Author Topic: Does Kirchhoff's Law Hold? Disagreeing with a Master  (Read 41833 times)

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Offline In Vacuo Veritas

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #25 on: November 07, 2018, 06:56:51 am »
A lot of older academics become kooks or cranks because they have been sniffing their own farts so long and no one dares to say they stink. (Because that would jeopardize their own path towards their auto-fart-sniffing career)

Old people should be viewed with suspicion, especially academics. It's sad but true. Look at the ideas that come out of 20-something brains versus the ossified crap and ego-stroking narcissism from older people.
 
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Offline ataradov

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #26 on: November 07, 2018, 07:57:06 am »
It is not like he taught advanced physics. From what I've seen his lectures were more like physically themed performances rather than actual lectures. This was fine for the first year EEs that have never seen physics before.
Alex
 

Offline T3sl4co1l

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #27 on: November 07, 2018, 08:40:03 am »
That said, even if we consider a perfect setup and a real discrepancy, my take here would be that KVL could still perfectly apply. We would just have to consider the loop having extra voltage sources from any induced voltage. That wouldn't defeat it, but make us consider that our circuit model is incomplete.

Simply put:

You don't get to pick and choose when you follow a law and when you don't.

This is physics, not politics~~

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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Online SiliconWizard

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #28 on: November 07, 2018, 01:41:25 pm »
That said, even if we consider a perfect setup and a real discrepancy, my take here would be that KVL could still perfectly apply. We would just have to consider the loop having extra voltage sources from any induced voltage. That wouldn't defeat it, but make us consider that our circuit model is incomplete.

Simply put:

You don't get to pick and choose when you follow a law and when you don't.

This is physics, not politics~~

Tim

Not sure I got your point, nor that you got mine.

I've yet to agree with KVL not being met, until I get a consistent and formal proof of that.

Any electrical circuit made of physical, non-ideal components and non-zero length, non-zero impedance connections between them will get inductive and capacitive coupling with its surroundings. If you devise the real, physical circuit that it actually is, including its surrounding environment, you get equivalents of transformers and capacitors added to the ideal circuit. Taking those into account makes up the real circuit IMO, and KVL is most likely met in any case. I'm saying "most likely" because this is what makes the most sense to me, but as others here, I would of course consider a different view as long as it's rigorously proven.

Not considering those "parasitics" (in the sense that they are not part of our idea of the ideal circuit we designed) is ignoring basic laws of physics. Still don't get how KVL would not apply, until I get a formal explanation and a rigorous experiment that shows that the observed discrepancies are not due to basic inductive and capacitive coupling.
 

Offline ajb

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #29 on: November 07, 2018, 01:54:37 pm »
I have a hard time believing that Lewin is "just trolling" or trying to get students to think as some have suggested, after reading his responses to Mehdi and others on his original video.  I think this link will work, if not just look for ElectroBOOM's comment on the video, it should be near the top: https://goo.gl/JsKHb8.

If Lewin has a more subtle point he's trying to make, he's doing a good job of hiding it.

[EDIT: link shortened to avoid the automatic Youtube embed]
« Last Edit: November 07, 2018, 01:57:44 pm by ajb »
 
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Offline T3sl4co1l

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #30 on: November 07, 2018, 03:58:30 pm »
Not sure I got your point, nor that you got mine.

I've yet to agree with KVL not being met, until I get a consistent and formal proof of that.

Any electrical circuit made of physical, non-ideal components and non-zero length, non-zero impedance connections between them will get inductive and capacitive coupling with its surroundings. If you devise the real, physical circuit that it actually is...

Precisely -- you can't construct a supposed circuit (following one law), then probe it with another circuit (following a different law).  While you can find such corruption in some domains, physics is a domain where this is strictly prohibited. :)

Tim
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Online RoGeorge

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #31 on: November 07, 2018, 04:25:14 pm »
 
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Offline SparkyFX

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #32 on: November 07, 2018, 05:42:54 pm »
physics is a domain where this is strictly prohibited. :)

Well, it is in violation of the law!  :-DD
 

Offline ANTALIFE

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #33 on: November 07, 2018, 06:30:10 pm »
Neato, I would imagine you need to remove the effects of the wires coming back to the scope to get a clearer picture. The only way I can think of is by using something like an LED. Would need to do a bunch of tests to figure out the LED intensity as a function of peak EMF induced voltage

Online RoGeorge

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #34 on: November 07, 2018, 07:30:35 pm »
Just to be clear, the Professor is correct.

