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

[Studiot]

You will find the links in posts 9 and 10 of this thread.

 

[yungman]

This is a lot more complicate than from the surface. It is so easy to just watch the video and form an opinion one way or the other. It is not until you actually set up the experiment and do the observation, then you realize the difficulty of this experiment.

So before you comment on the original video, please take the time, do the set up and play with it to get the insight. Please read post #224 and #227, read the two attachments that show in detail step by step my observation on the change in reading when I place the ground lead of the scope probe in very specific position. Then the attachment where I propose the loop that I form with the position of the ground leads. Use that as a starting point and then do your experiment to build on it before you make the conclusion. This is not something that you can sit back, watch the video and talk. I was surprised how sensitive the position of everything is to the result.



To the forum mentors and recognized contributors:

As I said, it is pretty black and white to me at this point. Please take a look at my post #224 and #227. Please tell me whether this is considered PATH DEPENDENT or just simply the knowledge required to measure the loop. If what I did is consider PATH DEPENDENT, then professor is right and I am wrong. If it is not consider path dependent, just require special technique to do the measurement, then I am right and the professor is wrong. And I am not strong enough in theory to determine this.

 

[Antiphon]

This really isn't complicated.

(first to Studiot's remark; power grid engineers don't use circuit theory on lines thousands of miles long because it doesn't work. They use transmission line theory from microwave cuicuit analysis. I know because I have done it.)

Yungman, you are analyzing a real circuit. Faradays law will apply, KVL might or might not give the right answer depending on the flux being linked to your circuit.

There is no such thing as path dependence in circuit analysis. A big loop on the chalkboard has the same inductance as a small one- zero.

When the professor put a magnetic field in the circuit he stopped doing circuit analysis and started doing electromagnetics. Not the same set of assumptions.

Yungman, you are doing electromagnetic experiments. The results are completely dependent on the paths of your wires. Your results confirm that the professor crossed two disciplines with differing assumptions to arrive at the mind blowing conclusion that KVL doesn't work. It doesn't except in the case of circuit analysis.

Edit: for extra clarity, the professor is wrong. You can't have a magnetic field in circuit theory. You can have inductors but you never see the magnetic field, only the terminal I and V.

 

[sokrates]

^^ good post

 

[yungman]

This really isn't complicated.

(first to Studiot's remark; power grid engineers don't use circuit theory on lines thousands of miles long because it doesn't work. They use transmission line theory from microwave cuicuit analysis. I know because I have done it.)

Yungman, you are analyzing a real circuit. Faradays law will apply, KVL might or might not give the right answer depending on the flux being linked to your circuit.
Yes, because the professor use a real circuit to make his case and me, being a long time engineer, I want to dispute his conclusion with the similar setup and detailly analyzing the circuit.
There is no such thing as path dependence in circuit analysis. A big loop on the chalkboard has the same inductance as a small one- zero.

When the professor put a magnetic field in the circuit he stopped doing circuit analysis and started doing electromagnetics. Not the same set of assumptions.

Yungman, you are doing electromagnetic experiments. The results are completely dependent on the paths of your wires. Your results confirm that the professor crossed two disciplines with differing assumptions to arrive at the mind blowing conclusion that KVL doesn't work. It doesn't except in the case of circuit analysis.

Edit: for extra clarity, the professor is wrong. You can't have a magnetic field in circuit theory. You can have inductors but you never see the magnetic field, only the terminal I and V.

Do you mean the professor cannot use his experiment to proof his point because in his circuit, he has magnetic field that make the probbing path dependent just like what I observed? That I agree. My whole thing is challenge his conclusion based on his experiment, I don't have enough theoractical background to charllenge the theory, just the experiment.

 

[Antiphon]

Yes. It is invalid on theoretical grounds to introduce any fields electric or magnetic into circuit analysis.

It goes like this: (for classical work)
1) Full field solutions using Maxwell and boundary conditions: always applicable
2) Problem is still electrically large but you define short line integrals on E and small closed loops enclosing H in selected locations: you have microwave circuit analysis. You talk about V and I by the arbitrary terminals defined by the short integrals but most of the rules of circuit analysis are different or don't apply.
3) you drop the physical extent of the problem to zero and eliminate cross coupling between L,R,C: circuit analysis.

 

[yungman]

Hey Antiphon

I want to clarify with you so there is no mis-understanding:

1) You imply the real circuit like what I did has real physical size. With physical size resistors and wires, electric and magnetic field come into play.

2)Where the professor only draw the circuit loop with two resistors, he automatically imply there is no physical size of the resistors and no length between the connections. He cannot just simply put it into a real circuit and hope that the real circuit is still only two resistor in a loop with no physical size. AND it just happen the method of measurement just happen to give the same result he was looking for.

3)Is that the reason in #2 above that you said the circuit model that the professor gave and his experiment don't match and he cross the line? That he mixed the theoractical circuit diagram ( with no physical size) and he did the experiment that the EM effect come into play.


I guess this is similar to what I said before that, if he want to use his experiment, he has to put in the extra "real life" circuit elements of the emf generator due to the transformer effect of the loop of wire that pick up the flux etc. AND his experiment was frauded with the EM interference.

Please reply point by point to my questions with different color fonds right below my questions so we have a clear understanding with each other. As I said so many time, I only challenge his experiment.

Thanks

PS: I think this is the first argument that make sense to me. Now we wait for the ones that disagree to come in and present their case.

