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Author Topic: Kirchhoff is for the birds...  (Read 24059 times)

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It's not as complicated as it may seem...
I would not be convinced that any shielding is taking place until I completely "shielded" the internal loop.

If one could devise a way of getting some indication that the loop has current, or does not, then one would know with 100% certainty shielding is taking place.

One idea might be to fully encase a wire loop where the wire loop terminates on a very small piezo buzzer. Anything that can make an audible noise would work. I'm sure there are other ideas that could work.

Is this a conclusive test? If there is no current, is this because shielding is taking place, or is there no current because of the big shorted ring?
OR, if there IS still current, is that because there is no shielding, or is it because of coupling between the shorted ring and the inner loop?

I suspect that even with 100% coverage of the "shield", an emf would still be induced in the inner loop. In other words, the so-called "shielding" in this case is not effective. This means that either the E field is inducing the current at the wire segment threading the toroid (as already proposed), or there is another elusive penetrating field or potential responsible for the induction (the Magnetic Vector Potential, A Field?)


Also, there is much talk about the toroid's E field making its way around to other parts of the loop to cause the induced emf. May I ask you to draw out your vision of what the E field looks like around the toroid coil and associated wire loop?
« Last Edit: 2019-06-28, 19:07:02 by poynt99 »


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Quote: A Faraday cage is an enclosure made of conductive materials which is capable of blocking external electric fields. In other words, it is a hollow conductor capable of keeping the charge or radiation on the external surface of the cage. Faraday cages are used in a wide range of applications, including protection of electronic devices from electrostatic discharge and external radio frequency interference.
A Faraday cage is also known as a Faraday shield.


Brad

I understand Faraday shielding.

I ran a quick bench test using a powdered iron toroid with a single shielded coax loop connected to a precision 1 ohm and 10 resistor which are connected in series to complete the loop.

The first scope pix has the scope grounds of both CH2 and CH3 connected at the junction of the two resistors and the resulting voltages are shown.  The Math channel shows the sum (actually difference to compensate for phasing) of the two channels to be 294.1mv rms.  Grounding the shield or leaving it unconnected makes no difference in the measurements. 

The second scope pix shows CH2 measuring the voltage across both resistors to be 293.1mv rms which is in close agreement with the Math channel above.  CH3 shows the voltage measured across the shield to be 295.5mv rms which is in phase and slightly higher than the loaded loop.

I find no evidence of shielding in this test.

Regards,
Pm

   
   
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Brad,

Here is another test using the same toroid but with a 4" diameter outer shield formed out of 1/8" diameter steel brake line with an insulated 18ga wire inserted for the main loop.  As can be seen from the pix, the ends of the loop are as close as possible and connect to a 10 ohm 1% resistor.

The scope pix shows the measurements with CH1(yel) being the input voltage to the toroid, CH2(blu) is the voltage across the 10 ohm resistor, and CH3(pnk) is the voltage across the outer shield.

If we consider the shield is actually working to shield the E field even though it appears as another secondary loop, then what is the method of induction for the inner wire?

Anyone?

Regards,
Pm
   

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It's not as complicated as it may seem...
Nice test PM.

I guess you've confirmed my assertion.

I would also assert that if you shorted across the tubing and re-ran the test, you would still have an induced emf in the inner loop, but the outer tubing would now be "hot" as well, emanating its own magnetic field.


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Yes, with the outer tube or shield shorted with a jumper that has some resistance, the voltage across the shield is 59.55mv rms and the voltage across the 10 ohm is 60.52mv rms.  This would indicate a rather tight coupling between the shield and inner wire.

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Back in the late 90s I was a sounstream car audio dealer, when it was the good stuff, and we carried monster cable. It was all coaxial rca cables back then.

I chose to buy the rca cable on rolls with, gold plated connectors that were soldered on, and made the cable to length for each system to avoid the wires from being tight or rolling up excess if the premade were too long or short.

