Textbook about Inductance - Page 1
Forum section: electronics / beginners

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 13, 2020, 08:34:11 am
content: I was reading this when I was confused by the introduction about inductance - How can we get to (2-52) from (2-51)?This is a page from an old book -- Basic Electronics for Scientists. I found this book very good.

author: Moriambar
author_rank: Unknown
timestamp: April 13, 2020, 12:00:41 pm
content: You basically multiply the first by the area occupied by the circuit (A).Ampère's law states that B, the magnetic field is proportional to the current (first equation). Then the total magnetic field that is contained within the area of the circuit is the first equation multiplied by the area

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 06:14:23 am
content: Quote from: Moriambar on April 13, 2020, 12:00:41 pmIn the face of it, it obviously is. The problem here is 2-51 is about a conductor (presumably straight), so how could it hook up with the area of a circular-shaped wire and get 2-52? Am I missing anything very obvious?I also struggle to understand 2-51 -- how could the magnetic field strength at apointsome distance away from the conductor dependent on the length of conductor(l)? Imagine a very long conductor the ends of which is very far away from the point of measurement. Will more length of the conductor add more magnetic field strength to the point (still proportional to the conductor length)?!

author: Circlotron
author_rank: Super Contributor
timestamp: April 14, 2020, 06:52:25 am
content: Quote from: max.wwwang on April 14, 2020, 06:14:23 amIf the inductor is a coil of wire then more turns with the same current = more field strength. Unroll it and the situation remains the same I expect.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 08:01:43 am
content: Quote from: Circlotron on April 14, 2020, 06:52:25 amAt least this is extremely unintuitive - I know it would be silly, or at least premature, to say the formula in the book is incorrect. Admittedly the situation seems a bit different for a coil comparing with a straight conductor (still not very helping though). But imagine a straight conductor, would an increase of one inch of wire 1,000 miles away have the same effect as the one inch of wire added very close to the point of measurement?! Crazy!I very much like the way this book introduces theideaof magnetic fields. It's very precise and responsible to say that -- albeit very much in line with the observations -- it's only an idea (or conceptual aid) of dealing with such things like electromagnetic induction. We cannot assume this reallyishow things work!I cannot get away from the impression that this topic is treated without clarity unlike the rest of the same book (I dare to speculate it's a result of the author's lack of full understanding hence lack of confidence?). But as a reader I cannot be very sure. I'm also conscious and cautious about the fact that English is not my first language. Do you also find the reference to "circuit" (underlined, especially the first one) very abrupt? (I would be surprised but still happy if you all say NO, but even this would be very helpful to me.)

author: westfw
author_rank: Super Contributor
timestamp: April 14, 2020, 08:22:30 am
content: QuoteWell, of course it does. At some fundamental level, the magnetic field is generated by moving electrons, and the longer the wire, the more moving electrons you have. Presumably you could derive the equation by integrating the field caused by each electron over all the electrons involved. (though I'm not exactly sure what "r" becomes in that situation - shortest distance? distance to each point?)As the length of the wire gets very long compared to the distance from the wire (as in almost any real electronic device), the r2term dominates.There's a lot of this sort of thing that happens in physics. Edge effects that are usually left unconsidered. Another obvious example is the electric field between the plates of a capacitor: assumed to be a pretty constant value. But it isn't, near the edges of the plates.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 08:33:27 am
content: Quote from: westfw on April 14, 2020, 08:22:30 amI see your point and agree that the length should have some effect (but my agreement is limited to this point). Its effect (as I understand), however, is not a stronger magnetic at the same given point, but rather more points (the same distance away from the conductor) you can have the same magnetic field (along more length of the wire).Of course, I could be wrong.Quote from: westfw on April 14, 2020, 08:22:30 amI take it as the shortest distance from the point (of measurement) to the (straight) conductor with the line along the distance perpendicular to the conductor.Quote from: westfw on April 14, 2020, 08:22:30 amI'm unable to see how r^2 would dominate. This does not change the fact that the magnetic field effect is always linearly proportional to length.Quote from: westfw on April 14, 2020, 08:22:30 amI don't have any problem with the edge effect. There is Saint Venant's Principle in mechanics and many other similar things in other disciplines. That's exactly why I use an extremely long wire (more than 1,000 miles long) as an example.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 09:00:12 am
content: At one point I asked myself - is it fundamentally incorrect to think of a straight conductor without a loop because current only exists in a loop? This isn't true - imagine a very long straight wire (one end in Beijing the other in Washington. And to make this precisely accurate it goes through the earth, not over its surface which would make it curve-shaped) and have a metal plate at the end of Washington. Suppose one man at this side approaches the plate with a ball with a great amount of negative charge. There will be a current flowing from Beijing to Washington along this wire, won't there?

