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

[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.

 

[yungman]

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

Claude

We never in agreement, I just use your example to build on it. I scan the drawing, don't know how long before people can see it, I know there will be a delay. If you have any easier way to post the picture, tell me how.

I label the points exactly the same as the example I gave on the single turn loop of the professor. The wire is the secondary of the transformer.

BTW, I agree the inductance is not in the picture just like you said your secondary internal impedance is 1ohm. I posted the calculations on the inductance of the 24 gauge wire taking into consideration of skin effect and all. Turn out that it is only a few ohms and not significant in the whole picture.

 

[yungman]

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

How can you not include the voltage source into the circuit. Without that, you cannot even generate the current to give the voltage across the two resistors. I guess you understand what I am driving at that there should be a voltage source in the drawing. I cannot see how you can not consider that is part of the circuit. KVL do use voltage source.

Induce EMF is a voltage and it is a source. It might be distribute over the whole loop but if you draw the equivalent circuit as all the books do, you replace the distribute source as a single source. Look at the equivalent schematic of Transmission line that represent the distribute L, R, G and C, people use discrete resistor, inductor etc. to represent the distributed element per unit length. How can you not consider the distribute emf on the loop as not a voltage source?

The professor was wrong. Until we can get over the way he described on his initial drawing, everything else is irrelevant.

Let me put it this way, ALL voltage source are generated, if you look at ALL the examples in books of KVL with voltage source and SHORT them all out! Everyone of the example failed! Then KVL is hot air if that is your point.

 

[yungman]

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.

I don't know that guy, I thought his presentation is just simply wrong. I am absolute amazed that people don't see the problem of his presentation and keep on arguing on the formulas behind the non conservative nature this and that. To me, until he make his model correctly, there is no merits on the rest.

I found him arrogant and condescending. Apparently people here respect him and his big name. I worked over 10 years in a company with over 60% PHDs as staff, this is nothing new. I spent years arguing with them! They are just human and they make mistake. And I notice a lot of them have such ego that they defend to death! We are no MIT, but you should see the draws of publications in scientific journals from our company. I myself have two paper published in the Review of Scientific Instrument in AIP and own a pattern on the detector of one of the product.

 

[cabraham]

How can you not include the voltage source into the circuit. Without that, you cannot even generate the current to give the voltage across the two resistors. I guess you understand what I am driving at that there should be a voltage source in the drawing. I cannot see how you can not consider that is part of the circuit. KVL do use voltage source.

Induce EMF is a voltage and it is a source. It might be distribute over the whole loop but if you draw the equivalent circuit as all the books do, you replace the distribute source as a single source. Look at the equivalent schematic of Transmission line that represent the distribute L, R, G and C, people use discrete resistor, inductor etc. to represent the distributed element per unit length. How can you not consider the distribute emf on the loop as not a voltage source?

The professor was wrong. Until we can get over the way he described on his initial drawing, everything else is irrelevant.

Let me put it this way, ALL voltage source are generated, if you look at ALL the examples in books of KVL with voltage source and SHORT them all out! Everyone of the example failed! Then KVL is hot air if that is your point.

How can we not include the voltage source is a good question. Dr. Lewin is merely illustrating that the definition of voltage across 2 points depends on the path chosen. Since integral E*dl is voltage, the path around the loop is the resistors & the conductors. There is no physical voltage source, or current source, present in the circuit.

We can, however, model an equivalent circuit which includes a voltage/current source (Thevenin/Norton). Then, KVL will be upheld. Without a source included in the circuit, KVL does not always hold.

It's that simple. In your sketch you attached, you are including a source, namely the xfmr secondary. When summing voltages around the secondary loop, the result is zero, regardless of path. In other words, KVL is valid here. But you have lumped the quantities. That in & of itself is not wrong, it just gives a different result.

If we drew a loop consisting of 2 resistors connected by wires in a closed loop, & do not lump the induction into a discrete source, then KVL does not hold. The sum of voltages around the loop equals the induced emf. But if we insert a lumped voltage source into the loop, whose value equals the induced emf, then the induced emf is canceled by the source, leaving zero around the loop. Thus KVL holds in this condition.

There seems to be universal consensus here that if the discrete lumped source equalling the induced emf is added to the circuit, KVL is upheld. If not, then KVL may not apply. I think that sums it up.

No need to argue. I'll gladly clarify further if needed. Again, I don't share Dr. Lewin's method of presenting his facts. His facts are correct, but I believe I could explain said facts in a manner which freshman & sophomores could relate to. Then again, I may be mistaken. I understand the material well. But the ability to convey it to a novice may not be easy. I might be overestimating my ability to convey info. It happens.

