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

[yungman]

How about some auto highZ ignition leads?

But that would be uniform resistance. YOu need to have something of two different resistance per unit length and combine together.

I think we might be onto something here, it would be very interesting to see the result. I bet if you marry half a turn of of 900 ohm total and half a turn of 100ohm total to form a loop, you would not get even close to 9:1 voltage ratio because if I am right about the micro sources, you will be measuring the resistor and the source instead of just the V=IR. Note that even if you don't get the V+IR relation, don't be too quick to say Ohm's law don't work under non conservative field etc. Because if you model the micro source in, KVL still work. Well talk is cheap, one experiment speak louder than anything else.

Maybe if you use one kind of resistor wire, half a turn make up of single wire and the other half turn make up of 9 wires in parallel, twist 9 wires together, use a meter to measure the length of the exact value of resistance you want (900ohm), then just cut the right length.

 

[yungman]

I'm open to donations :)

I thought about the carbon track, but I have no way of making it very regular. I was thinking of just writing with a graphite pencil on some transfer paper, but don't think that would come out very well.

Even if you build the loop with the discrete resistors like what I drawn, you should see reasonable results. If you make sure the wire junction between the two resistor is as short as possible(open twisted end is not part of the loop and don't matter like in my drawing), over 80% of the length of the loop will be of resistor material. You will see the effect if any. Don't sweat too much if you cannot find the resistent wires or carbon deposit. I have a suspicion that you are not going to get anywhere close to 0.9V on the 900 ohm resister if my theory of distributed source is correct, might not be even 0.5V. If you get close to 9:1 ratio, then I am blowing hot air. But in that case, still KVL holds and the voltage is not path dependent either.

We are looking forward to your result. Result speak louder than anything.

 

[yungman]

Guys, while we are waiting for the result, I have time just thinking about some theory that was thrown around in this case here:

Why are we talking about Lorentz force? My understanding about force only act on a charge that is moving. In this case, after the battery was removed, nothing is moving the electrons around the loop, only some thermal motion. Lorentz don't even apply here. It is very clear that

\vec F = q(\vec u X \vec B)\; \hbox { where } \;\vec u \;\hbox { is the velocity. }

And there is no force asseted on the charge if the charge is not moving. Recall magnetic field move the wire ONLY when there is a current passing through the wire? Also one more important point, the book very specificly said that the static magnetic field do not change the velocity of the particle, it only change the direction of the particle. So if the only motion of the electrons in the wire and resistors only change from random motion to random motion plus a few degree shift...still random, no current. Refer to P207 of Griffiths.


In my opinion, the formula in play in our case is :

V = \int _S (\nabla X \vec E) \;\cdot\; \hat n \; dS \;=\; \int _C \vec E \;\cdot\; \hat T \;dl \;=\; \int_S \frac{\partial \vec B}{\partial t} \;\cdot\; \hat n \;dS

From the experiment, the good professor use a changing magnetic field to induce the voltage into the loop. This is a time varying magnetic field and Lens law is in action in this case. And this is the voltage that drive the resistors. We'll have to see my distributed micro voltage inside resistors theory pan out or not.

 

[yungman]

Hey where is the enthusiasm?

 

[yungman]

Dr. Lewin is among the world's most qualified instructors regarding this material. I'm a little surprised at the EEs (or non-EEs) in the industrial community who are bashing Dr. Lewin. Those who do make me wonder how much e/m field theory they've had. Nothing personal, but will the critics of Dr. Lewin please state explicitly the errors in Dr. Lewin's teachings? He's a prof at MIT, an institution world renowed. Who are these critics anyway? What are their credentials? I'm just wondering.

One reason, first thing come to mind as I watch the first video is "How the hack he did the measurement"? How theoretical people that never held a probe will said you measure from the same two point and get two different reading. What? By hooking the scope probe from the left side or the right side to the same two points?

One of the difference between a physicist and engineer is the engineer has to produce something tangible, measuring at a real point, not an imaginary point like the professor did. I question the knowledge of electronics the professor has, and how many hours he spent on designing and building circuits. You understand this experiment is electronics?

In his experiment, I bet he connected the two resistor by wires, and that he missed the moon. I am waiting for Sarumonkee to come back with the result of the multiple resistors. But as I posted, I disagree that Lorentz force are in play in this case. It is FL that is in play.

Yes I notice the EM in EE is different from physics. We study a lot deeper into phasor, transmission line theory, smith charts etc. where physics (electro dynamics) get deeper into materials, potentials and more math. I follow the advanced EM courses of U of Santa Clara and pretty much finished what they taught. I did not attend any school, hack, I study mostly on my own in my whole career. I this is my third round studying EM, this time I study a lot of materials in “Intro to Electro Dynamics “ by Griffiths. There is a lot of stuffs that the EE books do not cover. BUT what we are arguing here is very basic laws like FL, KVL and conservatives. This are covered in the first 2 chapters. I still believe the professor did the experiment wrong. Nothing to do with the theory.

 

[cabraham]

Guys, while we are waiting for the result, I have time just thinking about some theory that was thrown around in this case here:

Why are we talking about Lorentz force? My understanding about force only act on a charge that is moving. In this case, after the battery was removed, nothing is moving the electrons around the loop, only some thermal motion. Lorentz don't even apply here. It is very clear that

\vec F = q(\vec u X \vec B)\; \hbox { where } \;\vec u \;\hbox { is the velocity. }And there is no force asseted on the charge if the charge is not moving. Recall magnetic field move the wire ONLY when there is a current passing through the wire? Also one more important point, the book very specificly said that the static magnetic field do not change the velocity of the particle, it only change the direction of the particle. So if the only motion of the electrons in the wire and resistors only change from random motion to random motion plus a few degree shift...still random, no current. Refer to P207 of Griffiths.


In my opinion, the formula in play in our case is :

V = \int _S (\nabla X \vec E) \;\cdot\; \hat n \; dS \;=\; \int _C \vec E \;\cdot\; \hat T \;dl \;=\; \int_S \frac{\partial \vec B}{\partial t} \;\cdot\; \hat n \;dS

From the experiment, the good professor use a changing magnetic field to induce the voltage into the loop. This is a time varying magnetic field and Lens law is in action in this case. And this is the voltage that drive the resistors. We'll have to see my distributed micro voltage inside resistors theory pan out or not.

The Lorentz eqn has 2 terms, 1 for electric, & 1 for magnetic. I've already stated said eqn as F = q(E + u X B).

Regarding the mag field influence on a charge, it can indeed change its direction, but not its speed or kinetic energy. By changing its direction, its "velocity" is also changing, as velocity is a vector quantity consisting of speed & direction.

I use momentum & kinetic energy when describing Lorentz force. An E field can change both, but a B field can only change momentum, not KE.

Did I help, or make matters worse?

Claude

 

[atyy]

Lewin is not a theorist.

 

[yungman]

The Lorentz eqn has 2 terms, 1 for electric, & 1 for magnetic. I've already stated said eqn as F = q(E + u X B).

Regarding the mag field influence on a charge, it can indeed change its direction, but not its speed or kinetic energy. By changing its direction, its "velocity" is also changing, as velocity is a vector quantity consisting of speed & direction.

I use momentum & kinetic energy when describing Lorentz force. An E field can change both, but a B field can only change momentum, not KE.

Did I help, or make matters worse?

Claude

No, since the only motion of electrons in the circuits with no current is random motion, changing the direction of a random motion is still random motion. You cannot make the random motion to become the direction of the wire to travel down the wire as current.

It is very obvious that FL is in play like what I wrote. The resistor body is still part of the loops. As I said before, if the professor use a 6" wire to connect the two resistor, most of the induced emf is on the wires. In case of the loop making up of resistors material, the result is the same where the micro voltage sources are embedded inside the resistors. Still waiting for the experiment result that if what I postulated is true, we are not going to see 9:1 voltage ratio on those resistors, not even close.

 

[yungman]

Lewin is not a theorist.

I can asure you he is not hands on!

