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Author Topic: Professor Walter Lewin's Non-conservative Fields Experiment  (Read 249978 times)
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The loop is located mid-way along the solenoid, and the solenoid length is >> greater than the loop length.

It becomes very difficult, because we are now with circuits in 3D.
My previous reasoning was based on a magnetic field as if it was generated by the circular circuit itself (i.e. generated in fact by an identical coil tighly positionned with the other).
I suggest to simplify the experimental setup by using a flat narrow coil of same diameter as the circuit with the resistances, the second being positionned just above the first one, otherwise I'm afraid that we are no more able to take into account all the parameters.

   

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It's not as complicated as it may seem...
On the contrary Ex, you're on the wrong track.

Go back to the basic principles of the experiment.

The requirement is for a UNIFORM (i.e. as close to pure vector as possible) changing magnetic field passing through the loop. As such, the absolute ideal configuration is to use a solenoid of infinite length, with the short loop (one wire width long) located precisely mid-way (assuming you can divide infinity in half, which of course you can't) along the length. In practice, we simply make the solenoid length >> the loop length, and the loop is located mid-way.

There are no "discrepancies" in the results I posted. They corroborate perfectly with the theory. The setup is correct and the results accurate.

Specifically, what is your objection to these results?


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Your solenoid is not infinite. You can't make the approximation of an infinite solenoid while connecting an oscilloscope above this supposed vertical infinite solenoid. In this case you have to take into account the end of the solenoid and the way that the wires escape from the proximity of the solenoid. Near a end of the solenoid, the field in not constant neither in intensity nor in direction, the field lines are curved in order to loop towards the other end. The oscilloscope doesn't measure anything else than the emf generating by the varying flux through the surface delimited by the measurement circuit (not only and not simply the circular circuit with the resistances), and this topology is not trivial.

« Last Edit: 2012-03-05, 09:57:49 by exnihiloest »
   

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It's not as complicated as it may seem...
As I said, in practice, we make the solenoid length >> the loop length.

This is sufficient to achieve a uniform field through the loop.

The decoupled lead configuration results in very minimal (<1%) induced voltage in the leads, despite the curl of the B field.

For the in-plane measurements (leads are in-plane), the scope leads measure the electric field only.


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Just an update....

Yes, I am assembling the experiment. I wish it to be set up the same as Lewin's but it won't be an exact duplication.

All along, I've been sensing some major holes in my memory on this experiment (the one I performed long before the Lewin version on YouTube).

I realized today that what he demonstrated is far from ground breaking, unique or even current. Memories are returning.

The sole purpose of the same experiment (by others) was to demonstrate the difference between conservative and non-conservative fields. By partial definition, a non-conservative field is path dependent. In other words, the value measured will depend upon the path the meter lead current takes, or probe orientation. This is well proven by Lewin and .99.

So, I'm left wondering what the hubbub is about. Is there an argument that the fields are not non-conservative? Is it still believed that KVL can be applied to such a field? If so, I don't and won't get it. Was it believed if the field is conservative then there is no non-conservative aspect - or vice-verse? Perhaps, there is an argument that non-conservative fields don't exist?

There are several things in physics that I prefer to check myself and a couple of things I completely disagree on. This isn't one of them. I suppose you could say that KVL still holds IF you are holding the probes the right way (& standing on one foot while whistling 'Dixie'?  ;D).

I'll lean on the same post used by some others here..... 'It has been proven repeatedly for eons and hasn't been wrong, yet. So, why do I need to argue it?'


Also remembered was the fact that my first version of that experiment was in 1977. Lewin didn't invent this experiment. I am convinced the same experiment was on a list of lessons at MIT in 77 except digital bench meters were used and the power supply was a variac. The class numbering was different- H20 or H30? Maybe that was just the military shortened version? In later demonstrations I used galvanometers so I could show the real fun aspects by rotating the meters around the transformer (solenoid primary / loop secondary). They didn't work at all like a scope sitting on a table would work. There is that path dependency, again  :)

Some interesting points I intend to confirm....

The ratio of voltage drop across the resistors is induction dependent.... The higher the voltage induced the wider the ratio. After all, that resistor loop is just a transformer secondary. The resistors resistances don't determine the induced fields. That is left up to Faraday with no concern for Kirchoff. They can only limit the current. Since they are part of the winding it was never a matter of simple voltage drop across the resistor because the resistor was also a section of the secondary.

Now, wouldn't that be interesting? If you crank the primary up with enough juice one resistor will show all the voltage dropped across it and apparently none across the other - all depending upon the path the current takes, of course  :o (Not my discovery)

The scope probes connected directly to the resistors had no effect upon the outcome of the Lewin version. This would be why he connected directly to the resistors - he knew it didn't matter when the scopes and probes weren't going to change relative orientation to the measured loop.

