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Author Topic: Professor Walter Lewin's Non-conservative Fields Experiment  (Read 250009 times)

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It's not as complicated as it may seem...
Surely an MITPHD would know this.
Every MITPHD SHOULD know this. Evidently, not all do.

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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?
That is EXTREMELY unlikely.

It's clear Lewin intended everything he said to be taken as the indisputable truth.


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I would then consider any time spent at MIT would be counter intuitive to thinking freely. Is this the type of professoring they cloisture?

Every MITPHD SHOULD know this. Evidently, not all do.
That is EXTREMELY unlikely.

It's clear Lewin intended everything he said to be taken as the indisputable truth.


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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.

I'm not sure if all got the explanation.  If we understand how it works, we should be able to calculate every situation that can throw at us.  It could be a mix of decoupled and in plane.  I told you that decoupled is just the same as in plane, so there is no true measurement... or more correctly, every measurement is correct if it cover by 1 method. 

The difference between measurement depends on how much flux passing through the loop.  Decoupled is no exception.  Half of the flux go up, but since the wire is 6' up, the field lines split half left and half right to return to the south pole. 

   

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It's not as complicated as it may seem...
I'm not sure if all got the explanation. 
Sometimes it's good to think about the things that were presented and figure out the details yourself.

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If we understand how it works, we should be able to calculate every situation that can throw at us.
Yes, and we do.

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It could be a mix of decoupled and in plane.
Anything in between fully decoupled or fully in-plane is irrelevant as far as I am concerned. What relevance does it have to you or this experiment?

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I told you that decoupled is just the same as in plane, so there is no true measurement... or more correctly, every measurement is correct if it cover by 1 method.
The decoupled and in-plane measurements are worlds apart. They are measuring completely different things.

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The difference between measurement depends on how much flux passing through the loop.  Decoupled is no exception.  Half of the flux go up, but since the wire is 6' up, the field lines split half left and half right to return to the south pole. 
The amount of flux passing through the measurement wire while it is decoupled (i.e. vertical), is inconsequential. The result is very little, if any induced emf.


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...
The amount of flux passing through the measurement wire while it is decoupled (i.e. vertical), is inconsequential. The result is very little, if any induced emf.
...

It's not little.  The decoupled measurement of .4V contained half of the flux through its loop.   It doesn't matter how big the loop is, the amount of emf is how much flux through that loop.  We can't say flux pass through a wire, it only makes sense if it pass through a loop. 



   
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Here is an example.  This is decoupled.  Half of the flux pass through the red loop. 


   

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

The measurement wires are NOT part of the induction loop when they are vertical, i.e. normal to the loop.


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Gibbs,

The measurement wires are NOT part of the induction loop when they are vertical, i.e. normal to the loop.


  My opinion is yes it is.  This is how I based all my calculations on anyway.



   

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

  My opinion is yes it is.  This is how I based all my calculations on anyway.

Opinions can't change the physics of reality.

From Wiki:
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Michael Faraday formulated that electromotive force (EMF) produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path.

Flux can not pass through a surface area that does not exist. When a loop is normal to the flux path, NO emf is induced in that loop, because there is NO surface area. You may wish to review your basic EM theory. It is well known that two coils situated 90º from each other have no coupling. This is no different

Here's an experiment for you to try or think about:

Take an air-core solenoid, roughly 4 inches long and 1/2' in diameter. Now loop a wire through the centre of the solenoid about 10 loops, say 15 inches in diameter. You now have two coils, one looped through the other, and they are normal to each other. Drive one coil with a sinusoid while monitoring the other "output" coil. What will you see on the "output" coil?

.99


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Opinions can't change the physics of reality.

From Wiki:
Flux can not pass through a surface area that does not exist. When a loop is normal to the flux path, NO emf is induced in that loop, because there is NO surface area. You may wish to review your basic EM theory. It is well known that two coils situated 90º from each other have no coupling. This is no different

Here's an experiment for you to try or think about:

Take an air-core solenoid, roughly 4 inches long and 1/2' in diameter. Now loop a wire through the centre of the solenoid about 10 loops, say 15 inches in diameter. You now have two coils, one looped through the other, and they are normal to each other. Drive one coil with a sinusoid while monitoring the other "output" coil. What will you see on the "output" coil?

