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Author Topic: Professor Walter Lewin's Non-conservative Fields Experiment  (Read 250005 times)

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It's not as complicated as it may seem...
Here's a hint:

Changing the diagram slightly, what would the indicated voltage be if the scope probes were placed across points 9 and D?


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Here's a hint:

Changing the diagram slightly, what would the indicated voltage be if the scope probes were placed across points 9 and D?



Hm.....  :( ..... 0 V ?
   

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It's not as complicated as it may seem...
Correct, 0V.

And what would be indicated if the probes were placed on points 9 and 1?


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Correct, 0V.

And what would be indicated if the probes were placed on points 9 and 1?


Depends,

If the scope loop eat all the resistors (contain within its loop), 1V.
If the scope loop leave the resistors outside its loop, 0V. 


   

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It's not as complicated as it may seem...
We've moved one probe from point D to point 1. That's all.

So 0V is again the correct answer.

Now, what is the voltage indicated when the probes are on points 1 and 1' when taken from the right side as per the diagram?


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We've moved one probe from point D to point 1. That's all.

So 0V is again the correct answer.

Now, what is the voltage indicated when the probes are on points 1 and 1' when taken from the right side as per the diagram?




I see... .9V
   
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..
Ex is wrong simply because there is no reversal of magnetic polarity on a setup identical to the Lewin experiment anywhere outside the coil except above and below the coil.
...

I gave the general case and Lewin's experiment is a particular case that of course is included in the first one. There is always somewhere reversal of magnetic polarity because a magnetic flux is looped. Naturally outside of the coil the B field is reduced but the flux is the same, just a question of crossed surfaces much larger outside than inside. As the emf depends on the flux crossing by unit of surface, the emf around the same loop surface will be much lower outside that inside of the coil. So outside of the coil, a more sensitive measurement must be done to show emf.
The Lewin's experiment doesn't show that the flux is non conservative because every observed thing is explainable quantitatively and qualitatively according to the current laws of electromagnetism.

   
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...
The scope impedance is so high in these tests that the induced current in those loops produces no appreciable EMF regardless of their orientation, size of the loop or distance from the solenoid. However it is needed to provide an accurate evaluation. But I am confident that it is well below my +/- 0.1V error margin offered in my answer.
...

Emf is a voltage around a looped pass, it doesn't imply any current. So the question of the probe impedance is irrelevant in this discussion (except if it was too low and made the voltage drop).

   

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The Lewin's experiment doesn't show that the flux is non conservative because every observed thing is explainable quantitatively and qualitatively according to the current laws of electromagnetism.

Very good point. Had he used only one meter with a fixed middle connection and simply moved the meter he would prove the non-conservative aspect. However, non-conservative fields ARE part of the current laws of electromagnetism - at least... this is what I was taught. Has that changed?

If so, who has the task of rewriting Maxwell's interpretation of Faraday's laws?

I am following the thread. Theories aren't a good thing to offer when the results are empiric. So, I'll comment on experimental results with experimental results as soon as possible  :)

Until then, assigning potential (a static aspect) to fields in motion (induced EMF) only leads to a contradiction. So, 'potential' means nothing with induced electric fields. Therefore, KVL can not be applied as it only deals with potential, not changing fields.

The difference between the non-motional EMF and motional EMF is the first produces current from an imbalance of charges and the latter produces current according to Lorentz. (I don't like the term 'non-motional EMF'. It makes no sense but is common).
Above: Incorrect in almost any sense. A quick check in the books indicates the term 'emf' was started by Volta and varied in real meaning since then. In the most agreed upon correct and current usage it has nothing to do with current and is not the voltage measured in a circuit. Circuit voltage is always less than emf (the potential source creating the circuit voltage).

Just thought I would correct comments that I suspected may be incorrect.

  

« Last Edit: 2012-02-29, 16:59:33 by WaveWatcher »


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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...
Until then, assigning potential (a static aspect) to fields in motion (induced EMF) only leads to a contradiction. So, 'potential' means nothing with induced electric fields. Therefore, KVL can not be applied as it only deals with potential, not changing fields.
The fact that we are dealing with changing fields has little to no relevance with certain aspects of the analysis, and discussing it is an unnecessary complication that only muddies the waters. It is important to address the dynamic aspect of the experiment, but not for the reasons you and Harvey have espoused. And we're getting to the theory part of this discussion very soon.

Quote
If so, who has the task of rewriting Maxwell's interpretation of Faraday's laws?
There is no need. Farady and Maxwell are fine.


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

I see... .9V

Correct. Now that's non-intuitive!

So going back to your original question posed here, what is your answer now?


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Correct. Now that's non-intuitive!

So going back to your original question posed here, what is your answer now?




The original question was what is the value as the loop pass into the changing magnetic field.  It would depends on how much flux the loop can contain.  It can have value range from .9V to -.1V .


  
   

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It's not as complicated as it may seem...
The original question was what is the value as the loop pass into the changing magnetic field.  It would depends on how much flux the loop can contain.  It can have value range from .9V to -.1V .
O0

So what are your thoughts regarding the dynamics involved with that particular situation?

Why does Ohm's law not hold in this particular case when the angle is beyond 90 degrees as we discussed?

Why DOES Ohm's law appear to hold when the angle is within that first 90 degrees?


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O0

So what are your thoughts regarding the dynamics involved with that particular situation?

