Walter Lewin 8.02 Lect 16 Super Demo (Correction) - Page 1
Forum section: electronics / beginners

author: TheOCDengineer
author_rank: Contributor
timestamp: November 25, 2017, 03:25:34 am
content: Dear AllI recently watched Walter Lewins lecture on Faraday's law. In this lecture at the end he does a super demo in which he measures the same point with 2 different voltmeters and gets 2 results that are different.Video Lecture hereHe has a pretty large youtube channel and I posted a comment under this videohttps://youtu.be/wz_GqO-Urk4Here is the dialogTheEngineer9 hours agoThe math and theory hold up but in the physical set up how did you determine which oscilloscope measured which path? If they are measuring the same exact spot would it be defined? If I added a third oscilloscope what would it measure? Does the orientation of the probes have any effect? Do you have a picture of the set up, with the placement of the probes? Thanks for your time.?REPLYLectures by Walter Lewin. They will make you ? Physics.Lectures by Walter Lewin. They will make you ? Physics.8 hours agowatch this video. The current through each voltmeter determines the reading of the voltmeter and those currents vary in my case by a factor of about 10. watch this video watch this video?REPLYTheEngineerTheEngineer8 hours agoThe video does not answer which meter is V1 and V2. If I were to repeat the experiment is it possible that they would switch places? If I add another meter could it measure something completely different, or effect the measurements of the others??REPLYLectures by Walter Lewin. They will make you ? Physics.Lectures by Walter Lewin. They will make you ? Physics.7 hours ago>>>The video does not answer which meter is V1 and V2>>>you are mistaken. Watch the video again. V1 and V2 are clearly indicated. I1 goes through V1 and I2 goes through V2 and my calculations show that the ration I2/I1 is about 9 and that the currents are in opposite direction; I1 goes through V1 in the -- to ++ and I2 goes in ++ to -- direction of V2. Thus the ratio of the magnitude of V2/V1 is 9 and the polarities are REVERSED This is my last response. Please learn to watch!?I found him to be quite rude "Please Learn to watch" and he also did not answer my question.So here is my question to everyone.1. Has anyone made a replica of his set up exactly? Was anyone able to replicate his results?2. What would you measure with only one voltmeter?3. Assuming the voltmeters are exactly the same how would you determine which voltmeter would be V1 and which would be V2.4. What would happen if I add a third meter? What would it measure?Please assume the theory is correct and that he is violating KVL. Also assume he is measuring the same exact point.Thanks

author: IanB
author_rank: Super Contributor
timestamp: November 25, 2017, 04:41:19 am
content: When you see something you do not understand, it means you did not understand what you saw.It is for you, as the one seeking knowledge, to go away and work on the problem, and think about it, and watch the video over and over, and think about it some more, until you get it.I think the question you asked means you really didn't understand the experiment, and therefore you asked the wrong question. If you ask the wrong question, how can it be answered to your satisfaction?

author: IanB
author_rank: Super Contributor
timestamp: November 25, 2017, 05:06:48 am
content: Let me give you a hint. If you set up two voltmeters to measure the same thing, they both will show the same reading.In the experiment demonstrated, the two voltmeters show different readings, therefore they are measuring different quantities. If you configured both meters to measure the V1 quantity they both would show the (same) V1 measurement. Equally, if you set them both up to measure the V2 quantity they both would show the (same) V2 measurement.So now, what your hypothetical third meter would measure would depend on how you configured it. If you configured it to measure V1 it would show V1. If you configured it to measure V2 it would show V2. If you configured it some other way it would show some other (unspecified) voltage.