The cable on the roll had the center signal wire, then the braided shield over that and an alum foil shield that was to be only connected on the source end connector.

But at times we still might get alternator noise(engine whine) in the audio. It became a must to ground the head unit and all other equipment to a single ground point instead of the radio grounded somewhere near the radio and the amps grounded to chassis in the area they were installed to avoid ground loops within the chassis. But still the cables were an issue at times in vehicles. These days I wire the power and gnd of the amps all the way to the battery. Helps in many ways.

But some years after that some companies came out with twisted pair rca cables. No shielding. And it has been a beautiful thing ever since and most all companies use that now. You can run your rcas with the large power and gnd amp wires and come out unscathed with noise issues.

So Im wondering if that would be a good approach here with test equipment to avoid these em interference issues.

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Brad,

Here is another test using the same toroid but with a 4" diameter outer shield formed out of 1/8" diameter steel brake line with an insulated 18ga wire inserted for the main loop.  As can be seen from the pix, the ends of the loop are as close as possible and connect to a 10 ohm 1% resistor.

The scope pix shows the measurements with CH1(yel) being the input voltage to the toroid, CH2(blu) is the voltage across the 10 ohm resistor, and CH3(pnk) is the voltage across the outer shield.

If we consider the shield is actually working to shield the E field even though it appears as another secondary loop, then what is the method of induction for the inner wire?

Anyone?

Regards,
Pm

Thank you for doing these tests Pm.

So i ask,is it not possible to shield the inner conductor from the E field?
-->Or,is it possible that i am right,and that the loop can be induced as long as 1 small part of it is exposed to the E field?.

Would you have not expected even a small reduction in voltage when you covered almost all of the secondary loop in a steel tube?.


Brad


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Also, there is much talk about the toroid's E field making its way around to other parts of the loop to cause the induced emf. May I ask you to draw out your vision of what the E field looks like around the toroid coil and associated wire loop?

See diagram below.


Brad


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It's not as complicated as it may seem...
Could have used this picture as well.



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It's not as complicated as it may seem...
is it not possible to shield the inner conductor from the E field?
Apparently not when the E field is created via a changing B field, and the wire is coincident with the E field.

Quote
-->Or,is it possible that i am right,and that the loop can be induced as long as 1 small part of it is exposed to the E field?.
In your opinion, if the loop was made 100 meters long (200 meters of wire) and elongated and pulled out taught, and all 100 meters from the toroid to the end was shielded, would there be an induced emf? (assuming essentially no wire is exposed to any significant E field)

Quote
Would you have not expected even a small reduction in voltage when you covered almost all of the secondary loop in a steel tube?.
I wouldn't.


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Could have used this picture as well.

I did  use that picture as well.
Perhaps you missed it?


Brad


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It's not as complicated as it may seem...
Yes, with the outer tube or shield shorted with a jumper that has some resistance, the voltage across the shield is 59.55mv rms and the voltage across the 10 ohm is 60.52mv rms.  This would indicate a rather tight coupling between the shield and inner wire.

Pm
OR, might this indicate that each loop has its own independently induced emf? (the likely answer in my opinion)


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Yes, with the outer tube or shield shorted with a jumper that has some resistance, the voltage across the shield is 59.55mv rms and the voltage across the 10 ohm is 60.52mv rms.  This would indicate a rather tight coupling between the shield and inner wire.

Pm

Pm

With your outer shield open (not shorted),do you see any load placed on the primary when you place a load on the secondary loop ?.


Brad


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Thank you for doing these tests Pm.

So i ask,is it not possible to shield the inner conductor from the E field?
-->Or,is it possible that i am right,and that the loop can be induced as long as 1 small part of it is exposed to the E field?.

I'm not sure about being able to shield the inner conductor.  I've been trying to think of a way to build a shield that would cancel any induced field in the shield itself and yet allow an induced field in the inner conductor.

In my mind, any part of a loop that passes thru the center hole of a toroid coil will experience induction where the E field is concentrated. 