 

[yungman]

The Lorentz eqn has 2 terms, 1 for electric, & 1 for magnetic. I've already stated said eqn as F = q(E + u X B).

Regarding the mag field influence on a charge, it can indeed change its direction, but not its speed or kinetic energy. By changing its direction, its "velocity" is also changing, as velocity is a vector quantity consisting of speed & direction.
BUt as I said, if there is no current in the loop, electrons are moving randomly. So applying a mag field just change the direction of the random movement and still is random. Not current around the loop created.
I use momentum & kinetic energy when describing Lorentz force. An E field can change both, but a B field can only change momentum, not KE.
Even if you argue it is a pulse and it is EM that consist of E field. If you put the loop on xy plane and the mag field is in z direction, the E field is propagating in z direction also because even though it is quadriture to the mag field, the direction of propatation still perpendicular to the loop. The loop being perpendicular to z direction will not be affected by E field in z direction. Remember induced E field is alway opposite to the external E field.
Did I help, or make matters worse?

Claude

I don't think the Lorentz law apply. THe only law in play is FL which is the induced voltage.

 

[yungman]

I was going to put in my comment on the professor's video. It was closed, or else I'll give him a piece of my mind.

 

[cabraham]

I don't think the Lorentz law apply. THe only law in play is FL which is the induced voltage.

Lorentz' law does apply. How can it not apply? If loop is immersed in a time varying mag field, it is also subjected to a time varying elec field. E & B are normal in space. The free electrons in the loop are acted upon by the E force. Once in motion the B force is incurred normal to the E force. Otherwise, how can the electrons ever start moving? In order to accelerate an electron you need an E field. Whenever a time varying B is present, so is E present. Under time changing (dynamic) conditions, neither one can exist independently. The E force can change not only the electron's direction, but its speed & KE as well. The B field can only change the electron's direction, & only if the electron is already moving.

Once the E field accelerates the electron, it is not random motion, but drift along the direction of the E field. The mag field B, acts upon the electron normal to its velocity. So, Lorentz law applies here. Otherwise, how would the electrons ever start moving? I can't believe that you don't see Lorentz' law as in effect here. Please elaborate. What gets the electrons initially moving if not Lorentz force? Just asking.

Claude

 

[yungman]

Lorentz' law does apply. How can it not apply? If loop is immersed in a time varying mag field, it is also subjected to a time varying elec field. E & B are normal in space. The free electrons in the loop are acted upon by the E force. Once in motion the B force is incurred normal to the E force.

Read the FL, E induced in the loop caused by B is not the same as the E that accompany the B. THis induced E is not the same as in the Lorentz equation. You have to be very careful about this. Vary B alway have E accompany along and is quad to the B, but this is perpendicular to the induced E in the loop. Only the B portion act on the loop.

Otherwise, how can the electrons ever start moving? In order to accelerate an electron you need an E field. Whenever a time varying B is present, so is E present. Under time changing (dynamic) conditions, neither one can exist independently. The E force can change not only the electron's direction, but its speed & KE as well. The B field can only change the electron's direction, & only if the electron is already moving.

Once the E field accelerates the electron, it is not random motion, but drift along the direction of the E field. The mag field B, acts upon the electron normal to its velocity. So, Lorentz law applies here. Otherwise, how would the electrons ever start moving? I can't believe that you don't see Lorentz' law as in effect here. Please elaborate. What gets the electrons initially moving if not Lorentz force? Just asking.

Claude

If you think of the loop is on the xy plane center at origin, the external EM field in +z direction, the EM is perpendicular the the loop. The E in the EM that propagate in +z direction is perpendicular to the loop and has NO effect on whatever E in the loop.

Remember induced E by an external E is always opposite in direction only. That is the reason we have the FL that stated only the B is in action to cause the induced E. There are two E here, you have to be careful not fixing them up.

 

[cabraham]

If you think of the loop is on the xy plane center at origin, the external EM field in +z direction, the EM is perpendicular the the loop. The E in the EM that propagate in +z direction is perpendicular to the loop and has NO effect on whatever E in the loop.

Remember induced E by an external E is always opposite in direction only. That is the reason we have the FL that stated only the B is in action to cause the induced E. There are two E here, you have to be careful not fixing them up.

Maxwell says otherwise. If the B is normal to the loop (x-y plane), then the E is in the x-y plane. You've placed both B & E on the z axis, which opposes Maxwell.

Either form of ME applies, integral or differential. For the diff form (or "at a point" form):

curl E = -dB/dt.

If B is non-zero & time varying, then curl E is non-zero as well. For that to happen, E is non-zero, since the curl of a zero vector is zero. This E field exerts a force on a free charge resulting in motion of said charge. In order for charge to circulate in x-y plane, E must have a component in x-y plane. Using your established reference with B along z axis, the curl of E is along the z axis only for E in the x-y plane.

Hence, E produces a force upon free electrons in the loop. Once they are in motion, they are subjected to the force due to B as well in addition to E.

Have I overlooked anything?

Claude

 

[yungman]

Maxwell says otherwise. If the B is normal to the loop (x-y plane), then the E is in the x-y plane. You've placed both B & E on the z axis, which opposes Maxwell.

Either form of ME applies, integral or differential. For the diff form (or "at a point" form):

curl E = -dB/dt.

If B is non-zero & time varying, then curl E is non-zero as well. For that to happen, E is non-zero, since the curl of a zero vector is zero. This E field exerts a force on a free charge resulting in motion of said charge. In order for charge to circulate in x-y plane, E must have a component in x-y plane. Using your established reference with B along z axis, the curl of E is along the z axis only for E in the x-y plane.

Hence, E produces a force upon free electrons in the loop. Once they are in motion, they are subjected to the force due to B as well in addition to E.

Have I overlooked anything?

Claude

You keep talking about the induced E in the loop. The external EM in +z direction has the component of E that is in +z direction and has no effect on the loop. You kept talking about varying B has E, this external E is not the induced E and is in +z direction.

The induced E in the loop is purely caused by the external B only. We are talking about two different E fields here.

Yes, the induced E in the loop cause the electrons to run, but that is because of the external B, and this the very essence of FL, not Lorentz.

 

[yungman]

To avoid confusion, I upload a drawing which show Lorentz force:

\vec F = q ( \hat z E_{(x)} +\vec u X \hat z H_{(y)})

Where the \hat z E_(x) \;&\; \hat z H_{(y)} are the electromagnetic wave.

The induced E is \; \vec E_{EXT} \;=\; \hat {\phi} E_{(r)} \; as shown.

As you can see, \hat z E_(x) is normal to the loop and has no effect.

It would be opposit if instead of the loop, we have a straight wire in z direction. In this case, the E assert force on the electrons inside the wire, but here, the B has no effect because B is parallel to the wire.

 

[cabraham]

To avoid confusion, I upload a drawing which show Lorentz force:

\vec F = q ( \hat z E_{(x)} +\vec u X \hat z H_{(y)})

Where the \hat z E_(x) \;&\; \hat z H_{(y)} are the electromagnetic wave.

The induced E is \; \vec E_{EXT} \;=\; \hat {\phi} E_{(r)} \; as shown.

As you can see, \hat z E_(x) is normal to the loop and has no effect.

It would be opposit if instead of the loop, we have a straight wire in z direction. In this case, the E assert force on the electrons inside the wire, but here, the B has no effect because B is parallel to the wire.

In an e/m wave, E & H (B) are normal. I'll double check tonight, but I'm perplexed by your inference that the external B & E fileds are both along the z axis. For a transverse e/m wave, E & H/B are perpendicular to each other, not coincident. I'll get back later.

Claude

 

[yungman]

In an e/m wave, E & H (B) are normal. I'll double check tonight, but I'm perplexed by your inference that the external B & E fileds are both along the z axis. For a transverse e/m wave, E & H/B are perpendicular to each other, not coincident. I'll get back later.

Claude

EM always goes in pair and field has to propergate along the z- axis in his experiment. Yes the E and in the EM wave are normal to each other, they just propagate in z direction.