Claude

 

[hikaru1221]

@yungman (& some other people): From what I understand, your transformer effect is actually included in Prof. Lewin's explanation. It's nothing else but electromagnetic induction. I don't understand where your transformer effect violates with Prof. Lewin's explanation. The "transformer" here is simply a very-non-ideal transformer, and thus, using the term "induction" is more appropriate.
Besides, since the wire's inductance is negligible, the 1-volt emf is mostly distributed on the 2 resistors (not evenly distributed over the whole loop). I don't really understand what to be in dispute here.

P.S.: I've just had a look at the file you attached in post #106. I think we cannot treat the circuit this way. The inductor (one of the coils of the transformer in particular) is MODELED to fit in simplified version of KVL; the emf is on the inductor. The circuit of Prof. Lewin is not that way; the emf is on the resistors.

P.S. #2: I think there is one thing that should be clarified. The induced emf due to the coil is NOT the only emf here; in other words, induced E-field is not the only E-field here. There is also E-field due to charge accumulation at the boundary between the resistors and the wires (since they are 2 different mediums with different resistivity!).
The induced E-field exists in the space regardless of the presence of the circuit; therefore, there is induced emf on the wire. However, the E-field due to charges cancels it. The final result is that the wire has zero emf on it, the 2 resistors have 1-volt emf.@Studiot: I think there is some other semantics here. From what I understand, your "Faraday's law" is the statement / the theory proposed by Faraday. The Faraday's law, as far as I've known, is the law that states induced emf is proportional to rate of change in magnetic flux (so "Faraday's law" is just a name). According to Wikipedia, in an attempt to explain the phenomenon, Faraday did propose a notion of line of force, which was rejected. Faraday's idea may come farther than the law, but the law which is named after Faraday has nothing to do with differential form or the notion of induced E-field.

 

[sarumonkee]

How can we not include the voltage source is a good question. Dr. Lewin is merely illustrating that the definition of voltage across 2 points depends on the path chosen. Since integral E*dl is voltage, the path around the loop is the resistors & the conductors. There is no physical voltage source, or current source, present in the circuit.

We can, however, model an equivalent circuit which includes a voltage/current source (Thevenin/Norton). Then, KVL will be upheld. Without a source included in the circuit, KVL does not always hold.

It's that simple. In your sketch you attached, you are including a source, namely the xfmr secondary. When summing voltages around the secondary loop, the result is zero, regardless of path. In other words, KVL is valid here. But you have lumped the quantities. That in & of itself is not wrong, it just gives a different result.

If we drew a loop consisting of 2 resistors connected by wires in a closed loop, & do not lump the induction into a discrete source, then KVL does not hold. The sum of voltages around the loop equals the induced emf. But if we insert a lumped voltage source into the loop, whose value equals the induced emf, then the induced emf is canceled by the source, leaving zero around the loop. Thus KVL holds in this condition.

There seems to be universal consensus here that if the discrete lumped source equalling the induced emf is added to the circuit, KVL is upheld. If not, then KVL may not apply. I think that sums it up.

No need to argue. I'll gladly clarify further if needed. Again, I don't share Dr. Lewin's method of presenting his facts. His facts are correct, but I believe I could explain said facts in a manner which freshman & sophomores could relate to. Then again, I may be mistaken. I understand the material well. But the ability to convey it to a novice may not be easy. I might be overestimating my ability to convey info. It happens.

Claude

I don't get your comment. You say if the model is changed to include an element that affects the circuit, you get the right answer. If the model ignores the "lumped element" that is the voltage source (or inductor), then it is not wrong? How can you justify ignoring the effect of the very thing that is coupling energy into the system? I just don't get it. Maybe it is a difference in thought between physicists and engineers...

Just because something is "spread out" around a circuit doesn't mean we can ignore it and remove it from the model. It seems like that is akin to saying a resistor can be ignored because it is a collection of atoms, and therefore the resistance is distributed.

 

[sarumonkee]

@yungman (& some other people): From what I understand, your transformer effect is actually included in Prof. Lewin's explanation. It's nothing else but electromagnetic induction. I don't understand where your transformer effect violates with Prof. Lewin's explanation. The "transformer" here is simply a very-non-ideal transformer, and thus, using the term "induction" is more appropriate.
Besides, since the wire's inductance is negligible, the 1-volt emf is mostly distributed on the 2 resistors (not evenly distributed over the whole loop). I don't really understand what to be in dispute here.

The wire's inductance is not negligible... It is what allows the energy to be coupled into the circuit. That's why Lewin has such a huge coil and says it "BLASTS" flux everywhere. Yeah, the inductance is small, but with enough energy, Lewin is coupling the needed energy to get the currents he does through the resistors.

