Is MIT Prof. Lewin wrong about Kirchhoff's law?

[atyy]

You really think MIT people can miss a factor of ten in this way?

I do. (It's only MIT, not Caltech.) But they didn't in this case.

 

[stevenb]

I do. (It's only MIT, not Caltech.) But they didn't in this case.

Good one!

Of course appeal to authority is not a good argument, but it's important to look at qualifications and evidence of due dilligence, as part of a debate like this. There seems to be enough horse power behind the experiment, and analysis to suggest that claims of fraud should be backed by a detailed analysis and/or a documented experiment, rather than vague hand-waving type arguments about parasitic inductance and transformer tapping.

 

[cabraham]

@ stevenb: look at the video part 2 at around 5:23. Freeze the video and look at his setup. The probes are NOT "effectively" connected to the same point. There is about 4-6" of wire on the secondary loop with the resistors on it. That means inductance to me. Your claim of moving the scope to the other side befuddles me, because the ENTIRE point of prof. Lewins lecture is not about the how to measure this system, but that across each resistor, a different voltage occurs, which I agree with.

Think of it this way: If 1mA amp is induced in the system, OF COURSE (as he puts it) there will be a -0.1V drop on the 100 Ω res, and 0.9V drop on the 900 Ω res. Now, if we simplify the system by saying "node A" is actually "effectively" the same point, we know have 1 V drop across "node D", a supposedly 0 Ω line (by Lewin's model). That means Lewin also denies that Ohm's Law applies in this situation, because that would mean infinite energy.
Ohm's law is hard to dispute, because this is more of a definition than a theory. Therefore, I contend we have a breakdown of his model.

Conversely, since you (stevenb) seem to be very sure of your answer (I am not 100% sure of mine, that's why I asked the question, and would like to learn), can you tell me how the energy got into the system from the main coil to his resistor circuit? Does it not involve inductance? I don't know of any case where you can couple energy like this without some kind of coupled inductor setup.

Honestly, I don't know where to begin. Ohm's law is in effect, & Dr. Lewin has not denied Ohm at all. You keep wanting to account for that extra volt of missing potential. If 2 points are connected by a short, then there is 0 volts if measured through the wire. If the wire loop is superconducting, then J = sigma*E, or E = rho*J. For rho=0, J= finite, then E=0. But v = integral E*dl, so v=0.

Of course, along differing paths outside the interior of the wire, results may vary. The potential from A to D is undefined unless a specific path is chosen. Vad is -0.1V along one path, & it is +0.9V along another path. The law of energy conservation always holds. If we compute the I^2*R loss converted to heat, it will never exceed the power in the mag field.

The product of the induced current & voltage is power. This induced power is less than or equal to the incident power on the loop in the mag field. If R is reduced power cannot increase w/o limit. A smaller R results in more induced current, which results in a larger mag field due to self inductance of the loop. The induced current has an associated mag field around the wire which opposes the external mag fiels per the law of Lenz.

Thus the induced I & V are subject to the laws of Lenz, Ohm, Faraday, Ampere, & conservation of energy. No paradox is present at all. Dr. Lewin's methods of demonstration differ from mine. I would explain it a bit differently. I understand his point & agree with him, as he is spot on re science. I would however, refrain from calling KVL "a joke". KVL, like all other laws, is defined under limited conditions, not universal conditions.

Since KVL is not taught as universal, I would reiterate KVL for what it is. It is valid w/ conservative fields. With non-con fields, it does not always hold, but can under specific conditions. Dr. Lewin has no contradictions w/ science. I would only offer him constructive criticism on the way he presents the material.

I've worked w/ magnetics for decades, & I understand what Dr. Lewin is saying, but I can see how an e/m novice could get blind sided by said material. Comments are welcome.

Claude

 

[yungman]

I'm not sure why you would say that I and the Professor have missed the "transformer effect". This effect is the source of the EMF that drives current in the loop. The main difference between this situation and a real transformer is that one does not usually put two large resistors in the loop winding of a transformer. Certainly the transformers you took measurements on were not built like this. But, this is a side issue.

The professor goes through a process of setting up the problem. First he describes the case with a 1 V battery in the loop, and then he erases the battery cell and uses changing flux in the center of the loop to replace the 1 V EMF. How can you say "he just miss the voltage source of the transformer"? He didn't miss it at all.

Faraday's Law in integral form just tells you that the 1V EMF exists somewhere in the loop. It doesn't specify where it is in the loop. Typically, a transformer is tapped to change the number of loops is the circuit, not to somehow tap a section of one loop. The details of what happens when tapping one loop need to be considered more carefully, as has been done in the provided analysis. In this case we know where the potential drops are (we can measure them with a meter that does not encircle the flux change) and we see that it adds up to 1V around the main loop. There is very little potential drop across the wires themselves. The transformer EMF in the main loop is 1V, so all is well with Faraday's Law. Also, all is well with the classical definition of KVL (given by Maxwell). Obviously, the version KVL that says the sum of potential drops is zero is violated, which makes the professor jump up and down and denounce his physics books for spouting bad physics.

In doing the proper measurement for potential on each resistor, you trace a path (through the meter and the resistor) that does not encircle the flux change and this tells you which resistor potential you are actually measuring. FL and KVL (both versions of KVL, mind you) work here. The Professor also reveals that if you consider the path through the other resistor, you see an apparent contradiction. You end up tracing a loop through the other resistor that encircles the flux change and you are not really measuring the potential on that resistor. Faraday's Law still works through that other path, and the classical definition of KVL also works, but the other definition of KVL fails yet again.

