#562 – Electroboom!

[bsfeechannel]

Yeah, it's getting more and more interesting with each new post.

Of course it is. Electromagnetism is interesting.

Now only Lorentz' force creates an electric field

Nope. Inside a moving wire perpendicular to a magnetic field, the Lorentz force will displace the charges inside a conductor. Since the magnetic field is static, it doesn't induce any other electric field. So you will have a net electric field inside the conductor produced by the charge displacement.

but not a changing magnetic flux (despite Maxwell-Faraday telling the opposite),

A changing magnetic flux will induce a voltage inside the wire, however the displacement of charges will counteract this field, resulting in a net electric field inside the conductor with zero intensity.

and now you only have an electric field in the space between the transformer windings.

Precisely. One of the main causes of failure in transformers is the breakdown of insulation between adjacent turns. This is because the field resides between the turns, and produces arching. That would not be possible if the electric field was inside the wire.

I really wonder how antennas ever worked.

It is easy. Study electromagnetism for real. It will take you years to master the trade. But its worthwhile. And forget those bloggers that pretend to teach you this complicated matter in a 12-minute video plagued with all kinds of pseudo-scientific claims.

Besides, this contradicts his own statement:

If the field "concentrates between the terminals" it should be nowhere else. Not between the windings either, because clearly they are connected to each other and thus there should not be a "boundary condition" where the "rotational electric field" (what?) is "forbidden to exist".

I said concentrate, not is absent elsewhere. This is very easy to understand. Look at the picture below. It is the "front view" of a turn. As you extend the left terminal on top of the right terminal to wind the second turn, you see that the electric field accommodates conveniently between the turns. It is not shown in the picture, but assume that the wires have an insulation.



And it is this property that is used for instance with a Variac. The electric field between the turns add up between the terminal of the first turn until the tap. And that's how you get your variable voltage. I'm omitting the resistive wiper that gets in contact with several adjacent turns, but you get the idea.

Next argument will be that the field only exists when you connect a meter to measure it, which would violate causality.

Your comment is violating causality, since I haven't seen yet what will be your next expression of absolute lack of understanding of electromagnetism.

[thinkfat]

So, let there be a single loop of wire and subject it to the same changing magnetic flux as the transformer in your example above. Let d be the length of wire needed for a single winding in said transformer. What is the voltage between two points of distance d on the single loop of wire?

[bdunham7]

Yes. When the wires are moving perpendicular to the magnetic field.

But in Lewin's circuit the wires are static in relation to the frame of reference. So the voltage across the wires will obey ohms law.

OK, so let's imagine I connect my scope to the ends of the top wire (A1 and A2 IIRC) and I repeat the experiment with the full loop in place and then with the resistors disconnected so that I only have the wire with open ends.  Then I do the same routine with my magic electroscopes.  What do I see?

[Sredni]

I no longer post here, but...

https://electronics.stackexchange.com/questions/506590/can-two-voltmeters-connected-to-the-same-terminals-show-different-values-circui?noredirect=1&lq=1
https://electronics.stackexchange.com/questions/551519/assigning-a-notion-of-voltage-even-when-there-is-a-changing-magnetic-field/551548#551548
https://electronics.stackexchange.com/questions/551244/what-would-a-voltmeter-measure-if-you-had-an-electromotive-force-generated-by-a/551428#551428

there are slight difference with the terminology, and I need to fix (at least) a missing sign but the gist is there.
For people who want to see: the key is in the fields.

[jesuscf]

KVL and KCL are special cases of the Maxwell's equations with some assumptions.
Maxwell's equations always hold, if we assume if there is no quantum physics shenanigans.
KVL always hold, if we assume a few things. How to derive it, what to assume? Google it, or open your university books, because in every half decent university they teach this shit, and I am amazed that people still talk about this. I am not surprised if a physics professor in some US university would go ahead and "discover on his own" that KVL suddenly doesn't hold. What I'm amazed that they let this professor teach electronics to students, and had nobody around tell him that "Da! It is in the coursework for the engineers, haven't you read it?"

This is by far the best summary on this 'debate' I've read!   I got tired of pointing out the literature on the debated subject and be dismissed out of hand by un-educated blockheads.  Even Lewin at some point in one of his videos says that every book in the matter is wrong and that is a "crime against humanity" or something along those lines not to teach his BS.  Somehow Lewin believes that he is better than hundred (if not thousands) of experts in the field!

[bsfeechannel]

I got tired of pointing out the literature on the debated subject and be dismissed out of hand by un-educated blockheads.

