Author Topic: #562 – Electroboom!  (Read 149719 times)

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Offline jesuscf

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Re: #562 – Electroboom!
« Reply #150 on: November 12, 2021, 04:59:25 am »
So how comes that when you eliminate the influence of the varying magnetic field on the measuring instrument KVL works perfectly?  Here are the results from 'fromjesse' (from his garage I guess?):
I have already explained how fromjesse and Mabilde obtain that reading. They are measuring the contribute of the Ecoul field in their probes. And that contribute is equal to the contribute of Ecoul in the arc they measure.
A good setup to find a partial component of the actual voltage.

It looks to me that your are in denial.  Here, read the transcript (or watch the video) of the conclusion from the video from Bob DuHamel from the YouTube channel RSD Academy.  He can explain it way better than me:

Dr. Lewin is disagreeing with the vast majority of the scientific establishment he's disagreeing with the vast majority of textbooks he is disagreeing with the vast majority of professors of both electrical engineering and physics and his postulation that Kirchhoff's voltage law doesn't hold is based on a incorrect application of ohm's law not knowing how ohm's law has to be applied to a voltage source.

Then he performs an experiment to prove his premise but he doesn't take precautions to make sure that the magnetic fields don't affect his measurements.  In the end he says “look at the math proves I'm right” well the math is right.  He is right about the non-conservative fields and the conservative fields, and if we put a loop of wire in that non-conservative field that loop of wire gets a current and the electric fields in the wire are still non-conservative.  But as soon as we either make a gap in the loop or put some resistance in there now we have a backup of energy where that gap is or where the resistors are, where, we get now a measurable voltage, a measurable difference in potential where we are now going up in potential and then back down in potential and we are no longer non-conservative.  We are now conservative so we end up with a conservative circuit or conservative fields in the circuit even though they're embedded in a non-conservative field.
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Offline bsfeechannelTopic starter

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Re: #562 – Electroboom!
« Reply #151 on: November 12, 2021, 05:32:22 am »
There are tons of EM books that agree with Lewin on this matter. Purcell is one of them. He treats the ring explicitly. Haus and Melcher is another one. Faria is another. I can go on an on... There are video from professors of MIT, Purdue University, Cornell University that apply Faraday the way Lewin does, and that say that voltage is no longer unique in a nonconservative settings.
(And who do you have? A comedian and three guys in a garage?)

This does not add to the argument. It is just an observation, but it is interesting to note that the KVLers' champions are always academic and engineering underdogs. They have no paper, scientific or engineering, published by a reputable scientific or engineering publisher with a sound and consistent argument that KVL holds for varying magnetic fields. Just a bunch videos with contradictory statements, Lewin bashing and the usual request for subscriptions, likes, and membership money.
 

Offline bdunham7

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Re: #562 – Electroboom!
« Reply #152 on: November 12, 2021, 05:36:10 am »
So, will your method give zero voltage between two points on the surface of a conductor, in electrostatic conditions? Can you prove it?

I misunderstood your question, thinking you were referring to a single absolute measurement, not a relative measurement.  My brass balls can give both!

Thank you for asking for proof!  This prevented me from stating what I thought was an obvious answer.  Don't ask what that was, it isn't obvious anymore!

I'm going to have to think about that question.  My only thought so far is that if the result were not zero, I would have a mechanism for perpetually extracting energy from a conservative field.

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Offline bdunham7

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Re: #562 – Electroboom!
« Reply #153 on: November 12, 2021, 05:47:48 am »
This does not add to the argument. It is just an observation, but it is interesting to note that the KVLers' champions are always academic and engineering underdogs. They have no paper, scientific or engineering, published by a reputable scientific or engineering publisher with a sound and consistent argument that KVL holds for varying magnetic fields. Just a bunch videos with contradictory statements, Lewin bashing and the usual request for subscriptions, likes, and membership money.