To understand why in the intuitive way, remember what is voltage: By definition, voltage between points A and B is the work necessary to move a unit of charge from A to B.

1. In a constant (conservative) field, it does NOT matter the path I choose to walk my charge between A and B. It will cost me the same amount of energy, no matter how straight or how twisted my way from A to B was. Imagine dropping a ball from a mountain. No matter the path of the ball from top to the bottom, the ball will gain the same energy. All it matters is the height between the top and the bottom. In electric engineering language we say, it does NOT matter how I twist or coil the leads of my voltmeter, the measured voltage will be the same.

2. In a variable (non-conservative) field, it DOES matter the path I choose to walk between A and B. Imagine the same ball and the same mountain from case 1, except this time, while the ball is going downhill, somebody is tweaking the knob of "gravity". Now, the final energy of the ball will vary not only with the height between top and bottom, but also with how much and when the "gravity knob" was adjusted, so it is not the same any more if the ball take a shorter or a longer path.
 
In electric engineering terms we say, it DOES matter how I twist or coil the leads of my voltmeter.

For case 2 we say "in a variable field a voltage is induced in the voltmeter's leads", or "the leads act as voltage sources, too", or whatever, which is nothing more than (a wrong way of) saying that "the path DOES matter when moving charges in a variable (non-conservative) field".

Offline EEVblog

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #35 on: November 07, 2018, 09:46:41 pm »
I have a hard time believing that Lewin is "just trolling" or trying to get students to think as some have suggested, after reading his responses to Mehdi and others on his original video.  I think this link will work, if not just look for ElectroBOOM's comment on the video, it should be near the top: https://goo.gl/JsKHb8.

If Lewin has a more subtle point he's trying to make, he's doing a good job of hiding it.

Wow, the spam link responses  :o
I posted a comment, will see if I'll get the spam link reply honor too.
 

Offline EEVblog

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #36 on: November 07, 2018, 09:49:35 pm »
In electric engineering terms we say, it DOES matter how I twist or coil the leads of my voltmeter.

In the real world it can (and in this case demonstrably does) matter how you twist or coil the leads. There is transformer coupling happening which is not shown on your theoretical circuit.
 

Offline KL27x

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #37 on: November 07, 2018, 10:05:48 pm »
Just trying to scroll through all the replies, I have learned something thru osmosis.
...in an induced EMF...
....loop...
...determined by path....
...Kirchoff's law is not valid....
...Special case...
...Thus, Faradays Law is always valid...
...basic physics...
...not going to argue about it....

see lecture 8, 5, 11, 7, 15
 

Offline EEVblog

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #38 on: November 08, 2018, 12:32:08 am »
He'll do a video when he gets back from vacation in a week  :popcorn:

 

Offline BrianHG

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #39 on: November 08, 2018, 01:25:57 am »
He'll do a video when he gets back from vacation in a week  :popcorn:


Thanks for you polite request on our behalf.
Yes, Dr. Lewin's says he will do a video, but, the way he worded his response, it is as if he is clearly refusing to watch or acknowledge ElectroBoom's video in any way.  This may be understandable as it is clearly possible that Dr. Lewin's may have been hit with so many disruptive responses over the years on the subject which he may have won his argument so many times that he is now immune to any new views on what ElectroBoom's has measured and it may unfortunately be a pure one tone response.  I hope he points out truly where ElectroBoom has made an error from his point of view, otherwise, we will most likely see a repeat of his early videos.

« Last Edit: November 08, 2018, 01:34:59 am by BrianHG »
__________
BrianHG.
 

Online RoGeorge

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #40 on: November 08, 2018, 03:01:56 am »
In electric engineering terms we say, it DOES matter how I twist or coil the leads of my voltmeter.

In the real world it can (and in this case demonstrably does) matter how you twist or coil the leads. There is transformer coupling happening which is not shown on your theoretical circuit.

Good point, let's simplify the circuit. Let's get rid of the voltmeter. We will use an electron to probe the voltage for each half of our loop. Even more, let's look only at the sign of the voltage for each half of the loop.