Also you can look at it this way, \hat z E_{x} is occilating along x direction and it affect the loop both directions and the result cancel out and will not push electron either direction. Only the magnetic field moving the electrons by inducing the electric field along the loop. as shown in arrow on the loop.

I am no expert in EM, this is my understanding. I would not dare to challenge the professor's knowledge on EM, I challenge him on his set up where he drawn the conclusion.

 

[yungman]

In an e/m wave, E & H (B) are normal. I'll double check tonight, but I'm perplexed by your inference that the external B & E fileds are both along the z axis. For a transverse e/m wave, E & H/B are perpendicular to each other, not coincident. I'll get back later.

Claude

You have a chance to look into this? I am no expert in EM, that is my understanding and I am willing to be wrong and learn.

 

[cabraham]

Yes I did look. For a transmission line (2 wire, parallel or coax), the wave propagates in TEM (transverse electromagnetic) mode. So does a space wave. But for a waveguide, TE (transverse electric) & TM (transverse magnetic) modes exist, no TEM mode at all takes place.

For TEM mode, if wave propagation is along z axis, then E is in x axis, & H is in y axis, or any orientation in x-y plane normal to each other. For TE mode, propagation remains along z axis, E is in x axis, but H is in y-axis & z axis. E is transverse (normal) to prop, but H has 2 components, 1 normal to prop, & 1 coincident with prop. So if wave prop is in z axis, E is in x axis, H is in y & z axes. Only E is transverse to prop direction.

For a TM it's vice versa. So in Prof. Lewin's setup, only the TEM mode takes place. If energy is propagating in z direction, then E & H/B are normal to each other in x-y plane, as well as normal to prop.

"Inducing" an E field into the loop is a colloquial phrase. This E field is present in space regardless of whether or not the loop is there. H & E cannot exist independently under time changing conditions. Sorry to be late responding. Christmas season, shopping, fixing up the house, you know.

Claude

 

[yungman]

Yes I did look. For a transmission line (2 wire, parallel or coax), the wave propagates in TEM (transverse electromagnetic) mode. So does a space wave. But for a waveguide, TE (transverse electric) & TM (transverse magnetic) modes exist, no TEM mode at all takes place.

For TEM mode, if wave propagation is along z axis, then E is in x axis, & H is in y axis, or any orientation in x-y plane normal to each other. For TE mode, propagation remains along z axis, E is in x axis, but H is in y-axis & z axis. E is transverse (normal) to prop, but H has 2 components, 1 normal to prop, & 1 coincident with prop. So if wave prop is in z axis, E is in x axis, H is in y & z axes. Only E is transverse to prop direction.
In the professor's case, it is a TEM because he generate a time varying magnetic field which automatically have E field accompany along from z direction. That is the reason I drew both E and H along the z direction. Your accessment is the same as my drawing, E in x and H in y( which I call E(x) and H(y) in my drawing. but they propergate at z direction. This is just a case of simple transverse electromagnetic wave (TEM) propagate in z direction.
For a TM it's vice versa. So in Prof. Lewin's setup, only the TEM mode takes place. If energy is propagating in z direction, then E & H/B are normal to each other in x-y plane, as well as normal to prop.z direction. In real life, the wave is usually polarized, either circular etc. It is not exactly straight E in x and H in y. But the result is the same as long as it propagate in z direction. So we just keep the discussion as E in x and H in y.

If you look at this way where E varying in x direction through the loop that is on xy plane, the effect cancel out because it affect in +ve x just as much as -ve x direction and the result is no effect on the loop due to the E field propagate up. Still only the H field only in play, which is Faraday's Law only, not Lorentz force.

"Inducing" an E field into the loop is a colloquial phrase. This E field is present in space regardless of whether or not the loop is there. H & E cannot exist independently under time changing conditions. Sorry to be late responding. Christmas season, shopping, fixing up the house, you know.
Yes, the source( external ) E and H cannot exist independently under time varying condition. My point is the E is normal to the resistor loop and has no effect. Therefore I claim only the H is in play and is 100% Faradays law, not Lorentz force.

Claude

That is what I have been driving at all this time that this is nothing more than magnetic induction into a closed loop consist of two resistors. Nothing more and the professor make a big sting out of nothing. AND he is wrong. I am not saying his conservative or non conservative ...well, don't know how other way to put it...BS... is wrong, it is just not in play in his experiment. I still say, it is so obvious that any praticing engineer can spot this mistake he made. I tried to put my comment in and want to challenge him out to join in, but problem is the comment section in youtube is closed. And yes I will confront him if possible. I deal with too many PhDs in my career. They are only human and they make mistake just as anyone else. Only difference, this guy is so arrogant about it. The nerve of him to make a video on youtube. He should at least be humble enough to summit a paper in AIP and let others to have a peer review first, then publish it in AIP instead of making a scene on youtube where 99.99% of the public have no idea what he is talking about and think he actually have something valid.

I wonder whether Sarumonkee have done the new experiment yet!

 

[Studiot]

I don't see the point of this protracted discussion about FL v Lorenz, or the castigating others for their point of view.

I pointed out, way back, that correct application of Kirchoff leads to a simple and unambiguous resolution of the issue.

All Proff Lewin did wrong was to offer an inappropriate version of Kirchoff.
There are many instances in mathematical physics where we can loose something if we equate to zero.

 

[yungman]

I don't see the point of this protracted discussion about FL v Lorenz, or the castigating others for their point of view.

I pointed out, way back, that correct application of Kirchoff leads to a simple and unambiguous resolution of the issue.

All Proff Lewin did wrong was to offer an inappropriate version of Kirchoff.
There are many instances in mathematical physics where we can loose something if we equate to zero.

Not castigating others, just one, the professor because of his arrogance. I think we have a good discussion here and I think we are all very civilize with each other. I think it is very important to establish that FL is in play here to make sure there is a voltage generator or distribute voltage generators in the loop, then everything make sense and KVL apply perfectly in this case...not in all cases, just this one.

 

[hikaru1221]

@yungman: Why does the wave propagate in the z-direction?

 

[Studiot]

not in all cases,

Would you like to provide an example?

 

[yungman]

Would you like to provide an example?

Don't know, some people here said it fail in some cases, all I want to say is I do not defend KVL, I only say KVL hold in this case.

Can you tell me your opinion with my assertion that the whole thing is just FL where a magnetic pulse induce a voltage in the loop and when taking into account of the voltage source, KVL hold.

 

[Studiot]

I explicitly showed how to apply Kirchoff to the problem in hand in my post#100.

I also displayed an example of where FL is inapplicable, but Kirchoff is applicable in my post#32.

Perhaps they were so short they slipped by notice?

To find examples of inapplicability, simply look at the conditions of validity of the theorem or equation or law.

FL requires a changing magnetic field. Hence post#32.

Kirchoff requires a complete loop.

 

[stevenb]

and when taking into account of the voltage source, KVL hold.

Which definition of KVL are you claiming is upheld in Prof. Lewin's example?

The version that the Prof gave was that the sum of potentials around a loop equal zero. Do you understand that the transformer EMF you are calling the lumped or distributed voltage source is not a potential? If you understood this you would not claim his definition of KVL is upheld.

Also, the version of KVL from the other MIT lecture is not upheld in the sense that the starting assumption is not true in this example.

So, this point you are trying to make is unclear to me. Telling us your accepted definition of KVL would help clarify and give us a chance of understanding your points.

 

[yungman]

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



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



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.
I think we are in agreement on this point that the mag field induce emf into the loop and is distributed around the whole loop. That was the reason I suggested to do the experiment over with resistors making up the whole loop instead of having long section of wire as part of the loop.
This brings out the difference between EMF (which is distributed around the circuit in this case) and Potential Difference or Potential Drop.
But as I said, the professor specifically said he measure the voltage across the resistors going clockwise and counter clockwise. That is potential. He claimed he can measure different voltages but he took the wire as a single point rather than part of the loop.
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.
I don't even think this is relavent for what he tried to claim. He used the voltage drop as an argument. It is PD that he was using.
The give away clue is in his statement about conservative and non conservative fields.

