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

Yes, okay. My contention is that the reason they're also valid in the limit of low frequency _at all_ also requires conservation of energy.

I'm not sure we're debating anything of significance here. The guy's argument in the video didn't make a lot of sense to me either; Prof Lewin is correct even though I don't much like him as an educator and he got into some trouble with MIT a while back:

"In December 2014, MIT revoked Lewin's Professor Emeritus title after an MIT investigation determined that Lewin had violated university policy by sexually harassing an online student in an online MITx course he taught in fall 2013."

there's foods to eat and I'm willing to drop it if you are. :)

Reply to
bitrex
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He's right generally-speaking but for the wrong reasons, his particular demonstration doesn't show what he claims it does

Reply to
bitrex

g, believes Prof Walter Lewin is wrong on Kirchoff's law not holding in non

-conservative circuits:

's correct.

at the same two points like Lewin, with minimum probe area -- so it isn't a probing problem. Yet he goes on to say:

mp in his set up; he needs to crack open a book on basic EM theory and stud y how shielding works.

Who, Electroboom? Hell, all you have to do is look at the setup and it is obvious the professor doesn't understand anything about it. The measuremen t is *LITERALLY* of the same two points only moving the probe wires. This has nothing to do with the voltage in the circuit but rather the voltage in troduced in the probe wires by the magnetic field.

The real issue is why the professor doesn't include the missing transformer ? Once you do that Kirchoff's law is obeyed clearly.

Rick C.

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Reply to
gnuarm.deletethisbit

KVL doesn't hold exactly in non-conservative circuits Lewin is pedantically-correct, but this little toy circuit probably doesn't have much to show in the way of how. Sorry I lost track of his argument somewhere a long the way must be that young-person short attention span. The most likely reason there is a discrepancy is some probing error, yeah, whatever it is exactly.

Get a 500kW 2 GHz pulsed doppler radar out of a Mig-25, put it on the table and be like "Okay class let's analyze this sumbitch using KVL" try not to vaporize the front row while you do so though that's not nice.

Reply to
bitrex

e:

ing, believes Prof Walter Lewin is wrong on Kirchoff's law not holding in n on-conservative circuits:

ds

et

in's correct.

es at the same two points like Lewin, with minimum probe area -- so it isn' t a probing problem. Yet he goes on to say:

amp in his set up; he needs to crack open a book on basic EM theory and st udy how shielding works.

s obvious the professor doesn't understand anything about it. The measurem ent is *LITERALLY* of the same two points only moving the probe wires. Thi s has nothing to do with the voltage in the circuit but rather the voltage introduced in the probe wires by the magnetic field.

Yes, there's an induced electric field in the wires, but there is also an o pposite internal static electric field created by surface charges to mainta in a net electric field close to zero -- enough to maintain the required cu rrent required by the rest of the circuit. Therefore the total work done in moving a charge from one end of an inductance to the other inside the wire is close to zero. Most EEs don't get this because they haven't thought abo ut the physics involved hard enough because they don't have to.

er? Once you do that Kirchoff's law is obeyed clearly.

There is no "missing" transformer once you realize the total electric field inside a wire is zero for "small" finite currents.

Larry Harson

Reply to
larryharson66

No, Lewin is totally correct; he says the voltage is UNDEFINED, not that it is accurately measured by any of his setups. The workaround of 'transformers' dotted around the circuit is a wacky approximation scheme, guaranteed only if you can characterize the flux coupling of every millimeter of wiring (and the internals of the resistors too). No circuit diagram can be useful with that many optional extra components...

The ElectroBOOM finale was a suggestion of short-straight-wires with twisted leads to the oscilloscope, BUT HE DIDN'T TEST THAT. A test would show that the suggestion does not solve the probing discrepancy.

Reply to
whit3rd

The "you have to think really hard about the _physics_!" style of didaction looks good on paper, the problem is that the bulk of first-year STEM students are attempting to learn (or re-learn) calculus and linear algebra simultaneously and your skill at grinding those squiggles quickly is mostly what you're graded on on a test

Reply to
bitrex

This, exactly.

In many cases shown by Youtube Guy, the sense wires form a partial turn (or full turn) in the magnetic field. They either add to or subtract from the voltage induced in the loop under test. In the case where the two test points are physically adjacent and the twisted pair goes to the scope, this is not the case.

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Reply to
Paul Hovnanian P.E.

Yes, that's the nature of the problem. But, it isn't 'a probing problem' entirely, the electrical potential at a point in space (general electrostatic case, NOT just circuit diagram) is not well-defined in the presence of changing magnetic flux. The circuit provides one way to illustrate this, but it also opens the possibility of adding in spurious circuit elements to compensate (and of mangling the circuit wiring to dodge around the magnetic flux). Those fixes are incomplete, you still don't have a well-defined energy change in moving charges from one place in the region to another.

There is still no function V(x,y.z) that fills the function of making voltage diferences that describe the physical situation.

Reply to
whit3rd

The voltage "induced" in the probe wires is Lewin's whole point.

