massive difference.
@@nullbeyondo No theory describes reality perfectly, it is pretty obvious he talks about science's continuous ability to find better and more precise theories by challenging its previous ones all the time.
@E it's not about being right or not, it's about testing, challenging and learning, that's the entire point of science. And most of the things ever tried don't work, but when they do, then we move forward, slowly but one step at a time. Any good teacher knows this.
@E I think he is, tbh. Old people become so bonheaded that they refuse to listen to younger people. And for Dr.Lewin to call him uneducated when his arguments are valid is immoral and i don't like him for that reason... It's like trying to tell an old mechanic that what he is saying is wrong, they will just scoff and say "i have been doing this all my life so you can´t teach me anything"
@@PonaHD ElectroBOOM said some things right and other things wrong. Here’s one example, said at least two times: 10:54 Nope, the voltage across two points in the presence of a time-varying magnetic field is not unique. Theoretically (if you’re computing the induced voltage), the induced voltage depends on the path taken to compute the line integral of the electric field. Practically, it depends on how you position the cables of the probes, as Lewin and EB showed in their respective videos. You may say “it’s bad probing”, but I’d reply the following. As was demonstrated in the videos by Lewin and EB, voltage depends on the two points as well as the path. Now I ask you: “what’s considered the correct path to measure an open-circuit voltage, and why?”. I think the answer is: there’s no wrong path, all paths are correct. 11:48 Wrong. There was *always* two different voltages across the two points. Again, voltage depends on the path taken when computing the line integral of the E field. What you claim to be "the only voltage between two points" is actually the voltage assuming a particular path; assume another path and you'll get a different voltage. --- Here's another example: 8:41 In the circuits being studied at this timestamp, certainly the RL circuit with the switch and battery acts as one winding of a transformer, and the circuit with the two resistors only acts as another winding of the transformer, so EB's explanation is correct here. But what if instead of creating the time-varying magnetic field with the RL circuit we instead create it with a moving permanent magnet? I wouldn't consider the magnet as a primary winding of a transformer, would you? So in this case we can no longer consider the circuit with the two resistors as the secondary winding of a transformer. So EB's explanation is no longer valid.
Well, I am surprised that Dr Lewin made this nonsense statement. Ofcourse Kurshoff laws hold. At least in this case, the problem is simply linked to the probing. The problem was clear from the beginning. And coming from a good experimental fellow like him, I am really disapointed. I spent my entire PHD developing a magnetic localisation system and if i have learned one thing during those years of experimental setups is to check and double check your wiring. Every bit of wire can act as an inductor and depending on which direction you wind it compared to the source of the varying magnetic field, it acts differently. I remmember running to a similar problem when building my first coils. A little untwisted wire absorbs a suplement of signal that completely falsifies the magnetic localisation by few centimeters (for a body parts localisation system, this is a big deal). Back then, I didn't even think of blaming Kirchhoff. I just started by checking my wiring and understood the problem in no time. Physics don't lie. Some laws are so established today that one must start by checking the experiment setup before even if it is Dr Lewin. But who doesn't make mistakes?? It happens to the best of us. I am a little disapointed though. Where are the sparks?!!!!!!!! Not even a little shock?!!! Is this electroboom or the school of electronics?
saifeddine ALOUI he’s not wrong. Mehdi puts it in such a way that it seems wrong, but KVL does actually fail when fields and physical structures are involved. Imagine a thick metal hollow tube carrying AC current. Due to skin effect the current in the outside surface is larger than the current on the inside surface. Due to resistance, the voltage drop along the outer wall is larger than that along the inner wall. Take a loop going into the tube at the top, all the way to the other side, and close it around the outside. Now the sum of voltages is not 0.
kirchhoff's law holds only in cases the circuit size is much smaller than the wave length that passes through the circuit from the source. This is called a Quasistatic approximation, wiki link below. The simplest example is an antenna: An antenna broadcasts an electromagnetic field to the environment by a changing current that runs inside it. If Kirchhoff's law was true, there would be no current running through the antenna (it is cut off by KCL KVL laws). en.wikipedia.org/wiki/Quasistatic_approximation
@@noamgraham9006 I understand the antenna example and the quasistatic approximation. But the experiment presented by Mehdi in this case is consistant with his explanation. The problem is not that the voltage difference is not the same if you measure it from the left or from the right. The problem here is that he used an extention of the wire that is embedded into the varying magnetic field. It is basically a setting problem and not a fundamental physical error. At least in this specific experiment. I 'm not arguing with the fact that when you pass a signal with a small enough wavelength, the Kirshhoff low doesn't hold. I'm just talking about this specific experiment with the specific outcome.
Here is a summary I found on the net on the limitations of the KVL and KCL laws: KCL is applicable on the assumption that current flows only in conductors and wires. While in High Frequency circuits where, parasitic capacitance can no longer be ignored. In such cases, Current can flow in an open circuit because in these cases, conductors or wires are acting as transmission lines. KVL is applicable on the assumption that there is no fluctuating magnetic field linking the closed loop. While, in presence of changing magnetic field in a High Frequency but short wave length AC circuits, the electric field is not a conservative vector field. So, the electric field cannot be the gradient of any potential and the line integral of the electric field around the loop is not zero, directly contradicting KVL. That’s why KVL is not applicable in such a condition. As you can see, both has limitations when it comes with high frequency (short wavelengths) which hopefully allow for open circuits like antenna to work.
This took some head scratching but I think I see the disagreement here. I usually explain KVL by analogy to elevation: if you walk in any closed loop here on planet Earth, you must necessarily have ascended exactly as much as you descended, because you end up at the same elevation. Dr. Lewin, I'm sure, would tell me that elevation is an incomplete analogy for voltage. If there's wind, for example, it'll be harder to walk into the wind than have the wind at your back. In this analogy, a changing magnetic field in a loop would be like trying to walk around a small cyclone: go with the cyclone's rotation and the wind is at your back, go against the cyclone's rotation and the wind is in your face. In that situation, "voltage" becomes harder to define because which path one takes between point A and B affects the work required to get from point A to B. Mehdi is arguing that the "wind" from the "cyclone" can be incorporated into the model, and so the "voltage difference between two points" becomes well-defined once more depending on where the cyclone's wind is worked into the circuit. In a sense they're both right and folks should really just go with whichever model helps them accomplish whatever they're trying to do. (But if you have two pieces of equipment measuring the same thing and they disagree, something's probably up and you need to recheck your experiment.)
There simply is no potential in this case. In your analogy, that's like trying to find the elevation of a point on Escher's staircase. Voltage is still defined, but depends on the path. That's the whole point.
@@Kalumbatsch I'd hesitate to use the Escherian Stairwell as an explanation for what's happening here, since it can't actually occur in the real world. Unless you mean to use the self-contradicting nature of the stairwell as a proof-by-contradiction that the elevation model isn't adequate for this situation? In that case it sounds like you're agreeing that the analogy is limited. (Sorry that I'm having difficulty understanding what you mean - the content of your reply appears to be a summary/reinforcement of my comment, but it reads more like a refutation/correction and that dissonance is throwing me off.)
Well, you said "Dr. Lewin, I'm sure, would tell me that elevation is an incomplete analogy for potential energy." It isn't, elevation is a perfect analogy for a scalar potential, because the gravitational potential is one. But in the case of a changing magnetic field, there is no electric potential. There is no way to assign one to every point because there is no potential that the electric field is the gradient of. Escher's stairwell is what you get when you apply the analogy to this situation.
Voltage is explained as a difference in Potential between two points. Here is the problem : Potential is a term that is mathematically defined. You can only have a potential if you are in a conservative Vector Field. A conservative vector field is equivalent to having any loop integral to be 0. *You can't just can't use the term potential in a non-conservative Field, because "Potential" is a well defined mathematical term !*
As someone uneducated in the field, I came to the same conclusion before you finished talking. I would love to learn why you would be wrong as I currently don't see it. Your arguments make perfect sense to me. The probe wires are part of this experiment when it is setup like this.
I'd go a step further and say that this is one of the cases where the fact that components are not, in fact, ideal, mathematical abstractions of components comes into effect. The wires have resistance and inductance.
I came to the same conclusion already when watching Dr. Lewins video. A wire going trough a magnetic field can't just be assumed to have zero voltage. No matter how low its resistance is, it will always have some inductance that reacts to the field. I could see however that looking at it from a theoretical point of view one could come to such an conclusion. In such theoretical examples you can never include all parasitic effects because the whole thing would just become a mess that literally takes weeks to calculate my hand. Its easy to miss a single significant parasitic effect and still have the math work out and seam logical. While on the other hand me being an engineer who deals with practical problems i know that a trace on my circuit board is not a perfect ideal connection between two nodes. I know from experience when a SPICE cirucit simulation behaves strangely that i need to model in some non ideal or parasitic effect. I have been bitten in the ass before by "simply ignore parasitics because they are so small they don't matter", my circuits didn't work how i wanted and every time it happened i learned how that particular parasitic effect is important and how to include it into my design process. You can't remove parasitics by simply ignoring them, you can only understand them and design them in as part of your product.
@@skonkfactory Even if the wires were ideal with zero resistance, there would still be a voltage across them. There isn't a voltage drop being measured across the wire. There is a voltage induced in the wire due to magnetic induction. If the wire were ideal, it would just give you a more accurate reading. The mistake that was made was not treating the circuit like a loop, but instead as separate parts. If you were to do the same with a transformer, you would conclude that KVL does not apply since the voltage applied is much higher or longer than the voltage read. But we know better since the change in voltage is found by measuring the coils.
This video illustrates why I subscribe to your channel. While the majority of your videos have an important entertainment value, they're based on scientific principles. I appreciate the level of critical thinking you are able to apply to the many scientific laws of electricity. Thank you for the work you put into your videos.
Sadly, he missed the point of the proffessor. By definition KVL wall is not akways true, BUT you can fix that in practice. From scientific point of view the proffesor is rigth, from practical point of view it does not matter or almost
The Kirchoffs Voltage Law used by Dr. Lewin and Electroboom are different. I think Dr. Lewin uses line_integral(E.dl)=0 as KVL, which definitely is incomplete, as this only says that the sum of voltages around a loop caused only by an external electric field is zero. According to this the voltage across an inductor is zero, as it is produced by a magnetic field and this is where Faraday's law comes into picture. But the KVL used by Electroboom is "the sum of voltages around a loop (caused both by magnetic and electric fields) is zero". This includes the induced voltage across the inductor (L*di/dt, which actually is a consequence of Faraday's or Lenz's law). So now it all comes down to which of the above versions was originally proposed by Kirchoff in his original paper.
_"it all comes down to which of the above versions was originally proposed by Kirchoff"_ Neither. Kirchhoff only talked about electromotive forces (emfs) and resistances. Remarkably, Walter Lewin also talks about an emf in his circuit, but he doesn't take it into account for his version. ElectroBOOM's version was first formulated by Maxwell and this is the version that is used to analyze circuits, see Feynman's lecture notes: www.feynmanlectures.caltech.edu/II_22.html#Ch22-S3
Take a circular loop, place an electromagnet and try to find the position of polarity of the induced emf- you can never find it, because it is not like a simple battery.
@@Theo0x89 "see Feynman's lecture notes: www.feynmanlectures.caltech.edu/II_22.html#Ch22-S3" Feynmann wrote: "by making the many approximations we have described in Section 22-2 and summarizing the essential features of the real circuit elements in terms of idealizations, it becomes possible to analyze an electrical circuit in a relatively straightforward way." And here is one of that approximations: "In particular, we assume that *the varying magnetic field is restricted to a definite region in the vicinity of the coil and does not appear outside the generator* in the space between the terminals." This assumption is clearly wrong in the case of the arrangement shown by Dr. Lewin. This is why no correct discrete model can be constructed. In strict meaning this assumption is not true for the majority of the real circuits. It is rather an approximation (as referred in 22-3). And the error of this approximation can be enormous in same cases as Dr. Lewin demonstrated. I have experience with even worse results. An error of these approximations can happen to be 40 dB higher than the quantity to be measured. This is why Lewin's example is not just correct and interesting, but important also for engineers.
9:56 "Why did we read 0V across the sense lines?" Because the path is different. At 9:26, your measurement loop encircles the flux, and the scope’s resistance allows a a path for the current to flow which arises from the induced emf from that flux. At 9:40, you’ve arranged your measurement loop such that it doesn’t encircle any flux, so there is no induced emf in that loop. In both cases, your scope is responding to induced current caused by the flux change thrugh the loop it forms with the probes and the wire arc. In the second case, there is little to no flux passing through the measurement loop as it doesn't encircle the solenoid -> no induced current -> no measured voltage. at 9:47 you find that you can get different results depending on how much flux you enclose. I can draw a picture if this isn't clear. 10:03 "If the voltage here is +V" But... It isn't. There isn't a localized potential difference there. Your previous measurement of +V depended on the path. Then you changed the path. This is covered in Belcher’s notes in the second video. But my intuition with this one is still shaky... Faraday’s law specifies that in the circle strictly around the loop, not including where the probe wires diverge, there is still am emf of 1V. In this particular circle, the impedance is high so there would be no resultant current across the gap, but how should we think of or visualize this EMF? imagebin.ca/v/4Qi4j6Awhq4w
This all sounds like simple oversight? I saw the problem immediately with the way the sensor wires were arranged at time of sensing, almost seems like he's intentionally trying to fool people. A magic trick..but terrible
Electrical engineers compensate dphi/dt change by taking into account inductance/emf on equivalent schematic. dPhi/dt exists as voltage induced in the coil/wire. So electricans actually use... Maxwell equation with dphi/dt "hidden" as emf and everything works perfectly. So... they both wrong?