For PD the line intergral \ointE.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.

I am not disagreeing with you on the points you make, I just disagree with the experiment he used to derive his argument. And the whole point is if the experiment was done wrong, there is no point of going any further and nothing can derive out of what he said. One can not over look the flaud of the experiment and continue the argument of the theory.

 

[yungman]

I explicitly showed how to apply Kirchoff to the problem in hand in my post#100.

I also displayed an example of where FL is inapplicable, but Kirchoff is applicable in my post#32.
Of cause FL is not applicable, there is no external magnetic field. I don't even see your point.
Perhaps they were so short they slipped by notice?

To find examples of inapplicability, simply look at the conditions of validity of the theorem or equation or law.

FL requires a changing magnetic field. Hence post#32.

Kirchoff requires a complete loop.

........

 

[yungman]

Which definition of KVL are you claiming is upheld in Prof. Lewin's example?

The version that the Prof gave was that the sum of potentials around a loop equal zero. Do you understand that the transformer EMF you are calling the lumped or distributed voltage source is not a potential? If you understood this you would not claim his definition of KVL is upheld.
It is my understanding that when you start measuring the voltage drop around the loop, it is potential. AND if he stop treating the connecting wire as a point or a note only, then he should measure the VOLTAGE drop along the wire also, then the sum of all VOLTAGE around the loop is zero and KVL hold. Once the EMF is induced into the loop, it become REAL VOLTAGE and should be treated accordingly.
Also, the version of KVL from the other MIT lecture is not upheld in the sense that the starting assumption is not true in this example.

So, this point you are trying to make is unclear to me. Telling us your accepted definition of KVL would help clarify and give us a chance of understanding your points.

My understanding of KVL is you include all the voltage drop across the components and the VOLTAGE is zero around a closed loop. I tread induced EMF as voltage source. How can you not treating this induced EMF a voltage source? No matter how it come about, as soon as you put it in the loop and start driving the circuits, it is a voltage source.

 

[stevenb]

How can you not treating this induced EMF a voltage source? No matter how it come about, as soon as you put it in the loop and start driving the circuits, it is a voltage source.

You keep side-stepping the issues with false premises on what is in disagreement. Nobody said you shouldn't treat emf as a source. But, the book used in the professor's class defines KVL to only consider potentials and says that the sum of potentials equals zero. So, both you and the Prof are saying the same thing. That is, the book is defining a version of KVL that does not apply to many cases.

So you and I and probably every one else here likes to use a definition of KVL that includes EMF in some form, while the Prof says the same, but does not call that KVL. As I said before, it's nothing but semantics, and making a big deal about anything lacking substance is a fools game.

You still have not clearly defined your KVL well enough for us to know whether you prefer the Maxwell definition, the Kraus version I posted, the MIT/Agarwal definition or some other variant. Not that it matters to me what you prefer to use, but knowing may help us to understand some of your posts better.

 

[yungman]

You keep side-stepping the issues with false premises on what is in disagreement. Nobody said you shouldn't treat emf as a source. But, the book used in the professor's class defines KVL to only consider potentials and says that the sum of potentials equals zero. So, both you and the Prof are saying the same thing. That is, the book is defining a version of KVL that does not apply to many cases.

So you and I and probably every one else here likes to use a definition of KVL that includes EMF in some form, while the Prof says the same, but does not call that KVL. As I said before, it's nothing but semantics, and making a big deal about anything lacking substance is a fools game.

You still have not clearly defined your KVL well enough for us to know whether you prefer the Maxwell definition, the Kraus version I posted, the MIT/Agarwal definition or some other variant. Not that it matters to me what you prefer to use, but knowing may help us to understand some of your posts better.

As I said before, I only concentrate on the way he did the experiment to which he made the assertion. I am not challenge the validity of any of the law. When come to EM, FL and KVL, I am still learning and don't even want to engage too heavily. I just want to make a very strong statement that what he did in the experiment was wrong, he treated the connection between the two resistor as a node or a point and he claimed that he measure different voltage from A to D and start making all the assertion of what is wrong where the very way he did the measurement was flaud. That is all I am saying. You cannot make any claim from a flaud experiment. No more, no less.

For all I care, what he claimed might be correct, but just not with this experiment. That is the one and only point I have been pushing over and over with 30 or 40 posts in this thread.

I only watched the first video where he made the claim, and I saw most of the second video to the point he show the wave form measured and I stop. All his other assertions and paper is just bla bla bla to me and I am not even interested. The experiment was wrong, and there is no point of carrying on any further. Show me an experiment I can buy, then we'll talk.

 

[stevenb]

All his other assertions and paper is just bla bla bla to me and I am not even interested. The experiment was wrong, and there is no point of carrying on any further. Show me an experiment I can buy, then we'll talk.

You've already done quite a bit of talking about an experiment you can't buy. The problem is that you claim to be right and refuse to listen to the reasons why you might be wrong. Anything that might reveal the truth looks to you like "bla bla bla". That, my friend, is the definition of arrogance.

 

[Studiot]

I really can't see how an 'experiment' can be right or wrong.

Professor Lewin did what he did. Right and wrong are value judgements and inappropriate to apply to bald facts.

In particular he assembled some apparatus, and displayed some meter readings when the meters were connected at certain points, over certain time intervals.

This is all a matter of factual record - it is neither right nor wrong.

Then we come to the matter of interpretation of the meter readings. Prof Lewin discusses these in terms of certain theories.

This part of the process can be open to interpretation and is a proper matter for discussion in this thread.

A clear and unambiguous statement of these theories helps the discussion no end.

I think I have made my position quite clear as regards to my version of these theories and to my interpretation of the facts presented.

I also afford Prof Lewin the respect due to a person who has done enough to make Professor at MIT, which must be considerable, even if I differ on some point or another.

It has already been pointed out that Kirchoff was developed for steady state conditions. So to try to apply it to a circuit where the loop EMF is decaying with time is worthy of proper discussion, and not an easy task. In particular the introduction of the time variable makes the definition of any 'voltage drop' difficult to say the least.

What do you mean by PD ? Is there a difference between Potential Drop and Potential Difference?

 

[yungman]

You've already done quite a bit of talking about an experiment you can't buy. The problem is that you claim to be right and refuse to listen to the reasons why you might be wrong. Anything that might reveal the truth looks to you like "bla bla bla". That, my friend, is the definition of arrogance.

Now we have a discussion here. I don't think you really responed to all my posts directly before. Now that I finally get your attention, let's just talk about the experiement first to establish the validity of the whole thing. Please don't call me arrogant, I am very careful not to call anyone on this forum here anything, this is a usually a rule in any forum. I called the professor because he is not in this forum. If he were to be on this forum, I would have address to him directly. I think we are too educated to call each other to the face here, please move on.

So you think the experiment was done right, and he was right to claim that point A and point D is only a point, that the voltage you measure from A to D through the 100 ohm resistor is different from D to A through the 900 ohm resistor?



BTW, I know about

\vec E = -\nabla V -\frac{\partial \vec A}{\partial t}

That in varying magnetic field, E is not conservative, that

\int_c \vec E \cdot d\vec l

is no longer path independent where the professor is driving. I know all that and that is not even in question.

 

[yungman]

I really can't see how an 'experiment' can be right or wrong.

Professor Lewin did what he did. Right and wrong are value judgements and inappropriate to apply to bald facts.

In particular he assembled some apparatus, and displayed some meter readings when the meters were connected at certain points, over certain time intervals.

This is all a matter of factual record - it is neither right nor wrong.

Then we come to the matter of interpretation of the meter readings. Prof Lewin discusses these in terms of certain theories.

This part of the process can be open to interpretation and is a proper matter for discussion in this thread.

A clear and unambiguous statement of these theories helps the discussion no end.

I think I have made my position quite clear as regards to my version of these theories and to my interpretation of the facts presented.

I also afford Prof Lewin the respect due to a person who has done enough to make Professor at MIT, which must be considerable, even if I differ on some point or another.