Since the professor is not actually acknowledging the classical definition of KVL, we can just ignore that aspect, and conclude that everything he is saying is correct. The results do not depend (other than small parasitic changes) on where you tap the node along the wire, but they do depend on how you route the leads of the meter. If we had the experiment in front of us, there would be a very simple way to prove this. Simply move the exact point where you tie into the nodes and see if the measurements change. It is my contention that they will not change very significantly. Anyone who doubts this should just do the experiment and convince themselves. Do you really think the people who set up this experiment would go through all this trouble, and then not verify this straightforward thing? You really think MIT people can miss a factor of ten in this way? That's hardly proof, but experiments are proof. I've done similar experiments in the past as part of my work. I can't ask anyone to accept my word, and if others don't want to accept the professor's capability, then just do the experiment.

I think we are talking in circles. Just look at the circuits as the secondary of the transformer with two resistors connect in series between the two end of the secondary. Then adjust the voltage at the primary to get EXACTLY 1V across the two resistor, you get what the professor was doing. Nothing more. YOu measure the 100ohm, you get 0.1V and 900ohm you get 0.9V. Problem is you miss the whole secondary of the transformer that generate the 1V to drive the 1V across the resistor.

Do you understand that if you connect the two resistors like what the professor draw, you form a loop EXACTLY like the secondary of the transformer with two resistors in series connected at the two end of the secondary? The length of the wire is not an issue here, it is the loop that create the transformer...that create a voltage when a varying magnetic field pass through the area enclosed by the loop. The professor has to draw in the voltage generator in his two resistor loop, with that, KVL hold.

You work with transformer before? You ever seen voltage created in just one or even half a loop of wire? If you don't believe me, just wind a 10 turn on a bobbin of some non magnetic material, then wind one turn on top as a secondary. You can even put the same two resistor in series. Hook up a scope onto the two end of the secondary. Use a 1.5 volt battery on the primary with a switch. Open and close the switch and see the meter needle jump, see how high ( voltage) it just! You will see that one little loop with only 1" or less can produce voltage.

 

[yungman]

Good one!

Of course appeal to authority is not a good argument, but it's important to look at qualifications and evidence of due dilligence, as part of a debate like this. There seems to be enough horse power behind the experiment, and analysis to suggest that claims of fraud should be backed by a detailed analysis and/or a documented experiment, rather than vague hand-waving type arguments about parasitic inductance and transformer tapping.

You mean you don't accept the idea of generating voltage of wire of less than 1" or 2" long forming a one turn loop? And you don't accept a 4" wire with two resistors connect at two ends is a loop? You think the 4" wire connecting the resistors to form the loop is insignificant? You really want me to go through the trouble to type out a simple transformer equation here?

If you ever design transformer for switching power supply, you will have no difficulty understanding what I have been talking. As switching frequency goes up, efficiency goes up, less turn is needed. That is the reason why the switching power supply is so so much smaller because less turn is needed, size of the core can be drastically reduced because core is lot more efficient at higher frequency. For a working engineer, I don't think it is hard to even get the idea of this. This is really simple!

I pretty much tell you how to reproduce the experiment in the other post, just wind the wire onto a Big ball point pen and you can do the experiment. Just be careful and wear rubber groves to avoid shock because when you open the switch, the coil can momentary generate very high voltage...Like ignition coil.

I can tell you, I designed high speed pulsing circuits with transformer driving MOSFETS. Because the design is 5KV switching, I had to stack 8 MOSFET in series to take the voltage. The driving circuit of each MOSFET has to be able to float. The transformer is the best approach. I actually design the transformer onto the PC board as trace. I only used 3 turns on the secondary to generate 15V to drive the gate of the MOSFET. 3 turns for 15V! The whole length of the secondary is less than 3". I hope you stop and think a little on this, this is real products been produced in the 90s. I am not a switching supply engineer, I was the manager and I came up with all the ideas on low turns, fast switching DC to DC converters that made our products exceptional at the time. I had my engineer did the detail calculation to get the turn number but the idea absolutely sound and was implemented on successful products.

 

[stevenb]

I think we are talking in circles.

That's exactly what we are supposed to do when discussing Faraday's Law and Kirchoff's Voltage Law.

 

[cabraham]

You mean you don't accept the idea of generating voltage of wire of less than 1" or 2" long forming a one turn loop? And you don't accept a 4" wire with two resistors connect at two ends is a loop? You think the 4" wire connecting the resistors to form the loop is insignificant? You really want me to go through the trouble to type out a simple transformer equation here?

If you ever design transformer for switching power supply, you will have no difficulty understanding what I have been talking. As switching frequency goes up, efficiency goes up, less turn is needed. That is the reason why the switching power supply is so so much smaller because less turn is needed, size of the core can be drastically reduced because core is lot more efficient at higher frequency. For a working engineer, I don't think it is hard to even get the idea of this. This is really simple!

I pretty much tell you how to reproduce the experiment in the other post, just wind the wire onto a Big ball point pen and you can do the experiment. Just be careful and wear rubber groves to avoid shock because when you open the switch, the coil can momentary generate very high voltage...Like ignition coil.

I can tell you, I designed high speed pulsing circuits with transformer driving MOSFETS. Because the design is 5KV switching, I had to stack 8 MOSFET in series to take the voltage. The driving circuit of each MOSFET has to be able to float. The transformer is the best approach. I actually design the transformer onto the PC board as trace. I only used 3 turns on the secondary to generate 15V to drive the gate of the MOSFET. 3 turns for 15V! The whole length of the secondary is less than 3". I hope you stop and think a little on this, this is real products been produced in the 90s. I am not a switching supply engineer, I was the manager and I came up with all the ideas on low turns, fast switching DC to DC converters that made our products exceptional at the time. I had my engineer did the detail calculation to get the turn number but the idea absolutely sound and was implemented on successful products.

This is irrelevant. Number of turns vs. frequency is not the issue here. I know xfmrs very well. This discussion involves the non-con nature of induced fields. Since you love to view things in terms of xfmrs, I'll do just that as illustration.