1. We have our loop of 2 resistors in an increasing magnetic field.
2. Electrons will flow through our loop, let's say clockwise.
3. Let's measure the voltage. By definition, voltage is the work required to move the unit of charge between our measuring points.
4. We don't have a voltmeter, so we grab an electron, and start moving it through each half of the loop, in order to see how much work do we need to accomplish that - or other said to probe the voltage for each half of the loop, left-hand half, and right-hand half.
5. Starting from top, when we circulate our grabbed electron through the left-hand half of the loop, we will need to put some work to move our electron against the flow of all the other electrons in the loop, so negative voltage on the left-hand half.
6. Starting from top, when we circulate our grabbed electron through the right-hand side of the loop, we don't need to put any work, our electron will move by itself, it will go with the flow of all the other electrons, it will generate some work, so positive voltage on the right-hand half.
7. From 5 and 6 we observe the voltage between the same points is once positive, once negative, depending on which half of the loop we measure. The sum of voltages in our closed loop is Vpositive - Vnegative, which is NOT zero. E.g. 3V - (-5V) = 8V.
8. We just seen the sum of voltages for our loop is NOT zero, yet Kirchhoff's Voltage Law predicts it to be ZERO, so Kirchhoff is broken for our setup.
« Last Edit: November 08, 2018, 03:35:09 am by RoGeorge »
 

Offline jancumps

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #41 on: November 08, 2018, 03:23:14 am »
He'll do a video when he gets back from vacation in a week  :popcorn:


There may be moderation. I see the prof’s answer in the thread, but not your comment.
 
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Offline wraper

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #42 on: November 08, 2018, 03:33:37 am »
As a relevant point of interest, you cannot have a half-turn transformer winding. It is always an integer number of turns. You have to connect the ends of your 'half turn' winding together somehow, for current to flow. Making the wire longer, as in including the test leads of a meter for example, does not alter the fact that the circuit still completes the same magnetic path as a complete turn would do. The longer wire intercepts a more diffuse magnetic field but over a longer distance, and the induced EMF is the same as if it were a tightly wound turn.
Fractional turns can be done. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.187.2764&rep=rep1&type=pdf
 
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Online rfeecs

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #43 on: November 08, 2018, 05:45:21 am »
Dr. Lewin repeats the same answer in the comments:

Quote
In the case of an induced emf the potentials in a circuit are no longer determined, they depend on the path and thus Kirchhoff's Loop Rule is not valid. Kirchhoff's Loop Rule is a special case of Faraday's Law (namely when phi/dt=0). Thus Faraday's Law is always valid.

The thing is, he's not wrong, and it is fundamental. 

That's the beginning and end of his argument.  He's not interested in "you are not measuring it right".

He even takes this argument so far to the point of saying that Kirchoff's voltage law does not apply to circuits with inductors, because they have a changing magnetic field:
http://web.mit.edu/8.02/www/Spring02/lectures/lecsup4-1.pdf

He claims this is where almost all the electrical engineering textbooks are dead wrong.

As Dave says, a case of theoretical physics vs practical engineering approaches.

 

Offline KL27x

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #44 on: November 08, 2018, 07:03:21 am »
Quote
For case 2 we say "in a variable field a voltage is induced in the voltmeter's leads", or "the leads act as voltage sources, too", or whatever, which is nothing more than (a wrong way of) saying that "the path DOES matter when moving charges in a variable (non-conservative) field".

He says "two voltmeters connected to the same two points can have two different readings." And he does an experiment where this is not only the case, but these two different reading are repeatable. If this was caused by consistently placing the probe wires (either intentionally or unwittingly) in a different orientation, most of the world, including physicists, would say this is a wrong way of saying that the path DOES matter when moving charges in a variable (non-conservative) field." Especially when he does not include the reason the scopes give the consistently (if you believe the results) different readings. For someone interested in the theory, as an educator or physicist, you would not make this comment (two voltmeters connected to the same two points can have two different readings) in the first place, unless the topic at hand is the potential deficiencies in common voltage measuring devices. You would say the voltage between those two points is in fact quantifiable and repeatable at any given point in time in the experiment.