It has already been pointed out that Kirchoff was developed for steady state conditions. So to try to apply it to a circuit where the loop EMF is decaying with time is worthy of proper discussion, and not an easy task. In particular the introduction of the time variable makes the definition of any 'voltage drop' difficult to say the least.

What do you mean by PD ? Is there a difference between Potential Drop and Potential Difference?

There is no right or wrong in the experiment itself. But together with the statement he made about conservative and non conservative using the experiment is wrong. As I said here, context is everything. You cannot use this experiment to prove conservative or non conservative.

I thought you use PD as potential difference. I have to say I never really think too hard about the difference between potential difference and emf, their origin of where they come from. If it is there, I count it as volt, whether it is induced into the loop in this case or put a battery in between the loop.

 

[stevenb]

I don't think you really responed to all my posts directly before.

I didn't respond to many of your post because you are so far off base I don't know where to begin on many of them. I did address your critical comments about emf on the wire and why this does not show up in the measurement and why this is the reason that the measurements nodes behave as nodes, but since everything beyond the experiment is "bla bla bla" you didn't notice that or understand the significance of what I said.

By the way, I'm not calling you arrogant. I just observed that some of your actions here fit the definition of arrogance. Other actions or yours seem to say you are not arrogant, so I can't really say one way or the other without knowing you. The experiment is there to display the theory, so you can't ignore the theory and say that you haven't even tried to understand it and call it "bla bla bla" and expect to be taken seriously. Claiming you're right while dismissing the very information that is critical to understand the deeper issue is just not acceptable in a discussion like this. I just think it's ironic that you call the Professor arrogant and yet ignore his detailed analysis. You don't offer any detailed analysis yourself and you don't show us where he is wrong. All you do is argue things that we already know and then claim the experiment is wrong. Then you post pointless theory on Lorentz force and give diagrams that have nothing to do with the discussion and show total incompetence on a theoretical level.

So you think the experiment was done right, and he was right to claim that point A and point D is only a point, that the voltage you measure from A to D through the 100 ohm resistor is different from D to A through the 900 ohm resistor?

Yes, the experiment was done correctly by all evidence that we have access to. As I mentioned above, you have not provided any analysis to show where it is wrong. His measurements obey Faraday's Law, while what you claim does not obey Faraday's Law. I dont' expect you to see that because although you claim to understand FL and seem to understand FL, you don't seem able to apply it to this situation correctly and see why he is right and you are wrong.

You keep worrying about the emf on the wire which you are correct to point out is there. You fail to recognize that his measurements that do not encircle flux change will not see the wire emf because it is canceled by an equal and opposite emf on the measurement probes. For this reason, you can slide the probe connection along that 4-6 inch length of wire that you are concerned about. This is also the reason why each meter reads a different potential. One correctly reads the potential on one resistor and the other correctly reads the potential on the other resistor.

 

[stevenb]

I have to say I never really think too hard about the difference between potential difference and emf, their origin of where they come from. If it is there, I count it as volt, whether it is induced into the loop in this case or put a battery in between the loop.

This is a major failing on your part. The fact that you can admit to this and not even realize that it reveals that your theoretical understanding of the physics is not even at a basic level speaks volumes on why you are so confused on this issue. Experiments without understanding are pointless.

 

[cabraham]

Yungman, please re-read your statements about the direction of E. TEM wave prop is in the z direction, we both agree on that. We both agree that this space wave is TEM. So if power is traveling along z axis, then E & H are normal to each other & mutually normal to the z axis. Thus E & H are in the x-y plane, normal to each other.

The loop is also in the x-y plane. When all charges in the resistors & conductors are at rest, an H field cannot move them since mag fields can only exert force on moving charges. To get these charges moving, an E field is required. Said E field must be in x-y plane tangential to the loop to move charge. The point of conflict seems to be how to associate E with H. I've already pointed out that E & H are normal in the x-y plane where the loop resides. Again, if the loop were removed from the vicinity where the TEM wave is located, there is still an E & an H field in the x-y plane.

Is H inducing E, or vice-versa? We get right back to a chicken & egg vicious circle. These forums have countless threads where this & similar issues perpetuate ad infinitum. E does not "induce" H, & vice-versa. When the loop is immersed into the TEM space wave, the H cannot move the charges since they are still. But the E field can. This E field per FL exerts a force on charges per Felec = q*E. This is Lorentz' law.

But the force will result in motion & once the charges are moving, the H field, which is normal to the charge velocity, exerts a force as well per Fmag = q*(u X B). The total force is F = Felec + Fmag = q*(E + u X B).

Again, under time changing conditions, i.e. the "ac" domain, E & H are Siamese twins as they only travel together. They are 2 arms on the same beast, 2 sides of the same coin, 2 domains of the same energy, etc. The phrase "induction" is used quite loosely. The E field which motivates the charges to move, is joined at the hip with its twin, the H field. Faraday & Lorentz both apply w/o exception.

As far as the lumped parameter emf source being added to the equiv circuit, vs. the distributed parameter version, both give the right answer. But the prof point is all important. I stated earlier that in order to add the emf source into the equiv circuit, we must know its value. To do that we must measure the sum of the voltage drops across each resistor in the loop.

This sum will not always equal zero. The measured non-zero value can then be added into the equiv circuit as an independent voltage source. Then, the sum of voltages around a loop does indeed equal zero.

In order to draw the lumped equiv circuit, you must measure the sum of voltages around the distributed loop, which is usually non-zero. So the distributed circuit non-zero measured value provides the correct value for the included voltage source.

This is way too long, & we've covered it all. No debate is needed, since all phenomena are accounted for. I'll clarify if needed. BR.

Claude

 

[yungman]

I didn't respond to many of your post because you are so far off base I don't know where to begin on many of them. I did address your critical comments about emf on the wire and why this does not show up in the measurement and why this is the reason that the measurements nodes behave as nodes, but since everything beyond the experiment is "bla bla bla" you didn't notice that or understand the significance of what I said.

By the way, I'm not calling you arrogant, I just observed that some of your actions here fit the definition of arrogance. Other actions or yours seem to say you are not arrogant, so I can't really say one way or the other without knowing you. The experiment is there to display the theory, so you can't ignore the theory and say that you haven't even tried to understand it and call it "bla bla bla" and expect to be taken seriously. Claiming you're right while dismissing the very information that is critical to understand the deeper issue is just not acceptable in a discussion like this. I just think it's ironic that you call the Professor arrogant and yet ignore his detailed analysis. You don't offer any detailed analysis yourself and you don't show us where he is wrong. All you do is argue things that we already know and then claim the experiment is wrong. Then you post pointless theory on Lorentz force and give diagrams that have nothing to do with the discussion and show total incompetence on a theoretical level.


1) Cabraham brought up the Lorentz and I responded to that, that show you really not reading very carefully.
2) So you think his point D is just a point?



Yes, the experiment was done correctly by all evidence that we have access to. As I mentioned above, you have not provided any analysis to show where it is wrong. His measurements obey Faraday's Law, while what you claim does not obey Faraday's Law. I dont' expect you to see that because although you claim to understand FL and seem to understand FL, you don't seem able to apply it to this situation correctly and see why he is right and you are wrong.
Did you read #153, it is one of the post I said and I have said repeatly that this is Faraday's Law issue. I think I must have close to 5 or 6 posts saying that this experiment is about Faraday's law. Please go back and read it first, find the post that I claimed that this does not obey Faradays Laws. If you read carefully, you would find I only said Lorentz law don't apply in my discussion with Cabraham.


You keep worrying about the emf on the wire which you are correct to point out is there. You fail to recognize that his measurements that do not encircle flux change will not see the wire emf because it is canceled by an equal and opposite emf on the measurement probes.


1) If you look at the size of the round thing shown in the second video, you know that the two resistors are connect through wires, do you agree?
2) Please look at the attachment in #123 that I posted. Ignor the probe setup, The loop that the professor made must be something like that in order to cover the area of that size. In the diagram, you see there is wire between B and C and that is part of the loop that generate most of the emf that drive the resistor. Point B and point C was lumped together by the professor and called a POINT D. This is the first thing that is wrong.