The secondary of a xfmr has 120V rms open circuited, with a secondary winding resistance of 0.10 ohm. The leakage inductance is very small, & the frequency is very small, meaning the reactance is much smaller than the 0.10 ohm resistance, so we can ignore it. An 11.9 ohm load is connected across the xfmr secondary. The current is of course 10 amp rms.

The terminal voltage at the secondary measured with a DVM is 119V rms. Thus we can say that if the secondary terminals are marked "a" & "b", then "Vab" is 119V rms. This is fine as long as it is understood we are measuring Vab along a path outside the xfmr core, so that the core flux does not influence the DVM reading. But consider the voltage from "a to b" along a different path, namely inside the secondary winding. We start at terminal a, integrate the E field along the path through the secondary wire, ending at terminal b. Now, Vab = Isec*Rsec = 10A*0.10 ohm = 1.0V rms. Thus "Vab" is 119V outside the xfmr assembly, & it is 1.0V inside the copper wire from which the secondary is wound.

This is the issue being discussed here. The fact that higher frequencies allow for a smaller xfmr & higher volts per turn is well known. Nobody is disputing that. My example shows that with non-con E fields, "Vab", the value of voltage from a to b, is not unique. The value depends upon the path of integration.

So what is the "real" value of Vab? Is it 119V, or 1.0V? The answer is that you must specify a path. Along the secondary conductor path it is "really 1.0V". Along an outer path, away from the conductor & core, it is "really 119V".

Think of this. Voltage is merely the math ratio of work expended moving a charge from a to b, per unit charge. To move 1.0 coulomb of charge through a path outside the core & winding, requires 119 joules of energy. But to move that same 1.0 coulomb along the path inside the wire requires only 1.0 joule of energy.

If the 11.9 ohm load resistor is a heater, it is emitting (10A)^2*(11.9 ohm) = 1190 watts of heat. But the xfmr secondary winding copper conductor is dissipating heat equal to (10A)^2*(0.1 ohm) = 10 watts of heat.

The 11.9 ohm heating element & the 0.10 copper wire secondary winding emit 1190 watt & 10 watt resp. But they carry the same exact current, 10 amp rms. What is going on here? If they carry the same current, yet have unequal powers, then the voltages must be unequal. But they are in parallel! How can parallelled elements have differing voltages?

Because the notion that 2 elements in parallel must be at the same potential is valid only with conservative fields, not so with non-con fields. KVL is the basis for 2 elements in parallel being at equal potentials. Not so with non-con conditions.

Does this help. I'll clarify if necessary. BR.

Claude

 

[yungman]

Claude, I am glad you use your example and we can build on this. If you remember the professor original drawing of replacing the 1V battery with a short circuit and then sent a magnetic field to get 0.1V on the 100ohm and 0.9V on the 900ohm.

Use your example. We use 110ohm in series with 9ohm across the secondary. Then we pulse the primary to get 9V on the 9ohm and 110V on the 110ohm. As in the professor case, let A be the junction between two resistors which in this case has 0 length. Where is point D?


He obvious know that the setup he had, by replacing the 1V battery with a short wire result in a loop formed by the wire and the resistors. Then he said he measure 0.1V across 100ohm and 0.9V across 900 ohm. Then he claimed he measure say clockwise from one point is 0.1V and counter clockwise as 0.9ohm! So where is the transformer that gave him the induced voltage in the picture?

Back to your example, the wire is the secondary of your transformer, point D is not a point, it consist of the winding (wire) of your secondary winding.


Is there any way for me to post a simple drawing of just two resistors and the secondary of the transformer without have to using a PDF and then attach, then have to wait for a day for other people to see it? Let me try this.


Let us redo the presentation again:

1) Let's arrange the components in counter clockwise.

2) Start with the 900ohm, then 100ohm and call the point between the two resistor is point A. I call the open end of the 900ohm resistor point C.

3) Then we connect one end of 4" wire to the 100ohm resistor and call this junction B. Remember I am still going counter clockwise now.

4) Then the other side of the 4" wire connect back to the open end of the 900ohm resistor which I call point C in step 2.

With this, we form a closed loop starting from point A between the two resistor, travel counter clockwise through 100 ohm resistor, to point B to the wire, through the wire to point C that connect to the other side of the 900ohm resistor.

Then you inject a pulse of magnetic field, you measure 0.1V across the 100 ohm resistor, and 0.9V across the resistor. Just like the good professor did.

Lets call point C is +ve and call point B as -ve. Let's travel from point B counter clockwise through point C to point A, you get 1V-0.9V=0.1V. If you travel from point B clockwise this time to point A, you get 0.1V! Why are they the same now? Because the transformer effect, this time I include the transformer in the picture and voltage come together! IN this case, KVL works beautifully.

 

[yungman]

I know I sounded vey bull headed and one track mind. But I watched very carefully his presentation and the drawing on his first video. AND the way he go from point A to D in one direction and from D to A in the opposite direction and totally ignore the transformer effect. In this case, context is very important.

Until someone explain to me how he can just make a big statement with just the drawing on his video one, everything else in 4 pages here, all the forumlas, the non conservative, integrations just become bla bla bla to me. I don't even want to go any further until I can be convinced that his first presentation make sense. Without that, any conclusion derive out of his drawing means nothing.

 

[sarumonkee]

@Claude: I totally agree with your example. Thank you for agreeing with Yungman and myself. A standard model of the secondary of a transformer with sufficient frequencies being put through it (not too high, not too low) is a voltage source (or current source depending on your take on things). I am contending that KVL still applies, because the secondary of the transformer still exists, and is a voltage source that Prof Lewin left out, thus making his example suspect.

