Author Topic: Textbook about Inductance  (Read 2183 times)

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Offline aneevuser

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Re: Textbook about Inductance
« Reply #25 on: April 15, 2020, 12:11:48 pm »
It may be helpful for you to work through Vol. 2 of the Feynman Lectures:

https://www.feynmanlectures.caltech.edu/II_toc.html

It's a very thorough and comprehensible treatment of EM theory, and is pretty easy reading to my mind - Feynman does a good job of handholding from the elementary stuff up to the pretty advanced. The only potential downside is that it's a very mathematical treatment but Feynman also develops a lot of the necessary vector calculus from scratch in the early chapters - OTOH if you're already familiar with div, grad, curl, and line, surface, volume integrals, it should be just up your street.
 

Offline pwlps

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Re: Textbook about Inductance
« Reply #26 on: April 15, 2020, 10:24:08 pm »
Not to sidetrack the discussion, but I watched this video which is also about electromagnetic induction and found he experiment from 48:20 or so is very eye-opening and thought-provoking.

Not only other professors referred to by professor Lewin, I deeply double the experiment (though, of course, I don't think he is cheating; and my doubt or difficulty in understanding is not a surprise). But I think he may have been misled by some physical setup of the experiment. How arrogant I sound!

There is nothing special or magic in this experiment. Look at the equivalent circuit I made and you will understand better.

Edit
Actually he did cheat a little because the circuit drawn on the blackboard does not show where the flux goes and where exactly the voltmeters are connected. I agree that his drawing was misleading, maybe deliberately to stimulate students to think about it.
« Last Edit: April 15, 2020, 10:44:02 pm by pwlps »
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #27 on: April 16, 2020, 05:05:50 am »
Ah sorry for this bad joke (but I couldn't resist :)), of course I have great admiration for EE too. I remember when trying to understand some RF devices I was wondering how they can have such a great insight into the working principles without the use of advanced mathematical calculations.

Haha no need to apologize! No one would doubt that's only a joke. :) And the smart and strong EEs don't need me to defend.

Quote
As already pointed out by T3sl4co1l there is always a loop and pure straight wires don't exist.
That said straight infinite wire concept can be a good approximation in some circumstances. Take a loop of very thin wire (wire radius much smaller than loop dimensions). Then the majority of the flux comes from the field very close to the wire which varies as 1/r so that it can be approximated by a straight wire: as the contributions of remote parts of the loop are negligible we can integrate to infinity getting an approximate inductance per unit length proportional to log(1/radius). For intermediate cases there are approximate formulas combining together different regimes, see e.g.

https://en.wikipedia.org/wiki/Inductance#Inductance_of_a_straight_single_wire

Thanks. I was aware of the Wiki practical formula but I didn't like it. I wanted an accurate theoretical one.

Quote
Didn't have time to look at the video yet, maybe this evening.

No obligations! It's too big an ask for an hour's time!
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #28 on: April 16, 2020, 05:12:12 am »
I don't think you can trust this book.

Sadly it seems I have to agree with you. I quite liked this book before this. And I had a blind-eye faith on books with some good age.

Quote
The first equation is not Ampere's Law. Ampere's Law is a relationship between the line integral of the magnetic field around a closed loop surrounding a conductor, and the current in the conductor.

I found a similar equation in my university physics textbook (University Physics, 5th Edition, by Sears, Zemansky & Young) on page 552. It's for the field at a point on the centre line through a circular loop of wire; r is the distance from a point on the loop to the measurement point, and theta is the angle between this line and the centre line.

Would it breach a law terribly if flicking one page for discussion? Thanks.

Quote
I can't make any sense of the second equation. If it's supposed to be for the total flux through the loop, then there shouldn't be an r or theta in it at all.