But he happens to be right. Its not hard to see that inducing current round a loop means the voltage also goes up round the loop one-way, so can never sum to zero.
Being a youtuber does not mean you know how to teach. The professor was giving a lecture to students, and showing them how not knowing the limitations of Kirchhoff, will produce wrong results. He made an experiment... he did not take the limitations into consideration (he did not correct the result by use of clever probing)... he obtained the wrong result... he showed his students this fact... his students now remember that the limitations need to be taken into consideration and do correct measurements or whatever. That is all. All these people on youtube showing how the experiment should have been done, are completely missing the point of the experiment.
The E field produced by a changing magnetic field is rotational and therefore can't have a defined potential and thus its values between two points depend on the path along which you make the measurements, as long as you believe in Maxwell's equations and vector calculus you must accept this even if you're able to come up with a specific example where they happen to be the same.
@@MarkTillotson Bur Kirchoff deals with not Electrical fields, it deals with voltages - so curl doesn't actually tell you much here since you don't need derivation; you need integration.
@@lux_expat But voltages ARE defined with respect to electric fields. V is defined to be the line integral of E, along a path. In electrostatics, V will be path independent, while if you include changing fields, V is no longer path independent. This is why "the way you probe" now matters. Now one way of "saving" KVL is to think of the emf as a "potential", which is ElectroBoom's point of view. "Saving" it works in practice; one can measure "voltages" such that KVL will hold. But on a fundamentally theoretical level, KVL already broke when the system included the changing magnetic field.
He's a damn good physics teacher..... that's it! He's not a good person at all, just watch his interviews( search physics wallah walter lewin interview)
Serious question: how is it possible that such a fundamental question doesn't yet have a solution that the scientific community agrees on? It looks very weird that nobody has encountered, and explained this before with some sort of peer review or it's just Lewin getting it wrong?
Seems like Lewin is the black sheep in this debate. As both Lewin himself and Mehdi note, the majority of literature suggests that KVL holds in general.
@@ElectroBOOM regarding your video for the charged comb No i don't think that current flows just because thecharge is moving .but the question is that if the current flows where would the charge flow to . there must be a circuit or path for the charges to flow and be considered current .for example take the case of me having a battery with me as i travel in a car the battery had charges and the car is moving that would not be current . I'm really confused can someone explain me where I'm wrong
I find it a little disconcerting that a professor at a leading university who is well accomplished in his field proposes a radical change to a commonly accepted theory and then doesnt want to argue it because he is tired of arguing. Extraordinary claims require extraordinary evidence. It is a basic tennant of science imo. You dont get to make a great claim and then refuse to have the conversation and thereby win by default.
sjm4306 Not comparing him to Lewin by any measure, but we had an EE professor at my somewhat reputable school in Canada who'd make outrageous and false claims (for instance, at one point trying to convince us KCL did not apply to a node in a circuit) to justify obvious mistakes in his notes and solutions, and then would get mad at people arguing with him, never backing off. He was also very corrupt grading wise. He was far from being the only professor that got me wondering how the hell they made it into academia. As part of a student group, I also once had a chat with a recent Nobel physics laureate that visited our school. He didn't make any outrageous claims (not that we'd be in any position to argue with him anyway,) but was also very full of himself and rudely dismissive of many comments. Not that I'm saying Lewin is like this, I personally loved his physics lectures and watched them all, but I always thought people at places like MIT didn't suffer from this attitude, and always looked at it in high regard for exactly this reason, and this is highly disappointing.
@@f4dy "Not that I'm saying Lewin is like this" No, Lewin is saying that through his replies, just look at the discussion at this link (highlighted comment being ElectroBooms): th-cam.com/video/nGQbA2jwkWI/w-d-xo.html&lc=UgwnKTFxT6OD-POhOc54AaABAg To me the worst part is that this means that any *other* faults in his lectures will remain without any corrections from Lewin since he is ignoring all other experts (textbooks, etc) and won't even look at a video that shows why he's getting the results he's getting.
he's a grumpy old physicist who is bestowed in his ways after teaching this numerous times. It would be a kick in the nuts to know that his theory was wrong all along due to shitty probing
This is a very good point, though several times before when I watched the video but didn't realize how important it is. The matter of this question is how to model the real physics system: we can model the magnetic field by inductances and transformers as electrical engineering, while physicists may look at PDEs and have less emphasis on lumped circuits. As for the probing, that's another vital lesson I have learnt, because I was lucky enough that I didn't burn the scope with ground circulation with two passive probe at a region of high di/dt, quite similar as here.
"If you do not agree you need to be educated...This is very very basic physics and I never argue with people who think they know but who do not... The stunning demo at the end of my lecture..." His attitude doesn't mean he's wrong about this particular thing, but it does mean he's not worth listening to.
In this case it is at least 11 women he was found to have sexually harassed, and my quick skim through Inside Higher Ed's January 2015 article on the matter shows that MIT was provided plenty of evidence and did a thorough investigation into said evidence
@@666aron not when the claim has supporting proof, Read this www.insidehighered.com/news/2015/01/23/complainant-unprecedented-walter-lewin-sexual-harassment-case-comes-forward
I am a law student who desperately wanted to study physics. I've searched for law stuffs, and TH-cam suggested me this. Thanks to TH-cam for realising my heartaches.
I watched that class a couple of years ago and I found it very weird. I assumed he was talking about some advanced definitions and that the model of the "imaginary" inductor as a voltage source was a way to make KVL work out for induction machines. Now I feel so much satisfied with your reasoning ♥. Also I like to imagine the straight wire as a collection of tiny inductors in series, even the resistor itself acting as one, so that is NOT ok to assume 0 voltage drop just because the resistance negligible
A better demonstration would be to magnetically shield the sense wires so that the measurements still read the same on either side of the circuit. That takes the variable out of the equation.
Unfortunately it only takes some of the variables out. EG What sort of scope was it measured on? EG some digital scopes have a problem with measuring some types of fast transients of different polarities due to them having Jfet inputs (to have very low pseudo capacitance). I found this out the hard way.
@@gordonlawrence4749 Perhaps, but didn't he state that the load of the scope shouldn't have a significant impact? The reason the numbers are so different (I believe) is because of the second loop that was unaccounted for from the sense wires.
@@macadameane gordon doesn't know what he's talking about. It's evident when he starts talking about different types of oscilloscope. You're right. it's all about the reversal of the sense wires. Just a rudimentary example. most of the flyback power transformers have just 5 windings for the secondary. EG the one that charges your phone. 5!! yet it can generate five volts. meaning 1 volt per winding. If you reverse that wire like the sense wire was reversed, you get -5 volts. # you can not violate lenz law, faraday's law or kirchoff's laws. The Dr forgot that if you reverse the direction of a conductor in a field, then the current reverses. We can therefore deduce that he didn't know what he was doing.
YES! I saw the transformer, too. The sense lines are part of the circuit! (I was just an ME, not an EE, although I am a licensed ham for what that's worth.)
Because there is no dispute. All he did was replace the changing magnetic field with a transformer to make kirchoff's laws work because you can remove the magnetic field from the circuit. Thats nothing new
Because he isn't proving anything new, like he said most of science agrees with him but Dr. Lewin does not (but he didn't write a paper on it), so simply he's disagreeing with Dr. Lewin's disagreement with this law in physics.
that's all well and good but perfectly acceptable and _useful_ papers have been written on less. Science needs rigor, and a paper isn't pointless just for confirming something widely believed to be true.
@@DrummerRF The argument is that there is a "hidden" inductor created through the measurement device, is it not? Which is why this is up for discussion in the first place. Proving that through math and experimentation surely could have a paper written on the results. Doesn't necessarily need to be a thesis paper.
I think one have to do more practicals than reading theories. But sadly our school curriculums have no time for practicals, so only 15% learning. ElectroBoom sir, you're a great master. Thanks for this.
Water flow is immensely more complex than this to me. Even as only a second year Electrical Engineering student, I look at Dr. Lewin's lecture and immediately sense something wrong, despite how experienced and knowledgeable he is. However, if literally anyone told me anything about pipes and flowrate and stuff I'd probably believe it because fluid mechanics is a complete enigma to me despite having had to take it twice
@@smlgd There are four main disciplines in CE. Structural (what you speak of) Transportation (roads, pavements), geotechnical (soils, landfills, underground structures) and Environmental (water, wastewater, air treatment). Had to study them all, focused on the latter.
@@BonJoviBeatlesLedZep Water and electricity are very much analogous on the basic levels, until you get to things like EM fields and capacitance and the higher stuff.
Very well described! I work with small signals from microphones and phono cartridges and your video explains extremely well what happens in low inductance and low resistance loops and induced currents. Well done!
I'm with you; your reasoning is solid. So you've basically done the same experiment and shown Lewin's conclusion to fail on reproducibility after accounting for a straightforward explanation. This is why replicating a result before accepting it is so important: especially from an experiment claiming well-established laws to have an exception that is not predicted --- Kirchhoff's law withstands scrutiny.
Intuitivly KVL and KCL form part of the conservation laws that underpin physics. That is current in equals current out and voltage in is dropped over the circuit. So if you are to prove these wrong even for one special case care must be taken. Induced EMF is difficult to measure experimentally because as others have said it is hard to remove from the measurement process. Always assume that long standing theories rule until conclusively proven wrong. Though Maxwell was a genius and his contributions to electronics cannot be understated.
@@alexandermilleriii499 great law? How great? Can it fit into a house? It has a graet reputation because of its simplicity. It is useful because of its simplicity. But it's too simple to be true generally, as demonstrated. Maybe it worth trying to expand its validity, but to make it valid generally its usage should have been made extremely complex, losing its main benefit.
@Kalum It is not a FACT that it does not hold. Electroboom shows how it does hold in this example -- with this fairly orderly flux by solenoid; KVL can still be accurately applied to the situation. That does not necessarily mean there may not be other more extreme configurations which may exist that it is possible where it could be argued KVL might or might not be able to be shown to hold or be consistent with measurements, such as what does it look like with a circuit when an irregular elongated magnetic field is applied to a segment that is growing and collapsing, not regularly behaved.
I am probably more wronger than both Mehdi and Lewin (I only have bachelor in applied physics), but here is my input: I would like to point out that it is not something groundbreaking. Basically Kirchhoffs law is a simplified case of Maxwell's laws of electrodynamics and/or electronic version of conservation of energy. Now what Lewin is doing here is saying "in case of external magnetic field, this law does not hold and needs an extended version", so he provides a Faraday's law that assumes, that system is not isolated and energy comes from a dynamic external magnetic field. Mehdi has another interpretation - he says "dynamic external magnetic field is a part of system and we can model it as a transformer". This is not unusual - different approaches - if correct (in this case - they all follow Maxwell's laws of electrodynamics) - they should yield the same results. Now for the correctness part - I would say Mehdi here is more correct (something that might come from experience) - physicists have a very hard time modeling their measurement instruments as a part of the system - at least in other way than providing possible errors - so I can easily see Lewin ignore the fact that wires leading to voltage meter form some kind of loop and are affected by external magnetic field. But both approaches should yield same results because they all are correct with Maxwell and we ignore this pile of crap that QED is.
That's a fair point. But, as I recall, the physics interpretation is that the voltage field is no longer single-valued when you have a changing magnetic field. In fact, the demonstration could have been changed pretty easily to get any value you want at all, theoretically -- just need to coil the wire a bit more in one direction or another, maybe add a few loops, and hey, you can get a spike of π, or -e, or 9001V if you really want. You can get the same numbers if you account for parasitic effects, but at a certain point, it's less worth it to cling on to your old concepts and more worth it to just invent new ones.
@@PersonaRandomNumbers Electric field has a very nice property: it's conservative. This means we can actually define an electric potential, which itself is a scalar field. This is what actually makes 2nd Kirchhoff's law possible: with no changing magnetic field, you are guaranteed to start from a point A that has some electric potential, walk over any loop back to point A and end up with the same electric potential value you started with - that translates to "to move charge q from A to B, you have to do work that is dependent on position of A and B, but not on the path taken between them". Now when you add a changing magnetic field to the mix, electric field loses that property. That means that walking with a charge around a closed loop requires doing work (positive or negative) so mathematically, Kirchhoff's law does not hold. Now there are ways around it. Magnetic field is very specific and has to satisfy some conditions which allow you to define a vector field that is called magnetic vector potential. After doing some math tricks you can define electric potential using electric field and magnetic vector potential - this is often used in quantum physics and electrodynamics because you have to deal with nabla operator a lot there anyway. Another one is a blackbox approach - you define a "transformer element" that "does something inside that isolates rest of the circuit from this violation of conservativeness and outputs voltage" - which is possible because this is physics and physics has only one constraint - it has to match the reality. Whatever happens in transformer, stays in transformer and the rest of the circuit can enjoy holding Kirchhoff's 2nd law.