 

[hikaru1221]

The wire's inductance is not negligible... It is what allows the energy to be coupled into the circuit. That's why Lewin has such a huge coil and says it "BLASTS" flux everywhere. Yeah, the inductance is small, but with enough energy, Lewin is coupling the needed energy to get the currents he does through the resistors.

What I meant by "wire" is the connecting wire of the circuit, the "secondary coil" of the transformer, not the wire of the primary one. Sorry for the confusion.

 

[atyy]

The wire's inductance is not negligible... It is what allows the energy to be coupled into the circuit. That's why Lewin has such a huge coil and says it "BLASTS" flux everywhere. Yeah, the inductance is small, but with enough energy, Lewin is coupling the needed energy to get the currents he does through the resistors.

No one is saying the wire's inductance is negligible.

It is the whole thing.

The question is how do you model the inductance?

By a kluge lumped element, or by Faraday's law?

A lumped element is fine for many purposes, but if you want to predict the different readings of voltmeters connected to exactly the same points, then you need Faraday's law.

 

[yungman]

How can we not include the voltage source is a good question. Dr. Lewin is merely illustrating that the definition of voltage across 2 points depends on the path chosen. Since integral E*dl is voltage, the path around the loop is the resistors & the conductors. There is no physical voltage source, or current source, present in the circuit.

We can, however, model an equivalent circuit which includes a voltage/current source (Thevenin/Norton). Then, KVL will be upheld. Without a source included in the circuit, KVL does not always hold.

It's that simple. In your sketch you attached, you are including a source, namely the xfmr secondary. When summing voltages around the secondary loop, the result is zero, regardless of path. In other words, KVL is valid here. But you have lumped the quantities. That in & of itself is not wrong, it just gives a different result.

If we drew a loop consisting of 2 resistors connected by wires in a closed loop, & do not lump the induction into a discrete source, then KVL does not hold. The sum of voltages around the loop equals the induced emf. But if we insert a lumped voltage source into the loop, whose value equals the induced emf, then the induced emf is canceled by the source, leaving zero around the loop. Thus KVL holds in this condition.
But in real life, the 6" wire is the secondary of the transformer. Yes the resistor body is part of the loop that pickup the magnetic field and generate part of the emf. But common sense is that a 1/4W resistor is about 0.3" length and two including the point to point solder is about 0.7" which is about 10% of the total length of the loop. So the error is 10% of the 9:1 ratio. This is within the error of the scope measurement easily. I just ignor the resistor body effect like you ignor the 1ohm internal impedance ( which I agree) of the secondary winding. We are arguing about the 9:1 ratio, not the 10% small stuff.
There seems to be universal consensus here that if the discrete lumped source equalling the induced emf is added to the circuit, KVL is upheld. If not, then KVL may not apply. I think that sums it up.

No need to argue. I'll gladly clarify further if needed. Again, I don't share Dr. Lewin's method of presenting his facts. His facts are correct, but I believe I could explain said facts in a manner which freshman & sophomores could relate to. Then again, I may be mistaken. I understand the material well. But the ability to convey it to a novice may not be easy. I might be overestimating my ability to convey info. It happens.

Claude

I consider this is a friendly debate here, no hard feeling at all. I just believe we have to include the voltage source no mater what, without the voltage source, where is the current come from? No voltage source, there is nothing for us to talk about. I model this as a single voltage source, this is in line with every electronics book that I read. Just use the model of the transmission that I described, you really don't have individual resistor, inductor etc. It is just a distributed element and is expressed as ohm/length, farad/length or henry/length. This transmission line model is in EM or ED textbook too!

 

[yungman]

No one is saying the wire's inductance is negligible.

It is the whole thing.

The question is how do you model the inductance?

By a kluge lumped element, or by Faraday's law?

A lumped element is fine for many purposes, but if you want to predict the different readings of voltmeters connected to exactly the same points, then you need Faraday's law.

Remember Sarumonkee repeat the experiment in #90 and he showed if he reference the ground of the probe in the middle of the wire ( 3” from each side) and measure the junction of the wire and resistor on both side, he got equal and opposite voltage on the wire. So we know FOR FACT there is induced emf on the wire and it is the emf that drive the resistors and get the 9:1 ratio of voltage.