The bosses keep jerking me a from one project to another. Don't be surprised if I must ask later - what are we trying to disprove?

   


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It's not as complicated as it may seem...
Strange that you're not sure what all the "hub bub" is/was about. I thought you were following along, but I guess you completely missed it. :D

I think Gibbs and Dumped got it, and Ex, well he is still undecided apparently.

Tomorrow, I'll try and spell it out for those who still don't know why we're here, or where we went.


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It's not as complicated as it may seem...
On second thought WW, I'd prefer to chat with you one on one first.

Lessons Learned: You and I always seem to go round and round in circles while discussing something.

So in an effort to mitigate the need for numerous posts on both our parts, we should chat on Skype, or Messenger or phone a couple times for an hour or so. Are you game?

Let me know via PM please, what medium you'd prefer, and when. I am available after 16:30 MST, and all day weekends.


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Quote from: WaveWatcher
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Is it still believed that KVL can be applied to such a field?
If so, I don't and won't get it.
... 

Aye, KVL does apply.  But we must recognize that the
circuit is not a DC circuit and therefore somewhat
different "rules" apply.

Has the pulse width of the observed waveforms been
established?  And the constituent frequency domain?

Are you familiar with reflectometry?


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Aye, KVL does apply. 

Only as far as KVL is a sub-set of Faraday. If you consider KVL to be the most frequent description, it doesn't apply at all.

Quote
But we must recognize that the
circuit is not a DC circuit and therefore somewhat
different "rules" apply.

Will you invent some new ones?

Quote
Has the pulse width of the observed waveforms been
established?  And the constituent frequency domain?

If one can read a scope, yes.

Quote
Are you familiar with reflectometry?

More than 'familiar'.
That has little to do with this experiment.



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On second thought WW, I'd prefer to chat with you one on one first.

Lessons Learned: You and I always seem to go round and round in circles while discussing something.

So in an effort to mitigate the need for numerous posts on both our parts, we should chat on Skype, or Messenger or phone a couple times for an hour or so. Are you game?

Let me know via PM please, what medium you'd prefer, and when. I am available after 16:30 MST, and all day weekends.

I posted since is saw you say something about this subject on the shout box, earlier yesterday.

I don't spend a lot of time on the web lately. So, maybe I did miss the point.

Ex will be back on the subject. I think it just takes a long time for him to filter everything through Google Translate.

It is obvious this subject will be another one of those not needing further discussion. What it needs is full demonstration of all aspects. When that is done there should be absolutely no doubt that the 'static/original' form of KVL applied is like measuring AC with a DC meter.

I intend to finish this demonstration on YouTube. With that built, I can have a reference point for a detailed conversation.

Sorry, Skype sucks on my connection.


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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
I think the point was indeed missed.

Quote
It is obvious this subject will be another one of those not needing further discussion. What it needs is full demonstration of all aspects. When that is done there should be absolutely no doubt that the 'static/original' form of KVL applied is like measuring AC with a DC meter.

Is this a round-about way of saying you don't believe my treatise thus far, and want to see proof? That's not a problem, will do.

Your other statement proves further that you are either not aware of, or don't understand the "problem" and the reason for this undertaking.

Do you have a phone that works?  ???


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It's not as complicated as it may seem...
Some interesting points I intend to confirm....

The ratio of voltage drop across the resistors is induction dependent.... The higher the voltage induced the wider the ratio.

 ??? That's truly a wild speculation. Absurd to be blunt.


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??? That's truly a wild speculation. Absurd to be blunt.

Very well. That will be part of the demonstration.

Quote
Is this a round-about way of saying you don't believe my treatise thus far, and want to see proof? That's not a problem, will do.

Absolutely not. I have never had doubts about your honesty.

Quote
Your other statement proves further that you are either not aware of, or don't understand the "problem" and the reason for this undertaking.

My comparison to using a DC meter to measure AC is not one of equipment misuse. It is the fact that a DC meter can be used to measure AC with a bit of circuitry. I'm saying KVL can be applied but only when you consider the battery source is replaced with the source of induction and probe orientation is a certain way. Yes, the sum can be zero.

I know my comments appear to be posted to p you off but that isn't the case.

If you don't mind... PM your time zone. I'm on 12H days right now but we'll work a call in somehow. I'm willing to try Skype but I'm on satellite. The lag is more than most can bear.