This makes me feel much better. Thanks.
« Last Edit: April 16, 2020, 05:37:46 am by max.wwwang »
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #29 on: April 16, 2020, 05:19:00 am »
It may be helpful for you to work through Vol. 2 of the Feynman Lectures:

https://www.feynmanlectures.caltech.edu/II_toc.html

It's a very thorough and comprehensible treatment of EM theory, and is pretty easy reading to my mind - Feynman does a good job of handholding from the elementary stuff up to the pretty advanced. The only potential downside is that it's a very mathematical treatment but Feynman also develops a lot of the necessary vector calculus from scratch in the early chapters - OTOH if you're already familiar with div, grad, curl, and line, surface, volume integrals, it should be just up your street.

This is some good stuff we can trust! Thanks. Ages ago I learned things like div, grad, and these sorts of integrals but I need some good head-scratches to pick them up! There used to be a Surely You’re Joking, Mr. Feynman! on the bookshelf but don't know where it is now.
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #30 on: April 16, 2020, 05:32:23 am »
There is nothing special or magic in this experiment. Look at the equivalent circuit I made and you will understand better.

Edit
Actually he did cheat a little because the circuit drawn on the blackboard does not show where the flux goes and where exactly the voltmeters are connected. I agree that his drawing was misleading, maybe deliberately to stimulate students to think about it.

Thanks for this but I'm not quite following. I wandered the same thing and posted a comment below the video wanting to know the 'physical' (not electrical) setup of the experiment. I got a long response to my comments, which I haven't followed up.

Some of my thoughts --

1) In this single (closed) loop, the place to attach the probe(voltmeter) matters. There are no longer things like equiv-potential points there any longer. Imagine a loop without resistors (or any components other than wire), there'll be a voltage drop between any two points along the wire, however close they might be. Superconductor does not work here because that would result in a current of infinity with any amount of EMF;

2) Don't be fooled by the word (and idea of) "measure". Attaching a voltmeter with wires is no different from adding another component (and one more loop) in the circuit. The only difference between the meter and a resistor is it's smart enough to tell you the potential difference across it. Since the geometry (including position) of the loop matters in this case, it of course also matters where to attach the meters!

Responding to my comments below the video, the professor mentioned the "insult" that was given to him by somebody referring to "bad probing". I sort of agree with this comment (on the problem of probing, or 'measurement' if in my term) though itself is tongue-in-cheek (when in this case crystal clarity is of grave importance).

[Edit] Having watched ElectroBOOM's video (see #37), I should say ElectroBOOM has made it crystal clear (by both experiment and analysis). It's not so clear only because of the brevity of reference (not an error).
« Last Edit: April 18, 2020, 12:18:18 am by max.wwwang »
 

Offline pwlps

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Re: Textbook about Inductance
« Reply #31 on: April 16, 2020, 02:40:12 pm »
There is nothing special or magic in this experiment. Look at the equivalent circuit I made and you will understand better.

Edit
Actually he did cheat a little because the circuit drawn on the blackboard does not show where the flux goes and where exactly the voltmeters are connected. I agree that his drawing was misleading, maybe deliberately to stimulate students to think about it.

Thanks for this but I'm not quite following. I wandered the same thing and posted a comment below the video wanting to know the 'physical' (not electrical) setup of the experiment. I got a long response to my comments, which I haven't followed up.

I did not search for your comment and answer but if you copy it here it might be interesting and might help to understand his lines of thought.

Quote
Some of my thoughts --

1) In this single (closed) loop, the place to attach the probe(voltmeter) matters. There are no longer things like equiv-potential points there any longer. Imagine a loop without resistors (or any components other than wire), there'll be a voltage drop between any two points along the wire, however close they might be.

Yes, you can put it simply (in my opinion)  considering  that the whole circuit is immersed in a varying  electric field (generated by rotE=-dB/dt).  The field outside an infinite solenoid is circular (not sure for the proper term, I mean only tangential component) therefore non-conservative for any path going around the solenoid (it would still be conservative for other paths). Since rotE=0 (there is no B there) it has to decrease as 1/r (solution of the Maxwell equation rotE=-dB/dt=0 with cylindrical symmetry and using the boundary conditions on the solenoid surface obtained from the solution inside the solenoid where dB/dt is uniform and E~r).