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 14, 2020, 09:22:51 am
content: Quote from: max.wwwang on April 14, 2020, 08:01:43 amthat formula is a good recap for me to remind/correct the fundamental idea of an "inductance" that it is actually proportional to the length and the idea of "magnetic flux". maybe we are confusing things with "flux density". flux density should be different when one wire is straight than when its coiled into compact geometry. but both will have the same amount of "flux". no? so for your wire washington to beijing, there will be a great deal amount of fluxes involved and a great "inductance" ie opposing to current change (di/dt) so when one applied a statics on the other end, you should feel it few seconds later on the other end, not instantaneously, due to inductance effect.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 09:29:13 am
content: Quote from: Mechatrommer on April 14, 2020, 09:22:51 amNo, I'm not mixing up flux density (i.e. magnetic field strength) with flux. I understand the inductance should be proportional to length - particularly easy to understand for a coil because similar more rounds of wires are working together to multiply the flux (and flux density). But the flux density for a straight conductor is different to me - the extra length of wire only creates another similar field (of the same density in its own surrounding space) but not adding to the field of its adjacent segment of wire.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 09:30:37 am
content: Not to sidetrack the discussion, but I watched this videowhich is also about electromagnetic induction and found he experiment from 48:20 or so is very eye-opening and thought-provoking.Not only other professors referred to by professor Lewin, I deeply double the experiment (though, of course, I don't think he is cheating; and my doubt or difficulty in understanding is not a surprise). But I think he may have been misled by some physical setup of the experiment. How arrogant I sound!

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 14, 2020, 09:48:16 am
content: Quote from: Circlotron on April 14, 2020, 06:52:25 amr is the distance of wire from the field being measured. if you add 1 inch wire in beijing, divided by great distance of r squared, it doesnt matter anyway, µ is already a very small constant.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 09:57:38 am
content: Quote from: Mechatrommer on April 14, 2020, 09:48:16 amThat's true. But the effect of this one more inch of wire hasexactlythe same effect of the inch of wire right crossing the plane containing the point and perpendicular to the conductor (i.e. closest to the point of measure), no matter how great 'r' might be and how small effect it is for any one inch of length. Let's assume the point of measurement has the same distance to both ends of the conductor (i.e. in a plane that is perpendicular to the wire and cuts the wire into two segments of equal length).Let me clarify, 'r' does not change for a given point if you add more wire along either or both ends of the wire (fixed relative to the point). In other words, adding another 1,000 miles of the wire at one end of it onlydoublesthe flux density at the same point, according to 2-51 (which does not seem right to me).[Edit - this is rightly presumed based on the wording of the book but it's incorrect. Refer to #18 for the correct meaning of "r".]

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 10:20:24 am
content: I guess perhaps formula 2-51 holds only for a coil (not applicable to a straight conductor) but this has been made clear in the book (I mean this page; this is the very beginning of the discussion about inductance in this book)?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 10:34:40 am
content: I found this formulahere, seems more convincing than 2-51 --It doesn't include the length of conductor in it, only I and r.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2020, 11:31:14 am
content: Quote from: max.wwwang on April 14, 2020, 09:00:12 amWhen this is done mathematically, the return path happens at infinity. There must always be a loop, yes. Magnetism is meaningless without loops (at least until monopoles are discovered).If you're looking for the inductance per length of wire in free space, it depends on geometry. A multi-turn coil has much higher inductivity than a loop, and a large loop has more than a small loop (um, hm, relative to wire diameter I think?). There is no single answer, but they are all proportional to the inductivity of free space, mu_0. For conductor pairs of prismatic cross-section, this is a transmission line property; together with e_0 and the geometry factors, you get Zo and c_0 (impedance and velocity).Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 14, 2020, 12:19:16 pm
content: and then there's permeability that can affect inductance... formula is for air... here if you want the more truth, it will involve derivation... have fun...http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/amplaw.htmland it seems somehow l.sin(Ø) = 4πr (assuming infinitely long wire?), and then it can be deduced to your later formula. go figure... i want to make circuit...