I would like to note that the universe doesn't care about our discussion.  My first impression of the matter was that it was much ado about nothing and just one of those quirks that I don't fully understand but don't need to.  But here we are.  I still think it is much ado about nothing--perhaps no more than a notational convention--but perhaps I and others will learn some stuff in the process.  Perhaps one reason Lewin chose to present this the way he did was to fire up his bored students, perhaps he likes the attention--as seems the case. 
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Offline Sredni

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Re: #562 – Electroboom!
« Reply #154 on: November 12, 2021, 05:54:26 am »
Here, read the transcript (or watch the video) of the conclusion from the video from Bob DuHamel from the YouTube channel RSD Academy.  He can explain it way better than me:

Dr. Lewin is disagreeing with the vast majority of the scientific establishment he's disagreeing with the vast majority of textbooks he is disagreeing with the vast majority of professors of both electrical engineering and physics and his postulation that Kirchhoff's voltage law doesn't hold is based on a incorrect application of ohm's law not knowing how ohm's law has to be applied to a voltage source.

Then he performs an experiment to prove his premise but he doesn't take precautions to make sure that the magnetic fields don't affect his measurements.  In the end he says “look at the math proves I'm right” well the math is right.  He is right about the non-conservative fields and the conservative fields, and if we put a loop of wire in that non-conservative field that loop of wire gets a current and the electric fields in the wire are still non-conservative.  But as soon as we either make a gap in the loop or put some resistance in there now we have a backup of energy where that gap is or where the resistors are, where, we get now a measurable voltage, a measurable difference in potential where we are now going up in potential and then back down in potential and we are no longer non-conservative.  We are now conservative so we end up with a conservative circuit or conservative fields in the circuit even though they're embedded in a non-conservative field.


And that's a lot of... alternative truths. To say the least.
But you probably won't find many confutations in the comments section, because Duhamel, like fromjesse, delete the comments that tell them they are wrong and why. For example, in his last video "Voltage in a loop is weird", a post by Silicon Soup stating that by putting the probes inside the magnetic region a voltage was induced in the loop has disappeared.

Here's the reason:


https://i.postimg.cc/vZ6BM4Wn/screenshot-3.png

Not so sure what is a "protected man", but I have some idea.

This has a lot in common with cults: silence the critical voices and surround yourself with bootlickers.

(and you have no idea how many other videos on this topic Duhamel has put out, only to redo them to correct the... alternative truths there were pointed out in the comment section. Now some of those videos are no longer listed and so the comments pointing out the... alternatively accurate statements are no longer visible.
« Last Edit: November 12, 2021, 06:21:13 am by Sredni »
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Offline jesuscf

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Re: #562 – Electroboom!
« Reply #155 on: November 12, 2021, 05:57:34 am »
There are tons of EM books that agree with Lewin on this matter. Purcell is one of them. He treats the ring explicitly. Haus and Melcher is another one. Faria is another. I can go on an on... There are video from professors of MIT, Purdue University, Cornell University that apply Faraday the way Lewin does, and that say that voltage is no longer unique in a nonconservative settings.
(And who do you have? A comedian and three guys in a garage?)

This does not add to the argument. It is just an observation, but it is interesting to note that the KVLers' champions are always academic and engineering underdogs. They have no paper, scientific or engineering, published by a reputable scientific or engineering publisher with a sound and consistent argument that KVL holds for varying magnetic fields. Just a bunch videos with contradictory statements, Lewin bashing and the usual request for subscriptions, likes, and membership money.

Suuure!  Tell that to this measurements from 'fromjesse' and see how they change based in your BS statement above:

Left resistor: -61mV
Right resistor: -660mV

Total voltage dropped: -721mV

Top copper: 378mV
Bottom copper 350mV

Total voltage induced 728 mV

Total sum: 7 mV (which is within the tolerance of the measuring instrument)

Also bsfeechannel, what are your credentials?  Are you an electrical engineer or not?  All of a sudden it became important to you to have a reputation to talk about the subject.