You got tired because you don't understand the theory and you dismiss the evidence that contradicts your claims.

[jesuscf]

I got tired of pointing out the literature on the debated subject and be dismissed out of hand by un-educated blockheads.

You got tired because you don't understand the theory and you dismiss the evidence that contradicts your claims.

Says the person with no credentials that dropped this jewel:

"A transformer winding is just a series of loops connected in series. The electric field resides in the space between each turn."

[bsfeechannel]

Let me summarize the discussion for you:

I trust Mehdi. Lewin is full of BS and everyone that shows any practical evidence that Mehdi's claims are false and Lewin's claim correspond to reality is a blockhead.

Only irrational arguments. No scientific or engineering arguments and no evidence.

You contribute nothing to any discussion.

[jesuscf]

Let me summarize the discussion for you:

I trust Mehdi. Lewin is full of BS and everyone that shows any practical evidence that Mehdi's claims are false and Lewin's claim correspond to reality is a blockhead.

Only irrational arguments. No scientific or engineering arguments and no evidence.

You contribute nothing to any discussion.

Once again... Says the person with no credentials that dropped this jewel:

"A transformer winding is just a series of loops connected in series. The electric field resides in the space between each turn."

[Sredni]

Once again... Says the person with no credentials that dropped this jewel:

"A transformer winding is just a series of loops connected in series. The electric field resides in the space between each turn."

Well, but that is true.
You can read and learn, if you want.
http://web.mit.edu/6.013_book/www/chapter10/10.1.html

[jesuscf]

Once again... Says the person with no credentials that dropped this jewel:

"A transformer winding is just a series of loops connected in series. The electric field resides in the space between each turn."

Well, but that is true.
You can read and learn, if you want.
http://web.mit.edu/6.013_book/www/chapter10/10.1.html

So if there is only one loop, no electric field?

[Sredni]

You can read and learn, if you want.
http://web.mit.edu/6.013_book/www/chapter10/10.1.html

So if there is only one loop, no electric field?

Oh, dear... you did not even open the link, did you?
If there is only one loop the field is still in the space. This time is mostly between the terminals.
There literally is a picture of the field for a single turn coil at that link.

I should have written "You can learn, if you can".

[bdunham7]

Once again... Says the person with no credentials that dropped this jewel:

"A transformer winding is just a series of loops connected in series. The electric field resides in the space between each turn."

What is the issue with that?

[bsfeechannel]

The issue is that he thinks that the voltage produced at the terminals of a loop of wire under a varying magnetic field is located inside the wire.

He doesn't know that the voltage is produced in the gap between the terminals. Typical KVLer, who thinks there must be some kind of battery inside the wire pulling the charges along.

He also doesn't understand that a coil of wire is just a bunch of loops in series. Go figure.

[jesuscf]

So which one is the correct one?  This one?

He doesn't know that the voltage is produced in the gap between the terminals. Typical KVLer, who thinks there must be some kind of battery inside the wire pulling the charges along.

Or this one?

A transformer winding is just a series of loops connected in series. The electric field resides in the space between each turn.

[jesuscf]

Once again... Says the person with no credentials that dropped this jewel:

"A transformer winding is just a series of loops connected in series. The electric field resides in the space between each turn."

Well, but that is true.
You can read and learn, if you want.
http://web.mit.edu/6.013_book/www/chapter10/10.1.html

There are a lot of idealizations in the link provided: "Magnetoquasistatic Electric Fields in Systems of Perfect Conductors".  I hope one day we have perfect conductors!

[bsfeechannel]

So which one is the correct one?  This one?

He doesn't know that the voltage is produced in the gap between the terminals. Typical KVLer, who thinks there must be some kind of battery inside the wire pulling the charges along.

Or this one?

A transformer winding is just a series of loops connected in series. The electric field resides in the space between each turn.

Both are correct. When you have just one loop, the electric field resides between the terminals. When you overlap the terminals to start the second loop on top of the first, the electric field accommodates to reside between the turns, as I showed you here.

[bsfeechannel]

I hope one day we have perfect conductors!

Haven't you heard? It's called superconductors. And any model is made of ideal components.

[thinkfat]

That's a lot of stuff to digest. But one aspect seems obvious, no matter which "cult" you adhere to: In order to actually observe the path dependence of the voltage between A and D, you need to probe in a way that allows "selecting" the path you want to observe. Watching the MIT video that was linked here by @rfeecs, it is obvious that this selection is done only by the layout of the probe wires. If I understood the reasoning correctly, the selector is not the magnetic flux, because the paths that lead to the instrument are outside of it. What is also clear from the MIT video is that there is no dependence on how closely the probe wires follow the loop, any path on the "same side" as the voltage to be observed is OK. "Same side" seems to be "geometrically on the same side relative to the magnetic flux". That means the chosen path must never encompass any area that is inside the magnetic flux. As long as you stick to that, you're good.