.99


The reality is I see a surface area and flux pass through it.  

The experiment you propose would yield no emf induced I know, but the in the Lewin's experiment the connection of measuring wire is physically connected to the loop with two resistors.  It itself formed its own loop.  The geometry is weird, but it's a loop and has surface area.



I'll try again with some picture.  The first one has no emf since it is normal to the loop.  The second one has some emf since it tilted.  The third one has half emf.

   

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It's not as complicated as it may seem...
The reality is I see a surface area and flux pass through it.  

The third one has half emf.

Incorrect on both counts.

The vertical portions of the wire (the measurement wires in the vertical plane) contribute NO emf to the measurement. Therefore, you are measuring ONLY what is induced in the horizontal loop at those two points.


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Incorrect on both counts.

The vertical portions of the wire (the measurement wires in the vertical plane) contribute NO emf to the measurement. Therefore, you are measuring ONLY what is induced in the horizontal loop at those two points.


Then how come in plane contribute emf? 


   

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Gibbs,

I say, you are on the right track.

I will also say, the majority of my posted opinions ARE based upon actually performing the experiments. I have no theories to promote regarding KVL. Faraday did that and Maxwell corrected Kirchhoff and Faraday while misusing the Volta's 'emf'.

No more time, at the moment. I'll be back.


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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

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No contribution to emf, yes.

It does contribute to the change of the surface area AND redefines that original loop area into TWO loop areas.


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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...
Then how come in plane contribute emf?  
Gibbs.

You seem lost in regards to how induction works?

Also, you seem to be ignoring the fact that the in-plane measurement becomes part of the experiment, and that the probes/leads in that position can ONLY measure the induced electric field, or if you want to think of it another way, the measurement leads sense what the electric field is doing between those two points when approached from a particular side.


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It's not as complicated as it may seem...
I will also say, the majority of my posted opinions ARE based upon actually performing the experiments.

Wavewatcher, based on your past performance throughout this thread, I have to say that your opinions are just that. Either it's been too long since you did the experiments, or you didn't perform the experiments I did, or you didn't realize the ramifications of the results you saw, or all of the above.

I'm guessing "all of the above".


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Gibbs.

You seem lost in regards to how induction works?

Also, you seem to be ignoring the fact that the in-plane measurement becomes part of the experiment, and that the probes/leads in that position can ONLY measure the induced electric field, or if you want to think of it another way, the measurement leads sense what the electric field is doing between those two points when approached from a particular side.

Poynt,

The reason I say it like that because the measurement is base on induction and not in plane or decoupled.  I agree with WW.  The loops now contain 2 planes.  One is vertical and the other is horizontal.  There is no flux through vertical plane but horizontal, yes.  I can't seems to find a better explanation.  Maybe someone else can describes it better.

   

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Wavewatcher, based on your past performance throughout this thread, I have to say that your opinions are just that. Either it's been too long since you did the experiments, or you didn't perform the experiments I did, or you didn't realize the ramifications of the results you saw, or all of the above.

I'm guessing "all of the above".

For your first guess, I'll just say I managed to find some notes that refreshed my memory.
Second, yes I didn't perform the experiments you did. The first time it was with two digital bench meters and a variac driving the primary. The last time, the measurements were with a galvanometer. In the first case the test circuit could be rotated between the measuring devices while they were connected to the secondary loop with slip-rings on the pivoting axis. The second case, the galvanometer rotated around the test circuit.
The third, well, I suppose more than one professor could be wrong. The ramifications were beaten into our heads by a very displeased associate professor (supposedly). He was always cranky after driving from Boston to Ft.Devens. It wasn't Lewin. We only knew him as 'Bernie'.