 

[Studiot]

Antiphon, for the record would you mind stating your (?the correct) version of Kirchoff's Law?

 

[Antiphon]

Antiphon, for the record would you mind stating your (?the correct) version of Kirchoff's Law?

The voltage around any closed circuit adds up to zero.

 

[Studiot]

Can you read German?

Because that is not what Kirchoff actually said or for that matter what appeared in Maxwell's translation of it.
His actual exposition make a huge difference to this problem.

 

[Antiphon]

Hey Antiphon

I want to clarify with you so there is no mis-understanding:

1) You imply the real circuit like what I did has real physical size. With physical size resistors and wires, electric and magnetic field come into play.

2)Where the professor only draw the circuit loop with two resistors, he automatically imply there is no physical size of the resistors and no length between the connections. He cannot just simply put it into a real circuit and hope that the real circuit is still only two resistor in a loop with no physical size. AND it just happen the method of measurement just happen to give the same result he was looking for.

3)Is that the reason in #2 above that you said the circuit model that the professor gave and his experiment don't match and he cross the line? That he mixed the theoractical circuit diagram ( with no physical size) and he did the experiment that the EM effect come into play.


I guess this is similar to what I said before that, if he want to use his experiment, he has to put in the extra "real life" circuit elements of the emf generator due to the transformer effect of the loop of wire that pick up the flux etc. AND his experiment was frauded with the EM interference.

Please reply point by point to my questions with different color fonds right below my questions so we have a clear understanding with each other. As I said so many time, I only challenge his experiment.

Thanks

PS: I think this is the first argument that make sense to me. Now we wait for the ones that disagree to come in and present their case.

Sorry, I don't know how to color the responses.

1) Yes. The assumptions of circuit analysis are that a circuit has zero physical extent and that the circuit elements types do not cross-couple. That means a capacitor only exhibits capacitance, not inductance or resistance. In a microwave circuit you assume that a capacitor has all three. In a field analysis of a physical capacitor you could construct an equivalent circuit that would have to have an infinite number of resistors, capacitors and inductors to model a physical capacitor.

2) Essentially, yes. It isn't just the two resistors but as you pont out also the wires and the voltage source, everything. Any time you draw a circuit it is implicit that it has zero physical size. The professor's sleight of hand was not precisely in assuming that the circuit was of zero size; all circuits in circuit theory are. His feint was in assuming that it was *not* of zero size by allowing a magnetic field to couple to it. He construced a hybrid circuit which was partly idealized as in circuit theory and partly an electromagentic induction loop, a field analysis.

3) Yes.

 

[Antiphon]

Can you read German?

Because that is not what Kirchoff actually said or for that matter what appeared in Maxwell's translation of it.
His actual exposition make a huge difference to this problem.

Ja, ich deutsch lesen und schreiben. Ich lebte in Bayern für ein Jahr.

But my fluency in German isn't germain. You aksed me for *my* version of KVL. Which is what is taught in today's schools.

What's *your* version Studiot?

 

[Studiot]

As Kirchoff originally stated, of course.

Here is an English translation

The conditions of a linear system

1) At any point of the system the sum of all currents which flow towards that point is zero.

2) 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 products of the currents in each conductor multiplied by the resistance of that conductor.


(2) is, of course the paragraph we are talking about here.

If you had bothered to read back in this thread you would have seen that I had already published this along with links to Maxwell's discussion of it where he explicitly states that he considers this is 'avoids consideration of potential' which is my objection to Professor Lewin's version.
As I said only a few posts ago I have already posted in this thread the simple application of Kirchoff's own words to this eliminate this problem.

 

[yungman]

Hey Studiot

I know you have been following this thread closely from day one. I wish you would join in and give your opinion. So far you only pick on the definition of emf and KVL etc. and not involve in the major point of discussion.

I think at this point it is very clear. I challenged the experiment and I proofed my point. StevenB insisted on this is still path dependent and proofed the professor was right. So since you are very into definition, tell us whether this is path dependent or not.

I said many times that I am not particularly strong in theory, but I do have a gift with my nose to smell out spin!( If you watch O'Rielly's no spin zoo, you know what I mean! lol!). And I worked a full career successfully trusting my nose. I am about making things work and find out why when it does not. I can't sit 3 days arguing about the definition of some abstract theory and definition. That's why when it came to this point, I stopped! I am not going to spent a day more to argue about the definition of path dependent! This is for the theractical physis to do.

Without the clear distinction like Antiphon, I think we charllenge the same thing about the real life experiment has more components that what was drawn in the professor's assumption of only two resistors. So you being very strong on definition, you should start putting in your opinion beyond what is KVL in german.



How about all the other mentors and contributors? This is down to definition now!

 

[Antiphon]

As Kirchoff originally stated, of course.

Here is an English translation

The conditions of a linear system

1) At any point of the system the sum of all currents which flow towards that point is zero.

2) 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 products of the currents in each conductor multiplied by the resistance of that conductor.


(2) is, of course the paragraph we are talking about here.

If you had bothered to read back in this thread you would have seen that I had already published this along with links to Maxwell's discussion of it where he explicitly states that he considers this is 'avoids consideration of potential' which is my objection to Professor Lewin's version.
As I said only a few posts ago I have already posted in this thread the simple application of Kirchoff's own words to this eliminate this problem.