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Offline jesuscf

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Re: #562 – Electroboom!
« Reply #156 on: November 12, 2021, 06:07:11 am »
And that a lot of... alternative truths. To say the least.
But you probably won't find many confutations in the comments section, because Duhamel, as fromjesse delete the comments that tell them they are wrong and why. For example, in his last video "Voltage in a loop is weird", a post by Silicon Soup stating that by putting the probes inside the magnetic region a voltage was induced in the loop has disappeared.

You can make out all the conspiratorial BS you want.  The results are there, KVL works perfectly when the circuit is in variying magnetic field and you measure correctly, as shown by 'fromjesse' experiment:

Left resistor: -61mV
Right resistor: -660mV

Total voltage dropped: -721mV

Top copper: 378mV
Bottom copper 350mV

Total voltage induced 728 mV

Total sum: 7 mV (which is within the tolerance of the measuring instrument)
Homer: Kids, there's three ways to do things; the right way, the wrong way and the Max Power way!
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Homer: Yeah, but faster!
 

Offline bsfeechannelTopic starter

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Re: #562 – Electroboom!
« Reply #157 on: November 12, 2021, 06:08:20 am »
I would like to note that the universe doesn't care about our discussion. 

Why would it?

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My first impression of the matter was that it was much ado about nothing and just one of those quirks that I don't fully understand but don't need to.

For most people, it is irrelevant. No doubt.

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But here we are.  I still think it is much ado about nothing--perhaps no more than a notational convention

It's more serious than that. It's about pseudo-scientific claims.

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--but perhaps I and others will learn some stuff in the process. 

You have no idea. I learned a ton with all of this.

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Perhaps one reason Lewin chose to present this the way he did was to fire up his bored students, perhaps he likes the attention--as seems the case.

Being a scientist, he obviously cares about rigor. But he is right. We engineers tend to cut corners, rely on rules of thumb, and make approximations. Sometimes we even give credit do certain myths. But it's a good thing to be reminded of the true nature of reality.
 

Offline bsfeechannelTopic starter

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Re: #562 – Electroboom!
« Reply #158 on: November 12, 2021, 06:30:25 am »
Dr. Lewin is disagreeing with the vast majority of the scientific establishment he's disagreeing with the vast majority of textbooks he is disagreeing with the vast majority of professors of both electrical engineering and physics and his postulation that Kirchhoff's voltage law doesn't hold is based on a incorrect application of ohm's law not knowing how ohm's law has to be applied to a voltage source.

Is there some objective data to substantiate this claim? I guess not. But it is interesting to note that guys like Niels Bohr and Richard Feynman disagreed with the vast majority of textbooks and professors of their time.

And they were right.

Quote
And that a lot of... alternative truths. To say the least.

You mean lies? Here's is one. He said that some say that the voltage in a loop of wire closed by a resistor under a varying magnetic field is in the resistor only. He said that this is philosophy and that no one can prove it. Not only the theory predicts that, but also you can hook up a meter across the resistor and measure its voltage, and across the wire and measure zero volts. The voltage is only across the resistor. No metaphysics. Only physics.
 

Offline bdunham7

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Re: #562 – Electroboom!
« Reply #159 on: November 12, 2021, 06:32:00 am »
It's more serious than that. It's about pseudo-scientific claims.

The noise-to-signal ratio is pretty high, but the perpendicular probing proponents are on to something IMO, although many of them may not know what it is.  And in trying to answer Sredni's last question, it may be slowly dawning on me how to show how the non-conservative example is different.  Hopefully not in a convoluted, roundabout way.
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Offline bdunham7

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Re: #562 – Electroboom!
« Reply #160 on: November 12, 2021, 06:41:47 am »
you can hook up a meter across the resistor and measure its voltage, and across the wire and measure zero volts. The voltage is only across the resistor. No metaphysics. Only physics.

Or 'bad probing'.  :-DD

But seriously, you can also hook up a meter and measure voltage on both.