Also, @stevenb. If orientation of your scope probes matters that much, what happens if the oscilloscope is directly above the table? Is there some voltage between 0.9 and -0.1V? Or does it just suddenly change between the two when you cross some threshold degree? I just don't see it yet. I am not being facetious, I really do want to understand what you are saying. If physical orientation of a voltage probe matters, why isn't it taught in school? Physical placement is important, and also taught in school.

Also, everyone who keeps harping on the fact that the Prof got the factor about 10 (actually exactly 9) correct, I never said he wouldn't. A resistor that is 1/9 the size of another resistor with the same current WILL have 1/9 the voltage drop on it. That is not impressive.

What IS impressive is calling a node a node even if it has supposedly two different voltages on it. I thought that the definition of a node is that it has one voltage on it.

 

[yungman]

@Claude: I totally agree with your example. Thank you for agreeing with Yungman and myself. A standard model of the secondary of a transformer with sufficient frequencies being put through it (not too high, not too low) is a voltage source (or current source depending on your take on things). I am contending that KVL still applies, because the secondary of the transformer still exists, and is a voltage source that Prof Lewin left out, thus making his example suspect.

Also, @stevenb. If orientation of your scope probes matters that much, what happens if the oscilloscope is directly above the table? Is there some voltage between 0.9 and -0.1V? Or does it just suddenly change between the two when you cross some threshold degree? I just don't see it yet. I am not being facetious, I really do want to understand what you are saying. If physical orientation of a voltage probe matters, why isn't it taught in school? Physical placement is important, and also taught in school.

Also, everyone who keeps harping on the fact that the Prof got the factor about 10 (actually exactly 9) correct, I never said he wouldn't. A resistor that is 1/9 the size of another resistor with the same current WILL have 1/9 the voltage drop on it. That is not impressive.

What IS impressive is calling a node a node even if it has supposedly two different voltages on it. I thought that the definition of a node is that it has one voltage on it.

I am actually approach Engineering from the opposite end. I never have an EE degree, my degree was organic chemistry, electronics was my hobby. I started as a technician and I studied on my own really hard. I got promoted to an engineer in two years and been a design engine and a manager of EE over 25 years. In the past 10 years, I decided to go back and study all the math including PDE, EM, RF design on my own.

That said, I am very glad I did this the other way around. A lot of the professor in EE never have a day of real engineering job. All the books never talked about the simple things like the ground connection, power supply V+ etc. In real life, these are the ones that give nightmare to engineers. Because in schematics, it is only one point like the good professor did with point A and point D. BUT in real life, it is a physical connection, a wire that can become a loop in this case, become inductance or resistance. Worst, the wire become an antenna and start picking up noise. The probe lead are part of the problem of the parasitic length that can form loop.

The circuits in the book really work! They just ignore the un-foresee stuffs like grounding and supply and the way to hook up your measuring equipment. In real life these "parasitic" are usually what kill the project. I have seen people from ivy league college cannot adapt to the real world and screw up on the job. The sad part is some have too much eagle to realize what they don't know. That is one thing I am impressed with a small university call U of Santa Clara. I actually contact with one professor call William Egan who wrote a very good phase lock loop textbook. He actually work over half the time in the private industry and teach part time. This is the kind of professor I want.

I always told my technicians, if they don't get the right measurement, 50% of time is the setup of the measurement is at fault. It goes higher as frequency goes up.

I have more books on RF, microwave and EM than the Stanford university book store, I was there buying books and gave up. I have one tall bookcase of books in these subjects and cauculus. I can tell you, only RF book talks a little about parasitic elements of components, nobody else does. Still none emphasis on power supply and grounding and NO BOOKS talk about the way to measure like what you are asking. This problem is getting worst when the speed of electronics getting higher. They actually have a special kind of engineer called "Signal Integrity" engineer to do nothing but to catch grounding, un-intention loop that pickup magnetic field, signal return path. In one of my contract with KLA Tencor, I worked on the signal integrity issue and help doing the pcb layout for their 3.3G bit CCD camera. That was in 2003. The un-foresee "parasitic" are the killer of a lot of electronics and you have to be very careful in the way you measure. A lot of times, we use a scope probe adapter that have very short ground lead and we solder it onto the circuit. Then we plug the probe into the adapter to measure the signal. This is to get rid of the ground loop caused by the ground lead hooking onto some distant ground point. It is all about grounding!

 

[stevenb]

Also, @stevenb. If orientation of your scope probes matters that much, what happens if the oscilloscope is directly above the table? Is there some voltage between 0.9 and -0.1V? Or does it just suddenly change between the two when you cross some threshold degree? I just don't see it yet. I am not being facetious, I really do want to understand what you are saying. If physical orientation of a voltage probe matters, why isn't it taught in school? Physical placement is important, and also taught in school.

I'm not taking your questions as facetious at all. I can see you are trying to come to grips with this difficult concept. The problem is that it hard for people to learn when they start off with lack of trust. You don't trust that the Prof is trying to teach you and is qualified to teach you. So, the learning process is going to be slower than it needs to be. Although your question to me is genuine, it also implies your lack of trust that I might know what I'm talking about too. Can't say I blame you since you don't know me at all, but it places me at a serious disadvantage in trying to help you. On top of that, this format is not terrible conducive to getting ones thought across clearly. As an example, I never said the orientation of the probes matter that much. What I said was that the path formed by the leads is the critical thing. So, I'm not hopeful that I can be of great help, and I think I've passed the frustration threshold for this thread in general.

The simplest answer I can give is that full understanding of Faraday's Law removes all mysteries here. Study FL thoroughly and when you feel that FL is telling you something that you just don't want to believe, then figure out how to do an experiment (yourself, since you don't trust others, no matter what their qualifications - not always a bad thing, by the way) to convince yourself of its truth. Your question about what happens if the meter is placed above the circuit is outstanding, and convinces me that you will understand this soon. Please explore the answer using FL. It's not difficult to answer, but the answer will help you come to grips with the concepts here.