author: pwlps
author_rank: Frequent Contributor
timestamp: April 14, 2020, 04:03:09 pm
content: Quote from: max.wwwang on April 14, 2020, 08:01:43 amYou are perfectly right, the explanation is misleading. But the problem is rather subtle and I have seen many bad explanations.Ampere's law is an integral version of a local law (Maxwell equation) : integral of B on a contour around a conductor is proportional to the current in the conductor. From this you can calculate the magnetic field only in a situation of infinite straight conductor where the cylindrical symmetry simplifies the problem. There shouldn't be any "l" in 2.52 if it is said to be a consequence of Ampere's law, period.QuoteActually 2.51 is the Biot-Savart law. This is also an integral law telling how to sum contributions from different portions of a circuit so that it is usually written as dB=const*I dl sin(theta)/r^2.In the limit of very long conductor the contributions from the ends will progressively vanish because of sin(theta) which goes to zero, therefore B is NOT proportional to the conductor length.The Biot-Savart law is ONLY correct (equivalent to Ampere law) if integrated over either an infinite or a closed loop conductor. Otherwise it would obviously violate the charge conservation.But the funny thing is that an incomplete (finite conductor length) integration has a well defined physical meaning, this comes from the fact that combining Biot-Savart with the continuity equation (charge conservation) yields Ampere's law:https://physics.stackexchange.com/questions/495625/derivation-of-amperes-law-from-biot-savartAs they point out, Biot-Savart "is inconsistent with the continuity equation. We don't know how to generalize Biot-Savart, but patching up the problem with the continuity equation in the simplest possible way yields the correct Ampere's law".What does this means physically ? A simple interpretation is that integration of Biot-Savart over a finite length conductor will yield a magnetic field that would be measured if we put e.g. two charged spheres (acting as a charge source/sink) at both ends of the conductor.The Ampere's law is recovered because we now have varying electric field from the spheres, this varying electric field gives a contribution to the total magnetic field which happens to compensate exactly for the missing part of the conductor, from the actual ends to infinity. Finally electromagnetism laws are very coherent and robust if we know how to interpret them correctly

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 08:53:49 pm
content: Quote from: pwlps on April 14, 2020, 04:03:09 pmThanks for seconding my understanding! That's very helpful and encouraging. I also found "l" in 2-52 very difficult to understand. With below in mind, I daresay both 2-51 and 2-52 are not only misleading; they areincorrect.QuoteThinking about 2-51 and theta again and again before I went to bed last night, without resorting to any textbook (proudly!), I reached the same thing (though only in the form of speculation)-1) both "B" and "l" should actually be differentials, i.e. dB and dl; and2) we must take the vector (referred to as "radius vector" in the book) as starting from the differential segment of conductor, dl, to the point of consideration.Without this, the inclusion of theta in the formula is meaningless - would always equal to 90 degrees otherwise. (And the term "radius vector" is very inaccurate and misleading, if not incorrect!)With the above understanding, I now perfectly understand that every unit length of conductor doesnotplay an equal role in contributing to the flux density measured at the given point. It's only proportional to sin(theta) which approaches sin(0)=0 when dl goes to infinity (relative to the point of measurement). This now makes sense to me.I need more time to digest the rest of your reply. Thanks a lot. I cannot overstate how helpful this is to me.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 14, 2020, 10:48:57 pm
content: I had more thinking about the experiment at the end of the MIT course when I was running this early morning. I've become reasonably confident now that the professor was misled by and hence misinterpreting the observations of the experiment. Of course, I don't believe he intended to cheat but, unfortunately, he was only cheated (by the experiment). Will follow up when I have the thoughts tidied up.

author: pwlps
author_rank: Frequent Contributor
timestamp: April 14, 2020, 11:23:16 pm
content: Quote from: max.wwwang on April 14, 2020, 08:53:49 pmYes, the Biot-Savart law is more clearly expressed in its vector form:https://www.quora.com/What-is-the-vector-form-of-Biot-Savart-lawQuoteI agree, I wouldn't recommend this book if you want to understand the underlying physics - notwithstanding its appealing title it seems lacking basic scientific rigour (Written by an EE ?)