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Offline bdunham7

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Re: #562 – Electroboom!
« Reply #161 on: November 12, 2021, 06:55:45 am »
In the KVLer approach, the probes form a loop drenched in the varying magnetic field. Funny that you mentioned a three dimensional world but you failed to consider the three dimensional loop that you configured with that arrangement.

I believe they choose their loop so that it bisects the solenoid so that there is no net flux.  Or at least that is what is attempted and claimed.

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I was using YOUR definition of voltage, which really fails in a non conservative field, you're right.

Fails at what?  The two definitions are irreconcilable as far as I can see in this specific instance.  That's the issue.
« Last Edit: November 12, 2021, 06:59:06 am by bdunham7 »
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Offline jesuscf

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Re: #562 – Electroboom!
« Reply #162 on: November 12, 2021, 07:03:29 am »
Not only the theory predicts that, but also you can hook up a meter across the resistor and measure its voltage, and across the wire and measure zero volts. The voltage is only across the resistor. No metaphysics. Only physics.

Wow, really! :palm: You hadn't learn anything after 7 pages of comments!   If your statement is true ("the voltage across the wire is zero volts"), please explain why the voltage across the copper strip in the picture below is 184mv.  Notice that the 'hands' of the Lewin clock are at angle of about 90 degrees; remember that the 'bottom copper' voltage is 350mV.  That is from the video from 'fromjesse' I linked in the previous messages.  By the way, that is exactly the same mistake Lewin makes, because under the influence of a varying magnetic field, a real piece of wire behaves as non-ideal voltage source.


« Last Edit: November 12, 2021, 07:07:07 am by jesuscf »
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Offline jesuscf

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Re: #562 – Electroboom!
« Reply #163 on: November 12, 2021, 07:13:32 am »
In the KVLer approach, the probes form a loop drenched in the varying magnetic field. Funny that you mentioned a three dimensional world but you failed to consider the three dimensional loop that you configured with that arrangement.
I believe they choose their loop so that it bisects the solenoid so that there is no net flux.  Or at least that is what is attempted and claimed.

bdunham7 you are technically correct.  The best kind of correct!!!
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Offline thinkfat

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Re: #562 – Electroboom!
« Reply #164 on: November 12, 2021, 08:07:37 am »
Humor me, guys. See the attached picture. All wires are actual wires and interact with fields, but have negligible resistance. R1 can be any value, I don't think it matters. Assume EMF=1V. Predict the voltages shown on V1 and V2, using KVL and Faradays Law.

V1 is zero, V2 is 1V. (didn't check the sign just saw the thumbnail), might as well be -1V
It's easy if you check if your measurement loop (voltmeter + probes + branch you want to know the voltage of) contains or does not contain the flux.

I'd say vice versa, because the path through V2 is not in the magnetic flux and the branch voltage is zero, so I'd think V2=0 and V1=1V (or -1V, I didn't check the field orientation either). If I'm wrong, I'd really like to hear your explanation.

Quote
Now I have a question for you.
Suppose I place a loop made with one resistor, the voltmeter, and its probes, near the magnetic flux region. Would you spend even a microsecond of your time to check that the induced field is being compensated? Or would you rather go: "nah, there is no flux in my measurement loop, I can apply KVL, no problem!"

Hm, I'd probably still picture the electric field vortex, because for me that's easier to visualize. But no I'd not spend any time actually checking, I'd know that if the field axis is outside of my loop area, the electric fields must sum up to 0. To me the electric field is somehow more "intuitive" because that's what the charge in the loop is interacting with.

Which is another thing I think I understood now: charge particles that are inert referenced to the magnetic flux are not interacting with the magnetic field, even if it's changing. Charge particles in motion (e.g. when you move the wire) interact with the magnetic field and experience Lorentz' force.

Quote
EDIT: Yep, the transformer trick would do. IIRC Big Clive used the whole secondary of a transformer to step down the voltage. From 240-->12V to 240 ---> 240-12V or something like that. Caveat: the current must be sustained by both windings.