 

[atyy]

What IS impressive is calling a node a node even if it has supposedly two different voltages on it. I thought that the definition of a node is that it has one voltage on it.

Yes, that is exactly what Lewin, stevenb, cabraham etc have all been saying - in the case of a time varying electric field, the electric field cannot be completely obtained from a scalar potential.

The part about 9:1 or whatever is just to show that in fact Lewin's results are consistent with his model, and not due to small factors he neglected, like the voltmeters not being connected at exactly the same points in his setup. If his model is wrong, then the voltmeter readings would be 1:1, and the small errors you have in mind would have to change this 1:1 to 9:1, basically the errors would have to be huge. The alternative is that the errors are small, and the 9:1 is the same 9:1 he predicts using his model.

 

[sarumonkee]

I'm not taking your questions as facetious at all. I can see you are trying to come to grips with this difficult concept. The problem is that it hard for people to learn when they start off with lack of trust. You don't trust that the Prof is trying to teach you and is qualified to teach you. So, the learning process is going to be slower than it needs to be. Although your question to me is genuine, it also implies your lack of trust that I might know what I'm talking about too. Can't say I blame you since you don't know me at all, but it places me at a serious disadvantage in trying to help you. On top of that, this format is not terrible conducive to getting ones thought across clearly. As an example, I never said the orientation of the probes matter that much. What I said was that the path formed by the leads is the critical thing. So, I'm not hopeful that I can be of great help, and I think I've passed the frustration threshold for this thread in general.

The simplest answer I can give is that full understanding of Faraday's Law removes all mysteries here. Study FL thoroughly and when you feel that FL is telling you something that you just don't want to believe, then figure out how to do an experiment (yourself, since you don't trust others, no matter what their qualifications - not always a bad thing, by the way) to convince yourself of its truth. Your question about what happens if the meter is placed above the circuit is outstanding, and convinces me that you will understand this soon. Please explore the answer using FL. It's not difficult to answer, but the answer will help you come to grips with the concepts here.

So after all that lead in, you don't answer my question about the scope above the table :)? If you know the answer, please tell me... Or at least hint at the direction I should look (but I think just hinting will make it look like you don't know the answer).

I still don't get what you mean by path. If a node is a node, the path to it, and through it are transparent. The two probes have the same path if connected to the same nodes. I just don't think these are actually nodes in this case.

Thanks.

 

[atyy]

So after all that lead in, you don't answer my question about the scope above the table :)? If you know the answer, please tell me... Or at least hint at the direction I should look (but I think just hinting will make it look like you don't know the answer).

I still don't get what you mean by path. If a node is a node, the path to it, and through it are transparent. The two probes have the same path if connected to the same nodes. I just don't think these are actually nodes in this case.

Thanks.

Hint:
integral(E.dl)~d(B.A)/dt
A is a vector perpendicular to the area in question.
There is a dot product, so there will be a cosine of some angle.
That angle is related to the angle in your question.

 

[yungman]

So after all that lead in, you don't answer my question about the scope above the table :)? If you know the answer, please tell me... Or at least hint at the direction I should look (but I think just hinting will make it look like you don't know the answer).

I still don't get what you mean by path. If a node is a node, the path to it, and through it are transparent. The two probes have the same path if connected to the same nodes. I just don't think these are actually nodes in this case.

Thanks.

The whole point is point D cannot be considered as a note. The wire IS a path. A note is a single point, no if and buts about it. The whole point of consider D as a note is not correct. The drawing that the professor should have at least 3 notes even if you solder the two resistor back to back. And if the other side of the two resistor at what so called point A is another short piece of wire, point A is actually a path with two notes in this one loop case, no if and buts about this. Some how, people here do not accept the idea the even that little piece of wire is part of the circuit and has to be accounted for. If you take the wire as a voltage source and work KVL around it, there is no mistake.

Path integration we are talking here is very simple, just integrate along the path. Just like the basic integration [itex]\int f(x)dx[/tex], this only mean that the integration is carry out on the path of the x-axis. In the $$\int_C \vec E \cdot d\vec l \hbox { is actually } \int_C \vec E \cdot \hat T dl$$ it is nothing more than integration of E along the path of the loop. THis is very well covered in vector calculus ( part in 3rd semester multi variable calculus).

In my post #78 in page 5, in step 2) and 3), points B and C are two notes with the wire in between which is part of the path of the closed loop. In the example, I made is simpler by soldering the two resistor back to back so the point A is truly a note with no wire length ( approximate only). So there are only 3 notes in that loop.

 

[kcdodd]

Any time a scalar potential is used to derive all the physics, it is being assumed that the electric field is conservative.

In situations where the electric field is not static, a scalar potential may still be useful if only approximate. This is the quasistatic approximation.

KVL uses a scalar potential, and is closely related to conservative fields.

My point is that in "normal" circuits, where KVL is applied, there are always non-conservative electric fields present. When applying the voltage law you get a -V on a resistor which represents an electric field pointing in the direction of current. However, the resistance itself is a non-conservative force and must be electric in nature. In an emf component you get a +V, which if we are consistent, represents an electric field opposing current. Of course this cannot be the whole story because for current to flow against the electric field there must be non-conservative forces present inside the emf too. Otherwise current would never flow anywhere in the circuit. And all of those forces are fundamentally electric in nature as well.

However, when dealing with KVL those non-conservative forces are "simply ignored", if for no other reason then they would be hard to calculate, and only the conservative part of the field is taken when calculating V. KVL in that sense basically just says that conservative forces are conservative, because it would always ignore non-conservative forces.