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 25, 2017, 07:20:59 pm
content: Looks like a double post? So I'll move my comments here.He is trying to amaze and impress his students by confusing them.At the end he claims that he connects two oscilloscope probes to exactly the same point and measures different voltages. "Lo Lehman" in the comments calls him out on this in a very long comment and points out that voltage is induced in the connecting wires, and Dr. Lewin responds:QuoteHere's his supplement to the lecture:http://web.mit.edu/8.02/www/Spring02/lectures/lecsup3-15.pdfClearly in the experiment there must be wires between the two resistors to form the loop. So the meters are not actually connected to the same physical points as he seems to insist in his comments.

author: IanB
author_rank: Super Contributor
timestamp: November 25, 2017, 07:38:37 pm
content: Quote from: rfeecs on November 25, 2017, 07:20:59 pmI think he is just saying that points A and D are wired as star connections, and therefore four wires meet together at a single point.

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 25, 2017, 07:45:43 pm
content: Quote from: IanB on November 25, 2017, 07:38:37 pmYes, then he is saying that two meters connected to the same points A and D measure different voltages. Absolute bullshit.

author: TheOCDengineer
author_rank: Contributor
timestamp: November 25, 2017, 09:10:16 pm
content: So I have been thinking about this problem the last couple of days and I think I have an idea to the confusion. Normally as engineers we think of the schematic as the components and their connections. Thus it holds no relationship of physical orientation. However in Mr. Lewin diagram it is also showing the physical relationship.I found this interesting video in which at the end he moved the pig tail loops from one side or another.https://youtu.be/-AjdUuq8JNYHis explanation is that there is a emf generated in the loop. But if we assume that there is no EMF generated in the loop ( most likely not the case) then this may be related to path selection. The problem that I have with Mr. Lewins demo is that meters and leads orientation aside there would be nothing to influence which meter reads what. I need to keep thinking about this.

author: IanB
author_rank: Super Contributor
timestamp: November 25, 2017, 09:41:31 pm
content: What Walter Lewin's demo shows is that the measuring instrument itself can be part of the circuit, in particular the wires that go from the test point to the instrument.This is actually not at all surprising, as it is commonly known that attaching a measuring instrument can affect the response of a circuit, and that the test leads can pick up signals that you are not intending to measure. In particular with oscilloscopes you have to be very careful to avoid picking up stray signals in the test probe, especially at higher frequencies. That is why the loop area of the probe tip and ground connection at the test point needs to be kept as small as possible.All Walter Lewin has done is to take this widely known aspect of measurement and dress it up to look like a party trick.

author: TheOCDengineer
author_rank: Contributor
timestamp: November 25, 2017, 09:44:35 pm
content: Maybe, but if you look closely to the lead the area is not large when in either the left most or right most. That is why it is very confusing.

author: IanB
author_rank: Super Contributor
timestamp: November 25, 2017, 09:45:01 pm
content: Quote from: TheOCDengineer on November 25, 2017, 09:10:16 pmThe lead orientation has absolutely everything to do with the outcome though. Consider that if a changing magnetic flux intercepts the test lead it will induce a current in that lead, and therefore the position of the test lead in space affects how and in what direction it is exposed to a changing magnetic field.

author: IanB
author_rank: Super Contributor
timestamp: November 25, 2017, 09:47:01 pm
content: Quote from: TheOCDengineer on November 25, 2017, 09:44:35 pmBut it must be enough. The test leads have to go all the way to the oscilloscopes, and the oscilloscopes are far away from the experiment. So you have a large loop area.

author: TheOCDengineer
author_rank: Contributor
timestamp: November 25, 2017, 09:52:08 pm
content: Not really the area is actually really small. The pigtail is on top of the resistor and the rest of the probe is a coax and thus no outside magnetic field will have much influence. That's why it is so interesting, and makes me think that it is path selection rather than induced emf.