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 15, 2020, 03:52:24 am
content: Quote from: pwlps on April 14, 2020, 11:23:16 pmDoh! Although I'm not an EE I have great respect for them, especially after I had a revisit, recently, of the achievements made by them during last century (especially the lower half) until today and the impact on the life of all of us. I couldn't help thinking, at least at one point, perhaps most of the smartest people were working in the field of EE.It's a pity the book I'm reading is so short of scientific rigour!Very keen to have your thoughts on the experiment in the MIT course.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 15, 2020, 08:58:32 am
content: Quote from: pwlps on April 14, 2020, 04:03:09 pmI have been looking for the theoretical formula for the inductance of a straight conductor but have not found one by Google search. According to what you are saying here, it seems the inductance of a conductor (integral) is meaningful only if either it's straight but infinitely long or it's a loop?

author: pwlps
author_rank: Frequent Contributor
timestamp: April 15, 2020, 10:47:27 am
content: Quote from: max.wwwang on April 15, 2020, 03:52:24 amAh sorry for this bad joke (but I couldn't resist), of course I have great admiration for EE too. I remember when trying to understand some RF devices I was wondering how they can have such a great insight into the working principles without the use of advanced mathematical calculations.QuoteAs already pointed out by T3sl4co1l there is always a loop and pure straight wires don't exist.That said straight infinite wire concept can be a good approximation in some circumstances. Take a loop of very thin wire (wire radius much smaller than loop dimensions). Then the majority of the flux comes from the field very close to the wire which varies as 1/r so that it can be approximated by a straight wire: as the contributions of remote parts of the loop are negligible we can integrate to infinity getting an approximate inductance per unit length proportional to log(1/radius). For intermediate cases there are approximate formulas combining together different regimes, see e.g.https://en.wikipedia.org/wiki/Inductance#Inductance_of_a_straight_single_wireQuoteDidn't have time to look at the video yet, maybe this evening.

author: gcewing
author_rank: Regular Contributor
timestamp: April 15, 2020, 11:32:58 am
content: I don't think you can trust this book.The first equation is not Ampere's Law. Ampere's Law is a relationship between the line integral of the magnetic field around a closed loop surrounding a conductor, and the current in the conductor.I found a similar equation in my university physics textbook (University Physics, 5th Edition, by Sears, Zemansky & Young) on page 552. It's for the field at a point on the centre line through a circular loop of wire; r is the distance from a point on the loop to the measurement point, and theta is the angle between this line and the centre line.I can't make any sense of the second equation. If it's supposed to be for the total flux through the loop, then there shouldn't be an r or theta in it at all.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 15, 2020, 12:11:48 pm
content: It may be helpful for you to work through Vol. 2 of the Feynman Lectures:https://www.feynmanlectures.caltech.edu/II_toc.htmlIt's a very thorough and comprehensible treatment of EM theory, and is pretty easy reading to my mind - Feynman does a good job of handholding from the elementary stuff up to the pretty advanced. The only potential downside is that it's a very mathematical treatment but Feynman also develops a lot of the necessary vector calculus from scratch in the early chapters - OTOH if you're already familiar with div, grad, curl, and line, surface, volume integrals, it should be just up your street.