Quote
Superconductor does not work here because that would result in a current of infinity with any amount of EMF;

No, the current wouldn't be infinite, it would just be enough to cancel the total magnetic flux across the superconducting loop. This also happens when you short the output of a transformer : at high enough frequency when the winding resistance can be neglected the current in the secondary winding will be such as to cancel the magnetic flux (the transformer then works as a "current transformer"). The only difference between a very good conductor and a superconductor is that such flux-cancelling current would also be observed for a DC magnetic field (see Meissner effect and London equations, but this is far beyond the actual problem). Note that such superconducting DC current transformers are used e.g. in SQUID magnetometers, they are called "flux transformers".

Quote
2) Don't be fooled by the word (and idea of) "measure". Attaching a voltmeter with wires is no different from adding another component (and one more loop) in the circuit. The only difference between the meter and a resistor is it's smart enough to tell you the potential difference across it. Since the geometry (including position) of the loop matters in this case, it of course also matters where to attach the meters!

Yes, but I still believe you can relatively well model such a circuit with discrete components, using transformers. In my schematics I supposed that voltmeters are connected very close to R1,R2 so that the magnetic flux across loops comprised of voltmeter connection wires can be neglected (usually we use twisted pairs to minimise magnetic flux in order to avoid this kind of error measurements).
Below is another variant, much closer to the schematics drawn on the blackboard. Note that L4 and L5 cannot be much smaller than L1 and L3 (if the circuit is to be physically realisable).

Quote
Responding to my comments below the video, the professor mentioned the "insult" that was given to him by somebody referring to "bad probing". I sort of agree with this comment (on the problem of probing, or 'measurement' if in my term) though itself is tongue-in-cheek (when in this case crystal clarity is of grave importance).

We can't judge on the quality of probing as long as we don't know the exact physical layout of all wires (if he refuses to show it I'm suspicious :)). If the wires are exactly as drawn on the blackboard it IS bad probing indeed. But, good or bad probing, it doesn't qualitatively change the result if I compare the two simulations.
« Last Edit: April 16, 2020, 06:38:51 pm by pwlps »
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #32 on: April 17, 2020, 06:20:32 am »
I did not search for your comment and answer but if you copy it here it might be interesting and might help to understand his lines of thought.

Copy and paste below. Wait -- did I say thank you?! My goodness! I really appreciate the time you took watching the video and doing the nice simulation!


max wang
3 days ago
I don't understand the experiment in the end!



Lectures by Walter Lewin. They will make you ? Physics.
Lectures by Walter Lewin. They will make you ? Physics.
3 days ago
you are not alone. Tough Physics!!!



max wang
max wang
2 days ago
?@Lectures by Walter Lewin. They will make you ? Physics. Simply I cannot believe that putting a meter at the other side of the circuit , while still connecting to the same to points, would change its reading. If anything, it's the loop of the wires connecting the meter that make the difference. I would like to see the physical (not electrical) setup of the experiment, not only the display of the scope or meter! :)