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 25, 2017, 10:04:08 pm
content: Quote from: TheOCDengineer on November 25, 2017, 09:10:16 pmThat video gives a very clear and complete explanation. It shows very clearly the voltage induced across the wires connecting the resistors.Dr. Lewin makes a very clearly incorrect statement in his lecture. At around 40:00, he claims that going from D to A by two different paths gives two different voltages. This is clearly wrong.It is a shame that his students must have walked out of that lecture scratching their head wondering what was going on, instead of getting a clear explanation for this simple concept.

author: chris_leyson
author_rank: Super Contributor
timestamp: November 25, 2017, 11:17:22 pm
content: What he is implies in his lecture is shown in the first schematic where the inductors represent the wire or the single turn secondary and V1 and V2 are the voltmeters. If this is the case then both voltmeters would read the same.I couldn't clearly see his experimental setup in the video but what he must be doing is shown in the second schematic.That second video was a very good explanation of what is really going on. It seems Walter Lewin was implying one thing but measuring another, it's no wonder people got confused. Also a very good demonstration of what happens if you've got stray magnetic fields and are using scope probe ground leads.Lectures by Walter Lewin. They will make you ? Physics. More like ? Walter Lewin

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 25, 2017, 11:48:10 pm
content: He's still at it:First he says Kirchoff's law is violated. So what? It is well known that Kirchoff's law has limitations.Then he goes on with the same old argument.This is one of those "paradox" problems like the "Capacitor paradox", where you take an ideal lumped circuit and get a strange result. You get strange results because the ideal circuit requires things like zero resistance and infinite current.In this video he says you have superconducting wires with no resistance, and you can't have E-fields in them or you get infinite current. Same problem.In any real circuit, you have electrical length in the wires, and the resistors too for that matter.There is no physical experiment that can reproduce what he is proposing: zero length wires and resistors; zero ohm zero inductance wires; zero inductance resistors; yet there is a finite loop with a magnetic field. It is a totally theoretical toy problem.You would think he would want to explain why his statement makes no physical sense. How do the meters know where they are placed? It's nuts. I'm wondering if he is going a little nuts as well.

author: IanB
author_rank: Super Contributor
timestamp: November 26, 2017, 12:08:35 am
content: The argument that there can't be a large voltage between the ends of a low resistance wire because of Ohm's law doesn't hold water if you consider that an ideal voltage source in theoretical models has zero impedance, yet still represents an arbitrary potential difference between its terminals.What's more, this theoretical model would allow an infinite current to flow if you shorted its terminals together.In the real world you can replace zero impedance with low impedance, and replace infinite current with large current, and everything still holds together.

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 26, 2017, 01:50:54 am
content: Where he makes his mistake in the "Is Kirchhoff's Loop Rule for the Birds?" video is he says that the wires have no resistance, so they can have no voltage across them. This is wrong.If he uses his favorite Faraday's law, and integrates from one end of the wire to the other, he will get an instantaneous voltage across it. For example, from the previous video, +0.5V across the top wire from end to end, then -.9V across one resistor, 0.5V across the bottom wire, and -.1V across the other resistor. He is wrong when he says that there is zero volts across the wire.Any electrical engineer knows that even an ideal transformer that has zero resistance in the secondary turn can generate a voltage across it. Same thing.

author: KE5FX
author_rank: Super Contributor
timestamp: November 26, 2017, 02:49:36 am
content: Didn't this guy get fired for hitting on freshman chicks, or something?

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 26, 2017, 03:13:11 am
content: Quote from: KE5FX on November 26, 2017, 02:49:36 amYes. He was already pretty much retired. They pulled down his online courses:http://news.mit.edu/2014/lewin-courses-removed-1208

author: orin
author_rank: Frequent Contributor
timestamp: November 27, 2017, 02:26:27 am
content: FWIW, Maxim agree with Walter Lewin:https://www.maximintegrated.com/en/app-notes/index.mvp/id/2238Also, from Kirk T. McDonald at Princeton:http://www.physics.princeton.edu/~mcdonald/examples/lewin.pdf"...which is particularly surprising in the case that points a and c are the same, and points b and d are the same" (i.e. a star wiring configuration for the resistors and voltmeters). Though he does use Kirchhoff with L*dI/dt + M*dIp/dt for the analysis, he effectively goes back to the the Faraday's law form equation (2). Kirchhoff and Faraday do give the same equations - if you account for _all_ inductances in the Kirchhoff analysis, but Dr Lewin considers it a cheat.Section 2.2 may or may not help explain the confusion of Voltage vs EMF.I do not consider the Mabilde experiment to be equivalent. Lewin has the secondary loop approximately half way up and around a solenoid. Mabilde puts his secondary loop on top of his solenoid and I'd tend to agree with "Icy Stranger"'s comments on the magnetic field lines.What one should take from this is that all bets are off when there is a time varying magnetic field inside a loop of your circuit. Indeed, the very first thing that was explained on MIT's 6.002 "Circuits and Electronics" course is the lumped circuit abstraction where one of the assumptions is that there are no time varying magnetic fields outside the circuit elements. An old version is here:BTW, the real fun starts in this lecture 35:30 in...