Why is this important? Because if one is consistent with "ignoring non-conservative forces" in this way for "normal" circuits, there is no reason to suddenly include them when you have a macroscopic non-conservative electric field which acts as an emf. If Lewin is to start including non-conservative forces in his loop integral then he should include all of them and not just pick and choose which ones to include. My guess is that in including all forces one would find the integral to in fact be zero.

 

[sarumonkee]

Hint:
integral(E.dl)~d(B.A)/dt
A is a vector perpendicular to the area in question.
There is a dot product, so there will be a cosine of some angle.
That angle is related to the angle in your question.

So you are saying pointing the "loop" of the voltage probe at a certain angle, I will get 0 Volts?

I actually rigged up this experiment, and sufficiently convinced myself that KVL is holding... I held the probes above, below, across, etc, and got the same numbers time and time again. I am seeing the inductance that yungman has been talking about, and a voltage drop across the wire, which definitely should not be a node. I think Lewin should have had at least a coupled inductor in his model (or two the way his experiment was setup).

 

[atyy]

So you are saying pointing the "loop" of the voltage probe at a certain angle, I will get 0 Volts?

I actually rigged up this experiment, and sufficiently convinced myself that KVL is holding... I held the probes above, below, across, etc, and got the same numbers time and time again. I am seeing the inductance that yungman has been talking about, and a voltage drop across the wire, which definitely should not be a node. I think Lewin should have had at least a coupled inductor in his model (or two the way his experiment was setup).

How did you ensure that the changing B field is confined to the central loop?

Also, what are the parameters of your setup?

 

[sarumonkee]

How did you ensure that the changing B field is confined to the central loop?

Also, what are the parameters of your setup?

I made my setup similar to what I think he did, except I made one connection from the 100Ω to 900Ω resistors very short. I then put ground of both probes between the two resistors on the short connection. The other sides of the resistors were connected with a 6" wire. I probed the resistors close to the actual resistor on the opposite side from the short "node".

The primary coil was about 40 turns of something like 14-16AWG wire, around a huge core I had laying around, with an effective core cross sectional area of probably just under 2 square inches. I then introduced a 10 V step onto the primary, while having the secondary (the resistors and wire) around the core, like Lewin's setup is presumably from 5:33 on part 2.

I observed a factor of about 1:9 as expected in the two voltages, since this was the ratio of the resistances. Now, the fun part. I connected the grounds of the probes to half way between the long wire, about 3" from both resistors, leaving the probe ends in the same location. Since this is a "node" in Lewin's analysis, I should not see any voltage across it if I make another step function on the primary.

Well, I introduced my step, and both probes read about the same magnitude (one was negative from the other, since it points the other way), and the sum of the two magnitudes (had to invert one because I wasn't using differential probes) equaled the sum of the previous points in standard KVL style, all adding to 0 if you do the loop. I was measuring a voltage across the 6" wire in two 3" segments.

I also held the probes above, across, and in many different orientations, and it still produced the same results. I plan on taking some pictures and maybe making a video this weekend if I have time.

Let me know if you have any measurements you would like me to make, or if you think my setup or analysis is flawed somehow... I'm here to learn.

 

[atyy]

Cool! So in the part where you got different readings of 9:1, did you connect the voltmeters to exactly the same two points?

 

[sarumonkee]

Cool! So in the part where you got different readings of 9:1, did you connect the voltmeters to exactly the same two points?

No, I just realized I need to show that one as well... I will next time I'm back in that lab. The time I got 1:9 ratio is when I had the probe leads as close as possible to the resistors, and the grounds on the short side (1 cm wire connection). The probe leads were separated by the 6" wire on one side direction around the loop, and the two resistors on the other.

I never actually connected the probes to the exact same point (probe on probe, and ground on ground). I will try to remember to do that next time, and spread the probes to opposite sides of the loop like I bet you will ask for.

I hope that paints the picture for you.

 

[yungman]

I made my setup similar to what I think he did, except I made one connection from the 100Ω to 900Ω resistors very short. I then put ground of both probes between the two resistors on the short connection. The other sides of the resistors were connected with a 6" wire. I probed the resistors close to the actual resistor on the opposite side from the short "node".

The primary coil was about 40 turns of something like 14-16AWG wire, around a huge core I had laying around, with an effective core cross sectional area of probably just under 2 square inches. I then introduced a 10 V step onto the primary, while having the secondary (the resistors and wire) around the core, like Lewin's setup is presumably from 5:33 on part 2.

I observed a factor of about 1:9 as expected in the two voltages, since this was the ratio of the resistances. Now, the fun part. I connected the grounds of the probes to half way between the long wire, about 3" from both resistors, leaving the probe ends in the same location. Since this is a "node" in Lewin's analysis, I should not see any voltage across it if I make another step function on the primary.

Well, I introduced my step, and both probes read about the same magnitude (one was negative from the other, since it points the other way), and the sum of the two magnitudes (had to invert one because I wasn't using differential probes) equaled the sum of the previous points in standard KVL style, all adding to 0 if you do the loop. I was measuring a voltage across the 6" wire in two 3" segments.

I also held the probes above, across, and in many different orientations, and it still produced the same results. I plan on taking some pictures and maybe making a video this weekend if I have time.

Let me know if you have any measurements you would like me to make, or if you think my setup or analysis is flawed somehow... I'm here to learn.

You are good man, you prove my point to the T! You grounded the probe in the middle of the wire 3" from each side and you keep the probe on the junction where you measure the voltage before and you get equal and opposite voltage. This is the transformer I am talking about and you prove my point from the post #2 on that the wire is the voltage source. The voltage source that the professor MISSED, and went on to trash others in such an arrogance manner. He need to go get a real job before he talked so loud.

I am surprised though that the probe is so insensitive to position. It would be nice if you take a few pictures. Placement is everything in the real world. It is almost pointless to talk about this subject on paper drawing resistors and nodes like the professor did.