Lectures by Walter Lewin. They will make you ? Physics.
Lectures by Walter Lewin. They will make you ? Physics.
2 days ago
I appreciate the fact that this Physics may not be familiar to you. Don't feel embarrassed about it; you are not alone. However, it's never too late to get educated. The choice is yours. Maybe this will help you.
I demonstrated at the end of my lecture #16 of my 8.02 E&M course at MIT that two identical voltmeters attached to the same 2 points in a circuit can show very different values. The reason is that in the case of an induced EMF (Faraday's Law) potential differences are no longer determined; they depend on the path. This also applies to the secondary windings of transformers as the EMF in the closed loop of secondary windings is induced.
Of course, in cases where Kirchhoff's loop rule (KVL) applies, 2 voltmeters attached to the same 2 points in a circuit will always show the same value.
My demo was first suggested and published by Romer in December 1982 in the American Journal of Physics. This demo has now become a classic; it's done all over the world at many colleges and universities.
Kirchhoff's original text can be found in the following link, pages 497-514: https://books.google.de/books?id=Ig8t8yIz20UC. Clearly he was fully aware of the prerequisite for his "loop rule". KVL is a special case of Faraday's Law. That's why Faraday's Law is one of Maxwell's equations and KVL is not.
By teaching students that KVL always works without telling them when it does not work, makes many believe that the closed loop integral of E dot dL is always zero. ElectroBOOM and Dirk Van Meirvenne therefore believe that 2 voltmeters attached to the same 2 points in a circuit must always show the same value which is not true as demonstrated in my lectures. They each posted a video (see @ footnote below) on their channel in which they claim to have proof for their wrong ideas which violate Maxwell's equations. Apparently they do not know, or do not understand, that in the case of an induced EMF potential differences are no longer determined; they depend on the path. MIT students who took my 8.02 course (Electricity and Magnetism) would not make this mistake! I therefore believe that to introduce a "modern version" of KVL and then teach students that KVL always holds is not advisable as you may set them up for making the same embarrassing mistake that both Dirk Van Meirvenne and ElectroBOOM made.
Also read Professor John Belcher's thoughts on Faraday's Law and on KVL. Professor Belcher is my former colleague at MIT. http://freepdfhosting.com/0813df09f5.pdf
(@) ElectroBOOM insulted me in his video by mentioning that the reason why my 2 voltmeters read different values was due to "bad probing". He stated that if I had done the demo correctly I would have found that both voltmeters would have read the same value. He could not have been more wrong 
This is my only msg on this issue. This is too basic. I do not argue either with people who believe that the Earth is flat.
This video may also help some of you to digest why 2 Voltmeters attached to the same 2 points in a circuit can give very different readings. https://youtu.be/T6S5VS03xZc
@max wang


Quote
Yes, you can put it simply (in my opinion)  considering  that the whole circuit is immersed in a varying  electric field (generated by rotE=-dB/dt).  The field outside an infinite solenoid is circular (not sure for the proper term, I mean only tangential component) therefore non-conservative for any path going around the solenoid (it would still be conservative for other paths). Since rotE=0 (there is no B there) it has to decrease as 1/r (solution of the Maxwell equation rotE=-dB/dt=0 with cylindrical symmetry and using the boundary conditions on the solenoid surface obtained from the solution inside the solenoid where dB/dt is uniform and E~r).

Thanks.

Quote
No, the current wouldn't be infinite, it would just be enough to cancel the total magnetic flux across the superconducting loop. This also happens when you short the output of a transformer : at high enough frequency when the winding resistance can be neglected the current in the secondary winding will be such as to cancel the magnetic flux (the transformer then works as a "current transformer"). The only difference between a very good conductor and a superconductor is that such flux-cancelling current would also be observed for a DC magnetic field (see Meissner effect and London equations, but this is far beyond the actual problem). Note that such superconducting DC current transformers are used e.g. in SQUID magnetometers, they are called "flux transformers".

My bad. This is so obvious! (But first time heard of SQUID. Nice.)

[Edit] I said this so soon though I should have known the current would not be infinite. It's not like - an EMF generates a fixed voltage across a conductor (then we get an infinity current on a superconductor with this FIXED voltage). This is NOT so obvious and is very subtle. I would not have said "it's so obvious". It's NOT very obvious.  |O

Quote
Yes, but I still believe you can relatively well model such a circuit with discrete components, using transformers. In my schematics I supposed that voltmeters are connected very close to R1,R2 so that the magnetic flux across loops comprised of voltmeter connection wires can be neglected (usually we use twisted pairs to minimise magnetic flux in order to avoid this kind of error measurements).
Below is another variant, much closer to the schematics drawn on the blackboard. Note that L4 and L5 cannot be much smaller than L1 and L3 (if the circuit is to be physically realisable).

I did think about using twisted wire for the meters as well.