author: orin
author_rank: Frequent Contributor
timestamp: November 27, 2017, 05:58:33 am
content: Quote from: rfeecs on November 26, 2017, 03:13:11 amIt was to do with some interaction with an online student. Having gone through "Anti sexual harassment awareness training"* on a previous job, I'd guess he'd been a bad boy before and had been warned.To be honest, and in my opinion, as I recall, some of his behavior in some of the online lectures I've seen was over the line... but I can't verify that it was on an edX course or opencourseware where I saw it as they took them down.None of the above is relevant to the Kirchhoff's vs Faraday's law issue where Faraday's Law always holds. Kirchhoff's law works at DC where there are no time varying magnetic fields and can be made to work at AC if you account for all inductances in a loop as L*dI/dt (or M*dI/dt for mutual inductances).Orin.*That really was the title and my take from it was: "You mean I can get away with that!?". But there again, I wasn't in any kind of supervisory position and you have to be much more careful in a supervisory position.

author: orin
author_rank: Frequent Contributor
timestamp: November 27, 2017, 06:19:54 am
content: Yes, I find this subject fascinating.For the record, another paper on the matter:http://www.phy.pmf.unizg.hr/~npoljak/files/clanci/guias.pdf

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 27, 2017, 05:52:41 pm
content: All three of these papers explicitly point out that the voltage measured on a voltmeter in the presence of a changing magnetic field depends on the path that the connecting wires to the voltmeter take. Move the wires and you will get a different measurement.Apparently this is what Dr. Lewin means when he says that tracing the loop around from A to D gives you two different voltages depending if you go around from the right or from the left. In the later response videos, he doesn't even show the meters at all. At best, his description is unclear. At worst he is being deceptive to make his magic trick look more puzzling and amazing.Sometimes he shows the direction of the magnetic field inside the main loop. But he never mentions that there is a field in the opposite direction outside the solenoid, and this field passes through the measurement loops. This field causes the difference in measurements between the right and left meter:To most of us, the EMF induced in the wires of the voltmeter represent a measurement error, and not an accurate measurement of the voltage from A to D. (How many times has Dave said "that's not a real signal, the oscilloscope is just picking up noise from the equipment and lights in the lab?) It's a matter of definition, as pointed out in some of the papers.

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 27, 2017, 10:15:15 pm
content: Quote from: orin on November 27, 2017, 06:19:54 amWow. I read over this paper more carefully and I think I finally get it.Mind blown. You don't even need the opposite field outside the solenoid. The outer loop containing the meters and the main inner loop both surround the same flux. So the induced EMFs cancel out.Thanks. Very interesting.

author: TheOCDengineer
author_rank: Contributor
timestamp: November 28, 2017, 03:24:07 am
content: Quote from: rfeecs on November 27, 2017, 10:15:15 pmI think that paper explains it very well. Still a little confused on how the path selection works physically. I wonder what the relationship is for lead position. Equation 3 suggests that for one loop path there should be a positive and a negative and experimentally we proved it.

author: TheOCDengineer
author_rank: Contributor
timestamp: November 28, 2017, 05:24:28 pm
content: I figured out how the path selection works. Imagine the magnetic field as a really long cylinder, when you go to connect the leads of the voltmeter you can only connect them on either side (cant be over or under the cylinder). Thus depending on the side that you connect them you get the different sign.