 

[yungman]

@yungman (& some other people): From what I understand, your transformer effect is actually included in Prof. Lewin's explanation. It's nothing else but electromagnetic induction. I don't understand where your transformer effect violates with Prof. Lewin's explanation. The "transformer" here is simply a very-non-ideal transformer, and thus, using the term "induction" is more appropriate.
Besides, since the wire's inductance is negligible, the 1-volt emf is mostly distributed on the 2 resistors (not evenly distributed over the whole loop). I don't really understand what to be in dispute here.
Not he did not include the transformer in his drawing, he just use point A and D between the two resistors. Double check part one of the video. That was when he start flying off the handle on trashing others. If there is not source, where is the 1mA that drive the resistors come from. You cannot ignor the source. If you look at every example of KVL and eliminate all the voltage source that don't conform to the definition of discrete element, KVL fail most of the time!
P.S.: I've just had a look at the file you attached in post #106. I think we cannot treat the circuit this way. The inductor (one of the coils of the transformer in particular) is MODELED to fit in simplified version of KVL; the emf is on the inductor. The circuit of Prof. Lewin is not that way; the emf is on the resistors.
Look at my post #115 in blue color. I gave the detail explanation about the resistor. The resistor only contribute about 10% of the induction, the wire generate the body of the emf.
P.S. #2: I think there is one thing that should be clarified. The induced emf due to the coil is NOT the only emf here; in other words, induced E-field is not the only E-field here. There is also E-field due to charge accumulation at the boundary between the resistors and the wires (since they are 2 different mediums with different resistivity!).
The junction potential are small, we are talking about the dispute of 9:1, I just simply ignor anything that is under 10%. The measurement by the professor is only accurate to about 10% in my estimate.

The induced E-field exists in the space regardless of the presence of the circuit; therefore, there is induced emf on the wire. However, the E-field due to charges cancels it. The final result is that the wire has zero emf on it, the 2 resistors have 1-volt emf.


@Studiot: I think there is some other semantics here. From what I understand, your "Faraday's law" is the statement / the theory proposed by Faraday. The Faraday's law, as far as I've known, is the law that states induced emf is proportional to rate of change in magnetic flux (so "Faraday's law" is just a name). According to Wikipedia, in an attempt to explain the phenomenon, Faraday did propose a notion of line of force, which was rejected. Faraday's idea may come farther than the law, but the law which is named after Faraday has nothing to do with differential form or the notion of induced E-field.

As I said, context is everything here.

 

[yungman]

I want to start this new post to remind everyone to read #90 from Samumonkee that he repeated the experiment and measure the voltage across the wire.

I really have to get back to my study, I'll try to resist checking this thread until I get some of my own work done!

 

[stevenb]

So we know FOR FACT there is induced emf on the wire and it is the emf that drive the resistors and get the 9:1 ratio of voltage.

Why do you think that emf on the wire is a surprise to us? Faraday's law tells us that there is a 1V emf around the loop. This new setup of sarumonkee has tiny leads on the resistor that are soldered right next to each other, and then a long length of wire to finish the loop. Wouldn't we expect emf in the wire?

Faraday's law tells us something else about this new measurement setup. There is also emf on the scope wires. Why? Because the scope leads are a valid completion of the loop with equal validity as the main circuit wire. For this reason, if you take a loop through your scope and probes and complete it through the wire, the net emf is zero because the wire and the scope leads have equal emf in opposite directions around this new loop. This makes sense in terms of Faraday's Law because there is no net flux change inside this new loop.

What is mysterious to me is that sarumonkee got a reading of emf on the scope when the loop emf being measured should be zero. Yes, there can be emf on the wire, but not in the measurement loop. This is the underlying reason that Lewin's analysis is correct. When you analyze a system with Faraday's law using loops, these details take care of themselves, and you don't need to identify the location of the emf. So, I'm highly suspect of the measurements of sarumonkee, but it's quite possible I'm misinterpreting the method he used. Unless I see the layout and know the method, I can't really judge. I am waiting for him to do more verifications and checking and then to post the details so we can be convinced of his methods. He should follow Lewin's lead and verify all loops obey Faraday's Law. This is a good check to help give confidence that the measurement techniques are valid. I commend sarumonkey for doing real measurements and getting to the bottom of this personally, and it's only fair to give him adequate time and not rush him.

 

[sarumonkee]

No one is saying the wire's inductance is negligible.

It is the whole thing.

The question is how do you model the inductance?

By a kluge lumped element, or by Faraday's law?

A lumped element is fine for many purposes, but if you want to predict the different readings of voltmeters connected to exactly the same points, then you need Faraday's law.

Sorry I didn't quote properly. That was a response to Hikaru's post #110 where s/he said:

"Besides, since the wire's inductance is negligible, the 1-volt emf is mostly distributed on the 2 resistors (not evenly distributed over the whole loop). I don't really understand what to be in dispute here."

Also, I didn't put the probes on the exact same spot yet, so I can't comment on that yet. I probably won't have time to do the testing again until this weekend, if that...