Apart from that the explanation in the video is pretty clear, I think: The paths C1 and C2 are not encompassing the magnetic flux, so the line integral over all electric fields along C1 or C2 sum up to 0. The only observable voltage there is whatever the resistor in the path drops. That is true even when you argue that C1 and C2 are still subjected to the magnetic flux change, because they don't encompass it. If you look at the rotational electric field, any path you walk that doesn't go "around the center" of the field sums up to 0.

What I still have a hard time coming to terms with is that there is no voltage contribution by the other possible path through the second resistor, and no contribution from the loop wire itself. But I think the other video posted by @rfeecs (from Silicon Soup) shows that there is in fact a contribution by the other possible path but as this path also captures the EMF (because it encompasses the magnetic flux), the sum of all voltages is again only the voltage dropped by the resistor in the "chosen" path.

Now, why is there no voltage contribution by the wire itself, this is the most counter-intuitive aspect and the core of the claim that KVL doesn't hold. "Silicon Soup" explains that the rotational electric field caused by the changing magnetic flux and the electric field inside the wire caused by charge separation cancel each other out and the net electric field "in the wire" is 0. D'oh! So it's there but you cannot observe it because there's another electric field of same "strength" just in the "opposite direction". I put that in quotes not to express doubt but to denote that I understand that it's a simplification and we're talking about vectors here. But it is still hard to stomach because you can observe the effect of the charge separation through the voltage measurable at the terminals.

So, is it really "not there" or is it just difficult to find a path along which it would be observable? If there is no field, then what did Mabilde measure in his setup? Because his setup modeled after the McDonald paper shows 0.25V across a quarter segment of the loop, and that is obviously way to much for just Ohms law in action.

I'm still not done thinking this through but at this point I have an inkling that when I'm done, I will have to apologize to @bsfeechannel

What I'm still chewing on is the Variac example I brought up and the claim that there is an electric field in the wire if it's not stationary, because then it's Lorentz' force and not EMF that is causing the charge separation. An experiment I'd like to see is what happens when you have e.g. a rotating magnet instead of a solenoid, iow a generator instead of a transformer.

But it's not a problem to be wrong, just being stubborn.

PS: after some more headscratching, I think I understood why you cannot see any voltage across the loop wire: every path that involves the piece of wire between the resistors and that does not go around the center of the electric field will sum up to 0. Therefore, if you just naively connect you voltmeter probes to the points "A1" and "A2", that path will only observe the voltage across the wire according to Mr. Ohm. No need to argue with electric charge being counteracted by another electric field.

[Sredni]

That's a lot of stuff to digest. But one aspect seems obvious, no matter which "cult" you adhere to: In order to actually observe the path dependence of the voltage between A and D, you need to probe in a way that allows "selecting" the path you want to observe.

It's very simple. If the measurement loop formed by voltmeter, probes, and that branch DOES NOT enclose (cut) any (appreciable) variable magnetic flux then the electric fields obeys E = -grad V without the additional term dB/dt and can be seen as conservative. If we limit ourselves to planar circuits to make it easier to see, all paths in the area delimited by that measurement loop give the same value for the path integral of E, so voltage between any two points inside that area is the same no matter how you choose to join them (it only depends on the endpoints): along the measured branch, in the space between them, or along the probes through the voltmeter. I.E. the value shown by the voltmeter is also the voltage computed on the path along the branch, and even across it if you stay in the space that is magnetic-free.

Watching the MIT video that was linked here by @rfeecs, it is obvious that this selection is done only by the layout of the probe wires. If I understood the reasoning correctly, the selector is not the magnetic flux, because the paths that lead to the instrument are outside of it.

The selector is the presence or better the absence of magnetic flux inside the measurement loop.

That means the chosen path must never encompass any area that is inside the magnetic flux.

Yes, but you need to think in terms of closed paths. When you put one voltmeter on the outside of the ring, it will form two loops: one with the nearest resistor - that does not enclose the dB/dt region, the other with the farthest resistor - that does enclose the dB/dt region.
The voltages between two points in the region of space delimited by the first loop are all equal, no matter the path - hence the reading on the voltmeter corresponds to the correct voltage of the nearest branch.
The other measurement loop is not reading the correct voltage of the farthest branch, though, because it is affected by the presence of the variable magnetic field linked.
The beauty of it is that [correct voltage of far branch] + [emf with correct sign] = [voltage read by multimeter]

What I still have a hard time coming to terms with is that there is no voltage contribution by the other possible path through the second resistor

Oh, but there is. The current flowing in the near resistor - the current that gives rise to the voltage you read on the the voltmeter - would not be there if not for the second resistor.

