Youtube guy with Masters in EE incorrectly believes Kirchoff's law holds in non-conservative circuits.

ElectroBOOM, a guy on Youtube with a Masters in electronic engineering, believes Prof Walter Lewin is wrong on Kirchoff's law not holding in non-conservative circuits:

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Larry Harson

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Larry Harson
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Yeah, I'd say he's right. I'm surprised anyone can't see that. While he gave a very good explanation, I could see the discrepancy right off the bat.

Rick C.

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Reply to
gnuarm.deletethisbit

In the 2-resistor case, it's easy to show what the voltages are across each resistor. One way is to measure their power dissipation. That loop of course makes its own magnetic field.

The professor has a probing problem.

Only vaguely related, I bonded some manganin current shunts to some temperature-controlled aluminum blocks to get PPM-stable pulsed current measurements. And got strange and unpleasant transient response from eddy-current effects from the currents induced into the aluminum. We found a fix for that.

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John Larkin

There are PhD's who believe boys are girls these days too.

Reply to
blocher

KVL is just conservation of energy in a dress. A non-conservative system doesn't conserve energy, by definition, so KVL is not strictly valid in one because it was derived with the assumption that conservation of energy holds strictly in a given system.

All real-world circuits are non-conservative to some degree or another; KVL functions as a heuristic for circuits which aren't too badly non-conservative and returns approximately-correct results.

Biot-Savart only applies strictly to DC currents but is used to analyze the magnetic fields generated by low-frequency AC currents all the time and it works out OK for practical work.

Lewin is formally correct he's just being a smugdog pedant of the type he is. He also probably doesn't design many circuits.

Reply to
bitrex

Well, yes, but every resistor has some inductance, too; there's no pure resistance possible in a current-carrying circuit (and heating tells you the current through the resistor, but leaves 'the voltage' undefined because of that inductance term).

It may be called a 'probing problem' but it isn't trivial nor completely solved by any rearrangement of wires. Minimized is possible, eliminated is not.

Reply to
whit3rd

elieves Prof Walter Lewin is wrong on Kirchoff's law not holding in non-con servative circuits:

The youtuber's mistake is he's not measuring instantaneous voltages all at the same time. He moves his probes and just gets a waveform without conside ration of what may be happening in the other circuit half at those same tim es. Just another contribution to the thesis that most of the information on the internet is misinformation.

Reply to
bloggs.fredbloggs.fred

Lewin _is_ wrong! He was deceived by his setup, where his scope leads were part of the circuit. He has been debunked quite thoroughly & yet is obstinate about being correct. One debunker is a professor at Princeton (I think), who published a rigorous paper on the subject.

Reply to
Bob Engelhardt

No, it's Coulomb's law plus Ampere's law. In a static system, the E field is irrotational (curl E = 0) and so the line integral of E around any closed loop is zero. (You should have taken the E&M elective at art school.) ;)

The Kirchhoff laws are valid in the limit of low frequency and small physical size, but not in general--a transformer violates the voltage law, and an antenna violates both the current and voltage laws. You have to patch them up by treating the transformer as a black box made of hyperspace or something.

Cheers

Phil Hobbs

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Phil Hobbs

You can correctly measure the voltage between any two nodes if the test leads have zero loop area relative to the mag field.

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John Larkin

Faraday, not Ampere.

Cheers

PH

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Phil Hobbs

Everything in a static system is Coloumb's law (or Gauss' law if you want to debate which is more fundamental...) and Ampere's law in a dress, but the thing in the video clearly isn't a static system so idk why it makes sense to talk about KVL being derived from those when applied to that use case.

Ya that's what I said. The classical EM equations for electrostatics/electrodynamics are not God-given laws that fall into neat, well-defined use-cases that you can always pick the perfect analysis tool for any given system, "as you know" they're classical analogs of a more complex behavior. A particular application of them is "true" when how they're applied returns predictions that jive with reality, more-or-less.

Reply to
bitrex

Faraday's law don't say much of interest in a static system. That the curl of E = 0 in a static system is an axiom, it's the same as saying the divergence of E = some constant. you can't derive that from anything AFAIK.

Reply to
bitrex

Hello, Earth to bitrex: E = grad V & curl E = 0 is precisely Kirchhoff's voltage law in differential form. (It's trivially convertible to integral form.)

If you don't think that's interesting, why bother posting about it?

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

Fog away. Or you could admit that you were wrong and actually learn something.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

I very much doubt a professor at Princeton 'debunked' Lewin since Lewin's correct.

In the video at 6:43 he confirms that he's measuring different voltages at the same two points like Lewin, with minimum probe area -- so it isn't a probing problem. Yet he goes on to say:

"maybe there's voltage across these wires, unlike what we thought"

God almighty, and this guy has a Masters in EE?

There is very little electric field in a typical conductor carrying 1 amp in his set up; he needs to crack open a book on basic EM theory and study how shielding works.

Larry Harson

Reply to
Larry Harson

If those equations your only assumptions, for what reason should KVL ever return anything like answers that jive with reality when applied to analyze circuits where dB/dt is not also precisely zero. idk beats me.

Reply to
bitrex

As I said above, Kirchhoff's laws are valid in the limit of low frequency and small physical size, but not in general. Induction and radiation break them. (So do capacitors, of course.)

Cheers

Phil Hobbs

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Phil Hobbs

Yes, I can see how the typical engineer might not understand this guy. You have to remember that this guy is as much a humorist as an engineer (I find him hilarious) and he says a number of things to be absurd expecting you to be clued in at that point.

Lewin is not correct, or at least that seems to be the consensus here.

Rick C.

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Reply to
gnuarm.deletethisbit

I lost his argument about halfway thru and his diagram is a non-interpretable mess. It doesn't give me great confidence in his conclusions.

Reply to
bitrex

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