In principle we could use the radial probing to extract a finely varying voltage from a solenoidal coil immersed in a magnetic field. Instead of going one turn at the time, we can have a continuous (well, only marred by asperities in contact) voltage from start to finish. Mechanical nightmare, of course.

I feel the need to add something to the discussion of the actual matter, Dr. Lewins experiment.

I believe what many of us participating in the discussion initially didn't understand, me explicitly included, is that Dr. Lewins experiment must be seen as a whole, including his choice of probing the voltages. The whole idea about formulating a "law" in science is to be able to explain and predict the outcome of an experiment, and as I already said but probably nobody really took notice, the test instrument setup including geometrical arrangement of probe wires is inevitably part of the experiment. Dr. Lewin proposed a circuit and the challenge was to explain the measurements, and quite honestly, you cannot do that with Kirchhoff. It doesn't work.

What Mabilde and "fromjesse" did was change the experiment until they could explain it with KVL. That's like, when you get some task assigned in a test and you cannot solve it, you change the task until you can. I don't think that'll earn you any points.

Mehdi came so close to the truth with his "+ V/2 - V/2" construct, but he was so fixated on saving Kirchhoff that he forgot what the task was.

For me that concludes the discussion. I've learned a lot in the course of it and I have a new understanding of electromagentism through it. I'll go back to the sidelines now for a while.
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Offline jesuscf

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Re: #562 – Electroboom!
« Reply #165 on: November 12, 2021, 08:54:55 am »
What Mabilde and "fromjesse" did was change the experiment until they could explain it with KVL. That's like, when you get some task assigned in a test and you cannot solve it, you change the task until you can. I don't think that'll earn you any points.

Nope!  I am pretty sure that Mabilde, fromjesse, electroboom, and many other people, including myself, saw Lewin's experiment and immediately thought "bad probing"!  Why?  Because all of us have been there before, many, many times.  If you are not careful setting your measuring equipment you'll pick-up all kinds of unwanted stuff.  You quickly learn that oscilloscopes and their proves are very good at that!

EDIT: in fromjesse experiment there is a moment in which he notices some unwanted spikes in the signal he is measuring.  Then he immediately turns off a light and presto:  the spikes are gone!!!   Raise your had if that has happened to you!
« Last Edit: December 02, 2021, 07:56:18 am by jesuscf »
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Offline thinkfat

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Re: #562 – Electroboom!
« Reply #166 on: November 12, 2021, 09:49:21 am »
What Mabilde and "fromjesse" did was change the experiment until they could explain it with KVL. That's like, when you get some task assigned in a test and you cannot solve it, you change the task until you can. I don't think that'll earn you any points.

Nope!  I am pretty sure that Mabilde, fromjesse, electroboom, and many other people, including myself, saw Lewin's experiment and immediately thought "bad proving"!  Why?  Because all of us have been there before, many, many times.  If you are not careful setting your measuring equipment you'll pick-up all kinds of unwanted stuff.  You quickly learn that oscilloscopes and their proves are very good at that!

If this is the mindset you're stuck with, that's not going to lead anywhere. It's not bad probing, the choice of probing was absolutely deliberate. The probes wires are part of the experiment. If you look at how Mabilde and also Mehdi are meticulously routing the probe wires so that they "hug" the inner loop because they think that's how they pick up "stray magnetic flux" or something, it's absolutely hilarious. They believe they have to, in order to reproduce Dr. Lewins "impossible" measurements. But in reality, you can route the probe wires anywhere you want, provided they don't enclose a non-conservative region. Also Mehdis careful twisting of the probe wires is superfluous, it doesn't change the outcome even slightly. It is not possible to understand the measurements if you don't look at the geometry of both the fields and the path described by the probe wires and where they attach.