@@NessHX Makes sense -- basically, either extend the old idea to be more general, or make up a new thing and throw it around to make the problems go away. One problem with the second is that it's really hard to figure out the parameters describing that new thing without resorting to doing the first, anyways.
@@PersonaRandomNumbers I immediately remembered the Feynman's lecture (somewhere on YT) on physicists vs. mathematics subject and him saying something like "guessing new laws seems to be an efficient way to make progress in physics". In case of Mehdi's video I would actually say that the blackbox approach (include transformer in the circuit) is more elegant and actually gets us somewhere - mostly because every time I had to include magnetic field somewhere I always felt annoyed by how it works. Good methods of hiding problems are always in demand for engineering purposes.
bro curl(E)=-B_t (where B_t is partial derivative of B with respect to t) is a maxwell eq. it is always valid. while applying kvl u assume that E is grad of some scalar function, if a vector field is grad of a scalar func, then it must be curl free. however we know that curl(E)=-B_t, thus kvl is not always valid, which assume curl(E)=0.
You've got confused by the probing issue - the fact remains that if you sum the voltage changes around the loop you get a non-zero value. Consider a uniform loop, current is flowing round the loop, so there is a voltage gradient around the loop, meaning that the electric field is not a conservative field. There has to be a voltage gradient to drive the current through the resistance of the loop wire, an EMF, but that EMF sums up to a non-zero value, breaking Kirchoff's law.
New to the channel (probably one of the best TH-cam channels yet). Love the video! and as an EE student, this totally makes sense, and I couldn't agree more. Keep them coming!
Does Kirchhoff's Law Hold? Yes. It always holds. Thanks for clearly shows that the induced emf by the magnetic field is actually an voltage source in the closed circuit.
john smith the power plant transmits at high voltage 500kv 250kv 115kv (69kv has been phased out in most regions of the US) then broken down at a substation for distribution in the 7.2kv range, in some areas 3.6kv this is primary distribution voltage, the secondary service output voltage from the distribution transformer is your AC voltage source. Surges and voltage drops are common, and must be factored in.
EB, professor Lewin is right! Your measurement is wrong based on the points you consider on the loop! Ill tell u why. Firstly, note that the TOTAL GENERATED EMF has to be the one generated by the changing mag flux, therefore by faradays law. At 11:07 you added V1+VR+V2 which is not correct because the induced EMF in the left branch is purely a Faradian EMF (path dependent), so is V1+V2. This is like a battery for that loop so VR must be VR=V1+V2 since the change in flux through the entire loop is used to generate the EMF in the circuit!! ( VR is the DROP IN POTENTIAL in that branch!!) Secondly, for an accurate measurement of the voltage across R1 you should connect the voltmeter right at the ends of the resistor, and since the wire going from R1 to R2 has negligable resistance we can consider them to be at the same potential( no voltage drop across). So if you were to measure right at the tips of the resistors you could neglect any additional voltage induced in the measuring wire , therefore the only voltage being measured would be VR1 for this purpose!!! You should try measure at the tips of the resistor and see that Dr. Lewin is right ! Please answer me back and tell me what you think
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The Wikipedia article on Kirchhoff's circuit laws explains this under Limitations: "The current law is dependent on the assumption that the net charge in any wire, junction or lumped component is constant. Whenever the electric field between parts of the circuit is non-negligible, such as when two wires are capacitively coupled, this may not be the case. This occurs in high-frequency AC circuits, where the lumped element model is no longer applicable. For example, in a transmission line, the charge density in the conductor will constantly be oscillating." "On the other hand, the voltage law relies on the fact that the action of time-varying magnetic fields are confined to individual components, such as inductors. In reality, the induced electric field produced by an inductor is not confined, but the leaked fields are often negligible."
I think it’s really a matter of technicality, if you are willing to look at the circuit as two separate components, a Transformer / voltage potential source, and a circuit not subject to changing magnetic flux, then you can use KVL and get an appropriate result, because you are factoring in the contribution due to Faraday’s law. However if you are to analyze the circuit in its entirety you must use Faradays law as using only KVL gives no indication of the potential difference between any points in the circuit without a source. I’ve watched Professor Lewis’s entire 8.02x lecture series, and really appreciate his contribution to education in physics. But I think this fight is rather pointless, we all agree on the results, we just don’t agree on the classification of our methods of analysis. Engineers like to think about KVL as just adding the potentials through a loop, and if the result isn’t correct it’s because you’ve neglected a term. Physicists such as Dr. Lewin like to think of KVL as a special case of Faraday’s law, simply because it doesn’t hold in every case unless you factor in the contributions from Faraday’s law, which happen to be zero most of the time.
" KVL gives no indication of the potential difference between any points in the circuit without a source. " No, KVL still holds true - at no point is it violated. The Problem is that Lewin is ignoring the effects his measurement-setup has on the outcome - when measuring with changing magnetic fields you need to make sure to account for that and - like Medhi has shown. "we all agree on the results" You say that commenting on a video that shows that Lewin is interpreting his faulty measurements as a breaking of a fundamental law of electromagnetism.... yeah no. Show us a single instance where it doesn't hold up and you'd be a candidate for the nobel price.
@@ABaumstumpf Kirchhoff laws don't hold when the current varies too rapidly. KVL is _not_ a fundamental law of EM. Maxwell's equations are (at least in the classical limit). When the current varies, you get radiation. And radiation takes out energy from your circuit, so the sum of the voltages comes out negative. Of course you have to get to very high frequencies to even notice the effect. Citing wikipedia: _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.[2] 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. It is often possible to improve the applicability of KVL by considering "parasitic inductances" (including mutual inductances) distributed along the conductors.[2] These are treated as imaginary circuit elements that produce a voltage drop equal to the rate-of-change of the flux._
ABaumstumpf or you could use Faraday’s Law which takes into account the voltage potential generated by changes in magnetic flux. I’m with you here, KVL is not violated, IF you account for the effect of Faraday’s law, but KVL does not take such effects into account by itself. Thus it simply is a matter of opinion, does KVL need to account for the contribution due to the changing flux for it to hold true, or is it the responsibility of the person using it to factor in that term? You seem to think that one needs to factor in the contribution or physics will break, but realize that Faraday’s law simply states that the closed loop integral of potentials is proportional to the rate of change of magnetic flux. In an instance where magnetic flux remains constant you get KVL saying that the potentials around the loop is zero. If you apply KVL with the contribution from Faraday’s law you get a modification of the equation Faraday’s law originally gave you with all the terms on one side and equal to zero.
KVL holds very true for conservative electric field. However, when you apply a time varying magnetic field through a loop , along with induced emf, a very special type of induced electric field is also generated which can form closed loops (thereby its non conservative) hence KVL cannot hold for this case and hence the whole notion of lumped circuit model fails . If the magnetic field is changing very slowly with time you can ignore these effects but if your magnetic field is largely changing with time, you cant ignore the non conservative electric field. The uniqueness in faraday's law is that it combines both results into a single equation. Hence faraday's law holds true for all such situations but not KVL. Speaking in more practical terms, all generators work by changing the orientation of magnetic field (not magnitude).
@@drawapretzel6003 This is not a conservative field. One way to interpret Faraday-Maxwell equation into words is to say that a time-varying magnetic field gives rise to an electric field WITH curl. Since the E-field has curl it is not conservative.
@@twbbrandon KVL cannot hold in the presence of a time-varying magnetic field, or a non-conservative E-field. Faraday-Maxwell equation tells us a time varying B-field gives rise to an E-field with curl.
Your "noise" measurements are the inductance of the two wires in parallel and EMF induced into the circuit and the untwisted portions of your lead. Reason why it differs is they are in different positions one time closer, another time farther away, or other times not totally parallel with each other. In order for you to get an accurate reading you should use shielded leads from point to point or have your untwisted leads, 90 degrees from the measurement point. And if you really want to get picky, the probe itself will affect your measurement as it becomes part of the circuit. Because, depending on the frequency and voltages being measured, you can introduce capacitance or attenuation. I see what the professor is saying... when a circuit is being effected by EMF due to the circuit component characters, wire and connection resistance, until the EMF finally peaks, the circuit will behave and measure differently in different spots and will not equal zero. Depending on the placement of the EMF and the measuring devices you can get, either a positive or negative peak. You can see it in your noise measurement, those ripples are the varying voltages until the circuit stabilizes. Due to each component characteristics and connecting wire length the circuit at times, the total voltages will not equal zero. In addition, EMF waves are formed in the circuit which can be out of phase until the circuit "catches" up. And you also have angular velocity, field flux, stray field and cross lines to account for until the field stabilizes. Plus, just arbitrarily setting the circuit over the coil doesn't allow for accurate readings... When the law was written, they did not understand I-V characteristics, nor could they measure it or the circuit accurately. Plus you are measuring with a single scope... measure it will multiple scopes and compare the timeline of the voltages measured in the circuit and you will see.
Hello team RECTIFIER! Make sure to watch the next video on the topic: th-cam.com/video/Q9LuVBfwvzA/w-d-xo.html
ElectroBOOM ok
Stop looking at the camera it feels like your looking deep into my soul.
I agree
I like how you challenge the norm, fight for the truth even if you have to oppose the majority! I hope you're indeed correct and get to have your own claim on humanity's understanding of all things electric!
HMM... there is no common earth, thus its unfair. - i bet your anomaly disappears. if you common eath the scope, too a fixed point on the input induction bolt coil.. for both tests.
You can tell he's serious because he doesn't shock himself in this video.
haha! +1
Well, I didn't understand anything but the fact that he still shocked some scientists out there
@@angelomartino4667 hehehe.
Also the altitude of his Eyebrow~s~
lol
"I couldn't be happier to be wrong and learn something new." -an important moral fiber rare these days.
So true. It's something that bothers me in almost every discussion I (try) to have with people, the rare exceptions excluded of course.
Yet it's so great to be wrong! Because now you now know better and have become a better person for it. What's not to love? :D
Yep
I literally read this the same time Mehdi said it
Are you Illuminati?
@@dimitriss.7954 I am working on it my friend.
"These days"
Nah that's a universal human bias
Very well done and diplomatic ;)
And the civil engineer shows up trying to play around with the electricals.
@@Token_Nerd You shouldn't anger a civil engineer. They usually design civil structures in a way that they can hide bodies inside.
@@_aullik As a Civil Engineer, no comment >;)
And civil engineers are experts in civility!
@@89rafa Very subtle pun.
Bad probing is almost always the number one source of error in electronics experiments. Kudos Mehdi
My thoughts exactly! Thanks for saving me from having to comment this.
@MikeDonaldson-eh2ru I've watched several of Lewin videos, the guy knows his stuff but often doesn't bother to explain....must be an MIT thing, gifted students expected to figure it out on their own.
I am surprised that Dr. Lewin did not consider the mutual coupling of the coils with each other in his experiments. In his test moving the probing wires around should change his results. I could not find anything wrong with your analysis. Dr. Lewin seems to verify his hypothesis through the demonstrated experiment which appears to be flawed in how it is setup and yields an incorrect result. First step would be to correct the setup before we even discuss the issue at hand.
God has spoken.
I think this is more an EE/Physics communication barrier. The physics view is that the EMF is not strictly a property of the circuit (and which points you choose), but of the full path enclosed by the probe wires and whichever part of the circuit forms the rest of the loop. Time-varying magnetic flux changes the situation from being a conservative potential (safely path-independent) to one where the loop integral is path-dependent.
When @ElectroBOOM talks about "bad probing" or "good probing" it essentially is defined as "arranging your probes to avoid encountering the effects of flux in the path segment outside the circuit. I.e., suppressing exactly the difference Levin is talking about. When he introduces a transformer into the model, it is incorporating the probe wires into the circuit as the secondary.
The controversy is about how one classifies these issues of measuring the voltage and whether they are included in the meaning of KVL or not.
he's reading a 1 turn coild with some wires that act exactly like a 1/2 turn antiparallel coil. Of course it's gonna negate half the reading, and you dont even need to flip it. Moving the wires most change the reading because he is litteraly adding 1 and substracting 0.5
I also think this is an EE/Physics communication barrier. Faraday's Law is very clear. The voltage in a closed loop is equal to the time derivative of the magnetic flux. If there is a time varying magnetic flux present, the voltage in a closed loop is not zero and Kirchhoff's Law doesn't hold.
@@ricardonunes6724 "If there is a time varying magnetic flux present, the voltage in a closed loop is not zero and Kirchhoff's Law doesn't hold."
no, that is simply false. It holds true for dynamic systems just as well.
"My mom thinks I'm mostly ok."
It's ok Mehdi. We're all in that boat together. Love you man. Keep up the great work!
My mom doesnt know what to do with me anymore.
My mom disowned me :c
Jk shes the best mom ever.
that ws sad
My mom does not have the same feeling about me Infact it's the complete opposite 😁
Can i just say that watching both this and your follow up video, i think the best lesson to be learned here is how to handle a disagreement like an adult. You found someone who had reached a conclusion that you disagreed with, and were still respectful of their findings and knowledge, while showing the reasoning that lead you to disagree. If more people could handle their disputes like this the world would be a happier place. Best wishes to you.
Couldn't agree more
He did everything he could to be tactful and respectful and still got gruff. You can really tell no one has disagreed with Lewin in at least 30 years. He handled it so poorly.
