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Author Topic: Professor Walter Lewin's Non-conservative Fields Experiment  (Read 154903 times)

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It's not as complicated as it may seem...
Thanks to Harvey and ION for bringing this experiment to light (again). [ION, this was the "treat" I was going to offer the other day].

Video demonstration: Part 1 and Part 2

We've discussed this experiment and concluded that Professor Lewin is incorrect in his assessment of his observations made regarding this experiment. The fields are indeed fully conserved, and it is more of a measurement error combined with an assumption which allowed him to come to this conclusion that the voltages measured are different depending on the direction taken.

Well, this is true, but Lewin has incorrectly assumed that he is measuring the same exact points in the circuit for each resistor. He is not.

I enjoy the challenge of being able to simulate various things using SPICE. This was no exception. I believe I have clearly shown that this experiment can be simulated, and that Professor Lewin was incorrect in his assessment. I also feel that Lewin may owe those other professors he talked about in his video-taped class regarding this experiment, an apology. They were correct in their disbelief, but they should have known why and where Lewin was in error.

Lewin_NCF_01.png shows the circuit and the resulting voltage measurements. Each probe is positive at the top. L3 is the solenoid, and L1, L2 represent the wiring between the resistors, and comprise part of the induction loop. Clearly Lewin overlooked the fact that the wiring represents an inductance in the loop and in a dynamic sense, points A, A' and D, D' are not the same at all. This is why the voltage is measured to be quite different.

Lewin must have had the scope probes set across each resistor as shown.

Lewin_NCF_02.png illustrates the probe locations had he actually placed them across the exact center of the wiring, as he has "implied".

Can anyone have a go at what the actual voltage measurement would have been, had he placed the probes in the center at position A and D (purple probes)?

Note that in this case, BOTH scope probes would have measured exactly the same voltage.

I'll post the solution after you've all had a chance to contemplate it a little and offer your answers.

.99
   

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No comments or answers?  :-X ?
hmm...
   
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No comments or answers?  :-X ?
hmm...

I've been very busy with other things but will post a reply as time permits.


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I drew a diagram to understand your point.  In my picture, I show that wires represent an exaggeration of coils.  The two points of measuring vary accordingly.  The difference is the inductance in betweens.  Am I close?
   

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I drew a diagram to understand your point.  In my picture, I show that wires represent an exaggeration of coils.  The two points of measuring vary accordingly.  The difference is the inductance in betweens.  Am I close?

Your drawing appears to be a rearrangement of mine, so I think it is ok. I'm not sure I understand your question though.

.99
   
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Your drawing appears to be a rearrangement of mine, so I think it is ok. I'm not sure I understand your question though.

.99


Question? What question?  ;D  Just want to understand your thinking.  No question... atleast not yet. >:-)
   

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The difference is the inductance in betweens.  Am I close?

This is what I am not sure about what you mean exactly.

The inductance of the wiring will be very close to the same in both sections between the resistors. Just as I have drawn it.

.99
   

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poynt99,

Your analysis is correct but I think you may wish to dig a bit deeper into the physics. Lewin is famous for dropping bombs to make you think

The big problem with determining if the good prof is wrong is you must take a stand on whether magnetic fields are truly conservative, in this sense.

By that, I mean conservative in equal distribution of energy on all points of the loop.

Since one side has a resistor 9x the other side, are both sides producing the same amount of waste heat? If they aren't then both sides are not equal in the initial eddy current or the current due to Ampere/Turns.

I'm not disagreeing with your ideas. There are some unknowns about his experiment causing me to wonder if he really did provide a true example of a non-conservative field.

Conservative fields store energy without loss.

The net work done by a non-conservative field taken around a closed loop is non-zero. The net work is invariably negative. Or, a non-conservative field dissipates energy.

Magnetic fields cannot gain or loose energy. Magnetic fields, or magnetic flux density in motion, will actually change motion (direction, density, etc.) because this is true.

So, what I'm wondering is this a special case where the uneven loss of energy from the uneven resistances is cause for also uneven eddy currents & CEMF?  If so, the minor inductance you show on your circuit may be the means for the separation between the loop sides and Lewin made use of those.