Professor stretched things. If this were intentionally done to make his students think about the problem, his video would congratulate the student who figures out the omission/trick of how he produced this result. Apparently we shall see if/when the promised video is published.

If you want to play this "path" game, and insist that voltage is not by default the lowest energy path between the two points, then you have to say that the voltage between those point is undefinable, no?
« Last Edit: November 08, 2018, 07:08:43 am by KL27x »
 

Offline JimRemington

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #45 on: November 08, 2018, 08:02:46 am »
Quote
If this were intentionally done to make his students think about the problem, his video would congratulate the student who figures out the omission/trick of how he produced this result

I agree completely, but Lewin's subsequent behavior suggests that he managed to fool himself, as well as the unwary student.

I always thought that the failure to take into account the probe placement was the problem, and am pleased to see this excellent video making that case!
 

Online SiliconWizard

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #46 on: November 08, 2018, 08:42:35 am »
Come to re-think of it, and as he's clearly not an idiot, I was then willing to believe that he actually did that on purpose, just to make young students aware of the question: using simplistic models while thinking they hold true in the real world, which is a very common pitfall. This would be all good if he made it clear in the end that it was his intent instead of making it even more confusing, to the point that he even managed to confuse some very experienced engineers, using his position of authority.

Now if he was genuinely trying to instill advanced physics notions in young heads, I think this was a very bad way of doing it from a pedagogical standpoint.
 
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Online SiliconWizard

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #47 on: November 08, 2018, 08:58:45 am »
He even takes this argument so far to the point of saying that Kirchoff's voltage law does not apply to circuits with inductors, because they have a changing magnetic field:
http://web.mit.edu/8.02/www/Spring02/lectures/lecsup4-1.pdf

Thanks for the reference. This helps making it much clearer what he was meaning - especially on page 3.

From what I got and summing it up quickly, his fundamental point is that we EE use an "adapted" version of Kirchhoff's law which actually describes things correctly but is NOT what Kirchhoff intended.
This is why we don't seem to agree - we actually do agree for all practical matters but for a professor in physics, we're just abusively using the term "KVL". The lecture extract that rfeecs posted explains that a lot more clearly than Lewin's oral lectures and misleading experiments.

« Last Edit: November 08, 2018, 09:18:30 am by SiliconWizard »
 

Online RoGeorge

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #48 on: November 08, 2018, 09:14:21 am »
If you want to play this "path" game, and insist that voltage is not by default the lowest energy path between the two points, then you have to say that the voltage between those point is undefinable, no?

Yes.
I'm intrigued, where can I find this definition of voltage as "the lowest energy path"? (With the battery, any path between two given points requires the same energy.)

The wires and the resistors are nothing more than a way to force the charges to flow through a specific path.  When there is no variable magnetic field, the path does not matter.  When there is a variable magnetic field, the path is essential.

A good intuitive understanding is the old analogy with the water flow.  If we have our loop inside the variable field, all the water will flow through our loop in a circle, always in the same direction.  If we swim from the top to the bottom of our loop against the water flow, we will spend a lot of energy.  If we swim again from top to bottom, but this time the other way around (with the water flow) we will gain some energy from the water flow.  The energy spent or gained is our voltage.  Once is negative, once is positive.  Obviously they are not equal, so it is important if we choose to swim clockwise or counterclockwise.

For the case where the current flow is caused only by the battery, it doesn't matter if we choose to swim clockwise or counterclockwise.  It matters only the start point and the end point.

Online rfeecs

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #49 on: November 08, 2018, 09:22:01 am »
He even takes this argument so far to the point of saying that Kirchoff's voltage law does not apply to circuits with inductors, because they have a changing magnetic field:
http://web.mit.edu/8.02/www/Spring02/lectures/lecsup4-1.pdf

Thanks for the reference. This helps making it much clearer what he was meaning - especially on page 3.

We've discussed this many times in previous threads.  Dr. Lewin gave his world famous SUPER DEMO as he refers to it in 2002, I guess.  But he didn't invent it.  It is an exact recreation of the experiment in this 1982 paper:
http://www.phy.pmf.unizg.hr/~npoljak/files/clanci/guias.pdf

In the paper, everything is explained very simply without drama.  It's no mystery.  The meter wires are part of the circuit and the orientation of the wires determines the results.
 
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