author: pwlps
author_rank: Frequent Contributor
timestamp: April 15, 2020, 10:24:08 pm
content: Quote from: max.wwwang on April 14, 2020, 09:30:37 amThere is nothing special or magic in this experiment. Look at the equivalent circuit I made and you will understand better.EditActually he did cheat a little because the circuit drawn on the blackboard does not show where the flux goes and where exactly the voltmeters are connected. I agree that his drawing was misleading, maybe deliberately to stimulate students to think about it.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 16, 2020, 05:05:50 am
content: Quote from: pwlps on April 15, 2020, 10:47:27 amHaha no need to apologize! No one would doubt that's only a joke.And the smart and strong EEs don't need me to defend.QuoteThanks. I was aware of the Wiki practical formula but I didn't like it. I wanted an accurate theoretical one.QuoteNo obligations! It's too big an ask for an hour's time!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 16, 2020, 05:12:12 am
content: Quote from: gcewing on April 15, 2020, 11:32:58 amSadly it seems I have to agree with you. I quite liked this book before this. And I had a blind-eye faith on books with some good age.QuoteWould it breach a law terribly if flicking one page for discussion? Thanks.QuoteThis makes me feel much better. Thanks.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 16, 2020, 05:19:00 am
content: Quote from: aneevuser on April 15, 2020, 12:11:48 pmThis is some good stuff we can trust! Thanks. Ages ago I learned things like div, grad, and these sorts of integrals but I need some good head-scratches to pick them up! There used to be aSurely You’re Joking, Mr. Feynman!on the bookshelf but don't know where it is now.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 16, 2020, 05:32:23 am
content: Quote from: pwlps on April 15, 2020, 10:24:08 pmThanks for this but I'm not quite following. I wandered the same thing and posted a comment below the video wanting to know the 'physical' (not electrical) setup of the experiment. I got a long response to my comments, which I haven't followed up.Some of my thoughts --1) In this single (closed) loop, the place to attach the probe(voltmeter)matters. There are no longer things like equiv-potential points there any longer. Imagine a loop without resistors (or any components other than wire), there'll be a voltage drop betweenanytwo points along the wire, however close they might be. Superconductor does not work here because that would result in a current of infinity withanyamount of EMF;2) Don't be fooled by the word (and idea of) "measure". Attaching a voltmeter with wires is no different from adding another component (and one more loop) in the circuit. The only difference between the meter and a resistor is it's smart enough to tell you the potential difference across it. Since the geometry (including position) of the loopmattersin this case, it of course alsomatterswhere to attach the meters!Responding to my comments below the video, the professor mentioned the "insult" that was given to him by somebody referring to "bad probing". I sort of agree with this comment (on the problem of probing, or 'measurement' if in my term) though itself is tongue-in-cheek (when in this case crystal clarity is of grave importance). Having watched ElectroBOOM's video (see #37), I should say ElectroBOOM has made it crystal clear (by both experiment and analysis). It's not so clear only because of the brevity of reference (not an error).