You really make my day. I spent the whole day today going back and forth trying to get this point across, make me missed the whole day in studying EM! I really need to get back to my studying. I better get back to the theoretical world!

 

[yungman]

No, I just realized I need to show that one as well... I will next time I'm back in that lab. The time I got 1:9 ratio is when I had the probe leads as close as possible to the resistors, and the grounds on the short side (1 cm wire connection). The probe leads were separated by the 6" wire on one side direction around the loop, and the two resistors on the other.

I never actually connected the probes to the exact same point (probe on probe, and ground on ground). I will try to remember to do that next time, and spread the probes to opposite sides of the loop like I bet you will ask for.

I hope that paints the picture for you.

You did get 9:1 read across the two resistor as the professor. As long as you move both grounds of the two probes to the middle of the wire and have the probes at the junction of the end of the wire to the resistor on both sides, you did the right thing. This is my understanding from your write up and that prove my point of the transformer effect. You should see 0.5V on each probe and add up to be 1V.

 

[hikaru1221]

Hikaru, thank you for being the only respondant to have the courage to post and answer to my question.


So did Farady state his law in 'Integral Form'


I think that what is happening here is that a non original form of Faraday's Law is being compared with a non original form of KVL. By non original I mean extended in the light of more modern knowledge.

Faraday's conclusion from his experiment was that a change in magnetic flux induces an induced current inside a closed circuit. The later speculation was that a change in magnetic flux induces an induced emf. Then Maxwell came in.
What I meant by "integral form" is that we consider a whole loop when applying Faraday's law. It is equivalent to the integral form of Maxwell-Faraday equation. Just the same as KVL. We write the equation for a loop, not a branch.
Please enlighten me with the modern knowledge you mentioned.

 

[Phrak]

Are you referring to a current measurement? The point of Prof. Lewin's experiment was not about voltmeter measuring errors, it was to supposedly show measuring across different points of two "nodes" can give you different voltages. I don't think they were actually "nodes" as inductance was not included.

I broke down and watched the first video. I found Lewin to be irritating. He seems to get too much delighted in generating confusion rather than clarity.

You can measure the same two physical points and get two different measurements because the leads of the measuring instrument enclose different regions of changing magnetic flux. It's really that simple.

The fact that this crazy thread has gone on so long is evidence of the guy's overwhelming success in creating confusion. Then he gets to be the genius-hero and rescue you from the confusion he, himself has so cleverly led you into. good grief.

Clear the indoctrination of this subversive screwball out of your head, learn about electromagnetic fields, then come back to it, and the confusion will have evaporated.

 

[sarumonkee]

Just to clarify, with my experimental setup, I was not getting a full 0.1V and 0.9V on the resistors. I don't think I have a large enough primary coil (read needs more turns), and I only did a 10V step, which was well away from the max current I could put through the thing. It was a quick first run, and I could do it with a stronger field next time.

The important thing still holds though, I got the right ratios, and the wires had voltages across them showing they weren't totally nodes as was originally contended.

 

[cabraham]

Claude, I am glad you use your example and we can build on this. If you remember the professor original drawing of replacing the 1V battery with a short circuit and then sent a magnetic field to get 0.1V on the 100ohm and 0.9V on the 900ohm.

Use your example. We use 110ohm in series with 9ohm across the secondary. Then we pulse the primary to get 9V on the 9ohm and 110V on the 110ohm. As in the professor case, let A be the junction between two resistors which in this case has 0 length. Where is point D?


He obvious know that the setup he had, by replacing the 1V battery with a short wire result in a loop formed by the wire and the resistors. Then he said he measure 0.1V across 100ohm and 0.9V across 900 ohm. Then he claimed he measure say clockwise from one point is 0.1V and counter clockwise as 0.9ohm! So where is the transformer that gave him the induced voltage in the picture?

Back to your example, the wire is the secondary of your transformer, point D is not a point, it consist of the winding (wire) of your secondary winding.


Is there any way for me to post a simple drawing of just two resistors and the secondary of the transformer without have to using a PDF and then attach, then have to wait for a day for other people to see it? Let me try this.


Let us redo the presentation again:

1) Let's arrange the components in counter clockwise.

2) Start with the 900ohm, then 100ohm and call the point between the two resistor is point A. I call the open end of the 900ohm resistor point C.

3) Then we connect one end of 4" wire to the 100ohm resistor and call this junction B. Remember I am still going counter clockwise now.

4) Then the other side of the 4" wire connect back to the open end of the 900ohm resistor which I call point C in step 2.

With this, we form a closed loop starting from point A between the two resistor, travel counter clockwise through 100 ohm resistor, to point B to the wire, through the wire to point C that connect to the other side of the 900ohm resistor.

Then you inject a pulse of magnetic field, you measure 0.1V across the 100 ohm resistor, and 0.9V across the resistor. Just like the good professor did.

Lets call point C is +ve and call point B as -ve. Let's travel from point B counter clockwise through point C to point A, you get 1V-0.9V=0.1V. If you travel from point B clockwise this time to point A, you get 0.1V! Why are they the same now? Because the transformer effect, this time I include the transformer in the picture and voltage come together! IN this case, KVL works beautifully.

Please provide a diagram. My xfmr example demonstrated that KVL does not always work. We are not in agreement here.

Claude

 

[sarumonkee]

Please provide a diagram. My xfmr example demonstrated that KVL does not always work. We are not in agreement here.

Claude

In my studies, the secondary resistance is modeled as a series resistance with the secondary inductance. It is the inductance that allows the energy transfer in a transformer, and is modeled as a voltage source when fed from the primary. I don't see a KVL error here.

Source: Fundamentals of Power Electronics 2nd ed., Erickson/Maksimovic

 

[Studiot]

Faraday’s and Kirchoff’s laws were developed for different circumstances and are therefore different.
Both sometimes apply to situations not covered by the other; neither is a special case of the other.