For this reason, you can slide the probe connection along that 4-6 inch length of wire that you are concerned about. This is also the reason why each meter reads a different potential. One correctly reads the potential on one resistor and the other correctly reads the potential on the other resistor.

No I don't concern on the mearsuring, you are an engineer, you know how to do common mode rejection and get a good enough measurement. I am concern about the professor treating the wire between point B and C as a single node and was called D in his video.

 

[yungman]

Yungman, please re-read your statements about the direction of E. TEM wave prop is in the z direction, we both agree on that. We both agree that this space wave is TEM. So if power is traveling along z axis, then E & H are normal to each other & mutually normal to the z axis. Thus E & H are in the x-y plane, normal to each other.

The loop is also in the x-y plane. When all charges in the resistors & conductors are at rest, an H field cannot move them since mag fields can only exert force on moving charges. To get these charges moving, an E field is required. Said E field must be in x-y plane tangential to the loop to move charge. The point of conflict seems to be how to associate E with H. I've already pointed out that E & H are normal in the x-y plane where the loop resides. Again, if the loop were removed from the vicinity where the TEM wave is located, there is still an E & an H field in the x-y plane.

Is H inducing E, or vice-versa? We get right back to a chicken & egg vicious circle. These forums have countless threads where this & similar issues perpetuate ad infinitum. E does not "induce" H, & vice-versa. When the loop is immersed into the TEM space wave, the H cannot move the charges since they are still. But the E field can. This E field per FL exerts a force on charges per Felec = q*E. This is Lorentz' law.

But the force will result in motion & once the charges are moving, the H field, which is normal to the charge velocity, exerts a force as well per Fmag = q*(u X B). The total force is F = Felec + Fmag = q*(E + u X B).

Again, under time changing conditions, i.e. the "ac" domain, E & H are Siamese twins as they only travel together. They are 2 arms on the same beast, 2 sides of the same coin, 2 domains of the same energy, etc. The phrase "induction" is used quite loosely. The E field which motivates the charges to move, is joined at the hip with its twin, the H field. Faraday & Lorentz both apply w/o exception.

As far as the lumped parameter emf source being added to the equiv circuit, vs. the distributed parameter version, both give the right answer. But the prof point is all important. I stated earlier that in order to add the emf source into the equiv circuit, we must know its value. To do that we must measure the sum of the voltage drops across each resistor in the loop.

This sum will not always equal zero. The measured non-zero value can then be added into the equiv circuit as an independent voltage source. Then, the sum of voltages around a loop does indeed equal zero.

In order to draw the lumped equiv circuit, you must measure the sum of voltages around the distributed loop, which is usually non-zero. So the distributed circuit non-zero measured value provides the correct value for the included voltage source.

This is way too long, & we've covered it all. No debate is needed, since all phenomena are accounted for. I'll clarify if needed. BR.

Claude

You put in a lot of materials, I'll read throught and really think about this before responding hopefully by tomorrow. Thanks

Alan

 

[stevenb]

I am concern about the professor treating the wire between point B and C as a single node and was called D in his video.

Being concerned is fine, but you have failed to show a good reason why approximating the wire as a node has any significant effect on his analysis. I can take any circuit design ever produced and claim that every node on the drawing is not truly a node. So, your comment is a red herring. If you want to get nit-picky then it's not correct to call it a node. But, do an analysis and post the numbers that you think are in play. Just as you were originally wrong about the significance of the wire inductance, you are wrong that the wire "transformer" emf enters into any measurement that does not encircle flux change. The wire emf is in fact significant, although the potential drop on the wire is insignificant. In this case the distinction is absolutely critical because this so-called circuit does not obey a critical assumption of usual circuit theory because that emf is there. A potential drop is measurable, but the emf of this type is not measurable in any loop that does not encircle a flux change. If it were, then Faradays law would fail in the measurement loop. So, if we can move the connection point anywhere along the wire and get the same reading, that wire is effectively a node in a Faraday Law analysis. The topology relative to encirclements of flux change is the important thing here, not physical lengths for connections along the wires.

Again, you need to do more than complain and claim. You need to demonstrate with experiment or with analysis. The OP at least started on the experiment path, which I respect very much, but we still do not have his results in any form than can be scrutinized, analyzed and reproduced. If he eventually posts results along the lines he thought he was seeing, I will show him how his measurements are inconsistent with Faraday's law. He would then have to explain this failing. You are in a position to post a full analysis with schematic, identified nodes, scope connections and "claimed" voltage readings on the scopes. If you do this, I will show you why Faraday's law is not upheld in your analysis. Then you will have to explain this failing, and then we would have to see if I could poke holes in that. This is how a productive discussion can be held. You telling us 15 different ways that "he is wrong and I am right" just doesn't cut it. He did the experiment and provided an analysis, while you've done neither.

 

[yungman]

Being concerned is fine, but you have failed to show a good reason why approximating the wire as a node has any significant effect on his analysis. I can take any circuit design ever produced and claim that every node on the drawing is not truly a node. So, your comment is a red herring. If you want to get nit-picky then it's not correct to call it a node. But, do an analysis and post the numbers that you think are in play. Just as you were originally wrong about the significance of the wire inductance, you are wrong that the wire "transformer" emf enters into any measurement that does not encircle flux change.
Which part of the wire forming the loop that the flux go through in the middle of the loop like the drawing the professor had in the first video you don't understand? That the loop encircle the flux change that you don't see?
The wire emf is in fact significant, although the potential drop on the wire is insignificant. In this case the distinction is absolutely critical because this so-called circuit does not obey a critical assumption of usual circuit theory because that emf is there.
This is what I proposed to look at this as a superposition of two circuit, one is the resistor voltage drop due to current passing through, the other is the emf generated along the closed loop. I have no way to do the experiment and Sarumonkee said he was going to do it with the resistor setup I proposed in #134. I don't claim I have the answer, I just suspect the experiment will not yield the normal voltage divider ratio because it is superpositon of the FL induced emf along the loop and the voltage drop across the resistors.
A potential drop is measurable, but the emf of this type is not measurable in any loop that does not encircle a flux change. If it were, then Faradays law would fail in the measurement loop. So, if we can move the connection point anywhere along the wire and get the same reading, that wire is effectively a node in a Faraday Law analysis. The topology relative to encirclements of flux change is the important thing here, not physical lengths for connections along the wires.It is the loop encircling the flux in his experiment. Again which part of his drawing on his first video to replace the battery with a short and pulse a magnetic field through the middle between the two resistor you don't understand? You do know he is trying to use the two resistors to form a loop. And my argument is there is no way he can form a loop with just two resistors without having wires to connect the two resistors to form a loop big enough in his second video. I hope you get at least that much before you calling me wrong.

Again, you need to do more than complain and claim. You need to demonstrate with experiment or with analysis. The OP at least started on the experiment path, which I respect very much, but we still do not have his results in any form than can be scrutinized, analyzed and reproduced. If he eventually posts results along the lines he thought he was seeing, I will show him how his measurements are inconsistent with Faraday's law. He would then have to explain this failing. You are in a position to post a full analysis with schematic, identified nodes, scope connections and "claimed" voltage readings on the scopes. If you do this, I will show you why Faraday's law is not upheld in your analysis. Then you will have to explain this failing, and then we would have to see if I could poke holes in that. This is how a productive discussion can be held. You telling us 15 different ways that "he is wrong and I am right" just doesn't cut it. He did the experiment and provided an analysis, while you've done neither.

Did you read #90, that Sarumonkee did the experiment and saw the voltage on the wire? He did see the 9:1 ratio on the resistors. He grounded both probe grounds in the middle of the loop and measure the two end of the wire which I called point B and C. He measure equal and opposite voltage. Then he move the probe around verticle and horizontal to prove the reading is consistence. Which part of this you don't understand?