BTW:

I made the comment about the changing voltage drop ratio because I found part of my notes from my first attempt. The changes are there and they were replicated - and expected by the prof. (I only remember him as our pet name - 'Bernie'.
 


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It's not as complicated as it may seem...
Tomorrow, I will try to post a summary of things that might help in our conversation.

We can always chat here in the chat engine if voice is not practical.

But this comment is getting there:

Quote
I'm saying KVL can be applied but only when you consider the battery source is replaced with the source of induction and probe orientation is a certain way. Yes, the sum can be zero.

And to comment briefly: Indeed, the battery source IS replaced by an induced emf in the experiment. But again, Lewin is mixing apples and oranges, and as a result of this oversight, his audience departs with an understanding that KVL never applies with induced circuits. Clearly it does. Faraday AND KVL are satisfied at the same time. Lewin does NOT espouse this fact at all with his treatise. In fact he diminishes KVL, when the truth is they are equally correct.

Hint: One approach to getting better clarity on this, is to forget about the measurement device and its leads, and focus on the physics and EM theory.


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Quote from: WaveWatcher

Quote from: Original Comment
Aye, KVL does apply.

Only as far as KVL is a sub-set of Faraday. If you consider KVL to be the most frequent description, it doesn't apply at all.

Quote from: WaveWatcher
Quote from: Original Comment
But we must recognize that the
circuit is not a DC circuit and therefore somewhat
different "rules" apply.

Will you invent some new ones?

No, the rules are now well established.

Quote from: WaveWatcher
Quote from: Original Comment
Has the pulse width of the observed waveforms been
established?  And the constituent frequency domain?

If one can read a scope, yes.

Those questions were meant to stimulate
thought.  Could the pulse width or the
representative frequencies in any way
account for the unusual nature of the
circuit performance?

Quote from: WaveWatcher
Quote from: Original Comment
Are you familiar with reflectometry?

More than 'familiar'.
That has little to do with this experiment.

That question was meant to stimulate some
unorthodox or unconventional thinking.
Does the circuit strangeness in any way
resemble a length of transmission line
unbalanced at each end with a transient
standing wave pattern in progress?

Do you recollect Tesla's experiments with
discharge excited parallel lines and the
resultant voltage patterns?


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Faraday AND KVL are satisfied at the same time.
...

Maybe we can save KVL but at the price of a complicated methodology. In KVL, the voltages are obtained from potential differences between two nodes. But a voltage from induced emf doesn't derive from a potential. It must be calculated by an intregral over a closed path. The mixing of the two forms emerges neither straightforwardly nor clearly from KVL.

   

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It's not as complicated as it may seem...
All true Ex, but that's not a problem, nor THE problem. ;)


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It's not as complicated as it may seem...
Lewin is brilliant with his delivery of this lecture. He has all the grace and sly of a good magician or illusionist. Most were captivated. But not all were fooled. ;)

At the very beginning, Lewin plants a seed that what you are about to see is "a very non-intuitive result". In reality, this of course is not the case, but you have already been set up to believe this is so. Call it "preconditioning".

Lewin begins by illustrating and discussing a simple DC circuit that contains an EMF source, and two potential drops across resistors. Take careful note that BOTH the EMF and the potential drops are illustrated. Also note that the potential differences calculated across the resistors is based on the circuit current. Ostensibly, his focus is on the potential difference between points D and A (VD-VA). See "DC_cct01.png" below.

Next, Lewin erases the battery and replaces the EMF source with an increasing magnetic field inside the loop. Here, Lewin notes that the resulting induced loop emf (peak) is 1V.

Lewin goes on to explain that the circuit current is still 1mA because the induced emf is 1V. Then he asks again; "what is VD - VA?" At this point, most viewers do not notice the slight of hand so brilliantly performed by the good professor. Did you?

Lewin is again calculating VD-VA using 1V emf and 1mA current, yet he has NOT illustrated the 1V emf source on his diagram. Why did he not illustrate the induced circuit emf?

This is in fact the faux pas that invalidates the rest of his lecture. Why? Because professor Lewin boisterously exclaims that Kirchhoff's rule (or law) does not hold in this case. He is absolutely incorrect with his assertion, and had he illustrated the induced circuit emf, it would be obvious why.

You can NOT have any potential difference across a circuit load, unless you ALSO have a circuit emf (or EMF in cases of DC) present. How can you have an emf induced on a piece of wire? I would ask, how can you not? This is Faraday's law.