I am in general agreement with Prof Lewin in his statements, with the exception that I have no trouble applying the original form of Kirchoff’s law to his apparatus.

It is instructive to consider the original form of both laws to see where they overlap and where they differ. It should be remembered that in their time magnetism was treated in terms of ‘lines of force’.

This is Maxwell’s translation of Kirchoff

“In any complete circuit formed by the conductors the sum of the electromotive forces taken around the circuit is equal to the sum of the currents in each conductor multiplied by the resistance of that conductor.”

They went on to state that this sum is known as the total EMF in a circuit (loop).


And this is Nightingale's record of Faraday

“Whenever a conductor cuts magnetic lines of force an EMF is generated. This EMF is proportional to the time rate at which the lines are cut.”

My understanding of Faraday's law is that it is in differential form as stated here. It is more far reaching than Kirchoff’s Law as it connects electric and magnetic effects. Kirchoff 's Law relates purely to electric effects. However the downside of this is that there must actually be magnetic flux to vary to yield the EMF.

Further differences are that Faraday does not require a closed loop, although he does not prohibit one either.
Kirchoff’s treatise concerned loops in meshes. He does not actually mention potential difference or drop and he does not distinguish between sources of EMF. They are all the same to him.

So to apply Kirchoff to Lewin proceed as follows:

The sum of the EMF's = The sum of the IR products

Used in this form it appears to me that the law is satisfied.

The total EMF in the circuit is 1 volt (the EMF induced by the coil) and the IR sum is 0.9 + 0.1 volt.

I believe it used to be phrased in this way to allow for just such a situation.

What has Lewin done then?

Well there is only one source of EMF in the loop and it is distributed around the whole loop. It is not lumped into any particular circuit element and cannot be applied at any particular point in the loop.

This brings out the difference between EMF (which is distributed around the circuit in this case) and Potential Difference or Potential Drop.

So Prof Lewin has demonstrated is that an EMF and a Potential Difference are not two names for the same thing. They are in fact different animals.

The give away clue is in his statement about conservative and non conservative fields.

For PD the line intergral $$\oint$$E.dl is zero around the loop.

For EMF it is not.

Another way to look at it is that an EMF is capable of introducing energy into the system, but PD is not.

A third way to look at it is to note that PD's result from the solution of Laplaces equation, EMF's result from the solution of Poissons equation, where there is a forcing function.

 

[stevenb]

Nice summary studiot.

Further differences are that Faraday does not require a closed loop, although he does not prohibit one either.

I'd like to make a clarification on this one aspect. I take it you are saying that a closed loop is not required because you are thinking about the differential equation form of Faraday's Law. However, the integral version, which is the more complete statement, does require closed loops for an analysis.

Even the differential version of FL is a limiting case of a small loop because the curl of the electric field is (by definition) the limit of the closed loop line integral of the field (per unit area) as the area the loop goes to zero.

I know you understand this very well, but I want to stress this point because I'm worried that those trying to learn will not grasp the importance of checking all measurements for consistency with FL using loops as the basis for the analysis.

 

[Studiot]

You can apply Faraday's law to an (infinite) straight conductor moving in an infinite parallel magnetic field.

It cuts lines of magnetic force, just as Faraday envisioned.

There are no loops involved.

Of course I am talking about mesh loops (as was Prof Lewin).

As another aside, other posters have mentioned other mesh loops created by the positioning of sensing leads and so forth.

This is irrelevant since Kirchoff's Law applies to all loops in the mesh and Prof Lewin has singled out one particular one so he is entitled to ignore other possible loops.

 

[cabraham]

In my studies, the secondary resistance is modeled as a series resistance with the secondary inductance. It is the inductance that allows the energy transfer in a transformer, and is modeled as a voltage source when fed from the primary. I don't see a KVL error here.

Source: Fundamentals of Power Electronics 2nd ed., Erickson/Maksimovic

I & others have stated such. You are making much ado about nothing. Including a voltage source is nothing but a construct. The "voltage source" (or current source per Norton/Thevenin equivalence principle) added to the circuit makes KVL valid. What does that mean? It means that w/o said voltage source, KVL is invalid. Dr. Lewin made this point, a correct point at that.

In reality, there is no "voltage source" (nor "current source) in series/parallel w/ the loop. The induced emf/mmf is distributed around the loop. The net voltage around the loop is not zero, but rather, the induced emf. KVL does not hold. Adding the voltage source to the loop modifies the problem by replacing distributed quantities w/ lumped quantities. Then KVL holds because we've transformed the problem from fields to circuits.

Dr. Lewin stated all this, & he has it right. His critics think they know more than him & other learned people. They don't. The problem with the critics is that they don't know what they don't know. They make much ado about things that are very well known. "Is Prof. Lewin wrong about Kirchoff's law?" is the title of this thread.

No, he is not wrong. He is right.

Claude

 

[Studiot]

Where exactly, and using Kirchoff's own words, did he state that a voltage source is needed?

Where, exactly and in Kirchoff's own words, did he state that all elements in a loop must posess 'lumped properties'?

I hold, and have displayed Maxwell's own view that he did neither of these things.

 

[stevenb]

You can apply Faraday's law to an (infinite) straight conductor moving in an infinite parallel magnetic field.

Maybe offline (to not distract the flow of the thread) you can show me how you do that. Personally, I would analyze this case using either the Lorentz force equation, or with Faraday's law by defining a hypothetical rectangular path to establish a mathematical closed loop and surface by which to define flux. This would then allow the derivative of flux to be calculated using the velocity of the wire. Either method would allow us to calculate an EMF per unit length on the wire. I don't know how to apply the integral version of FL without a closed loop and surface boundary to quantify flux change.