You cannot blanketly said the ground lead of the probe with cause a loop and pickup the flux no matter what. I hope you do understand that the area of any loop has to ENCIRCLE the flux in order for the loop to have induced emf. That the flux pattern of that set up is very predictable. Call the loop on xy plane, flux in +z direction, the flux will start bending and eventually going at -z direction and curl back to the other end of the flux generator. Or you just think you can shot down his finding no matter what!?

You follow? And if you move the probe and the ground lead so the loop formed by the probe is verticle or horizontal to the xy plane, flux enclosed will be different.You follow so far?

As I said, I did not see Saru's experiment, but by moving the probe in all different position and not seeing any change can tell you that there are at least voltage across the wire.

So you still say the wire is just one node D and need to proof more on this? BTW, what engineering field are you in?

In case you don't get it:

I show the flux pattern and you can see the area of the loop formed by the probe in green for verticle and red in horizontal. You don't get the same amount of flux through the loop in different position. And the flux will not be anywhere close to the flux through the circuit loop unless you intentionally make the loop of the probe onto the middle of the set up, and this is asking for it.

 

[stevenb]

there are at least voltage across the wire.

So you still say the wire is just one node D and need to proof more on this? BTW, what engineering field are you in?

Why do you belabor what is already known and accepted? Yes, there is at least voltage across the wire. This voltage is a good chunk of the loop emf. I already said that the emf is there in agreement with you. There are no arguments about this. If this, to you, means that node D is not a real node, then don't consider it a node. What do I care what you call it? Again, it's semantics. The issue is, what do you do with this wire with emf on it. With Faraday's Law, you don't have to think about it at all. With measurements, you won't see it if your measurement loop does not enclose flux. Why? Because Faraday's Law says so.

I really don't want to go through all your comments. You state many things that I know and accept, and I don't see the point of going through it. There is only one issue here. You still have not explained how a measurement loop that does not encircle any flux, has emf from the wire and an equal and opposite emf on the scope wires, and yet reads the wire emf. That violates Faraday's Law. How Sarumonkee made a measurement that violates Faraday's law is beyond my explanation because I wasn't there to witness it and see cross checks and verifications. I agree with his conclusion that there is emf on the wire, but I don't understand how the measurement revealed this.

What I can say is that after the holidays I'll bite the bullet and do the measurement with cross-checks and analysis and post it. Until then, I'll wait to see if something substantial in the form of checkable experimental results or an actual analysis comes forth.

 

[Born2bwire]

Yes I did look. For a transmission line (2 wire, parallel or coax), the wave propagates in TEM (transverse electromagnetic) mode. So does a space wave. But for a waveguide, TE (transverse electric) & TM (transverse magnetic) modes exist, no TEM mode at all takes place.

For TEM mode, if wave propagation is along z axis, then E is in x axis, & H is in y axis, or any orientation in x-y plane normal to each other. For TE mode, propagation remains along z axis, E is in x axis, but H is in y-axis & z axis. E is transverse (normal) to prop, but H has 2 components, 1 normal to prop, & 1 coincident with prop. So if wave prop is in z axis, E is in x axis, H is in y & z axes. Only E is transverse to prop direction.

For a TM it's vice versa. So in Prof. Lewin's setup, only the TEM mode takes place. If energy is propagating in z direction, then E & H/B are normal to each other in x-y plane, as well as normal to prop.

"Inducing" an E field into the loop is a colloquial phrase. This E field is present in space regardless of whether or not the loop is there. H & E cannot exist independently under time changing conditions. Sorry to be late responding. Christmas season, shopping, fixing up the house, you know.

Claude

I would clarify though that even in a waveguide the actual wave is TEM (though I'm sure somebody here knows of some kind of rare exception to this case). When we speak of TE and TM modes we are talking in reference to the GUIDED direction of propagation. The actual wave does not actually travel in the guided direction but it bounces back and forth off of the geometry of the waveguide such that the net direction of propagation is along the guided direction. A subtle clarification but I have seen it lead to misunderstanding regarding the situations where you can truly get a non-TEM mode (which is generally restricted to more esoteric situations like surface waves in inhomogeneous media).

As for some previous points about Faraday's Law and the Lorentz force, the two are inseparable. Anytime that you have an induction of current or potential difference this is always done via the Lorentz force since the Lorentz force is the mechanism by which the fields interact with charges and currents. Faraday's Law implicitly includes the effects of the Lorentz force (as do the other laws) when it relates the creation of an EMF via a flux (and this is shown explicitly in just about any given textbook in some basic examples).

It should also note that Faraday's Law incorporates this via two different mechanisms. The first mechanism is due to the motion of the circuit physically through a spatially varying magnetic field (in which case by moving the circuit we provide moving charges). The second mechanism is due to the fact that a time-varying magnetic field always means that we have an associated time-varying electric field. This electric field can provide a Lorentz force on stationary charges which is given as the EMF.

 

[yungman]

Why do you belabor what is already known and accepted? Yes, there is at least voltage across the wire. This voltage is a good chunk of the loop emf. I already said that the emf is there in agreement with you. There are no arguments about this. If this, to you, means that node D is not a real node, then don't consider it a node. What do I care what you call it? Again, it's semantics. The issue is, what do you do with this wire with emf on it. With Faraday's Law, you don't have to think about it at all. With measurements, you won't see it if your measurement loop does not enclose flux. Why? Because Faraday's Law says so.
why is that you don't look at the induced emf on the wire in the loop as part of the loop that we use Faraday's law to calculate the emf? I am absolutely confuse on this point. Why is the problem is not that simple as the wire have the induced voltage that drive the two resistor and and get the 9:1` ratio on the voltage? Far as I concern, you go through the whole loop and the voltage end up to be zero as KVL said. It is good for either direction.
I really don't want to go through all your comments. You state many things that I know and accept, and I don't see the point of going through it. There is only one issue here. You still have not explained is how a measurement loop that does not encircle any flux, has emf from the wire and an equal and opposite emf on the scope wires,

Which wire specificly you talked about that does not encircle any flux where I put the bold fonds on above?
This is the copy of Sarumonkee on post #90 in case you have not read it:

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.

What he was doing is putting the scope ground at the mid point of the 6" wire that connect the two resistors. He put one probe on one end and the other on the other end of the wire. Essentially he take the mid point of the wire as a reference and look at the voltages induced on both end of the wire and it was found equal and opposite. Does that mean anything to you? He actually went out of his way to move the probe up and down and every position to show the reading did not change to prove the probe loop did not significantly alter the reading. Have you stop and think about this? He did a very very good job and the result cannot be more clear that the voltage exist on that wire and that was the voltage that drive the two resistors. Plane and simple.


and yet reads the wire emf. That violates Faraday's Law. How Sarumonkee made a measurement that violates Faraday's law is beyond my explanation because I wasn't there to witness it and see cross checks and verifications. I agree with his conclusion that there is emf on the wire, but I don't understand how the measurement revealed this.

No, what he did did not violate the FL, in fact he proof the FL is in play here. That the voltage was induced into the loop and create a voltage source. AND if you take into account of the voltage, KVL apply perfectly.

What I can say is that after the holidays I'll bite the bullet and do the measurement with cross-checks and analysis and post it. Until then, I'll wait to see if something substantial in the form of checkable experimental results or an actual analysis comes forth.

If you can look at the drawing that I provided in #134 and Sarumonkee said he was going to try last week, I have a suspicion that you are not going to get the predicted voltage ratio across each resistor like 0.9V on the 900ohm. And don't say v=IR fail! I think you have to look at it as super-position of two event...1) voltage from the current passing through the resistors and 2) the induced emf onto the loop made up of the resistors. In this case, there are very little wire, the body of the loop made up of resistor material. I suspect the voltage source induced are distributed inside the resistors and make the reading different from prediction of V=IR. I am not absolutely sure, just a thought. But this will proof the point that if you consider induced emf as a voltage source, KVL work perfectly.

In my book, that prove the point. You have to care about what voltage induced along the wire.