@@pearz420 I kind of can see where prof. coming from. When people bother you with stupid questions and provide their demonstrations that doesn't make sense then after a while you just decide to stop going into any discussions. Maybe he had enough as educator. Of course he could have handled it better. I mean ElectroBoom certainly makes not only entertaining but also educational videos. And it would certainly benefit both of them to make a constructive argument.
I am really glad you did this video, not just cause I agree with you. It can be scary to challenge the findings of someone you respect but I think he would respect that challenge because we'll science.
Unfortunately dogma is big in science as well. Look at the oil drop fiasco.
Can anyone show me where the professor says that Kirchhoff should be changed because it is not correct, and where exactly is the experiment that he did and that he says shows this. Why are people pretending that the professor is attacking Kirchhoff. It is clear that what he is doing is showing his students that when you make an experiment where you don't take into considerations the very well known limitations of Kirchhoff, you will get the wrong results. Not probing in such a way as to make Kirchhoff work was the whole point of the experiment. It was to show that not adjusting for the effects caused by the limitations will produce bad results. I am absolutely dumfounded that people legitimately thing that the professor doesn't know how to probe a circuit, instead of thinking that he is doing so intentionally in order to show his students the dangers of not fully understanding Kirchhoff laws and their limitations.
Hi, I'm a theoretical physicist. I don't think Prof. Lewin was completely wrong, but I don't think your reasoning is wrong either. I agree with your calculations, but I think you are not applying Kirchhoff's law as is usually understood from the physicist's point of view. One may argue that Prof. Lewin is also wrong for the same matter when he says that Kirchhoff's law is sometimes wrong. It is never wrong: it's just that it does not apply on certain systems. In the end the problem, as I perceive it, is a semantic and not a physics one.
What I am certain, though, is that Lewin proved himself to act rude and arrogant in that comment box. Your objection was completely legit and he had no right to call you an uneducated.
As a mechanical engineer and amateur electrician, I was here for type a comment like this one literally. I totally agree with you. Dr. Lewin may right about KVL is not applicable on some circuits but that doesn't make the law "wrong" because it is not a theory. It is a "Law". Even if Mehdi is not right at all, it is not ethical to call him "uneducated".
As an asshole. you were all wrong.
@@tealiedie So Mr. Asshole. What is the truth? I am looking forward to.
@sudan suwal Mr. Suwal, please could you explain which argument that I posted above wrong? I am really wondering it. Actually not intend to kidding. I just want to know if I misunderstood something in electrical circuits. I'll be wait for your response. Good day.
You are completely right.
The professor is making the mistake that measurement of an experiment has to be the same regardless of the probes.
The fact that you get different results measuring the same thing means that something is wrong with the experiment,
automatically any conclusion is wrong.
Testing Kirchhoff's law in the professor ‘s experiment is the first mistake.
The conclusion is a ridiculous mistake.
I’m a professor so I can say what ever I want that is no mistake.
That is a fact.
And this is why science is based not on authority, but on peer review.
Or facts born out of evidence? You could argue that a bunch of professors could claim something erroneous and it would still be peer review (collective delusion or even collective error in methodology). But they cannot dispute facts.
@@youtubasoarus Be careful saying that. Anti-vaxxers and flat-earthers dispute facts all the time.
Consistently reproducible results i'd say.
"What if i test your theory this way" which is exactly what mehdi is doing and coming up with a different result. Hence, theory needs adjusting.
But then it isn't really is it? Never really has been, there's probably more politics in the scientific world than in the White House. Science is dead like in around the world in 80 days, it's all about theory and what theory is popular.
@@fordman7479 110% agree
What I got (reinforced) from this is that even the wire is a circuit component. Since the sense wire folds back on itself and follows about the same path back around, it induces nearly equal but opposite current from the wire that it's adjacent to, cancelling itself out. So you only read the effects of current through the opposite resistor. At least, that's what appears to be happening. He touched on that near the end when he drew in the hidden transformer.
"my mom thinks I'm mostly ok." 😂
r e l a t a b l e .
lol - yeah, I was glad to hear that too.
So I'm not the only one who has received that statement before 😂😂😂
the rectifierrr
Yeah my mom thinks I’m ok kinda.
Mostly lmao
Always express you thaughts. Just because he wrote 15 science books doesn't mean he Is right, or that he Is smarter than you.
Yes!
And if it is a teacher it also doesn't matter
Matija Lekovic there is no point of saying who's smarter or right, his point of view differ to that of ElectroBOOM, that's all. No matter who's correct in the end as you learn everyday, either of them will learn the truth that will change their perceptive positively 👍.
Tako je
We are taught the periodic table of elements when it's a world of isotopes, neither idea or the science is wrong, but it's complicated.
He's definitely smarter but smarter doesn't mean you know everything. He may know the math but can he make bread? Easy thing to do even a child can but I'll bet if you left him alone with all the ingredients he would have to mix them up many times before he came up with something resembling bread. Taught a lot of people to make bread and the amount of ways people find to screw it up is amazing.
Interesting to see you challenging Dr Lewin. Science drama is so much better than typical TH-cam drama. And this is science drama _on_ TH-cam! A new paradigm!
Id enjoy drama a lot more if all of it was like this!
I come to this video from time to time hoping I can understand all the concepts explained here a lot better. Out of all Mehdi's videos, this one does even more hard science than Mehdi usually does.
That being said, I have learned quite a bit from this great dude, and I do appreciate the fact that he shocks himself a lot just for the laughs and to enhance the learning experience.
I like how respectful this video was towards one of the greatest minds in out current time. Its not bad to disagree with someone and politely explain why. This is a great science video with awesome explanation AND a great guide to social communication. Good for you EB!
"Current time" !!!!
Smart guy yes, but one of the greatest minds of our time? Hmm. Hardly Stephen Hawking.
He is fuckin' smart, but far from the greatest minds.
@@88werwolfhun88 that why I said "ONE OF the greatest minds". Otherwise I would have said THE greatest mind.
If Lewin was so smart he wouldn't have been stripped of his emeritus professorship for sexually harassing students.
Arguing about incomparable achievements is pointless. So is the fetishization of hands-on knowledge. Takes all sorts to make the world go round.
I was working on this same experiment for a Book. And I couldn't find a Simple way of explaining this. You did it in just 15 minutes which is awesome.
This is very well done and quite diplomatic I must say.
Well he works in electronics & has an MS in it.
Do you really think that his explanation is right??
Not to be rude, but maybe you should wait with your book until you know what you're writing about. Mehdi is completely wrong, and what he is proposing is in direct contradiction with maxwell's third law. Kirchoff's voltage law does not hold under varying magnetic fields and has never in history been thought to. As a matter of fact, if it did hold generally we could not have engines, generators or electromagnetic waves. If you believe in the existence of those things I suggest you take another look at the theory.
What mehdi did was to 100% verify the effects and then without much of a reason just dismissed them as "probing errors". They were not probing errors, they were vital parts of the experiment. If only it was this easy to disprove electromagnetism. I bet he would "disprove" gravity by showing a falling ball and saying that if only the gravitation didn't cause a probing error it would just float there.
I like how your comment says "very well done and diplomatic" almost exactly the same way that other guy's comment says, and he posted his comment a day before you... COMPLIMENT THIEF AHA
You're stupid
You are correct I have experienced different measurement around the loop while performing some practical in my university even my professor were stoked to see that, but I realized later that I had bad probing.
Well explained Keep the videos coming and always express it good to see what other people think.
try probing shitty solder joints with a scope......you can get readings from 0 to 150V on a 5V circuit.....
90% of people who liked this comment saw just some smart words and liked even without reading
@@laharl2k yes
@@SpaceTimeBeing_ i though the comment was pretty clear to,weird.
Your regular, humour-filled videos with shocking situations that make you want to go Ohm, are nice and I love them, but this video was really refreshing. Seeing you explain a confusing topic and simplifying it down so those of us, not too familiar with electronics yet can understand...dude, I need more of this. I think that is a legitimate sign of intelligence.
would it be possible for you to start a lecture series about circuit analysis? i believe you are the best teacher i know. theory combined with actual applications/experimentations is the best way to learn.
i haven't been bored in any of your videos. you're so good! more voltage times current to you sir!
That would be great...
Bring this to the top, people! We need more Electro101 videos!
My god..... How about you stop writing cheesy comments and go and force times distance.
I'll support this ❤️
Ramon Cristopher Calam this would be highly appreciated!!!!!
Dr. Lewin is displaying behavior far too common in the veterened engineering academics in that he clearly believes his knowledge and opinion is higher than anyone else. It is an unfortunate side effect of hubris in this field and I've personally experienced it in many professors. Simply in the way he responded to your comments, insisting that any argument is the result of no education and only his video and lectures can educate you, all the way to that last clip you showed where he reveals that every other author and professor disagree with him and yet they're the ones that are wrong? I recall a professor refusing to allow us to use Thevenin's equivalence when analyzing BJT circuits simply because she didn't like it. Every single online tutorial, university, and textbook insists on its use over 8 KVL equations but she didn't care because her opinion with gospel. Knowledge =/= education and that's incredibly important to keep in mind. You can have all the knowledge in the world but if you don't or can't question that knowledge then you're not well educated.
" insisting that any argument is the result of no education and only his video and lectures can educate you, all the way to that last clip you showed where he reveals that every other author and professor disagree with him and yet they're the ones that are wrong?"
Sadly sounds like the attitude you get from the likes of anti-vaxers and other conspiracy theorists. Even really intelligent people can fall into this trap.
Very good point. You can get a nobel prize (in STEM that is) but still end up talking nonsense.
Pretty insulting how he approaches Mehdi's request. Theres such a thing as confidence in science, but there is also blatant arrogance.
I was lucky. When I changed career (I got bored with electronics as a profession as it takes 2 hours to design something then 8 months to do the damn paperwork). Several of the lecturers who were international leading experts in their fields said roughly the same thing: "This is my educated opinion, there have been many other educated opinions over the years that have been ultimately proven to be wrong. Remember the same is true of my opinion - just because I have a PhD does not mean I cannot be proven wrong at some point in the future." Basically we got marked on how many differing opinions we engaged with and how we analysed the evidence. In my dissertation I even got my supervisor questioning parts of his own PhD thesis.
Reminds me of medical academia. Most doctors today keep treating patients with medication that just makes them worse. Diabetes, for example. Countles and countless of cases where it's reverted by change in diet but they refuse to even take a look at it. Makes you wonder if theres a big pharma mafia after all. Fortunately there are more and more doctors leaving their ego at the door and raising their voice.
May I suggest an alternative test for probing this circuit:
Rather than having the probe wires in the same plane as the resistor loop, instead have the wires perpendicular to the plane of the loop (parallel to the changing magnetic field). Thus, no EMF would be introduced into them until they are sufficiently far away to make the effects negligible.
good girl
The alternative would be to use a real transformer and a AC supply...but clearly electrical engineers already know how transformers work.
also suggested that way, then prob won't affect by EMF
I will try this later this week
The probes in walter lewin's experiment are not affected by the changing magnetic field already, because in the external loops there's not much going on, there you can safely apply kirchhoff's loop rule and that's why you can measure Vr1 and Vr2, because the same voltage is applied on the scope
Teacher: The test isn't complicated
The test:
Teacher: predicts result,
Teacher: conducts experiment
Measurement: agrees with predictions
Former engineer: Nooooooo! You can't hit me!
Howwwwlyy Shhieeeett!!
When I heard "2 different Voltages across the same 2 points", I questioned my life and all circuits that I ever made ^^
Now that I saw the great explaination it all came together for me.
But I do agree with you. It does make a lot on sense when you think about it.
You're not alone.
This and the follow-up part 2 video are my two very favorite videos of yours! Your passion is for the science itself...finding the truth...This is the same passion, Faraday, Maxwell, Feynman, and the other greats all shared...You're in good company! Thank you for this series and for the inks to the counter-arguments by Lewin...
I fully agree that the model is missing an inductor. The ability for a wire to be able to have current induced from a changing magnetic field needs to be modelled in the circuit as an inductance. Just like the lumped element model for transmission lines.
Exactly.
Every wire with a length longer than zero has inductance, no matter how low its resistance is.
Had these been ideal wires with 0 inductance would mean they have to also have a length of 0, this would make the diameter of this circle also 0 giving it no magnetic loop area and making it impossible for a magnetic field to induce a voltage in it, hence voltage on all nodes would also be 0V. This then matches up with the circuit of two resistors, there is no component capable of creating a voltage.
But where does the inductance go in the circuit? It is distributed everywhere in the loop, including in the resistors. Lumping it in a specific place is not an accurate representation of what's going on.
@@Steve-du6ms Each resistor can be replaced by its resistance and a series inductor (ESL).
Each real life wire can be replaced by a resistor and a series inductor (ESR & ESL).
These elements are joined together with circuit diagram wires, which have no properties in real life.
@@@Steve-du6ms
Every length of wire in the physical circuit would be replaced by a inductor in the schematic with a value equal to the amount of total loop inductance it contributes.
Additionally all these inductors need to have a arrow drawn between them or a line along them to indicate they are coupled inductors that share the same magnetic field and each inductor should be given a dot at one end to indicate it going clockwise or counterclockwise to this field.