author: pwlps
author_rank: Frequent Contributor
timestamp: April 16, 2020, 02:40:12 pm
content: Quote from: max.wwwang on April 16, 2020, 05:32:23 amI did not search for your comment and answer but if you copy it here it might be interesting and might help to understand his lines of thought.QuoteYes, you can put it simply (in my opinion) considering that the whole circuit is immersed in a varying electric field (generated by rotE=-dB/dt). The field outside an infinite solenoid is circular (not sure for the proper term, I mean only tangential component) thereforenon-conservative for any path going around the solenoid(it would still be conservative for other paths). Since rotE=0 (there is no B there) it has to decrease as 1/r (solution of the Maxwell equation rotE=-dB/dt=0 with cylindrical symmetry and using the boundary conditions on the solenoid surface obtained from the solution inside the solenoid where dB/dt is uniform and E~r).QuoteNo, the current wouldn't be infinite, it would just be enough to cancel the total magnetic flux across the superconducting loop. This also happens when you short the output of a transformer : at high enough frequency when the winding resistance can be neglected the current in the secondary winding will be such as to cancel the magnetic flux (the transformer then works as a "current transformer"). The only difference between a very good conductor and a superconductor is that such flux-cancelling current would also be observed for a DC magnetic field (see Meissner effect and London equations, but this is far beyond the actual problem). Note that such superconducting DC current transformers are used e.g. in SQUID magnetometers, they are called "flux transformers".QuoteYes, but I still believe you can relatively well model such a circuit with discrete components, using transformers. In my schematics I supposed that voltmeters are connected very close to R1,R2 so that the magnetic flux across loops comprised of voltmeter connection wires can be neglected (usually we use twisted pairs to minimise magnetic flux in order to avoid this kind of error measurements).Below is another variant, much closer to the schematics drawn on the blackboard. Note that L4 and L5 cannot be much smaller than L1 and L3 (if the circuit is to be physically realisable).QuoteWe can't judge on the quality of probing as long as we don't know the exact physical layout of all wires (if he refuses to show it I'm suspicious). If the wires are exactly as drawn on the blackboard it IS bad probing indeed. But, good or bad probing, it doesn't qualitatively change the result if I compare the two simulations.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 17, 2020, 06:20:32 am
content: Quote from: pwlps on April 16, 2020, 02:40:12 pmCopy and paste below. Wait -- did I say thank you?! My goodness! I really appreciate the time you took watching the video and doing the nice simulation!max wang3 days agoI don't understand the experiment in the end!Lectures by Walter Lewin. They will make you ? Physics.Lectures by Walter Lewin. They will make you ? Physics.3 days agoyou are not alone. Tough Physics!!!max wangmax wang2 days ago?@Lectures by Walter Lewin. They will make you ? Physics. Simply I cannot believe that putting a meter at the other side of the circuit , while still connecting to the same to points, would change its reading. If anything, it's the loop of the wires connecting the meter that make the difference. I would like to see the physical (not electrical) setup of the experiment, not only the display of the scope or meter!Lectures by Walter Lewin. They will make you ? Physics.Lectures by Walter Lewin. They will make you ? Physics.2 days agoI appreciate the fact that this Physics may not be familiar to you. Don't feel embarrassed about it; you are not alone. However, it's never too late to get educated. The choice is yours. Maybe this will help you.I demonstrated at the end of my lecture #16 of my 8.02 E&M course at MIT that two identical voltmeters attached to the same 2 points in a circuit can show very different values.The reason is that in the case of an induced EMF (Faraday's Law) potential differences are no longer determined; they depend on the path. This also applies to the secondary windings of transformers as the EMF in the closed loop of secondary windings is induced.Of course, in cases where Kirchhoff's loop rule (KVL) applies, 2 voltmeters attached to the same 2 points in a circuit will always show the same value.My demo was first suggested and published by Romer in December 1982 in the American Journal of Physics. This demo has now become a classic; it's done all over the world at many colleges and universities.Kirchhoff's original text can be found in the following link, pages 497-514:https://books.google.de/books?id=Ig8t8yIz20UC.Clearly he was fully aware of the prerequisite for his "loop rule". KVL is a special case of Faraday's Law. That's why Faraday's Law is one of Maxwell's equations and KVL is not.By teaching students that KVL always works without telling them when it does not work, makes many believe that the closed loop integral of E dot dL is always zero. ElectroBOOM and Dirk Van Meirvenne therefore believe that 2 voltmeters attached to the same 2 points in a circuit must always show the same value which is not true as demonstrated in my lectures. They each posted a video (see @ footnote below) on their channel in which they claim to have proof for their wrong ideas which violate Maxwell's equations. Apparently they do not know, or do not understand, that in the case of an induced EMF potential differences are no longer determined; they depend on the path. MIT students who took my 8.02 course (Electricity and Magnetism) would not make this mistake! I therefore believe that to introduce a "modern version" of KVL and then teach students that KVL always holds is not advisable as you may set them up for making the same embarrassing mistake that both Dirk Van Meirvenne and ElectroBOOM made.Also read Professor John Belcher's thoughts on Faraday's Law and on KVL. Professor Belcher is my former colleague at MIT.http://freepdfhosting.com/0813df09f5.pdf(@) ElectroBOOM insulted me in his video by mentioning that the reason why my 2 voltmeters read different values was due to "bad probing". He stated that if I had done the demo correctly I would have found that both voltmeters would have read the same value. He could not have been more wrongThis is my only msg on this issue. This is too basic. I do not argue either with people who believe that the Earth is flat.This video may also help some of you to digest why 2 Voltmeters attached to the same 2 points in a circuit can give very different readings.https://youtu.be/T6S5VS03xZc@max wangQuoteThanks.QuoteMy bad. This is so obvious! (But first time heard of SQUID. Nice.) I said this so soon though I should have known the current would not be infinite. It's not like - an EMF generates a fixed voltage across a conductor (then we get an infinity current on a superconductor with this FIXED voltage). This is NOT so obvious and is very subtle. I would not have said "it's so obvious". It's NOT very obvious.QuoteI did think about using twisted wire for the meters as well.QuoteI don't want to hasten to a conclusion too soon (or be judgemental) as well. But stay critical. Maybe I will do the experiment myself.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 17, 2020, 08:42:56 am
content: I was very surprised when I saw Professor Lewin in his videowas still saying things like "voltmeters at different sides of the loop measuring exactly the same two points" ...In my opinion, the reference to "measure" or "same two points" is themost misleading.As he now correctly drew on the whiteboard (as below), the voltmeters, when "measuring", are now a part of the circuit. Given the (externally) induced current and, in turn, voltage drops, the voltmeters seemingly attached to the "same points" in the circuit but when at different sides of the varying magnetic field (which is vertical to the loop's plane), must show different values depending on the resistance of the resistor at the same side of the voltmeter in parallel. The wires connecting the meters now have also emf induced current.In my opinion, it would help a lot if he had stopped saying "measure", or saying "attached to the same points". These are meaningless now.That being said, what's in my mind was the second diagram which will also give different readings of "measurement" but with the difference without another induced loop current around the V/R1, or R2/V. I'm not saying his diagram is wrong - now only he knows the exact layout of his experiment.Also, he keeps talking about "people without decent/solid physics background" ... He really doesn't have to say this. I know my background is neither solid nor decent, but I'm now reasonably confident that, at the very least, he failed to point out the key point of why the experiment gives seemingly surprising observations (if he was not fooled).