For me the eyeopeners were the two videos linked by @rfeecs, the one by "Silicon Soup" that made me understand the rotational e-field, and the MIT courseware video where they basically showed Dr. Lewins experiment with a slightly different setup, using a toroidal core with one winding driven by a generator (like Mabilde) and just two resistors soldered together at their contact wires reaching around the core. Observe how carelessly they handle the probe wires going back to the oscilloscope, and how they explain the results on paper using C1, C2, CC as path designators.
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Offline bsfeechannelTopic starter

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Re: #562 – Electroboom!
« Reply #167 on: November 12, 2021, 02:08:46 pm »
Or 'bad probing'.  :-DD

But seriously, you can also hook up a meter and measure voltage on both.


Remember when I told you that engineers take classes in vector calculus? It is so as not to incur in errors in their reasoning when analyzing some phenomenon. Your configuration is measuring only the voltage across the resistor, exactly what you'd measure if your meter and probes were in the plane. In fact any position of the meter and probes in red in the picture below will do that. This is because the area they define together with the resistor is not traversed by the lines of the varying magnetic field. Faraday's law. KVL won't tell you that.

« Last Edit: November 12, 2021, 02:16:49 pm by bsfeechannel »
 

Offline bdunham7

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Re: #562 – Electroboom!
« Reply #168 on: November 12, 2021, 02:41:33 pm »
Forget nonconservative fields. Put your rod in a conservative field, an electrostatic field. It will 'charge up' by induction. You will have opposite charge at the extremes. Now measure your absolute voltage with your method. It should be zero.
Is it zero?

It should measure zero, simply because it needs to.  I have not come up with an elegant, direct proof, but I think I have one that is easy enough to understand.



I realized the field lines may not be exactly as I've drawn, but whatever they are, if I provide a path from each charged end out of the field along a path that is always perpendicular to the field lines and then connect them to a voltmeter and resistor, I can see if a current flows in the resistor.  If it does, I can then extract energy from a static, conservative and irrotational E-field, which violates conservation of energy, among other laws.  IOW, it just isn't done! 

I can do the same with my brass balls and I would expect the same result.  It takes no work to move them on a path that only cuts the field lines at right angles, so if they get charged up in the process, I could do it repeatedly and again, extract energy from a system that we know we can't get energy from.  Another way of looking at it is that the charges will have no force causing them to go from one point to another because the fields are balanced, (Eind + Ecoul = 0) but I'm unable to prove that more directly, especially in the universal case as you demanded. 

Now in the case of the straight wire tangent to a rotational, non-conservative field, things change.  Specifically, we know experimentally that we can take conductors (or brass balls, but I haven't proven that experimentally) out of the field and then extract energy from the system as long as the rotational E-field continues.  We keep hearing that things are different in a non-conservative field, well that is one of them.  If you look at my drawing in my last reply to bsfeechannel, you'll see what I mean--the loop is not 'closed' until you are far enough away that the field is negligible, so the act of closing it only allows current to flow.  If you had the ideal voltmeter and thus did not close the loop, you would still see the voltage.

In conclusion I guess I'm saying that the rules are different for conductors in static, irrotational, conservative fields than they are in dynamic, rotational, non-conservative ones--which we all knew already, right?   :)   Exactly how that manifests itself is something that perhaps someone else can expound on, but the basic facts have been shown experimentally.  The rule that a conductor is an equipotential in a the static, irrotational, conservative field may not apply here and adopting the 'integral loop' rule seems to me to be a crutch for a failed model.  The rotational E-field exerts its force locally and tangentially and the rest of the story is just the process of adding it all up.  The 'closed loop' requirement in general is a red herring and demonstrably false--I could, in theory, use multiple torii and construct a linear accelerator by lining up the relatively straight inner sections of the rotational E-field for as long as I like.  Sorry, the drawing isn't very good....