When the probe connections are moved around to the left or to the right of the circuit this causes them to flip from going clockwise to counterclockwise, this flips the dot on the inductor, indicating it will create a voltage in the opposite direction hence why the probes going to the left or to the right side produce a different result on the oscilloscope. Once you include all these inductors the sum of voltages equations you should get a pretty close match to what the oscilloscope is showing.
EDIT: Oh and you also need to include the inductor of the solenoid in the middle, it is also coupled to the common magnetic field and is connected to a voltage source. That is the voltage source that is powering this entire circuit.
@@berni8k The inductance needs to be added everywhere in the loop. This means that the voltage is induced everwhere in the loop, even within the physical resistors. So the model would include many, many mini-inductors, each with an induced voltage. The difficulty I have is that this cannot simply be lumped in a convenient place. So it is not obvious to me that the neat KVL circuit, with a source and lumped elements, emerges nicely from this situation.
From another Electrical Engineer- you got my vote👍
I've got an awesome relatively easy invention in electronics. would you be interested in a collab for improving the design, etc? maybe even patenting?
my vote too
As ways Mehdi>pedophiles
This is a risk one encounters when delving outside of ones field of expertise. By ignoring the transformer created in his model, Lewin made a mistake that you clearly identify. Well done!
Electrical Engineering is Applied Physics but I get your point
I came to the same conclusion before even watching this video:
Spooky phantom transformer
In true internet popcorn fashion, I'm trying to find out who's winning the argument here but I'm not reaching a quick answer. For example, I'm not certain Lewin ignores the transformer issue. See freepdfhosting.com/d5fc27ec92.pdf for the notes accompanying that lecture 16, check out Test 1 and Test 2.
@@jimmoriarty6964 but the dude's field of expertise is astronomy
@@clusterfork
I think where this came from is that Dr. Lewin forgot to also include the transformer coupling to the wires that go to his oscilloscope. He sort of assumed that his oscilloscope is directly observing the voltage at those points.
Or it could be that be knows what is wrong perfectly well and uses this as a way of finding the really bright students that figure out why this happens.
It was an excellent demonstration of the importance of considering all the details in a scientific experiment. In the demonstration, the hypothesis is raised that the consecrated Kirchhoff's Law could be nonsense, depending on the side where the instrument that measures the same induced voltage is positioned - an obviously absurd hypothesis. If the measuring instrument (oscilloscope) is to the right or left of the same circuit, the voltage reading should be the same - but in the demonstration it did not occur. Thus, the hypothesis that the said Law would be flawed was proven. The layman certainly went unnoticed that in both measurements, right and left, the circuit was not the same. The circuit, in fact, is not only what the demonstrator draws, but also the wires, cables and the internal impedance of the oscilloscope should be considered. As it is electromagnetic induction, any opening between wires will have voltage induction by the variation of the magnetic flux that surrounds them. The measuring circuit, to be the same with the instrument on the right and left, should be what was drawn by the demonstrator at 12:04. Soon after, he shows in practice that he did not follow what he drew; leaving again a new half turn wire near to the experimental loop over the magnetic field generator. It was an excellent joke of illusion. Thus, the hypothesis of failure of Kirchhoff's Law can not be confirmed.
Great scientists admit when they are wrong and let everybody learn from their mistakes; those other scientists get their ego punctured.
Indeed. Science is about the continually challenging and testing ideas and theories. Peer review is a cornerstone.
If you refuse to permit your claims to be challenged then you're a religion!
It doesnt necessarily have to be ego, but it can be very deeply rooted ways of understanding the principle. Especially for someone who does nothing but this for decades, you cant just step back and view it anew.
Nope. Walter Lewin is a great scientist, but great scientists are still human beings and still get caught up on personal bugbears that don't hold up to scrutiny.
If you don't understand anything, it's fine. He is not explaining it you, he is explaining it to Walter Lewin XD
This is the comment I was longing to hear....though I could grasp it up , still I didn't feel like I completely got everything he said
hahahahaha
You are just the alibi for the TH-cam revenue
@@karoly365 truth hurts man, but truth is truth
It's high school physics
I had been wondering about Dr. Lewin´s experiment since the first time I saw it. I watched it several times and had come to a similar conclusion.
Since this is an air core transformer, any nearby wire is part of that transformer. Magnetic fields can have very complicated effects.
Basically I´m glad you adressed this.
Kirchhoff's law holds only in cases the circuit size is much smaller than the wave length that passes through the circuit from the source. This is called a Quasistatic approximation, wiki link below.
The simplest example is an antenna:
An antenna broadcasts an electromagnetic field to the environment by a changing current that runs inside it. If Kirchhoff's law was true, there would be no current running through the antenna (it is cut off by KCL KVL laws).
en.wikipedia.org/wiki/Quasistatic_approximation
@@noamgraham9006 no that is not true at least not fully. Also there is also the possibility of "short" antennas that are smaller than a quarter or half waverlength. but that is not what lewin is trying to show. here he is not working in HF areas
Maybe he was trying to challenge some unknown genius to step forward and call him out??
wish that was the case, but no. Watch my next video on this... hey did you call me a genius?!
@@ElectroBOOM Good catch! I saw the next video, too. You have inspired me, after binge watching all of your videos in one week, to finally start my new channel I have been thinking about for about a year (and to do some of your builds starting with the rolled capacitor). I may not agree with all of your politics, but I still think you are a genius :)
@@ElectroBOOM CAN YOU PLEASE GIVE ME A MULTIMETER OR RØDE MICROPHONE PLEASE I BEG YOU !! PLEASE !!!
@@arshuarshaq5043 FO Beggar
@@Akarsh- mind your words ! And he is a teacher to me !
Uh, if you get different readings, dependent an moving your scope/sense wires around, that might be the hint, that your sense wires and scope position are not just sensing wires, but part of the circuit you created.
Exactly. Needs a bigger model
yep， as long as there is an alternating magnetic field through those wires. there is going to be an issue. Try with shielded wires and that will work
Yeah, I don't get how Walter Lewin didn't see this. It's something you learn really early on in physics, and something that definitely shouldn't make you think that something like Kirchhoffs Law is broken.
4:48 "I have a coil or solenoid"... Me: This is going to explode.
4:57 "The resistor limit the current to 10-12 Amps"... Me: This is going to explode.
5:30 "Now I'll measure across these two points..." Me: This time for sure.
not today my friend, not today.... he had to be serious when refuting against a distinguished professor
Whenever you see him use a capacitor and plug something into a power supply it's a done deal
That is true i though the same way 😂! I am not used to this kind of video from him !
Don't worry, the pulse is short.
I remember having a similar conversation with one of my instructors as well. He simply said, high tolerance applications, use Kirchhoff's Law. For low tolerance applications, use Faraday's Law. I doubt I would have caught this.
This clears up a lot for me.
"My mom thinks I'm mostly OK" Words to live by brother.
From Maxwells equations, the electric field E = minus grad V minus derivative of magnetic vector potential A. Obviously, mathematically, the sum of grad V around a loop is zero (~V2-V1+V1-V2). The question is, does the voltmeter measure grad V or E or something else in portions where A matters. It clearly doesn’t measure grad V, because the result depends on how you position its leads. The magnetic field affects the leads of the voltmeter and induces an additional E, thus current, inside them, which depends on how you position its leads in this magnetic field. The additional E is given by the rate of change of the total magnetic flux (magnetic field times area) through the closed loop formed by the meter leads connected to some circuit element. So, even if your leads follow the wires of the circuit, when you flip their position perfectly, grad V changes sign, while the additional E doesn’t flip sign. So, to eliminate it, the loop of your meter leads have to be parallel to the magnetic field (zero flux).
Oh, I worked in that field - years ago. The KVL is derived from "E = -grad( phi )" and the corresponding integration theorem.
If there is an time varying magnetic field, the true electric field is "E = - grad(phi) - (dA/dt)" . I.e. the KVL holds in the electro-quasistatic approximation assumption, that dA/dt is approximately 0. The KVL is false otherwise.
Kirchoff's Voltage Law always works under one of two possible conditions. The first condition is that you wait long enough for the system that you are measuring voltage from to stabilize, or alternatively that it is a lumped parameter system (this would assume that the wires connecting components are negligible). The concepts explaining why this is so are summarized under the topic of transmission lines.
With the help of the telegrapher equations, derived from Maxwell's equations considering a source-free space and the transmission lines that carry the signal, one can analyse the response of a signal in a circuit looking at it as a wave that travels at a fraction of the speed of light. This fast moving signal does not behave according to Kirchoff's voltage law until it has reflected and super-positioned onto itself sufficiently many times to stabilize to the expected Kirchoff's voltage. In other words, Kirchoff's voltage law is not wrong, it's only wrong to apply it to non-stabilized circuits or with non-zero length lines (which would not be a lumped parameter model) between components.
With regards to the measuring of two different voltages between the same two points I completely agree with ElectroBoom. Not even at the fast speeds of waves traveling on transmission lines, given no reflections, will you be able to measure two different voltages at the same points. This has less to do with KVL and more to do with simple equa-potential of a point.
The potential difference between two points in a medium, from Maxwell's equations, can be defined as the integral of the Electric field between the two points along some chosen simple path. Now, in the case where the B-field is changing, you can simply treat the total E-field as the sum of the regular E-field and the induced E-field (which is the partial time derivative of the magnetic vector potential) and voila, you have a net E-field from which a specific potential difference can be calculated. Having one equation and one unknown implies, from simple mathematics, that it can definitely be solved.
never argue with Dr. Walter Lewin.. he will Le Win
Nobody is arguing. It's more of a healthy scientific debate on an important topic.
😂
😂😂
@Daniel Stefanov Me right now: (chuckles) I screwed up.
What is this, a rage comic?
Dear Mehdi Mercury, I am (was) an electrical freshmen. After watching all your videos over the weekend, I decided to switch to Business & Management because I can no longer solder or plug in something to the outlet without imagining sparks.
Lol
Well that's a negative effect.
Don't worry, this panther will carry the EE torch.
I agree. I'm an OLD electronic tech/engineer and have seen this sort of thing come up as a problem in a industrial installation.
Medhi, I love your channel. I'm sorry to say both of you are arguing past each other... I'm a physicist who plays an EE at work so let me see if I can offer my 2 cents...
The secondary of the transformer you refer to is an effective lumped element representation of the mutual inductance between the solenoid and the loop with the resistors. There is a secondary for the sense wire loop as well.
However, the lumped element approach isn't general. The mutual inductance has to be calculated for a particular geometry, not a circuit, using you guessed it... Faraday's law. So you can only model the secondary if you have already solved the emf for the system. Lumped element then can be generalized for the flux profile so you can change the waveform, which makes it a powerful approach for circuit analysis, and makes the EE masters happy because they can just treat it like a circuit element and do normal AC analysis.
See the appendix of Clayton Paul's book Intro to EMC for a fully worked example of this exact treatment.
Now give me a damn oscope please.
Stand in line Cheyne..lol...I want the black siglent scope.. It would look good next to my Rigol...peace brother.
I ran that through Google translate...still no wiser :(
Cheyne Scoby or in other words, there are variables that apply in specific conditions.
Cheyne Scoby "physicist" tsss.. amateur
I love this about a person of science. Challenge even the most established idea. Your humble nature shows, and is really appreciated. Good vid, as you demonstrate how the exact positioning of sensory wires makes a massive difference.
Great discoveries are made by those who question the leaders of the field
@ That's the most ridiculous thing I've read today. Theories are never against reality. That kind of thinking bottlenecks our progress in science.
@@nullbeyondo No theory describes reality perfectly, it is pretty obvious he talks about science's continuous ability to find better and more precise theories by challenging its previous ones all the time.
@E it's not about being right or not, it's about testing, challenging and learning, that's the entire point of science. And most of the things ever tried don't work, but when they do, then we move forward, slowly but one step at a time.
Any good teacher knows this.
@E I think he is, tbh. Old people become so bonheaded that they refuse to listen to younger people.
And for Dr.Lewin to call him uneducated when his arguments are valid is immoral and i don't like him for that reason...
It's like trying to tell an old mechanic that what he is saying is wrong, they will just scoff and say "i have been doing this all my life so you can´t teach me anything"
@@PonaHD ElectroBOOM said some things right and other things wrong.
Here’s one example, said at least two times:
10:54 Nope, the voltage across two points in the presence of a time-varying magnetic field is not unique. Theoretically (if you’re computing the induced voltage), the induced voltage depends on the path taken to compute the line integral of the electric field. Practically, it depends on how you position the cables of the probes, as Lewin and EB showed in their respective videos.
You may say “it’s bad probing”, but I’d reply the following. As was demonstrated in the videos by Lewin and EB, voltage depends on the two points as well as the path. Now I ask you: “what’s considered the correct path to measure an open-circuit voltage, and why?”. I think the answer is: there’s no wrong path, all paths are correct.
11:48 Wrong. There was *always* two different voltages across the two points. Again, voltage depends on the path taken when computing the line integral of the E field. What you claim to be "the only voltage between two points" is actually the voltage assuming a particular path; assume another path and you'll get a different voltage.