I saw the discussion, but I didn't consider it in any way illuminating to the topic at hand. But I'll address them now since they are also your version of KVL and since you're advancing this version as the correct one. Don't build any circuits with it though or you'll be very disappointed.

As Kirchoff stated it he's totaling up the IR drops around the circuit and equating that to the available EMF. That way, if there is induction or batteries (or both) driving a current he's got that all in there. That's fine since circuit analysis hadn't yet been refined to the point it is today. Kirchoff was doing physics in the lab, not electrical engineering as we know it today. To see what I mean, try applying the as-stated Kirchoff Voltage law to a circuit consisting of a battery, resistor and a capacitor. It doesn't work.

Of course you can see what's happeneing here. When the authors of a new principle start fleshing it out, its often not as well defined as it is later on. That's why the KVL of modern electrical enegineering is the one we really need to be using in circuit analysis, where the only sources of EMF around the circuit are the voltage sources on the schematic, not the fields perpendicular to the blackboard.

 

[Studiot]

As Kirchoff stated it he's totaling up the IR drops around the circuit and equating that to the available EMF. That way, if there is induction or batteries (or both) driving a current he's got that all in there. That's fine since circuit analysis hadn't yet been refined to the point it is today. Kirchoff was doing physics in the lab, not electrical engineering as we know it today. To see what I mean, try applying the as-stated Kirchoff voltage law to a circuit consisting of a battery, resistor and a capacitor. It doesn't work.

Of course you can see what's happeneing here. When the authors of a new principle start fleshing it out, its often not as well defined as it is later on. That's why the KVL of modern electrical enegineering is the one we really need to be using in circuit analysis, where the only sources of EMF around the circuit are the voltage sources on the schematic, not the fields perpendicular to the blackboard.

How arrogant can you get?

Your example for analysis is easy. It does not conform to the boundary conditions which state "In any complete circuit formed by the conductors".
Of course a capacitor is not a conductor so there is no complete circuit formed by the conductors to analyse.

I have never claimed KVL to be universally applicable, in fact I stated the opposite a couple of posts back and posted a link to the (rather good for Wikipedia) article detailing one of the exceptions viz non planar circuits.

Yet this thread was entitled 'Was Prof Lewin Wrong?'

My answer is yes, not because of a sleight of laboratory handiwork, but because in this case correct application of KVL will yield a correct result.
This would not be the situation in every case.

So what my answer means is that Prof Lewin was correct to say that sometimes KVL does not work, but his IMHO his example was flawed.

 

[yungman]

why don't we concentrate on the professor's claim which he backed by the experiment. We cannot exactly separate the two. For all I care he cound be right in some cases, but not with his example and with the experiment he did. As Antiphon put is so nicely that if you look at it as a circuit model, you cannot have the wires and physical size of the resistors that can be acted on by the EM produced. If you use the circuit model, you are going to have to put in the parasitic components that come with the finite physical size. Using a physical wire and resistor around a physical coil get us immediately into a real life circuit and it is an electromagnetic experiment instead of a theoractical circuit model. That was the reason I jumped in because I smell the flaw. That is the reason I conclude the professor is wrong on his claim with his experment.

BTW, I think I mis-used the work Fraud instead of flaw. I don't mean he intentionally decieve people, I meant his experiment is flawed that don't back up what he claimed because of all the reason we presented. Excuse me on my English as this is not my primary language.

 

[Antiphon]

How arrogant can you get?

Your example for analysis is easy. It does not conform to the boundary conditions which state "In any complete circuit formed by the conductors".
Of course a capacitor is not a conductor so there is no complete circuit formed by the conductors to analyse.

I have never claimed KVL to be universally applicable, in fact I stated the opposite a couple of posts back and posted a link to the (rather good for Wikipedia) article detailing one of the exceptions viz non planar circuits.

Yet this thread was entitled 'Was Prof Lewin Wrong?'

My answer is yes, not because of a sleight of laboratory handiwork, but because in this case correct application of KVL will yield a correct result.
This would not be the situation in every case.

So what my answer means is that Prof Lewin was correct to say that sometimes KVL does not work, but his IMHO his example was flawed.

First let's correct your errors. A capacitor is a conductor. The current flowing in one end is the exact same current coming out the other. But since you prefer wires, the stated KVL doesn't work for a battery, resistor and inductor either. That's a circuit that can be constructed out of a battery and one big long piece of wire. So no, I'm not arrogant, I'm informed.

I guess I'm confused about what your view is exactly. If Kirchoff's own KVL applies in this case and gets the right answer, why is the professor wrong?

 

[Studiot]

guess I'm confused about what your view is exactly. If Kirchoff's own KVL applies in this case and gets the right answer, why is the professor wrong?

I can't put it any better than steveB did in post#4 of this thread it is an excellent summary.

We will have to agree to disagree about 'what comes out of the other end of a capacitor'.

I do, however, note that often your posts are just statements, without working or backup, although I have several times unsuccessfully invited you to provide the same.
I try to offer my working and backup when I make statements so that others may judge for themselves. Sometimes they have then proved me wrong and I have been the ultimate winner in that I have learned something new.

 

[Antiphon]

I can't put it any better than steveB did in post#4 of this thread it is an excellent summary.

We will have to agree to disagree about 'what comes out of the other end of a capacitor'.

I do, however, note that often your posts are just statements, without working or backup, although I have several times unsuccessfully invited you to provide the same.