author: pwlps
author_rank: Frequent Contributor
timestamp: April 17, 2020, 07:29:31 pm
content: QuoteHa, but his answer is not about physics at all, it really looks more like a police questioning of a suspect.Well, if he is attacked it is maybe because he is unable to give a clear and convincing explanation.I'm teaching physics myself (though didn't teach electromagnetism so far) and frankly I am shocked, this guy seems to be offended by a simple physics question, I can't believe it! After all your post is just a question about the interpretation of his "different paths" so I don't see why he feels offended.I know how hard it is to find where the student makes error in his reasoning, very often it needs many iterations trying to shed light on the problem from different perspectives etc. But he only blindly repeats"...depends on the path": we can recognize a bad professor when he just rewinds his speech repeating the same statements on any question. Good teachers are those whoencourageasking challenging questions, only bad teachers feel offended by them.I grew up with Feynman books which showed me what explaining things means, very often when something was not clear in a book or a lecture I would search in the Feynman lectures and my problem was soon solved. And Feynman himself was emphasising, in the book you cited, how all "stupid" students' questions were important and stimulating for his own understanding of physics, it seems many examples given in his lectures stem from such philosophical discussions with students.His comment"This is my only msg on this issue. This is too basic. I do not argue either with people who believe that the Earth is flat"is insulting and unworthy of a MIT professor.QuoteSomehow he forgot that HE is actually the one supposed to give such a solid background! I think I would fire himWhatever your background I find all your questions legitimate and well formulated. I myself had plenty of this kind of questions when I was a student and even today I would never pretend I cannot be fooled, physics is so subtle.QuoteNot sure he "correctly drew" because he still does not show where the magnetic flux goes and the voltmeters are not attached to the "same points" in the circuit as he pretends. Moreover, if there is magnetic flux across the loops labeled "I1" and "I2" then the probing is not good anymore.But at the end the important thing is that YOU now understand how to attack the problem.QuoteVery good idea! As an experimental physicist I often preferred to start by experiments when I had difficulties with understanding the theory.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 17, 2020, 10:26:11 pm
content: Quote from: pwlps on April 17, 2020, 07:29:31 pmI cannot be more agreeing with this. That all said, I really like professor Lewin's blackboard handwriting - that's not just amazing.Also, this discussion is the highlight of my life in the COVID-19 lockdown. Thanks to all involved.

author: Manul
author_rank: Super Contributor
timestamp: April 17, 2020, 11:32:44 pm
content: I remember watching a video from ElectroBOOM about this. And I learned that it is professor Lewin's famous experiment which is somehow causing big debate.Never understood what is even confusing about the result? His goal was to measure between points A and B. But his way of measuring basicaly modified the circuit and he measured something else which was orginal circuit + measuring circuit. So he did not measure between points A and B. This reminds me of math tricks where you are presented with a "proof" that for example 1=0. It is just a hidden mistake to get you tricked.What is shocking is that profesor seems to not admit, that he measured something else. Physics people are strongly familiar with concepts such as "measuring always affects system being measured" and so on. They normaly have very good understanding of how to get true measurements and not the effects of measuring itself. Until of course they meet Mr. Heisenberg at some point, but that is another story.