---
Here's another example:
8:41 In the circuits being studied at this timestamp, certainly the RL circuit with the switch and battery acts as one winding of a transformer, and the circuit with the two resistors only acts as another winding of the transformer, so EB's explanation is correct here. But what if instead of creating the time-varying magnetic field with the RL circuit we instead create it with a moving permanent magnet? I wouldn't consider the magnet as a primary winding of a transformer, would you? So in this case we can no longer consider the circuit with the two resistors as the secondary winding of a transformer. So EB's explanation is no longer valid.
It's very brave of you to challenge someone like that. Keep it up
Well, I am surprised that Dr Lewin made this nonsense statement.
Ofcourse Kurshoff laws hold. At least in this case, the problem is simply linked to the probing.
The problem was clear from the beginning. And coming from a good experimental fellow like him, I am really disapointed.
I spent my entire PHD developing a magnetic localisation system and if i have learned one thing during those years of experimental setups is to check and double check your wiring. Every bit of wire can act as an inductor and depending on which direction you wind it compared to the source of the varying magnetic field, it acts differently.
I remmember running to a similar problem when building my first coils. A little untwisted wire absorbs a suplement of signal that completely falsifies the magnetic localisation by few centimeters (for a body parts localisation system, this is a big deal). Back then, I didn't even think of blaming Kirchhoff. I just started by checking my wiring and understood the problem in no time.
Physics don't lie. Some laws are so established today that one must start by checking the experiment setup before even if it is Dr Lewin. But who doesn't make mistakes?? It happens to the best of us.
I am a little disapointed though.
Where are the sparks?!!!!!!!! Not even a little shock?!!! Is this electroboom or the school of electronics?
saifeddine ALOUI he’s not wrong. Mehdi puts it in such a way that it seems wrong, but KVL does actually fail when fields and physical structures are involved.
Imagine a thick metal hollow tube carrying AC current. Due to skin effect the current in the outside surface is larger than the current on the inside surface. Due to resistance, the voltage drop along the outer wall is larger than that along the inner wall.
Take a loop going into the tube at the top, all the way to the other side, and close it around the outside.
Now the sum of voltages is not 0.
Wish i could draw it.
kirchhoff's law holds only in cases the circuit size is much smaller than the wave length that passes through the circuit from the source. This is called a Quasistatic approximation, wiki link below.
The simplest example is an antenna:
An antenna broadcasts an electromagnetic field to the environment by a changing current that runs inside it. If Kirchhoff's law was true, there would be no current running through the antenna (it is cut off by KCL KVL laws).
en.wikipedia.org/wiki/Quasistatic_approximation
@@noamgraham9006 I understand the antenna example and the quasistatic approximation.
But the experiment presented by Mehdi in this case is consistant with his explanation. The problem is not that the voltage difference is not the same if you measure it from the left or from the right. The problem here is that he used an extention of the wire that is embedded into the varying magnetic field. It is basically a setting problem and not a fundamental physical error. At least in this specific experiment. I
'm not arguing with the fact that when you pass a signal with a small enough wavelength, the Kirshhoff low doesn't hold. I'm just talking about this specific experiment with the specific outcome.
Here is a summary I found on the net on the limitations of the KVL and KCL laws:
KCL is applicable on the assumption that current flows only in conductors and wires. While in High Frequency circuits where, parasitic capacitance can no longer be ignored. In such cases, Current can flow in an open circuit because in these cases, conductors or wires are acting as transmission lines.
KVL is applicable on the assumption that there is no fluctuating magnetic field linking the closed loop. While, in presence of changing magnetic field in a High Frequency but short wave length AC circuits, the electric field is not a conservative vector field. So, the electric field cannot be the gradient of any potential and the line integral of the electric field around the loop is not zero, directly contradicting KVL. That’s why KVL is not applicable in such a condition.
As you can see, both has limitations when it comes with high frequency (short wavelengths) which hopefully allow for open circuits like antenna to work.
This took some head scratching but I think I see the disagreement here.
I usually explain KVL by analogy to elevation: if you walk in any closed loop here on planet Earth, you must necessarily have ascended exactly as much as you descended, because you end up at the same elevation.
Dr. Lewin, I'm sure, would tell me that elevation is an incomplete analogy for voltage. If there's wind, for example, it'll be harder to walk into the wind than have the wind at your back. In this analogy, a changing magnetic field in a loop would be like trying to walk around a small cyclone: go with the cyclone's rotation and the wind is at your back, go against the cyclone's rotation and the wind is in your face. In that situation, "voltage" becomes harder to define because which path one takes between point A and B affects the work required to get from point A to B.
Mehdi is arguing that the "wind" from the "cyclone" can be incorporated into the model, and so the "voltage difference between two points" becomes well-defined once more depending on where the cyclone's wind is worked into the circuit. In a sense they're both right and folks should really just go with whichever model helps them accomplish whatever they're trying to do.
(But if you have two pieces of equipment measuring the same thing and they disagree, something's probably up and you need to recheck your experiment.)
More people need to read this comment!
There simply is no potential in this case. In your analogy, that's like trying to find the elevation of a point on Escher's staircase. Voltage is still defined, but depends on the path. That's the whole point.
@@Kalumbatsch I'd hesitate to use the Escherian Stairwell as an explanation for what's happening here, since it can't actually occur in the real world. Unless you mean to use the self-contradicting nature of the stairwell as a proof-by-contradiction that the elevation model isn't adequate for this situation? In that case it sounds like you're agreeing that the analogy is limited. (Sorry that I'm having difficulty understanding what you mean - the content of your reply appears to be a summary/reinforcement of my comment, but it reads more like a refutation/correction and that dissonance is throwing me off.)
Well, you said "Dr. Lewin, I'm sure, would tell me that elevation is an incomplete analogy for potential energy."
It isn't, elevation is a perfect analogy for a scalar potential, because the gravitational potential is one. But in the case of a changing magnetic field, there is no electric potential. There is no way to assign one to every point because there is no potential that the electric field is the gradient of. Escher's stairwell is what you get when you apply the analogy to this situation.
Voltage is explained as a difference in Potential between two points. Here is the problem : Potential is a term that is mathematically defined. You can only have a potential if you are in a conservative Vector Field. A conservative vector field is equivalent to having any loop integral to be 0.
*You can't just can't use the term potential in a non-conservative Field, because "Potential" is a well defined mathematical term !*
As someone uneducated in the field, I came to the same conclusion before you finished talking. I would love to learn why you would be wrong as I currently don't see it. Your arguments make perfect sense to me. The probe wires are part of this experiment when it is setup like this.
I'd go a step further and say that this is one of the cases where the fact that components are not, in fact, ideal, mathematical abstractions of components comes into effect. The wires have resistance and inductance.
I came to the same conclusion already when watching Dr. Lewins video.
A wire going trough a magnetic field can't just be assumed to have zero voltage. No matter how low its resistance is, it will always have some inductance that reacts to the field.
I could see however that looking at it from a theoretical point of view one could come to such an conclusion. In such theoretical examples you can never include all parasitic effects because the whole thing would just become a mess that literally takes weeks to calculate my hand. Its easy to miss a single significant parasitic effect and still have the math work out and seam logical. While on the other hand me being an engineer who deals with practical problems i know that a trace on my circuit board is not a perfect ideal connection between two nodes. I know from experience when a SPICE cirucit simulation behaves strangely that i need to model in some non ideal or parasitic effect. I have been bitten in the ass before by "simply ignore parasitics because they are so small they don't matter", my circuits didn't work how i wanted and every time it happened i learned how that particular parasitic effect is important and how to include it into my design process. You can't remove parasitics by simply ignoring them, you can only understand them and design them in as part of your product.
@@skonkfactory Even if the wires were ideal with zero resistance, there would still be a voltage across them. There isn't a voltage drop being measured across the wire. There is a voltage induced in the wire due to magnetic induction. If the wire were ideal, it would just give you a more accurate reading. The mistake that was made was not treating the circuit like a loop, but instead as separate parts. If you were to do the same with a transformer, you would conclude that KVL does not apply since the voltage applied is much higher or longer than the voltage read. But we know better since the change in voltage is found by measuring the coils.
@@megamixa Right, exactly- the wires have inductance (specifically, mutual inductance with the driving coil).
@@megamixa An ideal wire would have zero loop area and zero mutual inductance. It would literally be a circuit node of zero dimension.
This video illustrates why I subscribe to your channel. While the majority of your videos have an important entertainment value, they're based on scientific principles. I appreciate the level of critical thinking you are able to apply to the many scientific laws of electricity. Thank you for the work you put into your videos.
Sadly, he missed the point of the proffessor. By definition KVL wall is not akways true, BUT you can fix that in practice. From scientific point of view the proffesor is rigth, from practical point of view it does not matter or almost
As a medical student I have no idea what I'm doing here lol
It’s the mystical power of his eyebrows 😂
you are not alone dude..hahaha
😆 well you did learn something!
Same.
When you have to use a defibrillator this might come in handy.
The Kirchoffs Voltage Law used by Dr. Lewin and Electroboom are different. I think Dr. Lewin uses line_integral(E.dl)=0 as KVL, which definitely is incomplete, as this only says that the sum of voltages around a loop caused only by an external electric field is zero. According to this the voltage across an inductor is zero, as it is produced by a magnetic field and this is where Faraday's law comes into picture.
But the KVL used by Electroboom is "the sum of voltages around a loop (caused both by magnetic and electric fields) is zero". This includes the induced voltage across the inductor (L*di/dt, which actually is a consequence of Faraday's or Lenz's law).
So now it all comes down to which of the above versions was originally proposed by Kirchoff in his original paper.
Wwwwhaaaaatttttttttttttt?
_"it all comes down to which of the above versions was originally proposed by Kirchoff"_
Neither. Kirchhoff only talked about electromotive forces (emfs) and resistances. Remarkably, Walter Lewin also talks about an emf in his circuit, but he doesn't take it into account for his version. ElectroBOOM's version was first formulated by Maxwell and this is the version that is used to analyze circuits, see Feynman's lecture notes: www.feynmanlectures.caltech.edu/II_22.html#Ch22-S3
Take a circular loop, place an electromagnet and try to find the position of polarity of the induced emf- you can never find it, because it is not like a simple battery.
@@Theo0x89
"see Feynman's lecture notes: www.feynmanlectures.caltech.edu/II_22.html#Ch22-S3"
Feynmann wrote: "by making the many approximations we have described in Section 22-2 and summarizing the essential features of the real circuit elements in terms of idealizations, it becomes possible to analyze an electrical circuit in a relatively straightforward way."
And here is one of that approximations:
"In particular, we assume that *the varying magnetic field is restricted to a definite region in the vicinity of the coil and does not appear outside the generator* in the space between the terminals."
This assumption is clearly wrong in the case of the arrangement shown by Dr. Lewin. This is why no correct discrete model can be constructed. In strict meaning this assumption is not true for the majority of the real circuits. It is rather an approximation (as referred in 22-3). And the error of this approximation can be enormous in same cases as Dr. Lewin demonstrated. I have experience with even worse results. An error of these approximations can happen to be 40 dB higher than the quantity to be measured. This is why Lewin's example is not just correct and interesting, but important also for engineers.
Kirchoff original paper does not have varying electric or magnetic fields. So Lewin is right.
9:56
"Why did we read 0V across the sense lines?"
Because the path is different. At 9:26, your measurement loop encircles the flux, and the scope’s resistance allows a a path for the current to flow which arises from the induced emf from that flux. At 9:40, you’ve arranged your measurement loop such that it doesn’t encircle any flux, so there is no induced emf in that loop.
In both cases, your scope is responding to induced current caused by the flux change thrugh the loop it forms with the probes and the wire arc. In the second case, there is little to no flux passing through the measurement loop as it doesn't encircle the solenoid -> no induced current -> no measured voltage. at 9:47 you find that you can get different results depending on how much flux you enclose. I can draw a picture if this isn't clear.
10:03
"If the voltage here is +V"
But... It isn't. There isn't a localized potential difference there. Your previous measurement of +V depended on the path. Then you changed the path. This is covered in Belcher’s notes in the second video.
But my intuition with this one is still shaky... Faraday’s law specifies that in the circle strictly around the loop, not including where the probe wires diverge, there is still am emf of 1V. In this particular circle, the impedance is high so there would be no resultant current across the gap, but how should we think of or visualize this EMF? imagebin.ca/v/4Qi4j6Awhq4w
This all sounds like simple oversight? I saw the problem immediately with the way the sensor wires were arranged at time of sensing, almost seems like he's intentionally trying to fool people. A magic trick..but terrible
Kvl is a simplified form of Maxwell's equation obtained by lumped matter discipline in which we assumed dphi/dt = 0 I.e zero change in flux.
Electrical engineers compensate dphi/dt change by taking into account inductance/emf on equivalent schematic. dPhi/dt exists as voltage induced in the coil/wire. So electricans actually use... Maxwell equation with dphi/dt "hidden" as emf and everything works perfectly. So... they both wrong?
Being an accomplished physicist does not preclude him from being wrong.
But he happens to be right. Its not hard to see that inducing current round a loop means the voltage also goes up round the loop one-way, so can never sum to zero.
Being a youtuber does not mean you know how to teach. The professor was giving a lecture to students, and showing them how not knowing the limitations of Kirchhoff, will produce wrong results. He made an experiment... he did not take the limitations into consideration (he did not correct the result by use of clever probing)... he obtained the wrong result... he showed his students this fact... his students now remember that the limitations need to be taken into consideration and do correct measurements or whatever. That is all. All these people on youtube showing how the experiment should have been done, are completely missing the point of the experiment.