SteveB did sum it up nicely but he drew the wrong conclusion (to include Faraday's law in KVL.) The distinction he's not making is the one I've been pointing out.

I don't know what's coming out the back of *your* capacitor, but I can tell you mine is clean as a whistle. :)

I missed your invitations but ok. My favorite treatment of this topic begins on page 264 of "Electromagnetic Fields, Energy and Forces" by Fano, Chu, and Adler. This is an out-of-print MIT texbook from 1963 so I'll do you and everyone the courtesy of making a trip to the library unnecessary. I've added some of my comments in bracket in caps. I'm not shouting, its just that I don't want what I wrote to be confiused with the book's text.]

"6.10 The Concept of Voltage and Kirchoff's Laws
[...] Kirchoff's voltage law states that the sum of the branch voltages along any closed path in the circuit (measured in the same direction) must be equal to zero. This law is the equivalent of Maxwell's first equation, i.e. of Faraday's induction law. This equivalence, however, is not as directly evident as the relation between Kirchoff's current law and the conservation of charge. Indeed, the voltage law depends on how the branch voltages are defined in herms of the electromagnetic field. Although the concept of voltage has already been discussed in Sec. 6.8 in connection with inductive fields, it deserves some further, careful consideration in view of its key role in circuit theory.
To obtain a better feeling for what is involved in in the circuit concept of voltage, it is helpful to consider its definition from an experimental point of view. A little thought will make it obvious that all voltmeters are designed to measure the line intrgral of the electric field along the path formed by the connecting leads. This is evident in the case of electrostatic voltmeters whose operation depends directly on the forces exerted by the electric field. Other more common insturments measure actually the current through a resistor of known value; the current desnity in any such resistor is proportional, by Ohm's law, to the elctric field and, therefore the total current is proportional to the line integral of the electric field between the terminals of the resistor. On the other hand, there are implicit limitations on the use of voltmeters. For instance, nobody in his right mind would wrap the leads of a voltmeter around the core of a transformer in determining the voltage between two points in a circuit. Furthermore, it is understood that the leads of a voltmeter should be kept reasonably short and that little meaning should be attached to an indications which depends on the exact position of the leads. [YOUNGMAN, THIS IS YOUR EXPERIMENT]
These limitations on the use of voltmeters indicate that the voltage between two points has meaning only wjen the line integral of the electric field between two points is closely independent of the path of integration for all reasonably short paths. In mathematical terms, this amounts to saying that a voltage can be defined only between between points of a region in which there exists a scalar potential whose negative gradient is closely euqal to the electric field [VIOLATED BY PROFESSOR LEWIN'S EXPERIMENT]. Thus the concept of voltage in the presence of of time-varying currents is strictly an extension of the concept of voltage as defined in electrostatic systems; this extension is valid only when the path of integration used in the computation of the voltage is contained in a region of space in which the electric field behaves approximately as an electrostatic field."[THIS IS WHY A CIRCUIT HAS TO BE OF INFINITESIMAL SIZE;]

There is much more on the topic but I'll only type it if there is interest.

 

[stevenb]

SteveB did sum it up nicely but he drew the wrong conclusion (to include Faraday's law in KVL.) The distinction he's not making is the one I've been pointing out.

I feel you are slightly misrepresenting what I said here, but I don't blame you because of the length of this thread and certainly it's difficult to absorb it all.

I wasn't really trying to include Faraday's Law in KVL, but mentioned that I prefer a version of KVL which is in some sense consistent (at least more consistent than Lewin's version) with FL. It's not until post number 23 that I clarify this by posting 2 pages from Krauss and clearly state the definition I mean. Interestingly, it's not until post #144 where we bring in the version of KVL you are stressing - the modern circuit version. So, in that post I try to express the main difference between the 3 versions by stating them in an order that clarifies the assumptions.

As to why it took so long to bring in this version to the thread, I'll give my opinion. Essentially, Lewin is not discussing circuit theory at all. You really need to watch his entire course and understand the level of students in the class to understand his point of view. These students (although extremely bright and talented) are freshman level students - most of whom are not heading to be physicist and electrical engineers. This is the general class that all students take along with basic mechanics. So Lewin is not discussing circuit theory, but field theory. His definition of KVL (although I also don't like it) is a field definition. It says that the line integral of electric field is zero. Classical circuit theory is not implied in his discussion. We may not like this, but we should respect the substance of what he is saying, even if we want to point out a criticism of the definition and foundation he applies.

As I mentioned a few times in this thread, personally I have no interest in debating semantics, and I won't go any further down this road than this. So, my position is that if we accept his definitions and previous classwork in full context, he is essentially correct.

The real point of this thread, in my mind, is the issues the OP raised. He objected to the Prof's assertions and made his own prediction that the Prof was not measuring the voltages the way he said he was (basically an accusation of fraud, or at least extreme incompetence). He also made his own predictions of what a proper measurement would yield. He then did an experiment (improperly, mind you) that supporting his conclusions. Then he left thinking he was right. Later, once given enough time, I did the measurements and analysis and posted a full report on the proper way to do the measurements, the causes of error and a clear indication of the mistakes the OP made. I stand by all of this, and am quite confident in what I've put forward, with the motivation of helping others.

 

[yungman]

I feel you are slightly misrepresenting what I said here, but I don't blame you because of the length of this thread and certainly it's difficult to absorb it all.