Quote
We can't judge on the quality of probing as long as we don't know the exact physical layout of all wires (if he refuses to show it I'm suspicious :)). If the wires are exactly as drawn on the blackboard it IS bad probing indeed. But, good or bad probing, it doesn't qualitatively change the result if I compare the two simulations.

I don't want to hasten to a conclusion too soon (or be judgemental) as well. But stay critical. Maybe I will do the experiment myself.
« Last Edit: April 17, 2020, 10:33:28 pm by max.wwwang »
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #33 on: April 17, 2020, 08:42:56 am »
I was very surprised when I saw Professor Lewin in his video

was still saying things like "voltmeters at different sides of the loop measuring exactly the same two points" ...

In my opinion, the reference to "measure" or "same two points" is the most misleading.

As he now correctly drew on the whiteboard (as below), the voltmeters, when "measuring", are now a part of the circuit. Given the (externally) induced current and, in turn, voltage drops, the voltmeters seemingly attached to the "same points" in the circuit but when at different sides of the varying magnetic field (which is vertical to the loop's plane), must show different values depending on the resistance of the resistor at the same side of the voltmeter in parallel. The wires connecting the meters now have also emf induced current.

In my opinion, it would help a lot if he had stopped saying "measure", or saying "attached to the same points". These are meaningless now.

That being said, what's in my mind was the second diagram which will also give different readings of "measurement" but with the difference without another induced loop current around the V/R1, or R2/V. I'm not saying his diagram is wrong - now only he knows the exact layout of his experiment.

Also, he keeps talking about "people without decent/solid physics background" ... He really doesn't have to say this. I know my background is neither solid nor decent, but I'm now reasonably confident that, at the very least, he failed to point out the key point of why the experiment gives seemingly surprising observations (if he was not fooled).
« Last Edit: April 17, 2020, 08:52:50 am by max.wwwang »
 

Offline pwlps

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Re: Textbook about Inductance
« Reply #34 on: April 17, 2020, 07:29:31 pm »
Quote
Copy and paste below.

Ha, but his answer is not about physics at all,  it really looks more like a police questioning of a suspect  ;D.
Well, if he is attacked it is maybe because he is unable to give a clear and convincing explanation.

I'm teaching physics myself (though didn't teach electromagnetism so far) and frankly I am shocked, this guy seems to be offended by a simple physics question, I can't believe it!  After all your post is just a question about the interpretation of his "different paths"  so I don't see why he feels offended.

I know how hard it is to find where the student makes error in his reasoning, very often it needs many iterations trying to shed light on the problem from different perspectives etc. But he only blindly repeats "...depends on the path" :  we can recognize a bad professor when he just  rewinds his speech repeating the same statements on any question. Good teachers are those who encourage asking challenging questions, only bad teachers feel offended by them.
I grew up with Feynman books which showed me what explaining things means, very often when something was not clear in a book or a lecture I would search in the  Feynman lectures and my problem was soon solved. And Feynman himself was emphasising, in the book you cited, how all "stupid" students' questions were important and stimulating for his own understanding of physics, it seems many examples given in his lectures stem from such philosophical discussions with students.

His comment "This is my only msg on this issue. This is too basic. I do not argue either with people who believe that the Earth is flat" is insulting and unworthy of a MIT professor.
 

Quote

Also, he keeps talking about "people without decent/solid physics background" ... He really doesn't have to say this. I know my background is neither solid nor decent, but I'm now reasonably confident that, at the very least, he failed to point out the key point of why the experiment gives seemingly surprising observations (if he was not fooled).

Somehow he forgot that HE is actually the one supposed to give such a solid  background!  I think I would fire him :)
Whatever your background I find all your questions legitimate and well formulated. I myself had plenty of this kind of questions when I was a student and even today I would never pretend I cannot be fooled, physics is so subtle.