Gibbs,

With your current understanding (more than one plane, etc.), what do you think the measurement would be if the vertical section (normal to the transformer secondary loop) was still vertical but below the measure loop? (This would be with the measurement points midway between resistors - not across the resistors like Lewin shows, just for clarity.)

Should it still be +.4V, -.4V or another number entirely different?

No hurry. As soon as my double shifts are over I'll provide the answer with something better than opinions.







Because of those fine memories (now), I seriously doubt Lewin was doing more than reciting a lecture script from the 70's.


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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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Gibbs,

As an attempt to assist you with your thoughts on the many ways to change the value of the measurements....

If you are familiar with Faraday's sliding bar experiment, look at your latest posted drawing showing the two planes and imagine what it would look like from above.

To create induction or change the measurement you need not only change B. You may also choose to change A by sliding the bar (the Faraday sliding bar experiment) or change the relative angle between the loop or the test probe circuit and the flux.

 


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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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Gibbs,

With your current understanding (more than one plane, etc.), what do you think the measurement would be if the vertical section (normal to the transformer secondary loop) was still vertical but below the measure loop? (This would be with the measurement points midway between resistors - not across the resistors like Lewin shows, just for clarity.)

Should it still be +.4V, -.4V or another number entirely different?

No hurry. As soon as my double shifts are over I'll provide the answer with something better than opinions.







Because of those fine memories (now), I seriously doubt Lewin was doing more than reciting a lecture script from the 70's.

I think the value is still +/- .4V .  I'm just thinking would it reverse polarity, but my conclusion is it should not.

   
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Gibbs,

As an attempt to assist you with your thoughts on the many ways to change the value of the measurements....

If you are familiar with Faraday's sliding bar experiment, look at your latest posted drawing showing the two planes and imagine what it would look like from above.

To create induction or change the measurement you need not only change B. You may also choose to change A by sliding the bar (the Faraday sliding bar experiment) or change the relative angle between the loop or the test probe circuit and the flux.

 

Thanks WW,

This is the main point of my argument.  In-plane and decouple are just the extreme points between 0-90 degrees.  They still obey induction law no matter which position they're in.  We just need to know how much flux passing through our define loop and use KVL/Faraday on the defined loop. 
 
   

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

The reason I say it like that because the measurement is base on induction and not in plane or decoupled.  I agree with WW.  The loops now contain 2 planes.  One is vertical and the other is horizontal.  There is no flux through vertical plane but horizontal, yes.  I can't seems to find a better explanation.  Maybe someone else can describes it better.

I really don't understand you Gibbs. You appear to be a answering your own question every time you say that there is no flux through the vertical plane.  :-\

I don't understand what your confusion is.  ???

Also, I don't understand what you mean by this:
Quote
the measurement is base on induction and not in plane or decoupled

That makes no sense to me whatsoever. Throw me a bone here man, I'm trying to understand whats preventing you from understanding this very simple concept.  :)

.99


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

I'll also emphasize this once more:

I have no interest in any measurement angle other than 0 degrees, and 90 degrees. Anything in between is irrelevant to this experiment.


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It's not as complicated as it may seem...
...what do you think the measurement would be if the vertical section (normal to the transformer secondary loop) was still vertical but below the measure loop?

Should it still be +.4V, -.4V or another number entirely different?
The answer is +0.4V. The same as it was before. Nothing has changed.

And that is the whole point I am trying to get across to Gibbs; a vertical (decoupled) measurement lead is not influenced by the experiment, therefore it does not alter the true (emf and PD) readings taken from the conductive loop.

If there is no coupling (that's why I call it "decoupled"), how can the leads be getting influenced by the experiment? They can't. If they can't, then placing them above or below the solenoid at 90 degrees makes absolutely no difference in the measurement.

.99


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.99,

Your use of the term 'decoupled' is a fair usage for this case. Translation of it for a none native English speaker may be an issue.

When normal to the loop, the metering leads are not inductively coupled to the solenoid coil.

I'll leave the other angles and relative positions alone until I can provide more than an opinion. That information really goes beyond the Lewin demonstration of a non-conservative field, anyway.


---------------------------
"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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