I wasn't really trying to include Faraday's Law in KVL, but mentioned that I prefer a version of KVL which is in some sense consistent (at least more consistent than Lewin's version) with FL. It's not until post number 23 that I clarify this by posting 2 pages from Krauss and clearly state the definition I mean. Interestingly, it's not until post #144 where we bring in the version of KVL you are stressing - the modern circuit version. So, in that post I try to express the main difference between the 3 versions by stating them in an order that clarifies the assumptions.

As to why it took so long to bring in this version to the thread, I'll give my opinion. Essentially, Lewin is not discussing circuit theory at all. You really need to watch his entire course and understand the level of students in the class to understand his point of view. These students (although extremely bright and talented) are freshman level students - most of whom are not heading to be physicist and electrical engineers. This is the general class that all students take along with basic mechanics. So Lewin is not discussing circuit theory, but field theory. His definition of KVL (although I also don't like it) is a field definition. It says that the line integral of electric field is zero. Classical circuit theory is not implied in his discussion. We may not like this, but we should respect the substance of what he is saying, even if we want to point out a criticism of the definition and foundation he applies.

As I mentioned a few times in this thread, personally I have no interest in debating semantics, and I won't go any further down this road than than this. So, my position is that if we accept his definitions and previous classwork in full context, he is essentially correct.

The real point of this thread, in my mind, is the issues the OP raised. He objected to the Prof's assertions and made his own prediction that the Prof was not measuring the voltages the way he said he was (basically an accusation of fraud, or at least extreme incompetence). He also made his own predictions of what a proper measurement would yield. He then did an experiment (improperly, mind you) that supporting his conclusions. Then he left thinking he was right. Later, once given enough time, I did the measurements and analysis and posted a full report on the proper way to do the measurements, the causes of error and a clear indication of the mistakes the OP made. I stand by all of this, and am quite confident in what I've put forward, with the motivation of helping others.

Are you referring me as the OP? I never admit I was wrong, I just loss interest when I saw what we are arguing is just the definition of path dependent. I alway asserted that there are additional elements like the distributed emf generator along the loop of wires and resistors. I showed very clearly how I get different reading by making the ground of the probe traveling at different path. You said this is path dependent.

What your report said was only one way of your mearsurement, you show nothing of the different position of the ground leads that cause different reading. I had a very detail experiment and detail explanation of my observation. I thought you agree to my finding and you call that path dependent, so I did not argue any further. It was what it was. The result showed.

Now Antiphon talked about his opinion that I totally agree. That what the professor drawn is a too simplistic of a drawing. Even if it is hard to measure the voltage correctly, it ABSOLUTELY don't imply the voltage sources are not there. So don't say I saw I was wrong.

I think you should speak for yourself to proof Antiphon is wrong on his assertion first.

 

[stevenb]

Are you referring me as the OP?

No, you are not the OP. OP is basically the original post or poster.

 

[stevenb]

.

I think you should speak for yourself to proof Antiphon is wrong on his assertion first.

I am speaking for myself. I'm not trying to prove anyone wrong. There are too many shades of grey and side issues for me to have any motivation for that. I'm just putting my opinion forward.

 

[cabraham]

I believe that the questions presented have all been answered. There seems to be disagreement regarding how to define voltage across 2 points in a non-conservative E field, like that encountered w/ induction. The voltage from a to b is unambiguous when the field is conservative, as it is independent of path of measurement. Voltage is a quantity defined as the work done per unit charge transporting said charge from a to b, along a specific path for a non-conservative E field, & independent of path for conservative E fields.

In the non-conservative case, the voltage from a to b can be defined & have valid meaning if a path is specified. Otherwise it's ambiguous. Prof. Lewin was only pointing that out, which he did do correctly. Of course his measurement techniques could have introduced error. But he was emphasizing that one cannot assume that KVL holds. Two circuit elements in parallel do not necessarily have the same voltage across them when the E field is non-conservative.

I believe it has been affirmed that Prof. Lewin is correct in his teachings, but most on this forum feel he did not explain it as well as it could be explained. I certainly explain it a little differently than Prof. Lewin, but he is spot on technically. As far as a voltage source is concerned, it could be added to the equivalent circuit, so that KVL would then apply. But Prof. Lewin has to inform the students that this equivalent independent voltage source does not show up in measurements directly. Rather, the non-zero sum of voltages around the loop are the value of said voltage source.

That has to be known & he explained it. Is there anything in Prof. Lewin's lecture that is technically wrong? I have not found it, but feel free to say so if you are still at odds w/ him technically. BR.

Claude

 

[yungman]

I just watch the two part video of the Levin and I have to reconfirm...I meant everything I said about him other than the mis use of the "Fraud", it should be "flawed".

After weeks on this subject and did the whole experiment myself and did all the ground lead placement and recording the observation. AND listen the part 1 of the video from 4:20 to the end 5 times and noting down what he said. HE IS FULL OF IT AND FULL OF HIMSELF.

1) He mentioned Lens Law at 4:30 that induce I, he drew the magnetic source and consequence generated 1v emf. But he never put the equivanlent voltage source into the circuit. If he acknowledge there is an induced emf according to Lens Law, why he fail to put in the equivalent voltage source?

2) In part 2, he recognize that the area of the loop consists of the two resistors is 10cm^2. So he obviously know that what he draw is not just a circuit model. Then he fail to put the emf source into the drawing in part 1.