To be sure you know what I am talking about, this is the picture. The wire is between point B and C which the professor call it a node or point D. As shown in red, the ground of the two probes attached in the middle 3" from B and from C and the probe measure point B and C which I called Probe B and Probe C. The professor claimed he measure 0.1 on the 100 ohm and 0.9 on the 900ohm. Of cause, I never disagreed, it is the wire from B to C. As in the drawing,

 

[yungman]

Yungman, please re-read your statements about the direction of E. TEM wave prop is in the z direction, we both agree on that. We both agree that this space wave is TEM. So if power is traveling along z axis, then E & H are normal to each other & mutually normal to the z axis. Thus E & H are in the x-y plane, normal to each other.

The loop is also in the x-y plane. When all charges in the resistors & conductors are at rest, an H field cannot move them since mag fields can only exert force on moving charges. To get these charges moving, an E field is required. Said E field must be in x-y plane tangential to the loop to move charge. The point of conflict seems to be how to associate E with H. I've already pointed out that E & H are normal in the x-y plane where the loop resides. Again, if the loop were removed from the vicinity where the TEM wave is located, there is still an E & an H field in the x-y plane.

Is H inducing E, or vice-versa? We get right back to a chicken & egg vicious circle. These forums have countless threads where this & similar issues perpetuate ad infinitum. E does not "induce" H, & vice-versa. When the loop is immersed into the TEM space wave, the H cannot move the charges since they are still. But the E field can. This E field per FL exerts a force on charges per Felec = q*E. This is Lorentz' law.

But the force will result in motion & once the charges are moving, the H field, which is normal to the charge velocity, exerts a force as well per Fmag = q*(u X B). The total force is F = Felec + Fmag = q*(E + u X B).
You know, I never thought of it this way, this is interesting. I don't dare to agree or disagree. I just want to present to you my understanding why E field have no effect. Take a look and tell me your feeling.

In the diagram below, I drew the loop in blue, the E of the TEM in red. Notice it is on the x-axis occilating back and fore as the red arrow pointing both ways. Induced E is opposite direction to the applied E, so I drew the orange arrow of the vector component along the loop for the +ve part of the E wave ie when the wave is moving towards the +ve x direction. As you can see, the induced E want to go both ways, one towards the CCW direction and the other go towards CW. This is shown as \vec E_{+y} \hbox { and } \vec E_{-y}. The result is cancellation. I am not making a strong statement here, this is just how I look at this. Please give me your feedback.

I can see your point even if the E don't keep the current flow, but it would give it a kick start and the B take over. that is a very good question on the FL that who started the motion if B cannot change the velocity. AND the motion of the electrons in the loop is random motion initially.
Again, under time changing conditions, i.e. the "ac" domain, E & H are Siamese twins as they only travel together. They are 2 arms on the same beast, 2 sides of the same coin, 2 domains of the same energy, etc. The phrase "induction" is used quite loosely. The E field which motivates the charges to move, is joined at the hip with its twin, the H field. Faraday & Lorentz both apply w/o exception.

As far as the lumped parameter emf source being added to the equiv circuit, vs. the distributed parameter version, both give the right answer. But the prof point is all important. I stated earlier that in order to add the emf source into the equiv circuit, we must know its value. To do that we must measure the sum of the voltage drops across each resistor in the loop.

This sum will not always equal zero. The measured non-zero value can then be added into the equiv circuit as an independent voltage source. Then, the sum of voltages around a loop does indeed equal zero.

In order to draw the lumped equiv circuit, you must measure the sum of voltages around the distributed loop, which is usually non-zero. So the distributed circuit non-zero measured value provides the correct value for the included voltage source.

This is way too long, & we've covered it all. No debate is needed, since all phenomena are accounted for. I'll clarify if needed. BR.

Claude

I'll look at the second half of your write up tomorrow as this is getting really late. I'll edit this post to up date. You spend the time writing this long one and I want to really read it over first.

 

[stevenb]

If you can look at the drawing that I provided in #134 and Sarumonkee said he was going to try last week, I have a suspicion that you are not going to get the predicted voltage ratio across each resistor like 0.9V on the 900ohm. And don't say v=IR fail! I think you have to look at it as super-position of two event...1) voltage from the current passing through the resistors and 2) the induced emf onto the loop made up of the resistors. In this case, there are very little wire, the body of the loop made up of resistor material. I suspect the voltage source induced are distributed inside the resistors and make the reading different from prediction of V=IR. I am not absolutely sure, just a thought. But this will proof the point that if you consider induced emf as a voltage source, KVL work perfectly.

In my book, that prove the point. You have to care about what voltage induced along the wire.

To be sure you know what I am talking about, this is the picture. The wire is between point B and C which the professor call it a node or point D. As shown in red, the ground of the two probes attached in the middle 3" from B and from C and the probe measure point B and C which I called Probe B and Probe C. The professor claimed he measure 0.1 on the 100 ohm and 0.9 on the 900ohm. Of cause, I never disagreed, it is the wire from B to C. As in the drawing,

Again, pretty pictures do not replace a full analysis. Also, stating known things that we agree on does nothing productive. You still have not answered my critical question.

It seems you don't even understand the question so I may need to use your drawing to indicate the measurement loop that violates Faraday's law. I'm on mobile now so I'll have to do that later.

I can make one more verbal attempt. Just as you say the wire emf in the main loop completes your version of KVL (not Lewin's version mind you), I can say that the scope leads themselves also have emf that completes your version of KVL. Why? Because Faraday's Law says any loop is valid, and the scope provides a new loop once you hook it up. Do you understand this point? If not, wait for my diagram. If you do understand this, then analyze the loop formed by the scope and the main wire. This loop does not encircle much flux, as you show in your diagrams. FL says that the emf in this loop is zero. Hence, I ask again, how does the scope register voltage in a loop with two equal and opposite EMFs that add to zero (or small really)? If you can answer this question then you would convince me you are right. So, I recommend you concentrate on this critical point.

EDIT: I've attached a pdf for a drawing which may make my point more clear. Note that loop 1 and loop 2 encircle the full flux change. Loop 3 does not encircle very much flux change.

 

[yungman]

Again, pretty pictures do not replace a full analysis. Also, stating known things that we agree on does nothing productive. You still have not answered my critical question.

It seems you don't even understand the question so I may need to use your drawing to indicate the measurement loop that violates Faraday's law. I'm on mobile now so I'll have to do that later.
Your drawing is wrong, you really need to read things more carefully before you write. You missed the most important thing...THE SCOPE PROBE GROUNDS IN THE MIDDLE OF THE WIRE C AND D !
I can make one more verbal attempt. Just as you say the wire emf in the main loop completes your version of KVL (not Lewin's version mind you), I can say that the scope leads themselves also have emf that completes your version of KVL. Why? Because Faraday's Law says any loop is valid, and the scope provides a new loop once you hook it up. Do you understand this point? If not, wait for my diagram. If you do understand this, then analyze the loop formed by the scope and the main wire. This loop does not encircle much flux, as you show in your diagrams. FL says that the emf in this loop is zero. Hence, I ask again, how does the scope register voltage in a loop with two equal and opposite EMFs that add to zero (or small really)? If you can answer this question then you would convince me you are right. So, I recommend you concentrate on this critical point.

EDIT: I've attached a pdf for a drawing which may make my point more clear. Note that loop 1 and loop 2 encircle the full flux change. Loop 3 does not encircle very much flux change.

The scope probe ground take to loop 3 out all together. Why? In the scope, each channel measure the DIFFERENTIAL voltage, the voltage between the probe head and it's OWN ground leads. Did you even look at my diagram that I have the ground leads on, AND Saru have detail description about the probe ground leads?

If you really want to argue the loop between the two probes. The only loop will be from E to F to the probe ground at D, then to the middle of the wire CD to the ground lead of probe at D. Then go back to E. That is not in the signal path.

BTW, there is really no internal resistance between E and F. They are of separate channel. Your resistance is differential mode resistance between the two channel.

I know most of the scope have isolated return( ground) on the probe input BNC connectors. This is to address these kind of problems.

Can you please read Saru's #90 again before we go any further? I marked up your drawing to show you what is missing and my comments.