Quote
Would you have not expected even a small reduction in voltage when you covered almost all of the secondary loop in a steel tube?.

Actually no, I didn't expect any reduction because I viewed the shield as another secondary with a rather high coupling factor.  However, I'm not sure if we are truly shielding the inner conductor because we do have induction in the outer shield and with the apparent tight coupling to the inner conductor, we would then see induction in that inner wire.  So in my mind at this point, the jury is still out!

Pm

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Pm

With your outer shield open (not shorted),do you see any load placed on the primary when you place a load on the secondary loop ?.


Brad

Brad,

There was no measurable change in the primary current when adding a 10 ohm load to the shield.  So, I removed both loads to see what the primary current measured and again no change which means the core has a high magnetization current compared to the reflected current from the secondary to the primary.

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OK, here is a test that was an attempt to cancel the induced field in the shield by applying an equal and opposite current to the shield from another source.  The source in this case was another toroid with a single turn that was coupled 180 degrees out of phase.  The second toroid is an identical core but with a different wire size and turn count on the primary so there was a slight phase adjustment in the driving signals from the power amplifier.

We do see a significant reduction in the inner wire voltage on CH3(pnk) but it is higher than the shield voltage on CH2(blu) so this reduction may be a result of the tight coupling rather than a shielding effect.  So, these results are inconclusive IMO. 

Also note the current on CH4(grn) in the cancelling turn on toroid 2 and the unbalanced voltage across the shield is the 3rd harmonic.

Regards,
Pm
   
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Been thinking about the Prof. Lewin video's since they were put on you tube, done the research, read the papers and done the tests. My conclusion is that a point source like a battery in a closed loop is in effect an electron pump creating a high/low electron density which must distribute itself along any length of resistance... equal and opposite. We could use a water pump and restrictions in a loop of pipe and produce the same results using the same line of reasoning.

Now we take the same conductor loop which is induced from an external source and the only real difference is that the Electromotive Force propelling the electrons along the conductor is now distributed along the entire length of the conductor versus a point source. In effect this would be equivalent to using an impeller within a circular casing and imparting a uniform force on every part of the water loop along the outer circular path of the casing. If we then added restrictions along this path then it would seem reasonable that the sum of pressure drops must be equal to the sum of pressure gains... equal and opposite.

In my mind all the equations and math, all the measurements and debate over what they mean is less important than the line of reasoning used to explain what we think we are seeing. Thus to say this system is non-conservative is to say something came about which was not present in the first place. An electromotive force or pressure on the free electrons came about which caused a change in the difference in potential which was measured across the resistances not supplied by a point source or uniformly distributed source. In conclusion, I say it cannot be non-conservative within a simple closed loop however we should acknowledge that Kirchhoff said the rule only applies to a closed loop with good reason.

Personally... I think Lewin, possibly about to retire, may have decided to throw a monkey wrench into the works just to see what would happen. I mean look at his clothes, that hair and how intelligent and witty he is for a man of his age. You have to respect the fact he's a troublemaker and isn't afraid to offend a few people. I think he just wanted to stir the pot and make a few people think and that is not necessarily a bad thing in this day and age. That or he's been smoking crystal meth, it's uncertain at this point.

Regards
AC



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

I am quite sure that Prof. Lewin is quite serious in his assertion.

He would be correct if he would separate the discussion into E field measurements, and emf/voltage drop measurements. They are two distinct animals, but he intermixes them in his lectures, and it has caused a great deal of confusion and misunderstanding with his audiences, both students and academics alike.

If one is talking about the induced E field and the measurement thereof (as in Lewin's experiment), then yes, the "voltages" are non-conservative. In this case KVL does not hold, and it is not expected to.

If one is concerned with the induced emf and voltage drops, then one must measure them accordingly by effectively isolating the measurement device from the experiment. In this case KVL holds, and it is expected to.