3) I gave it more thoughts, just because you cannot easily measure the voltage because of the loop created by the measuring probe don't imply the voltage source is not there. Path dependent voltage don't imply anything about how you can measure it. IF YOU CAN MEASURE VOLTAGE ACROSS THE RESISTORS, THEN YOU HAVE TO HAVE A VOLTAGE SOURCE SOMEWHERE IN THE LOOP. Or else where is the voltage come from?

4) If Levine miss the voltage source in the drawing of the loop, what is the point of even talking about conservative and non conservative and path dependent.



Please stop arguing about the definition of the KVL, let's concentrate on the totality of the experiment and the lecture. Watch the lecture again and please read post #224 and #227. Tell me where is the voltage source?

As I specified, I am not arguing about what he claimed KVL don't hold in certain case. I just determine the whole lecture and experiment he did was flawed and don't mean anything about KVL and conservative path dependent.

 

[yungman]

I believe that the questions presented have all been answered. There seems to be disagreement regarding how to define voltage across 2 points in a non-conservative E field, like that encountered w/ induction. The voltage from a to b is unambiguous when the field is conservative, as it is independent of path of measurement. Voltage is a quantity defined as the work done per unit charge transporting said charge from a to b, along a specific path for a non-conservative E field, & independent of path for conservative E fields.

In the non-conservative case, the voltage from a to b can be defined & have valid meaning if a path is specified. Otherwise it's ambiguous. Prof. Lewin was only pointing that out, which he did do correctly. Of course his measurement techniques could have introduced error. But he was emphasizing that one cannot assume that KVL holds. Two circuit elements in parallel do not necessarily have the same voltage across them when the E field is non-conservative.
Can you show me a case of two parallel circuits do not have the same voltage across it. Please put in the equvalent voltage source also.

I believe it has been affirmed that Prof. Lewin is correct in his teachings, but most on this forum feel he did not explain it as well as it could be explained. I certainly explain it a little differently than Prof. Lewin, but he is spot on technically. As far as a voltage source is concerned, it could be added to the equivalent circuit, so that KVL would then apply. But Prof. Lewin has to inform the students that this equivalent independent voltage source does not show up in measurements directly. Rather, the non-zero sum of voltages around the loop are the value of said voltage source.
No, he did not inform anything to the student. I watched the part 1 5 times. He mentioned about the Lens law induce current and mentioned the induced emf. But he fail to incoporate into the drawing and went right into telling the student the magic of his finding. Of cause if you include the voltage source, then I won't be complaining. The next question would be where do you measure the voltage as the emf source is distributed along the wire? That proofed to be the tricky part of the experiment.

That has to be known & he explained it. Is there anything in Prof. Lewin's lecture that is technically wrong? I have not found it, but feel free to say so if you are still at odds w/ him technically. BR.

Claude

Watch the video over and listen to him again.

You've been gone before me and StevenB did the experiment and wrote the detail write up. Please look at #224 and #227 with the attachments.

 

[vanhees71]

In the meantime I browsed through this thread in a bit more detail. Here are some quotes from a textbook by Antiphon which are plain wrong, and Prof. Lewin's explanation is way better. I think all this has been answered already in the first few postings of this thread, but let's summarize it again.

"6.10 The Concept of Voltage and Kirchoff's Laws
[...] Kirchoff's voltage law states that the sum of the branch voltages along any closed path in the circuit (measured in the same direction) must be equal to zero.

That's plain wrong according to Faraday's Law. Kirchoff's Laws are strictly valid only for DC circuits. As soon as you have time-dependent magnetic fields they do not hold anymore. Faraday's Law says (in its local form)

\vec{\nabla} \times \vec{E}=-\partial_t \vec{B},

i.e., es soon as you have a time-varying magnetic field the electric field is not conserved anymore, and the "sum of the voltages" along the circuit is not 0. Of course "sum of the voltages" here means the integral along the closed circuit, and this value is according to the above equation the negative time derivative of the magnetic flux through any surface with the boundary given by this loop.

To make things easy, and I think that's also the case discussed in this thread, let's only discuss circuits without moving parts, i.e., in the following all surfaces and their boundaries are assumed to be at rest.

Now suppose, we have the most simple case of one resistor in a closed loop, and a volt meter measuring the voltage drop across that resistor. What we have then are effectively two loops, namely the resistor loop and the volt-meter loop of this parallel circuit. I assume a simple volt meter which I can treat as another resistor (of high resistance). Then we can use Faraday's Law for these two loops. Let's start with the resistor loop and integrate Faraday's Law over an arbitrary surface with a boundary given by this loop. The left-hand side can be taken as path integral along that path, using Stoke's Law. This gives

U_1=R i_1=-\dot{\Phi}_1,

where \Phi_1 is the magnetic flux through the surface. The integral is independent of the particular choice of this surface due to Faraday's Law. So there's no ambiguity here.