There doesn't seem to be any close-ups of the exact setup. So, saying he is wrong or right are only words, unless you are convinced there is no true example of non-conservative induction.   :)

In any case, those other professors mentioned don't deserve an apology. They should have corrected Lewin or agreed with his experiment. They did neither.


« Last Edit: 2011-02-21, 10:55:42 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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WW, Thanks for responding, and your points have been considered.

However, I stand by the analysis as being accurate. I also stand by the notion that professor Lewin measured each resistor directly across the resistor terminals.

If we eliminate the possibility of induction in the measurement probe cables, two probes placed literally across precisely the same two points anywhere in the circuit, will measure precisely the same voltage on each.

The amount of power dissipated in each resistor is dependent on Ohm's law, and varies according to R value and circuit current. The current in the circuit is always equal for both resistors, therefore calculating the power is a simple task.

If the two resistors are increased to 1k and 9k respectively, what parameters change?

The eddy current idea is interesting, but I think the effective dissipation resulting from any eddy currents in those resistors would be magnitudes lower than that from I2R dissipation.

Maybe Lewin is fully aware of this illusion, which is why there are no close-up views of the experiment? Illusion: 2. The condition of being deceived by a false perception or belief.

Lewin seems to have a very strong personality, and I doubt many, if any question him. In one of his lectures he makes an obvious error with simple Ohm's law, and there was not a single peep from his class about it. Those other professors are probably "afraid" of Lewin, or out or respect, don't question him, just as his students don't.

.99
   

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Lewin's experiment didn't show the rise time on his scope display. I suspect this is due to the settings for trigger & capture.

Is there a reason for the same not shown on your simulation?

Just curious.
 


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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...
Lewin's experiment didn't show the rise time on his scope display. I suspect this is due to the settings for trigger & capture.

Is there a reason for the same not shown on your simulation?

Just curious.
 

With reference to the schematic diagram, I would be happy to post any wave form you wish.

In terms of rise time, you can see the rise and fall time of the generator driving the solenoid coil, and you can see the inductance and DCR values of the solenoid coil. Is it the solenoid current that you want to see? No problem.

I posted the wave forms I thought were relevant, and I've shown much more than Lewin showed in terms of the actual measurement points and "hidden" components that actually make up the circuit. I am not trying to obscure anything, if that is what you are getting at.  ???

.99
   

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I had no thoughts of you hiding anything.

Now that I'm on something besides my phone, I can see the red and green trace going from zero. So the rise time (on the loop, not the solenoid) may be a couple of ns?

With Lewin's lectures is is easy to spot his frequent mistakes. It isn't so easy when he is trying to emphasize something with his usual grandstand methods. There always seems to be something, aside from the experiment, he tries to sneak in. Normally you have to go through what he says in great detail.

One such detail is his emphasis on the solenoid being in the center of the loop. Visually, you can see the magnetic coupling is rather loose. The oddity of this setup is he is using the equivalent of a loose coupled transformer with a single winding secondary. The odd thing about the secondary is one half of that single turn has nine times the resistance of the other half. This is not something you would find in the real world.

In this case, references to non-conservative action has nothing to do with free energy. My interest is due to the odd transformer layout.

When I get some time I'll perform the experiment to satisfy my curiosity.


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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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I can't watch the videos here at work, but I'm wondering if this is not the old tricky experiment shown to physics students at most universities.

From what I remember, the voltage measurements at the same point are different depending on what side the probe wires lie.  It has to do with induction and loops,  since the circuit is subjected to magnetic fields, or is sitting over a coil fed AC voltage.

EM
   

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Here are two plots:

sol_loop_current01.png shows both the solenoid and loop currents.

R_power01.png shows the instantaneous power in each resistor.

.99
   

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Another showing rise time.

@EM, I don't think this experiment involves induction into the measurement probes. As you can see, the results work out perfectly at the most fundamental level with no mumbo-jumbo required.

WW, I'm glad you are going to perform the experiment; it saves me from having to do it to prove it to you guys.  O0

Is anyone going to take a stab at the now two or three un-answered questions?