The "induct_cct01.png" diagram illustrates what Lewin SHOULD have drawn on the board before he went off leading his audience down the garden path. Take careful note that the polarity of each emf source (red lines and polarity markers) is in opposition to the potential differences (PD) across the loads (green polarity markers). This is very important to understand. This is ALWAYS the case in every circuit, regardless if it is a DC or an induction circuit such as this. The total induced emf is equal to the total potential drops across the circuit non-reactive loads (i.e. the two resistors); the sum of the two being equal to zero. In other words, the closed loop integral equals 0V. And at ANY instant of time.

What this means, is that Kirchhoff's Law DOES hold in this case, contrary to what Lewin erroneously espouses in his lecture.

The simple matter of HOW to prove this and properly measure the potential drops AND emf's in the loop have been illustrated in several posts prior to this. It's important to realize that Lewin never made one emf nor PD measurement in his experiment. He only measured the electric field, and that's all he could measure considering the way his probes were situated. When you see this, you realize professor Lewin was confused about what he was measuring. It is clear he was mixing apples and oranges.

This should answer the question as to what the purpose of all this testing and discussion has been about.

btw, what is the closed loop integral of the induced electric field in this experiment? It is 1V of course!  O0

.99
« Last Edit: 2012-03-30, 00:57:52 by poynt99 »


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Quote from: poynt99
...

You can NOT have any potential difference across a circuit load, unless you ALSO have a circuit emf (or EMF in cases of DC) present. How can you have an emf induced on a piece of wire? I would ask, how can you not? This is Faraday's law.
...

And there you have it!

Well done.


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You can NOT have any potential difference across a circuit load, unless you ALSO have a circuit emf (or EMF in cases of DC) present. How can you have an emf induced on a piece of wire? I would ask, how can you not? This is Faraday's law.

......

tw, what is the closed loop integral of the induced electric field in this experiment? It is 1V of course!  O0


Great work and fine detail, as I have come to expect from you.  O0

However (  :D  ), with the problems in nomenclature, in general, and the lack of agreement for the definition of 'emf' throughout all facets of the sciences and industries, I can only express my complete agreement with Ex:

"Maybe we can save KVL but at the price of a complicated methodology. In KVL, the voltages are obtained from potential differences between two nodes. But a voltage from induced emf doesn't derive from a potential. It must be calculated by an intregral over a closed path. The mixing of the two forms emerges neither straightforwardly nor clearly from KVL."

My problem with all of this ( and I do see it as my problem ) is that I understand KVL as only being applicable to electric fields. There are no electric fields from one end of an inductor to another except where winding resistance is considered.

To me, the closed loop integral of the electric field produced by induction is 1V. Therefore, KVL is not applicable.

Sorry  :)


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"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
My problem with all of this ( and I do see it as my problem ) is that I understand KVL as only being applicable to electric fields. There are no electric fields from one end of an inductor to another except where winding resistance is considered.

To me, the closed loop integral of the electric field produced by induction is 1V. Therefore, KVL is not applicable.

Sorry  :)


"emf" is any source of voltage or current, manifest either by physical placement (eg. battery) or induced. It is unambiguous. In this case it results from the induced electric field. Yes, the CLI of the induced electric field is 1V, I've stated that.

You believe KVL only applies to electric fields? I would disagree. The CLI of the electric field in this case is 1V, and is therefore non-conservative. KVL applies only to emf's and PD's, NOT to E-fields. Lewin didn't measure emf's nor PD's in his experiment, so he is incorrect when he states that KVL does not hold. btw, Lewin does not mention electric fields at all; he relates everything to the circuit current and potential drops. In that, it would seem you are in disagreement with the professor.

What is your definition of an electric field?

You may disabuse yourself of your beliefs or understanding by making the same measurements I have made and shown here in this thread. Bench measurements trump all beliefs and theories.
« Last Edit: 2012-03-30, 14:11:07 by poynt99 »


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Bench measurements trump all beliefs and theories.

Well said, poynt.   

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My simple proposal is that we need to find out why measurement difference between decoupled and in plane.  What method can we use to solve both cases? 

 
   

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It's not as complicated as it may seem...
Gibbs,

The reason for the difference in the measurements has already been explained. What is it that you don't understand?

In terms of making the measurement, there is no one way to get the same results for both all around, but the decoupled measurement is the only TRUE measurement method IF you are interested in the emf's and PD's.

Clearly professor Lewin was interested in illustrating the existence of a non-conservative field, and he would have completely succeeded had he not decided to mix in PD calculations.

To re-emphasize; professor Lewin was measuring nothing more than the induced electric field in his experiment.


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Frequency equals matter...


Buy me a drink
Surely an MITPHD would know this.

Maybe he is making his students think?, Make them question? Find a flaw in what he is telling them like what has been done in this thread?


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