Now by the same argument we can integrate Faraday's Law across the area with the boundary given by the volt-meter loop, giving

-R i_1+R_V i_2=\dot{\Phi}_1+U_V=-\dot{\Phi}_2,

where R_V is the resistance of the volt meter and U_V the corresponding voltage. Thus, what you measure is

U_V=-\dot{\Phi}_1-\dot{\Phi}_2,

and of course the voltage, measured by the volt meter, depends on both fluxes, i.e., the volt-meter reading will change when the volt-meter loop is changed. [This result you can of course also get, if you integrate along the outer loop, containing only the volt meter as a resistance. The total magnetic flux is of course the sum of the fluxes through the resistor and the volt-meter loops.] If you want to measure the magnetic flux through the resistor loop alone, you not only have to make the resistance of the volt meter, R_{V} \gg R (as would be sufficient for DC circuits) but also make sure that the magnetic flux through the volt-meter loop can be neglected (by either arranging it to be outside the relevant time-varying magnetic field or making it as small as possible).

This law is the equivalent of Maxwell's first equation, i.e. of Faraday's induction law. This equivalence, however, is not as directly evident as the relation between Kirchoff's current law and the conservation of charge. Indeed, the voltage law depends on how the branch voltages are defined in herms of the electromagnetic field. Although the concept of voltage has already been discussed in Sec. 6.8 in connection with inductive fields, it deserves some further, careful consideration in view of its key role in circuit theory.
To obtain a better feeling for what is involved in in the circuit concept of voltage, it is helpful to consider its definition from an experimental point of view. A little thought will make it obvious that all voltmeters are designed to measure the line intrgral of the electric field along the path formed by the connecting leads. This is evident in the case of electrostatic voltmeters whose operation depends directly on the forces exerted by the electric field. Other more common insturments measure actually the current through a resistor of known value; the current desnity in any such resistor is proportional, by Ohm's law, to the elctric field and, therefore the total current is proportional to the line integral of the electric field between the terminals of the resistor. On the other hand, there are implicit limitations on the use of voltmeters. For instance, nobody in his right mind would wrap the leads of a voltmeter around the core of a transformer in determining the voltage between two points in a circuit. Furthermore, it is understood that the leads of a voltmeter should be kept reasonably short and that little meaning should be attached to an indications which depends on the exact position of the leads. [YOUNGMAN, THIS IS YOUR EXPERIMENT]

This is the same thing with words as I derived for this most simple example above, but it's wrong to call this "voltage". A voltage is a difference of an electric potential. In the case of time-dependent fields, there is no electric potential. In this case, one must use not only a scalar but also a vector potential to describe the electromagnetic field, i.e.,

\vec{E}=-\vec{\nabla} \Phi-\frac{\partial}{\partial t} \vec{A}, \quad \vec{B}=\vec{\nabla} \times \vec{A}.

For a given electromagnetic field the electromagnetic potentials (relativistically the four-vector potential) is not unique but only determined up to a gauge transformation and have not a clear physical meaning except of giving the fields in a way such that the homogeneous Maxwell equations (i.e. Farday's Law and the absence of magnetic monopoles) is fulfilled, but that's not the point here.

In any case, if you have a time varying magnetic field, \vec{E} is not a conserved vector field, which however is already clear from Faraday's Law in terms of the electromagnetic field itself, without using the potentials. "Voltage" thus doesn't make sense here. Of course sometimes, one calls L \frac{\mathrm{d} i}{\mathrm{d} t} a "voltage", but that's at least misleading and precisely the reason for unnecessary confusion as in this thread. Prof. Lewin is right to stress this point as in http://ocw.mit.edu/courses/physics/...netism-spring-2002/lecture-notes/lecsup41.pdf (which has already been quoted in #9 of this thread).

These limitations on the use of voltmeters indicate that the voltage between two points has meaning only wjen the line integral of the electric field between two points is closely independent of the path of integration for all reasonably short paths. In mathematical terms, this amounts to saying that a voltage can be defined only between between points of a region in which there exists a scalar potential whose negative gradient is closely euqal to the electric field [VIOLATED BY PROFESSOR LEWIN'S EXPERIMENT]. Thus the concept of voltage in the presence of of time-varying currents is strictly an extension of the concept of voltage as defined in electrostatic systems; this extension is valid only when the path of integration used in the computation of the voltage is contained in a region of space in which the electric field behaves approximately as an electrostatic field."[THIS IS WHY A CIRCUIT HAS TO BE OF INFINITESIMAL SIZE;]

No, it's simply a wrong statement, as explained in detail above and in much more detail in the above quoted lecture note by Prof. Lewin. Volt meters simply have to be used in the right way to measure the very quantity you are interested in. Circuit theory can be used for any circuit as long as the quasistationary limit is applicable (i.e. as long as the typical wave length of the em. fields under consideration are much larger than the size of the circuit and thus Maxwell's displacement current can be neglected) and as long as all emf's from time-varying magnetic fluxes are taken into account properly.

 

[yungman]

Tell me whether I am wrong:

Sounds like a lot of physicist only talk about circuit that is physically there. It seems they really don't get the idea about using equivalent circuits. Equivalent circuit in this case is the induced emf in the loop can be represented by a voltage source or better yet, a differential voltage source ei. mini voltage source per unit length.

If people cannot comprehend this concept, they really have no place to talk circuit. They are going to bang their head on the wall when they deal with any physical circuits in microwave frequency.

People should really take a class in RF circuit design which is an extention of EM. I study both and I can tell you that the electrodynamics in physics class miss the whole thing on transmission lines where we deal with equivalent circuits. That a little section of transmission line can be made to behave like a capacitor or an inductor depend on the length of the section. That we can design all sort of filter network, impedance matching by just using sections of lines of different width and length that to physicist is only a line or worst yet only a note like Levine called point "A" and "D".