The E field produced by a changing magnetic field is rotational and therefore can't have a defined potential and thus its values between two points depend on the path along which you make the measurements, as long as you believe in Maxwell's equations and vector calculus you must accept this even if you're able to come up with a specific example where they happen to be the same.
Exactly, the E-field is non-conservative if B is changing, ie curl(E) depends on dB/dt - mathematically that breaks Kirchoff's law by definition.
@@MarkTillotson Bur Kirchoff deals with not Electrical fields, it deals with voltages - so curl doesn't actually tell you much here since you don't need derivation; you need integration.
@@lux_expat But voltages ARE defined with respect to electric fields. V is defined to be the line integral of E, along a path. In electrostatics, V will be path independent, while if you include changing fields, V is no longer path independent. This is why "the way you probe" now matters. Now one way of "saving" KVL is to think of the emf as a "potential", which is ElectroBoom's point of view.
"Saving" it works in practice; one can measure "voltages" such that KVL will hold. But on a fundamentally theoretical level, KVL already broke when the system included the changing magnetic field.
@@DamianDailisan That's my intention to emphasise too actually :) Voltage is potential (at least it is how we are taught :) )
@@lux_expat voltage is like a difference in potencial electric energy only in electrostactics or in low frecuencies
8:53 Dr. Lewin ended up not being a nice guy
Nah he’s a jackass like all old smart fucks who think they know everything.
@@outros5062 is it real
@@anantapadmanabhmyatagiri you have internet. Try look it up. As far as the time goes, it is real.
@ your reply made me laugh so hard that my ribs are paining now.
He's a damn good physics teacher..... that's it! He's not a good person at all, just watch his interviews( search physics wallah walter lewin interview)
The first serious video in this channel 😂
A lot of love to Elctro Boooom
Serious question: how is it possible that such a fundamental question doesn't yet have a solution that the scientific community agrees on? It looks very weird that nobody has encountered, and explained this before with some sort of peer review or it's just Lewin getting it wrong?
Yes
Seems like Lewin is the black sheep in this debate. As both Lewin himself and Mehdi note, the majority of literature suggests that KVL holds in general.
Makes sense then. Cheers
@@ElectroBOOM /r/inclusiveOr
@@ElectroBOOM regarding your video for the charged comb
No i don't think that current flows just because thecharge is moving .but the question is that if the current flows where would the charge flow to . there must be a circuit or path for the charges to flow and be considered current .for example take the case of me having a battery with me as i travel in a car the battery had charges and the car is moving that would not be current .
I'm really confused can someone explain me where I'm wrong
I find it a little disconcerting that a professor at a leading university who is well accomplished in his field proposes a radical change to a commonly accepted theory and then doesnt want to argue it because he is tired of arguing. Extraordinary claims require extraordinary evidence. It is a basic tennant of science imo. You dont get to make a great claim and then refuse to have the conversation and thereby win by default.
sjm4306 Not comparing him to Lewin by any measure, but we had an EE professor at my somewhat reputable school in Canada who'd make outrageous and false claims (for instance, at one point trying to convince us KCL did not apply to a node in a circuit) to justify obvious mistakes in his notes and solutions, and then would get mad at people arguing with him, never backing off. He was also very corrupt grading wise.
He was far from being the only professor that got me wondering how the hell they made it into academia.
As part of a student group, I also once had a chat with a recent Nobel physics laureate that visited our school. He didn't make any outrageous claims (not that we'd be in any position to argue with him anyway,) but was also very full of himself and rudely dismissive of many comments.
Not that I'm saying Lewin is like this, I personally loved his physics lectures and watched them all, but I always thought people at places like MIT didn't suffer from this attitude, and always looked at it in high regard for exactly this reason, and this is highly disappointing.
@@f4dy "Not that I'm saying Lewin is like this"
No, Lewin is saying that through his replies, just look at the discussion at this link (highlighted comment being ElectroBooms): th-cam.com/video/nGQbA2jwkWI/w-d-xo.html&lc=UgwnKTFxT6OD-POhOc54AaABAg
To me the worst part is that this means that any *other* faults in his lectures will remain without any corrections from Lewin since he is ignoring all other experts (textbooks, etc) and won't even look at a video that shows why he's getting the results he's getting.
@Steve Yzerman Fan what do you mean?
He's retired and old as hell.
he's a grumpy old physicist who is bestowed in his ways after teaching this numerous times. It would be a kick in the nuts to know that his theory was wrong all along due to shitty probing
1:20 everyone.
Imagine this world if just more people had Mehdi's scientific and philosophical humbleness
@@therealb888 that's racist
This is a very good point, though several times before when I watched the video but didn't realize how important it is. The matter of this question is how to model the real physics system: we can model the magnetic field by inductances and transformers as electrical engineering, while physicists may look at PDEs and have less emphasis on lumped circuits. As for the probing, that's another vital lesson I have learnt, because I was lucky enough that I didn't burn the scope with ground circulation with two passive probe at a region of high di/dt, quite similar as here.
Every wire in a practical circuit may be considered an ideal resistor and inductor, no matter how short it is or how low resistance it may be.
"If you do not agree you need to be educated...This is very very basic physics and I never argue with people who think they know but who do not... The stunning demo at the end of my lecture..."
His attitude doesn't mean he's wrong about this particular thing, but it does mean he's not worth listening to.
His attitude also means I'm not at all surprised about this:
news.mit.edu/2014/lewin-courses-removed-1208
@@87knox meh... nowadays everything can be considered sexual harassment if one lady decides it is one.
This attitude is simply badge flashing. Which is sad for a scientist.
In this case it is at least 11 women he was found to have sexually harassed, and my quick skim through Inside Higher Ed's January 2015 article on the matter shows that MIT was provided plenty of evidence and did a thorough investigation into said evidence
@@666aron not when the claim has supporting proof,
Read this www.insidehighered.com/news/2015/01/23/complainant-unprecedented-walter-lewin-sexual-harassment-case-comes-forward
Why do I watch this? I understand nothing.
I've asked myself that very question.
I'm as smart as a bag of rocks. All I hear is scientific gibberish
@@johnfarris6152 If you don't understand the terms you're learning nothing our minds are great but they're not magical
just learn 4Head
I’m trying to learn through osmosis.
lol
Two truly good and decent men struggling to teach the next generation!.. they should be proud of each other!.. Im proud of you ElectroBOOM!
Mehdi is soo near to 2M...
He is a electronics channel not chemistry how can he be near to 2 moles?!
@@alanwolf313 hahaha
And every single one of his subscribers is probably a uni student
@@alanwolf313 Lol, That's BRILLIANT!
@@alanwolf313 M actually refers to the concentraion c , M = Molarity
You just invented Polite Roasting
He lives in Canada, eh? :D
Electric Blanketing, then?
That's been a staple of good science since forever..
Like a true Canadian!
I will watch this in front of my family, so they will think I'm smart
😂😂
@@animeshpathak3921 Really? Come on there is no need for that, Apologize.
Ok.
😂😂i think that i am the only one
Clever man😂😂😂😂😂
I am a law student who desperately wanted to study physics. I've searched for law stuffs, and TH-cam suggested me this. Thanks to TH-cam for realising my heartaches.
Ouch, love of physics is really strong
So did you quit law to study physics?
I sent this to some of my science teachers, im curious of what they are gonna say about it.
You have all my support👊
I watched that class a couple of years ago and I found it very weird. I assumed he was talking about some advanced definitions and that the model of the "imaginary" inductor as a voltage source was a way to make KVL work out for induction machines. Now I feel so much satisfied with your reasoning ♥. Also I like to imagine the straight wire as a collection of tiny inductors in series, even the resistor itself acting as one, so that is NOT ok to assume 0 voltage drop just because the resistance negligible
A better demonstration would be to magnetically shield the sense wires so that the measurements still read the same on either side of the circuit. That takes the variable out of the equation.
Unfortunately it only takes some of the variables out. EG What sort of scope was it measured on? EG some digital scopes have a problem with measuring some types of fast transients of different polarities due to them having Jfet inputs (to have very low pseudo capacitance). I found this out the hard way.
@@gordonlawrence4749 Perhaps, but didn't he state that the load of the scope shouldn't have a significant impact? The reason the numbers are so different (I believe) is because of the second loop that was unaccounted for from the sense wires.
@@macadameane gordon doesn't know what he's talking about. It's evident when he starts talking about different types of oscilloscope. You're right. it's all about the reversal of the sense wires. Just a rudimentary example. most of the flyback power transformers have just 5 windings for the secondary. EG the one that charges your phone. 5!! yet it can generate five volts. meaning 1 volt per winding. If you reverse that wire like the sense wire was reversed, you get -5 volts. # you can not violate lenz law, faraday's law or kirchoff's laws. The Dr forgot that if you reverse the direction of a conductor in a field, then the current reverses. We can therefore deduce that he didn't know what he was doing.
YES! I saw the transformer, too. The sense lines are part of the circuit! (I was just an ME, not an EE, although I am a licensed ham for what that's worth.)
Why don't you churn out a thesis paper on this?
Because there is no dispute. All he did was replace the changing magnetic field with a transformer to make kirchoff's laws work because you can remove the magnetic field from the circuit. Thats nothing new
Some of the biggest idiots I know have doctoral degrees.
Because he isn't proving anything new, like he said most of science agrees with him but Dr. Lewin does not (but he didn't write a paper on it), so simply he's disagreeing with Dr. Lewin's disagreement with this law in physics.
that's all well and good but perfectly acceptable and _useful_ papers have been written on less.
Science needs rigor, and a paper isn't pointless just for confirming something widely believed to be true.
@@DrummerRF The argument is that there is a "hidden" inductor created through the measurement device, is it not? Which is why this is up for discussion in the first place. Proving that through math and experimentation surely could have a paper written on the results. Doesn't necessarily need to be a thesis paper.
I think one have to do more practicals than reading theories. But sadly our school curriculums have no time for practicals, so only 15% learning.
ElectroBoom sir, you're a great master. Thanks for this.
The way you explain it, it seems so obvious, but I have a Master's in Civil Engineering, so what do I know? I studied water.
Solomon Parker There must be something wrong. I was under the impression that civil engineers studied only concrete and steel
Water flow is immensely more complex than this to me. Even as only a second year Electrical Engineering student, I look at Dr. Lewin's lecture and immediately sense something wrong, despite how experienced and knowledgeable he is.
However, if literally anyone told me anything about pipes and flowrate and stuff I'd probably believe it because fluid mechanics is a complete enigma to me despite having had to take it twice
@@smlgd There are four main disciplines in CE. Structural (what you speak of) Transportation (roads, pavements), geotechnical (soils, landfills, underground structures) and Environmental (water, wastewater, air treatment). Had to study them all, focused on the latter.
@@BonJoviBeatlesLedZep Water and electricity are very much analogous on the basic levels, until you get to things like EM fields and capacitance and the higher stuff.
Water is important too. Possibly more important.
WoW ! - I feel like I'm watching two gods fighting each other on mount Olympus and I have a BSEE degree.....
Very well described! I work with small signals from microphones and phono cartridges and your video explains extremely well what happens in low inductance and low resistance loops and induced currents. Well done!
I completely agree, im a janitor and i got shocked once so i know my stuff.
hehehe. Use kirchoff's law to determine the total number of people leaving in your apartment.
Are you a Janitor who works at MIT and anonymously solves problems given to students?
Yes that would be me friend, i then sell the answers to students.
I'm with you; your reasoning is solid. So you've basically done the same experiment and shown Lewin's conclusion to fail on reproducibility after accounting for a straightforward explanation. This is why replicating a result before accepting it is so important: especially from an experiment claiming well-established laws to have an exception that is not predicted --- Kirchhoff's law withstands scrutiny.
Dracolith1 Lewin was too quick to think one of the greatest laws do not hold true
Intuitivly KVL and KCL form part of the conservation laws that underpin physics. That is current in equals current out and voltage in is dropped over the circuit. So if you are to prove these wrong even for one special case care must be taken. Induced EMF is difficult to measure experimentally because as others have said it is hard to remove from the measurement process. Always assume that long standing theories rule until conclusively proven wrong. Though Maxwell was a genius and his contributions to electronics cannot be understated.
No. KVL does not hold if there is a changing magnetic flux through the loop. That's a simple fact.
@@alexandermilleriii499 great law? How great? Can it fit into a house? It has a graet reputation because of its simplicity. It is useful because of its simplicity. But it's too simple to be true generally, as demonstrated. Maybe it worth trying to expand its validity, but to make it valid generally its usage should have been made extremely complex, losing its main benefit.
@Kalum It is not a FACT that it does not hold. Electroboom shows how it does hold in this example -- with this fairly orderly flux by solenoid;
KVL can still be accurately applied to the situation. That does not necessarily mean there may not be other more extreme configurations which may exist that it is possible where
it could be argued KVL might or might not be able to be shown to hold or be consistent with measurements, such as what does it look like with a circuit when an irregular elongated magnetic field is applied to a segment that is growing and collapsing, not regularly behaved.
i like this way of experimentally trying to prove someone wrong. It's a nice and a humble way to disagree with someone of such high caliber.