Quote
In my opinion, the reference to "measure" or "same two points" is the most misleading.As he now correctly drew on the whiteboard (as below), the voltmeters, when "measuring", are now a part of the circuit. Given the (externally) induced current and, in turn, voltage drops, the voltmeters seemingly attached to the "same points" in the circuit but when at different sides of the varying magnetic field (which is vertical to the loop's plane), must show different values depending on the resistance of the resistor at the same side of the voltmeter in parallel. The wires connecting the meters now have also emf induced current.

Not sure he "correctly drew" because he still does not show where the magnetic flux goes and the voltmeters are not attached to the "same points" in the circuit as he pretends. Moreover,  if there is magnetic flux across the loops labeled "I1" and "I2" then the probing is not good anymore. 
But at the end the important thing is that YOU now understand how to attack the problem.


Quote
I don't want to hasten to a conclusion too soon (or be judgemental) as well. But stay critical. Maybe I will do the experiment myself.

Very good idea! As an experimental physicist I often preferred to start by experiments when I had difficulties with understanding the theory.
« Last Edit: April 17, 2020, 08:14:44 pm by pwlps »
 
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Offline max.wwwang

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Re: Textbook about Inductance
« Reply #35 on: April 17, 2020, 10:26:11 pm »
Not sure he "correctly drew" because he still does not show where the magnetic flux goes and the voltmeters are not attached to the "same points" in the circuit as he pretends. Moreover,  if there is magnetic flux across the loops labeled "I1" and "I2" then the probing is not good anymore. 
But at the end the important thing is that YOU now understand how to attack the problem.

I cannot be more agreeing with this. That all said, I really like professor Lewin's blackboard handwriting - that's not just amazing.

Also, this discussion is the highlight of my life in the COVID-19 lockdown. Thanks to all involved.
« Last Edit: April 17, 2020, 10:54:15 pm by max.wwwang »
 

Online Manul

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Re: Textbook about Inductance
« Reply #36 on: April 17, 2020, 11:32:44 pm »
I remember watching a video from ElectroBOOM about this. And I learned that it is professor Lewin's famous experiment which is somehow causing big debate.

Never understood what is even confusing about the result? His goal was to measure between points A and B. But his way of measuring basicaly modified the circuit and he measured something else which was orginal circuit + measuring circuit. So he did not measure between points A and B. This reminds me of math tricks where you are presented with a "proof" that for example 1=0. It is just a hidden mistake to get you tricked.

What is shocking is that profesor seems to not admit, that he measured something else. Physics people are strongly familiar with concepts such as "measuring always affects system being measured" and so on. They normaly have very good understanding of how to get true measurements and not the effects of measuring itself. Until of course they meet Mr. Heisenberg at some point, but that is another story.
 

Offline rfeecs

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Re: Textbook about Inductance
« Reply #37 on: April 17, 2020, 11:48:19 pm »
 
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Online Manul

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Re: Textbook about Inductance
« Reply #38 on: April 17, 2020, 11:51:47 pm »
I guess what is different about EE people is that after:

1. Crystal oscillator is not working! Oh.. wait.. my cheap probe is 20 pF...
2. Voltage is too low! Oh...voltage source is high impedance...
3. Current is less then calculated! Oh.. so that is called burden voltage...
4. Enter your failed measurement experience

we get better at measuring :)
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #39 on: April 17, 2020, 11:59:57 pm »
What is shocking is that profesor seems to not admit, that he measured something else. Physics people are strongly familiar with concepts such as "measuring always affects system being measured" and so on. They normaly have very good understanding of how to get true measurements and not the effects of measuring itself.

That's the hardest part (and a sad disillusionment to me - I have GREAT admiration for MIT), but beyond what we can control.
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #40 on: April 18, 2020, 12:21:35 am »
FYI, already discussed with only 42 pages worth of posts:
https://www.eevblog.com/forum/chat/does-kirchhoffs-law-hold-disagreeing-with-a-master/

Thanks, have just watched. Nice analysis and experiment. The judgement of "bad probing" is very accurate to me.