Reply to
boB

te:

ring, believes Prof Walter Lewin is wrong on Kirchoff's law not holding in non-conservative circuits:

ads

yet

win's correct.

ges at the same two points like Lewin, with minimum probe area -- so it isn 't a probing problem. Yet he goes on to say:

1 amp in his set up; he needs to crack open a book on basic EM theory and s tudy how shielding works.

is obvious the professor doesn't understand anything about it. The measure ment is *LITERALLY* of the same two points only moving the probe wires.

ltage introduced in the probe wires by the magnetic field.

OK, but the loop area of the probe wires is minimized so there shouldn't be any voltage induced.

ElectroBOOM can't bet his head around the simple idea that the total electr ic field inside a wire, including one picking up induced electric fields, i s close to zero when carrying a finite current -- it has to be from j = s igma E. But he can't be blamed where there's many EEs on here who have the same problem, and don't understand the physics involved.

mer? Once you do that Kirchoff's law is obeyed clearly.

Reply to
larryharson66

Yes... But Faraday's law can induce voltage in the leads that are measured by the meter. V = N dPhi/dT

Isn't that what is being measured ? I may need to go back to watch both videos again.

Reply to
boB

On Thursday, November 22, 2018 at 2:16:43 PM UTC-5, snipped-for-privacy@gmail.com wro te:

ote:

ering, believes Prof Walter Lewin is wrong on Kirchoff's law not holding in non-conservative circuits:

eads

yet

.

ewin's correct.

ages at the same two points like Lewin, with minimum probe area -- so it is n't a probing problem. Yet he goes on to say:

1 amp in his set up; he needs to crack open a book on basic EM theory and study how shielding works.

is obvious the professor doesn't understand anything about it. The measur ement is *LITERALLY* of the same two points only moving the probe wires. T his has nothing to do with the voltage in the circuit but rather the voltag e introduced in the probe wires by the magnetic field.

opposite internal static electric field created by surface charges to main tain a net electric field close to zero -- enough to maintain the required current required by the rest of the circuit. Therefore the total work done in moving a charge from one end of an inductance to the other inside the wi re is close to zero. Most EEs don't get this because they haven't thought a bout the physics involved hard enough because they don't have to.

rmer? Once you do that Kirchoff's law is obeyed clearly.

ld inside a wire is zero for "small" finite currents.

So how is the current induced in the circuit if there is no transformer? T he transformer secondary is the single turn of the circuit loop. It is dis tributed around the circuit but it is still present.

Rick C.

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Reply to
gnuarm.deletethisbit

When he makes his measurements between the same two points simply by flopping the probe wire from one side to the other it becomes perfectly clear why he is seeing what the video shows.

Rick C.

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Reply to
gnuarm.deletethisbit

So his point is to demonstrate poor probing? His claim is that there are different voltages between the same two points. That is not correct.

Rick C.

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Reply to
gnuarm.deletethisbit

But the loop area isn't minimized. the two curved ends of the probe wire form a semi circle which constitutes a fair sized loop.

Do you really not see that?

Rick C.

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Reply to
gnuarm.deletethisbit

On Saturday, November 24, 2018 at 6:30:21 AM UTC, snipped-for-privacy@gmail.com w rote:

rote:

neering, believes Prof Walter Lewin is wrong on Kirchoff's law not holding in non-conservative circuits:

leads

& yet
t

ct.

Lewin's correct.

ltages at the same two points like Lewin, with minimum probe area -- so it isn't a probing problem. Yet he goes on to say:

"

ng 1 amp in his set up; he needs to crack open a book on basic EM theory an d study how shielding works.

it is obvious the professor doesn't understand anything about it. The meas urement is *LITERALLY* of the same two points only moving the probe wires. This has nothing to do with the voltage in the circuit but rather the volt age introduced in the probe wires by the magnetic field.

an opposite internal static electric field created by surface charges to ma intain a net electric field close to zero -- enough to maintain the require d current required by the rest of the circuit. Therefore the total work don e in moving a charge from one end of an inductance to the other inside the wire is close to zero. Most EEs don't get this because they haven't thought about the physics involved hard enough because they don't have to.

former? Once you do that Kirchoff's law is obeyed clearly.

ield inside a wire is zero for "small" finite currents.

The transformer secondary is the single turn of the circuit loop. It is d istributed around the circuit but it is still present.

The induced EMF in the looped wire can be modelled by a single turn seconda ry of a transformer, and then used to create the illusion of KVL being obey ed. In reality, there is no *net* EMF inside the conducting wire of the loo p carrying a finite current, giving a zero EMF contribution to KVL. But by taking the rate of change of flux term to the LHS in Faraday's law, it crea tes a pseudo-KVL with this term confused with an EMF term that's zero.

Larry Harson

Reply to
Larry Harson

No, it isn't. It's to demonstrate the effect of changing B field.

That's not his claim. His claim is that voltages don't describe the physical situation. A shorted secondary on a transformer doesn't have voltage variation along its length, but does have current. How do you reconcile that with Ohm's law and Kirchoff's rules?

Reply to
whit3rd

You haven't described the circuit well enough? You haven't analyzed the entire circuit?

Reply to
krw

A shorted secondary is a wire ring, around the core of the transformer next to the primary winding. What 'analyzed the entire circuit' breakdown should apply to that ring?

Reply to
whit3rd

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