Why does Ohm's law not hold in this particular case when the angle is beyond 90 degrees as we discussed?

Why DOES Ohm's law appear to hold when the angle is within that first 90 degrees?




I think Ohm's law holds. 

One can either use Faraday or KVL and the answer is the same.  When the angle is within 90 degrees, one solve it in two ways and either one would give correct answer:

1/ The  loop that contain no magnetic field (probe through 900 Ohms back to probe)
2/ The loop that contain all magnetic field (probe through 100 Ohms back to probe)

For no magnetic field, voltage of the scope has to equal the 900 Ohms.  No magnetic flux in that loop so wherever you move the loop, as long as it does not contain flux, the answer is always .9V . 

For the magnetic flux case, the voltage has to be 100 Ohms and compensated by the induced voltage by the flux, depends on how much flux.  If we contain all the flux, then it's 1V minus 100Ohms potential which is still .9V. 

The reason it departs from .9V is the flux in the loop that we set up is partial.  This happens when we move the loop pass 90 degrees.

Either case, if you do KVL/Faraday on any closed loop, you will get the right answer. 
   

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It's not as complicated as it may seem...
You're getting the idea Gibbs, but you're not quite correct yet.

For the following discussion, unless otherwise stated, assume the probes are in the same plane as the loop. Now, when we place the probes in such a way that they encircle part or all of the loop, what are we actually measuring?


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You're getting the idea Gibbs, but you're not quite correct yet.

For the following discussion, unless otherwise stated, assume the probes are in the same plane as the loop. Now, when we place the probes in such a way that they encircle part or all of the loop, what are we actually measuring?




I'm not sure what is your thinking but I think the probe plane always has a component in the same plane as the loop.  It's the cosine part of that plane.  When 90 degrees, it means cos 90= 0 . 

When it encircle part or all of the loop, we measure the total EMF minus resistance produced by the field flux, depends on how you define your loop. Other than that, I'm not sure what you mean. 

   

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It's not as complicated as it may seem...
Reference my question of what exactly we are measuring;

The answer is, we are measuring the electric field E (or part thereof) produced by the changing magnetic field B.

That's why Ohm's law does not hold in some cases.


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I see.  I think Ohm's law is just that V=IR . 

The boundary condition for this system is encircle just the resistor and not anything outside of that. 

So yes, we're measuring the E field due to changing B, but it's outside of the boundary of how we defined Ohm's law.



   

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It's not as complicated as it may seem...
I see.  I think Ohm's law is just that V=IR .  
Yes that's Ohm's law. But Ohm's law appears to be broken when we measure the voltage as shown in the diagram, agreed?

Quote
The boundary condition for this system is encircle just the resistor and not anything outside of that.  
The boundary condition of this experiment is the entire loop when considering that plane.


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The boundary condition of this experiment is the entire loop when considering that plane.



Yes, the boundary condition for this experiment is the entire loop, but the boundary condition for Ohm's law is just the resistor.  That's why if you look into the entire loop you have a value of the scope, but when you reduce the boundary to just the resistor, you have the potential across the resistor.  It is indeed different from scope and obey Ohm's law. 


   

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It's not as complicated as it may seem...
OK, I'll buy that.

So do you now have a clear understanding of the dynamics involved when the measurements are taken in-plane vs. decoupled? Can you with confidence predict the voltage indicated on the scope across any two points and in both scenarios? Do you understand why the indicated scope voltage is the same for points 9-9' and D-A when measured in-plane?


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So do you now have a clear understanding of the dynamics involved when the measurements are taken in-plane vs. decoupled? Can you with confidence predict the voltage indicated on the scope across any two points and in both scenarios? Do you understand why the indicated scope voltage is the same for points 9-9' and D-A when measured in-plane?



Not sure what decoupled means, but yes, if you put the probes on any two points and show your probe wiring geometry, I can give you the exact value.  The indicated value is the same for 9-9' and D-A are all in accordance with KVL/Faraday.  So if one understand what KVL/Faraday is, then he should understand why. 



   

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It's not as complicated as it may seem...
Not sure what decoupled means,
This is in reference to how the first 3 diagrams are depicted. The measurement device's leads are normal to the loop, therefore decoupled from the experiment, vs. the in-plane configuration, where the leads ARE coupled to the experiment.

Quote
but yes, if you put the probes on any two points and show your probe wiring geometry, I can give you the exact value.  The indicated value is the same for 9-9' and D-A are all in accordance with KVL/Faraday.  So if one understand what KVL/Faraday is, then he should understand why.  
Indeed? Alright.  8)

What will be the indicated voltage on the scope for this decoupled configuration?


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It's not as complicated as it may seem...
The indicated value is the same for 9-9' and D-A are all in accordance with KVL/Faraday.  So if one understand what KVL/Faraday is, then he should understand why.  

Please explain it.  :)

If you knew KVL/Faraday and that is all you require to know the dynamics of this experiment, why did you get most of the answers wrong?  ???


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What will be the indicated voltage on the scope for this decoupled configuration?




I see.  Decouple means not letting the magnetic field effect it.  Thanks.


 ;D  Testing me out?  It's fair... lol

.5V  (forget polarities, I hate it lol )  


I need to formulate my thoughts again for KVL/Faraday.  Maybe sometimes later.

BTW Poynt.... Thanks.   :)

I got it wrong because I didn't trust your experiment.  But it's absurd to think you would lie. lol 
   
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