If this is how the physicist look at thing, I don't think they should even talk about this problem here. They need to study a few books in EM for engineering like "field and Wave Electromagnetic" by Cheng. There are detail theory about equivalent circuits.


A wire in microwave is equivalent to a series of inductors and capacitors. Induced voltage become a voltage source. Without these kind of knowledge, you really cannot talk about circuits. Sorry that circuits has to work in AC, not just DC. Don't tell me physicist sweep all these into "non conservative"! that would be really discouraging for me. I was planning to pursue advanced electrodynamics, but if this is what end up to be, I think I'd change my mind!


THis sum up my observation. Seem like We are talking in different languages and I think this seems to be the problem right here. That might be the reason why some people find it so hard to comprehend the induced voltage concept here. People that work in high speed microwave electronics look at this as cake walk! The non existing induced emf source really deliver power, those non existing capacitor really behaving like a cap that filter out high frequency and the non existing inductors really work as inductors. And these are all swept under " non conservative" behaviors?

 

[cabraham]

Tell me whether I am wrong:

Sounds like a lot of physicist only talk about circuit that is physically there. It seems they really don't get the idea about using equivalent circuits. Equivalent circuit in this case is the induced emf in the loop can be represented by a voltage source or better yet, a differential voltage source ei. mini voltage source per unit length.

If people cannot comprehend this concept, they really have no place to talk circuit. They are going to bang their head on the wall when they deal with any physical circuits in microwave frequency.People should really take a class in RF circuit design which is an extention of EM. I study both and I can tell you that the electrodynamics in physics class miss the whole thing on transmission lines where we deal with equivalent circuits. That a little section of transmission line can be made to behave like a capacitor or an inductor depend on the length of the section. That we can design all sort of filter network, impedance matching by just using sections of lines of different width and length that to physicist is only a line or worst yet only a note like Levine called point "A" and "D".If this is how the physicist look at thing, I don't think they should even talk about this problem here. They need to study a few books in EM for engineering like "field and Wave Electromagnetic" by Cheng. There are detail theory about equivalent circuits.


A wire in microwave is equivalent to a series of inductors and capacitors. Induced voltage become a voltage source. Without these kind of knowledge, you really cannot talk about circuits. Sorry that circuits has to work in AC, not just DC. Don't tell me physicist sweep all these into "non conservative"! that would be really discouraging for me. I was planning to pursue advanced electrodynamics, but if this is what end up to be, I think I'd change my mind!


THis sum up my observation. Seem like We are talking in different languages and I think this seems to be the problem right here. That might be the reason why some people find it so hard to comprehend the induced voltage concept here. People that work in high speed microwave electronics look at this as cake walk! The non existing induced emf source really deliver power, those non existing capacitor really behaving like a cap that filter out high frequency and the non existing inductors really work as inductors. And these are all swept under " non conservative" behaviors?

But this lecture by Dr. Lewin is from an undergraduate physics class. Transmission lines have not been covered at that point. So an undergrad probing a circuit where induction is happeneing will notice that the voltage summation around a loop does not always equal zero. Dr. Lewin is informing the students that it is perfectly normal to get a non-zero loop voltage summation when induction is taking place.

Regarding the equivalent circuit approach, I've already covered it. You may add the measured loop summation voltage to the equiv circuit as an independent voltage source. Then KVL holds. As far as "distributed emf sources" go, this is already covered in the Lorentz force law. The source that is giving rise to the non-zero loop emf is the external circuit generating the time varying fields. A portion of, or nearly all of the magnetic flux generated by the primary circuit links the secondary circuit. A transformer can serve as an example.

We can model the xfmr referring to either the primary or secondary. In this case we are viewing the secondary equiv circuit. The primary power source which drives the primary circuit generating the magnetic field, gets reflected to the secondary in accordance w/ the turns ratio & coupling coefficient. The secondary circuit undergoes induction, i.e. "non-conservative" E field, & a measurement of the loop emf summation will result in non-zero value.

But the equiv circuit ref secondary includes a voltage source which is really a reflection of the one driving the primary. As far as RF goes, & "distributed parameters", that is EE course material for junior & senior EE majors. An undergraduate physics class does not have the time to delve into RF/T-lines & distributed parameters. Dr. Lewin is correct that a loop voltage summation measurement will not be zero-valued when induction happens.

But distributed parameters like L, C, R, equiv emf sources, etc., is beyond the scope of said course. Dr. Lewin's lecture is not all encompassing, he is dealing w/ undergrads in elementary physics, many of them sophomores. Most will not major in EE, but ME, CE, ChE, physics, etc. Dr. Lewin gave them good info. Those majoring in EE will later learn about distributed parameters & t-line concepts.

The critics of Dr. Lewin are making too much ado over nothing. His thesis is correct. But now the focus has moved to RF & t-lines, which are too advanced & specialized for an undergrad general physics class. For such topics, the 2nd semester or third quarter of e/m fields is a good place to learn. Yungman, you are taking the view that a wire being an inductance, resistance, & capacitance, distributed per unit length, is some earth-shattering revelation nobody but you is aware of.

I learned that in e/m fields in the 70's & it was ancient news then! Sir Oliver Heaviside pretty much summed up t-lines in the 1870's, also the same decade Maxwell published his cornerstone equations. It is too well known to be giving us lectures.

Anything else that needs to be clarified?

Claude