Now, why is there no voltage contribution by the wire itself, this is the most counter-intuitive aspect and the core of the claim that KVL doesn't hold. "Silicon Soup" explains that the rotational electric field caused by the changing magnetic flux and the electric field inside the wire caused by charge separation cancel each other out and the net electric field "in the wire" is 0. D'oh! So it's there but you cannot observe it because there's another electric field of same "strength" just in the "opposite direction". I put that in quotes not to express doubt but to denote that I understand that it's a simplification and we're talking about vectors here. But it is still hard to stomach because you can observe the effect of the charge separation through the voltage measurable at the terminals.

It is exactly that charge separation that produces the coloumbian field that obliterates the induced field in the wire. If the wire is a perfect conductor, then the total electric field inside is zero. If the wire is a real conductor with high conductivity such as copper then there is a tiny electric field of the order of a handful of microvolts per meter that is compatible with the local form of Ohm's law.

So, is it really "not there" or is it just difficult to find a path along which it would be observable?

It's not there because the electric field has been cancelled (exactly in a perfect conductor, almost entirely in a good conductor.)

If there is no field, then what did Mabilde measure in his setup? Because his setup modeled after the McDonald paper shows 0.25V across a quarter segment of the loop, and that is obviously way to much for just Ohms law in action.

Mabilde has put its probes INSIDE the variable magnetic field region. He is cutting flux on purpose to induce a voltage in his measurement loop so that it can cancel the contribute of the induced electric field. This leaves only the contribute of the coloumbian field that is a conservative field. It's a nice technique, but he does not understand that he is measuring a partial contribute only. The field the electrons in the wire and the space experience is the total field

Etot = Eind + Ecoul

He is measuring the effects of Ecoul alone which admit a scalar potential phi. Which is fine if you realize that phi alone is not sufficient to completely describe the system. You also need the vector potential A, as McDonald shows.
What McDonald did was to apply the Helmoltz decomposition of fields to the total electric field and then associate the scalar potential phi to Ecoul and the vector magnetic potential A to Eind.
What the KVLers understood is only half of it. They stopped at phi and thought: "see? the potential is uniquely defined" without realizing that such potential is referred to a part and not all of the electric field.

In one of my answers on EESE I quote a paragraph of Popovic & Popovic where the actual expression of voltage in the presence of a variable magnetic field is given.

I'm still not done thinking this through but at this point I have an inkling that when I'm done, I will have to apologize to @bsfeechannel

From time to time you read, on Lewin's YT channel, the posts of someone apologizing to him after realizing he has been right the whole time.

an electric field in the wire if it's not stationary,

Get hold of Purcell. It has pictures for that.

And for the variac... we know the field is in the space between turns, and that is what you measure with a voltmeter on the outside. But if you have trouble computing that, why don't you use the other measuring loop, the one following the conductor around the core? You just have to apply Faraday and the concept that there is no appreciable voltage drop in the conductor: [voltage measured outside] = [negligible voltage drop in the turns tapped] + [emf with correct sign multiplied by number of turns]

That also explains why you cannot measure partial turn voltage, but only integer multiples.

(Edit: corrected a sentence to make it clear which area, which loop, which points)

[Sredni]

Once again... Says the person with no credentials that dropped this jewel:

"A transformer winding is just a series of loops connected in series. The electric field resides in the space between each turn."

Well, but that is true.
You can read and learn, if you want.
http://web.mit.edu/6.013_book/www/chapter10/10.1.html

There are a lot of idealizations in the link provided: "Magnetoquasistatic Electric Fields in Systems of Perfect Conductors".  I hope one day we have perfect conductors!

Magneto-quasistatics refers to the way electric and magnetic fields are changing. And it is the settings one uses for all non radiating circuits. Transformers operate in a magneto-quasistatic settings. Antennas do not.
That leaves us with the perfect conductors. Which are used everywhere in physics and electronics to simplify computations - such as... all wires in a circuit schematic are considered perfect conductors. Do you have superconducting ceramics at nearly 0 K in your cellphone charger? I don't think so, and yet I bet the SMPS has been modeled in a SPICE-like program assuming perfect conductors, and modelling losses via lumped resistors.