It calls for a epic rap battles of history
Martin lewis versus Electroboom
🤣🤣 that would be awesome
He is Walter Lewin
Who is Martin Lewis? It's Walter Lewin
Hahaha lol
can you show how to change a lightbulb? I already tried 2 times and almost killed myself with resistance and capacity
Have you tried keeping an anglerfish, you know the fish which grows a bulb?
I think it was the voltage that almost killed you ;)
He already did this th-cam.com/video/-oRCvLtnYMY/w-d-xo.html
have you tried turning it off and on again?
@@tobywenman4769 well that was not helpful at all. now I'm dead from all the electrons and what not. why you let me do this by myself?
I am probably more wronger than both Mehdi and Lewin (I only have bachelor in applied physics), but here is my input:
I would like to point out that it is not something groundbreaking. Basically Kirchhoffs law is a simplified case of Maxwell's laws of electrodynamics and/or electronic version of conservation of energy. Now what Lewin is doing here is saying "in case of external magnetic field, this law does not hold and needs an extended version", so he provides a Faraday's law that assumes, that system is not isolated and energy comes from a dynamic external magnetic field. Mehdi has another interpretation - he says "dynamic external magnetic field is a part of system and we can model it as a transformer". This is not unusual - different approaches - if correct (in this case - they all follow Maxwell's laws of electrodynamics) - they should yield the same results.
Now for the correctness part - I would say Mehdi here is more correct (something that might come from experience) - physicists have a very hard time modeling their measurement instruments as a part of the system - at least in other way than providing possible errors - so I can easily see Lewin ignore the fact that wires leading to voltage meter form some kind of loop and are affected by external magnetic field.
But both approaches should yield same results because they all are correct with Maxwell and we ignore this pile of crap that QED is.
That's a fair point. But, as I recall, the physics interpretation is that the voltage field is no longer single-valued when you have a changing magnetic field. In fact, the demonstration could have been changed pretty easily to get any value you want at all, theoretically -- just need to coil the wire a bit more in one direction or another, maybe add a few loops, and hey, you can get a spike of π, or -e, or 9001V if you really want. You can get the same numbers if you account for parasitic effects, but at a certain point, it's less worth it to cling on to your old concepts and more worth it to just invent new ones.
@@PersonaRandomNumbers
Electric field has a very nice property: it's conservative. This means we can actually define an electric potential, which itself is a scalar field. This is what actually makes 2nd Kirchhoff's law possible: with no changing magnetic field, you are guaranteed to start from a point A that has some electric potential, walk over any loop back to point A and end up with the same electric potential value you started with - that translates to "to move charge q from A to B, you have to do work that is dependent on position of A and B, but not on the path taken between them".
Now when you add a changing magnetic field to the mix, electric field loses that property. That means that walking with a charge around a closed loop requires doing work (positive or negative) so mathematically, Kirchhoff's law does not hold.
Now there are ways around it. Magnetic field is very specific and has to satisfy some conditions which allow you to define a vector field that is called magnetic vector potential. After doing some math tricks you can define electric potential using electric field and magnetic vector potential - this is often used in quantum physics and electrodynamics because you have to deal with nabla operator a lot there anyway.
Another one is a blackbox approach - you define a "transformer element" that "does something inside that isolates rest of the circuit from this violation of conservativeness and outputs voltage" - which is possible because this is physics and physics has only one constraint - it has to match the reality. Whatever happens in transformer, stays in transformer and the rest of the circuit can enjoy holding Kirchhoff's 2nd law.
My name Jeff
@@NessHX Makes sense -- basically, either extend the old idea to be more general, or make up a new thing and throw it around to make the problems go away. One problem with the second is that it's really hard to figure out the parameters describing that new thing without resorting to doing the first, anyways.
@@PersonaRandomNumbers I immediately remembered the Feynman's lecture (somewhere on YT) on physicists vs. mathematics subject and him saying something like "guessing new laws seems to be an efficient way to make progress in physics". In case of Mehdi's video I would actually say that the blackbox approach (include transformer in the circuit) is more elegant and actually gets us somewhere - mostly because every time I had to include magnetic field somewhere I always felt annoyed by how it works. Good methods of hiding problems are always in demand for engineering purposes.
bro curl(E)=-B_t (where B_t is partial derivative of B with respect to t) is a maxwell eq. it is always valid. while applying kvl u assume that E is grad of some scalar function, if a vector field is grad of a scalar func, then it must be curl free. however we know that curl(E)=-B_t, thus kvl is not always valid, which assume curl(E)=0.
Richard Feynman would be proud of you for disagreeing with a master
You've got confused by the probing issue - the fact remains that if you sum the voltage changes around the loop you get a non-zero value. Consider a uniform loop, current is flowing round the loop, so there is a voltage gradient around the loop, meaning that the electric field is not a conservative field.
There has to be a voltage gradient to drive the current through the resistance of the loop wire, an EMF, but that
EMF sums up to a non-zero value, breaking Kirchoff's law.
Where we you when i was studying? That's some beautiful explanation.
Studying electrical engineering and watching your videos is the best combination
New to the channel (probably one of the best TH-cam channels yet). Love the video! and as an EE student, this totally makes sense, and I couldn't agree more. Keep them coming!
Does Kirchhoff's Law Hold? Yes. It always holds. Thanks for clearly shows that the induced emf by the magnetic field is actually an voltage source in the closed circuit.
john smith the power plant transmits at high voltage 500kv 250kv 115kv (69kv has been phased out in most regions of the US) then broken down at a substation for distribution in the 7.2kv range, in some areas 3.6kv this is primary distribution voltage, the secondary service output voltage from the distribution transformer is your AC voltage source. Surges and voltage drops are common, and must be factored in.
@john smith I know I’m late as hell, but this really is the best argument for why Medhi’s approach to interpreting the results is correct.
EB, professor Lewin is right! Your measurement is wrong based on the points you consider on the loop! Ill tell u why.
Firstly, note that the TOTAL GENERATED EMF has to be the one generated by the changing mag flux, therefore by faradays law.
At 11:07 you added V1+VR+V2 which is not correct because the induced EMF in the left branch is purely a Faradian EMF (path dependent), so is V1+V2. This is like a battery for that loop so VR must be VR=V1+V2 since the change in flux through the entire loop is used to generate the EMF in the circuit!! ( VR is the DROP IN POTENTIAL in that branch!!)
Secondly, for an accurate measurement of the voltage across R1 you should connect the voltmeter right at the ends of the resistor, and since the wire going from R1 to R2 has negligable resistance we can consider them to be at the same potential( no voltage drop across). So if you were to measure right at the tips of the resistors you could neglect any additional voltage induced in the measuring wire , therefore the only voltage being measured would be VR1 for this purpose!!!
You should try measure at the tips of the resistor and see that Dr. Lewin is right !
Please answer me back and tell me what you think
One of my subscribers just turned me onto your videos. Absolutely magnificent stuff your brilliant! Going through most of your videos now it's going to take me awhile, but I'm having a blast. Thanks for sharing
The Wikipedia article on Kirchhoff's circuit laws explains this under Limitations:
"The current law is dependent on the assumption that the net charge in any wire, junction or lumped component is constant. Whenever the electric field between parts of the circuit is non-negligible, such as when two wires are capacitively coupled, this may not be the case. This occurs in high-frequency AC circuits, where the lumped element model is no longer applicable. For example, in a transmission line, the charge density in the conductor will constantly be oscillating."
"On the other hand, the voltage law relies on the fact that the action of time-varying magnetic fields are confined to individual components, such as inductors. In reality, the induced electric field produced by an inductor is not confined, but the leaked fields are often negligible."
I think it’s really a matter of technicality, if you are willing to look at the circuit as two separate components, a Transformer / voltage potential source, and a circuit not subject to changing magnetic flux, then you can use KVL and get an appropriate result, because you are factoring in the contribution due to Faraday’s law. However if you are to analyze the circuit in its entirety you must use Faradays law as using only KVL gives no indication of the potential difference between any points in the circuit without a source. I’ve watched Professor Lewis’s entire 8.02x lecture series, and really appreciate his contribution to education in physics. But I think this fight is rather pointless, we all agree on the results, we just don’t agree on the classification of our methods of analysis. Engineers like to think about KVL as just adding the potentials through a loop, and if the result isn’t correct it’s because you’ve neglected a term. Physicists such as Dr. Lewin like to think of KVL as a special case of Faraday’s law, simply because it doesn’t hold in every case unless you factor in the contributions from Faraday’s law, which happen to be zero most of the time.
" KVL gives no indication of the potential difference between any points in the circuit without a source. "
No, KVL still holds true - at no point is it violated. The Problem is that Lewin is ignoring the effects his measurement-setup has on the outcome - when measuring with changing magnetic fields you need to make sure to account for that and - like Medhi has shown.
"we all agree on the results"
You say that commenting on a video that shows that Lewin is interpreting his faulty measurements as a breaking of a fundamental law of electromagnetism.... yeah no.
Show us a single instance where it doesn't hold up and you'd be a candidate for the nobel price.
This is the best response actually. It's a matter of how much you take into account
@@ABaumstumpf Kirchhoff laws don't hold when the current varies too rapidly. KVL is _not_ a fundamental law of EM. Maxwell's equations are (at least in the classical limit). When the current varies, you get radiation. And radiation takes out energy from your circuit, so the sum of the voltages comes out negative. Of course you have to get to very high frequencies to even notice the effect.
Citing wikipedia:
_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.[2] 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.
It is often possible to improve the applicability of KVL by considering "parasitic inductances" (including mutual inductances) distributed along the conductors.[2] These are treated as imaginary circuit elements that produce a voltage drop equal to the rate-of-change of the flux._
To be precise, the general formula for the electric field is: *E* = - *∇* V -∂ *A* /∂t
ABaumstumpf or you could use Faraday’s Law which takes into account the voltage potential generated by changes in magnetic flux. I’m with you here, KVL is not violated, IF you account for the effect of Faraday’s law, but KVL does not take such effects into account by itself. Thus it simply is a matter of opinion, does KVL need to account for the contribution due to the changing flux for it to hold true, or is it the responsibility of the person using it to factor in that term? You seem to think that one needs to factor in the contribution or physics will break, but realize that Faraday’s law simply states that the closed loop integral of potentials is proportional to the rate of change of magnetic flux. In an instance where magnetic flux remains constant you get KVL saying that the potentials around the loop is zero. If you apply KVL with the contribution from Faraday’s law you get a modification of the equation Faraday’s law originally gave you with all the terms on one side and equal to zero.
A schooling in Metrology101. Step1: Check stray inductance, capacitance & noise. Step2:
Marry me and give me scope.
KVL holds very true for conservative electric field. However, when you apply a time varying magnetic field through a loop
, along with induced emf, a very special type of induced electric field is also generated which can form closed loops (thereby its non conservative) hence KVL cannot hold for this case and hence the whole notion of lumped circuit model fails . If the magnetic field is changing very slowly with time you can ignore these effects but if your magnetic field is largely changing with time, you cant ignore the non conservative electric field. The uniqueness in faraday's law is that it combines both results into a single equation. Hence faraday's law holds true for all such situations but not KVL. Speaking in more practical terms, all generators work by changing the orientation of magnetic field (not magnitude).
But shouldn't KVL still work if you lumped the wire's inductance into a "secondary coil" like Medhi has?
Other people have stated however that this IS a conservative field, and has no curl. KVL holds because the problem is with the sensing, not KVL
@@drawapretzel6003 This is not a conservative field. One way to interpret Faraday-Maxwell equation into words is to say that a time-varying magnetic field gives rise to an electric field WITH curl. Since the E-field has curl it is not conservative.
@@twbbrandon KVL cannot hold in the presence of a time-varying magnetic field, or a non-conservative E-field. Faraday-Maxwell equation tells us a time varying B-field gives rise to an E-field with curl.
Your "noise" measurements are the inductance of the two wires in parallel and EMF induced into the circuit and the untwisted portions of your lead. Reason why it differs is they are in different positions one time closer, another time farther away, or other times not totally parallel with each other. In order for you to get an accurate reading you should use shielded leads from point to point or have your untwisted leads, 90 degrees from the measurement point. And if you really want to get picky, the probe itself will affect your measurement as it becomes part of the circuit. Because, depending on the frequency and voltages being measured, you can introduce capacitance or attenuation.
I see what the professor is saying... when a circuit is being effected by EMF due to the circuit component characters, wire and connection resistance, until the EMF finally peaks, the circuit will behave and measure differently in different spots and will not equal zero. Depending on the placement of the EMF and the measuring devices you can get, either a positive or negative peak. You can see it in your noise measurement, those ripples are the varying voltages until the circuit stabilizes. Due to each component characteristics and connecting wire length the circuit at times, the total voltages will not equal zero.
In addition, EMF waves are formed in the circuit which can be out of phase until the circuit "catches" up. And you also have angular velocity, field flux, stray field and cross lines to account for until the field stabilizes. Plus, just arbitrarily setting the circuit over the coil doesn't allow for accurate readings...
When the law was written, they did not understand I-V characteristics, nor could they measure it or the circuit accurately.
Plus you are measuring with a single scope... measure it will multiple scopes and compare the timeline of the voltages measured in the circuit and you will see.