We did the job with only 2 pages of posts (and only as a sidetrack of another discussion). I can say we excelled in terms of the (less) number of posts. :-DD
« Last Edit: April 20, 2020, 06:28:53 am by max.wwwang »
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #41 on: April 18, 2020, 12:24:02 am »
I guess what is different about EE people is that after:

1. Crystal oscillator is not working! Oh.. wait.. my cheap probe is 20 pF...
2. Voltage is too low! Oh...voltage source is high impedance...
3. Current is less then calculated! Oh.. so that is called burden voltage...
4. Enter your failed measurement experience

we get better at measuring :)

 :-DD
 

Offline max.wwwang

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Re: Textbook about Inductance
« Reply #42 on: April 26, 2020, 11:49:23 pm »
I came across another video of ElectroBOOM--

which refers to two useful articles , one of which has been specifically written on the experiment that ElectroBOOM did as a reaction to Profession Lewin's one.

These two articles can be found here and here.

Just for records. I have not read either of these articles, so I'm not suggesting they are correct or not (though probably correct).
« Last Edit: April 27, 2020, 12:20:43 am by max.wwwang »
 

Offline pwlps

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Re: Textbook about Inductance
« Reply #43 on: April 27, 2020, 12:04:00 pm »

These two articles can be found here and here.

Just for records. I have not read either of these articles, so I'm not suggesting they are correct or not (though probably correct).

Romer's paper is very nice! It is quite subtle indeed (eqs. 6 and 7 are the keypoint, though easy to overlook) but the discussion in the section V and Fig.6 shows clearly why Lewin's experiment cannot be analyzed without knowing the exact topology of the system.

BTW I had missed this question:

[Edit] I said this so soon though I should have known the current would not be infinite. It's not like - an EMF generates a fixed voltage across a conductor (then we get an infinity current on a superconductor with this FIXED voltage). This is NOT so obvious and is very subtle. I would not have said "it's so obvious". It's NOT very obvious.  |O

It is obvious if you write the transformer equations, something like:
V1=-L1 dI1/dt-M12 dI2/dt
V2=-L2 dI2/dt-M12 dI1/dt

output open: I2=0 -> V2=M12/L1  * V1
output shorted: V2=0 -> dI2/dt=-M12/L2*dI1/dt

If there is no flux leak (as in a perfect iron-core transformer), with n1,n2 being the numbers of primary/secondary windings,  then L1~n1^2 (B~n1*I1 and the flux~n1*B), L2~n2^2 and M12~n1*n2 hence M12/L1~n1/n2 and M12/L2~n2/n1
« Last Edit: April 27, 2020, 12:42:05 pm by pwlps »
 

Offline bsfeechannel

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Re: Textbook about Inductance
« Reply #44 on: May 13, 2021, 04:28:15 am »
What is shocking is that profesor seems to not admit, that he measured something else. Physics people are strongly familiar with concepts such as "measuring always affects system being measured" and so on. They normaly have very good understanding of how to get true measurements and not the effects of measuring itself.

That's the hardest part (and a sad disillusionment to me - I have GREAT admiration for MIT), but beyond what we can control.

It never ceases to amaze me the damage caused by Mehdi Sadaghdar, the ElectroBOOM, in the minds of his audience by accusing Dr. Lewin of "bad probing".

Mehdi is incapable of understanding electromagnetism and in a recent video he stated that he lives off Youtube and he has to do everything he can to get views. And that apparently includes passing his misunderstandings as facts.

It's disheartening to see that kind of phenomenon, that we are used to see predominantly in politics, for instance, set a foot in engineering. Really troubling.
« Last Edit: May 13, 2021, 04:30:50 am by bsfeechannel »
 

Offline Electro Fan

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Re: Textbook about Inductance
« Reply #45 on: May 13, 2021, 05:07:12 am »
I was reading this when I was confused by the introduction about inductance - How can we get to (2-52) from (2-51)?

This is a page from an old book -- Basic Electronics for Scientists. I found this book very good.

Hi, who is the author of this book (Brophy or Eggleston) and what year was it published? Thx
 


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