Edit:  "Linear accelerator" is a flawed statement because I had a conductor in mind, not a loose charged particle.  I'll try to refine what I'm trying to say when I get time.  But the point will be that I can develop an observable, usable and perhaps measurable phenomenon at the ends of the straight conductor without ever contemplating any loop at all, something that can't be done in a static, irrotational conservative field.
« Last Edit: November 12, 2021, 03:34:28 pm by bdunham7 »
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Offline bdunham7

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Re: #562 – Electroboom!
« Reply #169 on: November 12, 2021, 02:52:57 pm »
Remember when I told you that engineers take classes in vector calculus? It is so as not to incur in errors in their reasoning when analyzing some phenomenon. Your configuration is measuring only the voltage across the resistor, exactly what you'd measure if your meter and probes were in the plane. In fact any position of the meter and probes in red in the picture below will do that. This is because the area they define together with the resistor is not traversed by the lines of the varying magnetic field. Faraday's law. KVL won't tell you that.

I don't know why, at this point in the discussion, you think this is a revelation to me.  I deliberately drew that diagram to illustrate that the position of the voltmeter doesn't matter, as you've drawn.  I don't need Faraday's Law or KVL to tell that, all I need to know is that I'm out of the region of flux and thus there are no rotational E-fields to become tangential to my wires and push the charges around.  That's the whole point of extending out the partial turn.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline bsfeechannelTopic starter

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Re: #562 – Electroboom!
« Reply #170 on: November 12, 2021, 03:08:47 pm »
I believe they choose their loop so that it bisects the solenoid so that there is no net flux.  Or at least that is what is attempted and claimed.

Do the math. You'll see that there is no way to position the probes if their projection crosses the area of the varying magnetic field that can cancel its effects. The only way to do this was shown by Mehdi himself and it is to twist half of the loop so that the line integral of the electric field in one of the halves will cancel out the line integral of the other half.

In which case your meter will measure zero. Duh!

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I was using YOUR definition of voltage, which really fails in a non conservative field, you're right.

Fails at what?  The two definitions are irreconcilable as far as I can see in this specific instance.  That's the issue.

Nope. YOUR definition of voltage is just a special case of the more general definition of voltage which is the line integral of the electric field along a certain path, be it the field conservative or non-conservative.

They're not irreconcilable.
« Last Edit: November 12, 2021, 03:11:53 pm by bsfeechannel »
 

Offline Sredni

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Re: #562 – Electroboom!
« Reply #171 on: November 12, 2021, 03:09:15 pm »
Humor me, guys. See the attached picture. All wires are actual wires and interact with fields, but have negligible resistance. R1 can be any value, I don't think it matters. Assume EMF=1V. Predict the voltages shown on V1 and V2, using KVL and Faradays Law.
V1 is zero, V2 is 1V. (didn't check the sign just saw the thumbnail), might as well be -1V
It's easy if you check if your measurement loop (voltmeter + probes + branch you want to know the voltage of) contains or does not contain the flux.
I'd say vice versa, because the path through V2 is not in the magnetic flux and the branch voltage is zero, so I'd think V2=0 and V1=1V (or -1V, I didn't check the field orientation either). If I'm wrong, I'd really like to hear your explanation.

The explanation is that I need glasses. I only looked at the thumbnail and assumed you had numbered the voltmeters from left to right.  You are correct, and you have understood how to solve this kind of problems.
Here's a graphical depiction of the key, with an open circuit in lieu of the resistor


Source: https://i.postimg.cc/5yLyGRy0/Voltage-along-a-single-turn-coil.jpg

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Now I have a question for you.
Suppose I place a loop made with one resistor, the voltmeter, and its probes, near the magnetic flux region. Would you spend even a microsecond of your time to check that the induced field is being compensated? Or would you rather go: "nah, there is no flux in my measurement loop, I can apply KVL, no problem!"
Hm, I'd probably still picture the electric field vortex, because for me that's easier to visualize. But no I'd not spend any time actually checking, I'd know that if the field axis is outside of my loop area, the electric fields must sum up to 0. To me the electric field is somehow more "intuitive" because that's what the charge in the loop is interacting with.

Not sure how you will find an axis with more complex configurations of the fields. My suggestion is to stick with the flux: does it cut the surface bounded by your measurement loop? If yes, then kiss KVL goodbye.