.99
   

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The odd thing about the secondary is one half of that single turn has nine times the resistance of the other half. This is not something you would find in the real world.

It only appears strange. The fact that there are two "load" resistors rather than one is the only "oddity". With a single 1k resistor placed across the ends of the loop, the resulting induced circuit current would be identical, and the total power dissipation would be as well.

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Is this professor saying something heretical about this experiment?    Is he claiming that it can't be explained?    just currious.
   

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Is this professor saying something heretical about this experiment?    Is he claiming that it can't be explained?    just currious.

I'm not sure if he knows what he is claiming. Have a look at his lecture supplement (attachment). His first diagram and assumption that all the currents are clockwise can not be correct.



.99
   

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I'm not sure if he knows what he is claiming. Have a look at his lecture supplement (attachment). His first diagram and assumption that all the currents are clockwise can not be correct.

.99

 ;D

Ah! I never think about reading the supplements.

This is why I wish to perform the experiment.

When he made the statement about Kirchhoff not applying but Faraday always applies, I had an idea of what may be going on.

If I understand him correctly, KVL has limitations. This experiment is supposed to be an example.

What I want to know.... Is it a true example? I'll have to find out  :)
  


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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...
Notice that Professor Lewin has shown his volt meters connected to the resistors, not to points A and D. Again I'll ask: What voltage would he measure if the meters were connected to points A and D?

I really fail to "get" his point of this experiment. All seems well and good in EM Theory land. There will be -0.1 and 0.9 volts across those resistors regardless if there is a volt meter (and it's associated leads) connected or not (assuming the meters are very high impedance as he noted).

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I doubt KVL has any limitations, and I think it holds in this case as well.

Can anyone tell me how Professor Lewin has skewed this experiment and thus claimed KVL does not work or apply?

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I finally watched the video, and I assumed correctly, it's that experiment they love to show, to confuse those young minds.

KVL does not hold, because adding all the voltage drops around a loop should give zero, but here it does not.  The reason is that we have magnetic induction and KVL assumes no induction.  That's all he's trying to say.  And yes you measure different voltages from different sides, I saw that years ago.  It's really and important experiment to ponder.  

Hope that helps.

EM
« Last Edit: 2011-02-24, 15:30:02 by EMdevices »
   
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@Poynt99
Quote
Can anyone tell me how Professor Lewin has skewed this experiment and thus claimed KVL does not work or apply?
First Professor Lewin did not skew the measurement and second KVL never has applied, the professor has simply provided proof that there is an exception to the rule as there usually is if we look hard enough. The law does not apply to open systems such as this whereby the energy has a "choice" as to the path it takes, that is we have not forced it around a defined path in which the results must always be the same. I should also add that this has nothing to do with the conservation of energy and the issue is in the effects which change the measurement and the fact it does not work out as the law stipulates in this case. I also like this because it follows another truely fundamental law --- If we do things differently sometimes we get different results.
Regards
AC


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It's not as complicated as it may seem...
AC, EM.

I respect both you guys and your opinions, but these responses are a little disappointing. :-\

EM, sorry but you're wrong.

AC, you're the one always lecturing us on how we should be looking beyond what we think we see, and how our minds should be open to "other" possibilities; well free your minds guys, you're not seeing this for what it truly is.

Put on your thinking caps and try again.  :D

.99
   

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It's not as complicated as it may seem...
Start by reviewing KVL: ;)

Kirchhoff's Voltage Law (KVL)


      The voltage law says that the sum of voltages around every closed loop in the circuit
      must equal zero. A closed loop has the obvious definition: Starting at a node, trace
      a path through the circuit that returns you to the origin node. KVL expresses the fact
      that electric fields are conservative: The total work performed in moving a test charge
      around a closed path is zero. The KVL equation for our circuit is
      

        
v1+v2−v=0

      (4)



      In writing KVL equations, we follow the convention that an element's voltage enters
      with a plus sign if traversing the closed path, we go from the positive to the
      negative of the voltage's definition.
    
   
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