author: rfeecs
author_rank: Frequent Contributor
timestamp: April 17, 2020, 11:48:19 pm
content: FYI, already discussed with only 42 pages worth of posts:https://www.eevblog.com/forum/chat/does-kirchhoffs-law-hold-disagreeing-with-a-master/

author: Manul
author_rank: Super Contributor
timestamp: April 17, 2020, 11:51:47 pm
content: I guess what is different about EE people is that after:1. Crystal oscillator is not working! Oh.. wait.. my cheap probe is 20 pF...2. Voltage is too low! Oh...voltage source is high impedance...3. Current is less then calculated! Oh.. so that is called burden voltage...4. Enter your failed measurement experiencewe get better at measuring

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 17, 2020, 11:59:57 pm
content: Quote from: Manul on April 17, 2020, 11:32:44 pmThat's the hardest part (and a sad disillusionment to me - I have GREAT admiration for MIT), but beyond what we can control.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 18, 2020, 12:21:35 am
content: Quote from: rfeecs on April 17, 2020, 11:48:19 pmThanks, have just watched. Nice analysis and experiment. The judgement of "bad probing" is very accurate to me.We did the job with only 2 pages of posts (and only as a sidetrack of another discussion). I can say we excelled in terms of the (less) number of posts.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 18, 2020, 12:24:02 am
content: Quote from: Manul on April 17, 2020, 11:51:47 pm

author: max.wwwang
author_rank: Frequent Contributor
timestamp: April 26, 2020, 11:49:23 pm
content: I came across another video of ElectroBOOM--which refers to two useful articles , one of which has been specifically written on the experiment that ElectroBOOM did as a reaction to Profession Lewin's one.These two articles can be foundhereandhere.Just for records. I have not read either of these articles, so I'm not suggesting they are correct or not (though probably correct).

author: pwlps
author_rank: Frequent Contributor
timestamp: April 27, 2020, 12:04:00 pm
content: Quote from: max.wwwang on April 26, 2020, 11:49:23 pmRomer's paper is very nice! It is quite subtle indeed (eqs. 6 and 7 are the keypoint, though easy to overlook) but the discussion in the section V and Fig.6 shows clearly why Lewin's experiment cannot be analyzed without knowing the exact topology of the system.BTW I had missed this question:Quote from: max.wwwang on April 17, 2020, 06:20:32 amIt is obvious if you write the transformer equations, something like:V1=-L1 dI1/dt-M12 dI2/dtV2=-L2 dI2/dt-M12 dI1/dtoutput open: I2=0 -> V2=M12/L1 * V1output shorted: V2=0 -> dI2/dt=-M12/L2*dI1/dtIf there is no flux leak (as in a perfect iron-core transformer), with n1,n2 being the numbers of primary/secondary windings, then L1~n1^2 (B~n1*I1 and the flux~n1*B), L2~n2^2 and M12~n1*n2 hence M12/L1~n1/n2 and M12/L2~n2/n1

author: bsfeechannel
author_rank: Super Contributor
timestamp: May 13, 2021, 04:28:15 am
content: Quote from: max.wwwang on April 17, 2020, 11:59:57 pmIt never ceases to amaze me the damage caused by Mehdi Sadaghdar, the ElectroBOOM, in the minds of his audience by accusing Dr. Lewin of "bad probing".Mehdi is incapable of understanding electromagnetism and in a recent video he stated that he lives off Youtube and he has to do everything he can to get views. And that apparently includes passing his misunderstandings as facts.It's disheartening to see that kind of phenomenon, that we are used to see predominantly in politics, for instance, set a foot in engineering. Really troubling.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 13, 2021, 05:07:12 am
content: Quote from: max.wwwang on April 13, 2020, 08:34:11 amHi, who is the author of this book (Brophy or Eggleston) and what year was it published? Thx