But, anyway... What do you think would change in the Lewin ring if, instead of a perfect conductor, you had a highly conductive copper conductor?
I tell you what: almost nothing. And certainly nothing of relevance. The only difference is that, instead of zero electric field and zero voltage drop in the conductors, you will see an almost negligible electric field E = j /sigma_copper and an almost negligible voltage drop of a handful of microvolts. Against the hundreds of millivolts of drops at the resistors.

[bsfeechannel]

"Same side" seems to be "geometrically on the same side relative to the magnetic flux". That means the chosen path must never encompass any area that is inside the magnetic flux. As long as you stick to that, you're good.

You got that right. It all boils down to geometry. Not electricity. Electricity works as usual.

I will have to apologize to @bsfeechannel

No worries.

[bsfeechannel]

OK, so let's imagine I connect my scope to the ends of the top wire (A1 and A2 IIRC) and I repeat the experiment with the full loop in place and then with the resistors disconnected so that I only have the wire with open ends.  Then I do the same routine with my magic electroscopes.  What do I see?

I need to understand what is really bugging you. But before I ask you some questions, allow me to suggest a different analogy.

Mehdi said in this episode of the AmpHour that someone gave him an explanation full of math so he dismissed it. The advantage of math is that it is rigorous. It doesn't admit contradictions, so you are sure your reasoning has no flaws. However math is not very intuitive, so I can understand him.

An analogy is intuitive, but not rigorous. So, let's start with an analogy to give us some insight about what is going on. Then we leave the math as an exercise to cover for any loopholes.

Picture yourself in a boat on a river. This river has an accelerating stream of water at constant rate, which produces a constant force throughout the stream.



Your boat experiences this force, but there's no friction so your boat can move perpendicularly to the stream without effort.

It doesn't matter if your boat chugs from D to A through path 1 or path 2. It'll experience the same force all along. You're path independent. Moreover, if you want to return from A to D, the force produced by the stream will take you there. No need to row your boat.

That's why we can say that point A has a positive potential in relation to D. Because if you are at point A you don't need to spend energy to get to point D. We can say that D has a negative potential in relation to A, because you'll have to overcome the force of the stream. If you go from point D to point A and return to point D again, the sum of the potentials will be zero.

Now your boat is in swirling accelerating waters. It is pretty obvious that if you go from point D to A trough path 1 you will get there effortlessly, but if you choose path 2 your up the creek and you'll need a paddle.



You're now path dependent. We could say that A has a positive "potential" in relation to D, because you can go from A to D through path 2 without spending energy. But, hey, D also has a positive "potential" in relation to A, because you can choose path 1 and get there for free. But if in each case you choose the other path, the potentials are negative. What is the "potential" between A and D? Undefined. Its path dependent.

So if you calculate the sum of the potentials, from D to A through path 1 and from A to D through path 2, it will give you a positive number. You can go anti-clockwise if you will and you'll get a negative number. But NEVER zero.

This has nothing to do with electricity. This is a property associated with any force field. That's why I said we take at least two years in an electronics engineering degree studying with the due rigor all the implications of this kind of thing.

Alright. Now replace the boat with an electric charge, and the moving water with an electric field. Nothing changes. Moving a charge in a circle or circuit of any shape through a conservative field will give us a net zero potential. That's KVL in all its glory.

Moving a charge through a rotational field around a complete circuit will give us a potential that's different than zero. And here is where we pronounce the words that is a blasphemy to any KVLer: KVL becomes bird seed.

In the conservative field, you'll need to spend energy along some path to get from D to A. In the rotational field you don't. Just take path 1 and you're good to go.

That's why in the conservative electric field, you'll need some kind of EMF along the path between D and A. But not in the case of the rotational field.

So you can have two resistors connected in a circuit without any kind of component to overcome the field.

Because in this case the electric field is already circuital.

So in this case where does the energy come from? It comes entirely from the electric and the varying magnetic fields. And here we have to resort to the Poynting vector.

[thinkfat]

"Same side" seems to be "geometrically on the same side relative to the magnetic flux". That means the chosen path must never encompass any area that is inside the magnetic flux. As long as you stick to that, you're good.

You got that right. It all boils down to geometry. Not electricity. Electricity works as usual.

I will have to apologize to @bsfeechannel

No worries.

[jesuscf]

And Lewin's supporters went the obfuscation way again!  Paragraphs and paragraphs of gibberish.  I almost felt for that again. Lets go back to the original problem at hand:  probing the circuit correctly as electroboom did shows that KVL works perfectly in Lewin's experiment.  It is that simple.