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I believe what many of us participating in the discussion initially didn't understand, me explicitly included, is that Dr. Lewins experiment must be seen as a whole, including his choice of probing the voltages. The whole idea about formulating a "law" in science is to be able to explain and predict the outcome of an experiment, and as I already said but probably nobody really took notice, the test instrument setup including geometrical arrangement of probe wires is inevitably part of the experiment.

well, yes and no. When there is no flux cut, the choice of the probes is irrelevant. But you realized that.
What one sees by looking at the fields is that, if we limit ourselves to the only two possible paths in the circuit joining the points A and B on the diameter (don't know what Lewin calls these points, maybe A and D) it is clear that the path integrals along these two different paths HAVE TO BE different.


source: https://electronics.stackexchange.com/questions/551244/what-would-a-voltmeter-measure-if-you-had-an-electromotive-force-generated-by-a/551428#551428

(I don't understand why the images from postimage.cc do not show up inline, like this one).

Of course you can put voltmeters on the sides, but only the voltmeter forming a measurement loop that does not cut the flux will measure the correct voltage. If voltmeter and resistor run aroung the mag zone, then the voltage shown will be altered by the intercepted EMF.


Source: https://i.postimg.cc/QtVW6mc9/Two-measuring-loops-one-consistent-result.jpg


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For me that concludes the discussion. I've learned a lot in the course of it and I have a new understanding of electromagentism through it. I'll go back to the sidelines now for a while.

Welcome to the dark side.
All instruments lie. Usually on the bench.
 

Offline Sredni

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Re: #562 – Electroboom!
« Reply #172 on: November 12, 2021, 03:21:07 pm »
Remember when I told you that engineers take classes in vector calculus? It is so as not to incur in errors in their reasoning when analyzing some phenomenon. Your configuration is measuring only the voltage across the resistor, exactly what you'd measure if your meter and probes were in the plane. In fact any position of the meter and probes in red in the picture below will do that. This is because the area they define together with the resistor is not traversed by the lines of the varying magnetic field. Faraday's law. KVL won't tell you that.

I don't know why, at this point in the discussion, you think this is a revelation to me.  I deliberately drew that diagram to illustrate that the position of the voltmeter doesn't matter, as you've drawn.  I don't need Faraday's Law or KVL to tell that, all I need to know is that I'm out of the region of flux and thus there are no rotational E-fields to become tangential to my wires and push the charges around.  That's the whole point of extending out the partial turn.

What do you think the voltmeter would read, in that picture?
All instruments lie. Usually on the bench.
 

Offline bsfeechannelTopic starter

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Re: #562 – Electroboom!
« Reply #173 on: November 12, 2021, 03:27:10 pm »
I don't know why, at this point in the discussion, you think this is a revelation to me.  I deliberately drew that diagram to illustrate that the position of the voltmeter doesn't matter, as you've drawn

But you said that you are measuring the voltage across both the wire and the resistor with that configuration. Or did get it wrong?

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I don't need Faraday's Law or KVL to tell that, all I need to know is that I'm out of the region of flux and thus there are no rotational E-fields to become tangential to my wires and push the charges around. 

Unless you discovered that independently, it is generally attributed to Michael Faraday the discovery of the phenomenon you've just described above.
 

Offline bdunham7

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Re: #562 – Electroboom!
« Reply #174 on: November 12, 2021, 03:40:40 pm »
Nope. YOUR definition of voltage is just a special case of the more general definition of voltage which is the line integral of the electric field along a certain path, be it the field conservative or non-conservative.

They're not irreconcilable.

By irreconcilable I mean that they aren't going to be the same number in a non-conservative field.  You say "be it conservative or non-conservative", but I think perhaps the Magneto-Quasi-Static idea has fooled you into thinking that the concepts from conservative fields can be applied to non-conservative fields as long as they hold steady for some period of time.  I'm pretty sure that is untrue, but I'm struggling to demonstrate that.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 


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