When discussing or lecturing on this experiment, one must be keenly aware of the differences between these two perspectives and not to get them confused with each other or equate one with the other. Unfortunately, Lewin does a poor job of this, and is not even aware of his error.


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"Some scientists claim that hydrogen, because it is so plentiful, is the basic building block of the universe. I dispute that. I say there is more stupidity than hydrogen, and that is the basic building block of the universe." Frank Zappa
   
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https://www.insidehighered.com/news/2015/03/10/despite-mits-decision-delete-walter-lewins-lecture-videos-live

I like this response...
Quote
Those who oppose taking the videos down, meanwhile, often bring up the transgressions of other famous scientists. “I don’t care that Newton was a petty asshole, an alchemist and a religious nut; I don’t care that Feynman was misogynist; I don’t care that Heisenberg worked for the Nazis; and I don’t care about Lewin either,” another commenter wrote. “I care about their knowledge that they gave to the world. I am not responsible for their lives, that’s their goddamn headache, but don’t punish me for their sins by restricting my access to their material.”

What a sorry state of affairs.


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

Leftist knee-jerk reactions and protests (and the not-well-thought-out knee-jerk responses) are creating unnecessary damage to many a reputation and society in general.


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I also totally agree with what Allcanadian quoted.
Should we also delete the films of Roman Polanski and Woody Allen accused in a similar way?
Nobody's perfect. Men's faults must not make us destroy the works of their talented part, especially when it is not proven.



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AC,

I am quite sure that Prof. Lewin is quite serious in his assertion.

He would be correct if he would separate the discussion into E field measurements, and emf/voltage drop measurements. They are two distinct animals...
...

I wouldn't say that. We have to go back to definitions.
An E field is a property of a point in space, where a charge q is subjected to a force F such that E=F/q (I can't put arrows but F and E are vectors).
There is therefore only one electric field, defined from a mechanical force.

Only the way to obtain it changes depending on whether it is by a potential difference, or by induction.

In the first case, the electric field is created through a conductor or dielectric by a potential difference, i. e. by forcing an imbalance of the charges between the two ends. The electric field E at a point therefore depends on the differences in charge accumulation at distant locations.

In the case of induction, the field is directly created in any point of space by moving charges, those of the current of the source of the magnetic field, because their motion creates an anisotropy of their coulombian field that do no more cancel the positive charges at rest.
The induced charges in a conductor move through it because they are subject to this resultant field. If there is a resistance in the induced circuit, then the displacement of the induced charges is no longer uniform and will create differences in the accumulation of charges that are seen as potential differences.

This is the opposite effect of the previous one: the induction field E generates the potential difference in the resistance, while in the other case the potential difference generates the field E through the resistance.


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It's turtles all the way down
Indeed,

Leftist knee-jerk reactions and protests (and the not-well-thought-out knee-jerk responses) are creating unnecessary damage to many a reputation and society in general.

Agreed

From F6FLT

Quote
I also totally agree with what Allcanadian quoted.
Should we also delete the films of Roman Polanski and Woody Allen accused in a similar way?
Nobody's perfect. Men's faults must not make us destroy the works of their talented part, especially when it is not proven.

Agreed again.

I don't know the full truth of the matter, but from what I have read it seems IMO that MIT is caving in to a few politically correct feminist staff members yielding more power than they deserve to have.  Oh that's right we are supposed to believe at all costs a mentally ill person's word without proof even if it destroys the reputation and works of Lewin. Now where have we seen this pattern before?

We are in very sick times.
« Last Edit: 2019-07-08, 17:09:17 by ion »


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An E field is a property of a point in space, where a charge q is subjected to a force F such that E=F/q (I can't put arrows but F and E are vectors).
Several of my books on EM theory use bold characters to represent vectors, and you will have seen that I follow that practice.  Perhaps I should have made that clear before posting any formula.  Anyhow its easy to do here.
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What i find interesting is that we seem to be unable to shield the secondary from the electric field.
Dose no one else find this interesting ?.


Brad


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