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
Suppose you put the probes of a voltmeter across the terminals of an inductor (with very small resistance) in a circuit. What will you measure? What you will measure on the meter of the voltmeter is a "voltage drop" of Ldi/dt. But that is not because there is an electric field in the inductor! It is because putting the voltmeter in the circuit will result in a time changing magnetic flux through the voltmeter circuit, consisting of the inductor, the voltmeter leads, and the large internal resistor in the voltmeter
a case of theoretical physics vs practical engineering approaches.
The meter wires are part of the circuit and the orientation of the wires determines the results.
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.
The meter wires are part of the circuit and the orientation of the wires determines the results.
What happens if we remove the voltmeters? How do we measure the voltage without the voltmeters? We will need to take an electric charge and drag it through the circuit from A to B. How much energy do we need to move our charge from A to B?
The answer is: it depends. If we go from A to B through the left side, or through the right side. Not only that the energy is different for each half of the circuit, but it has an opposite sign. No chance to get a zero for the whole loop.
As a relevant point of interest, you cannot have a half-turn transformer winding.
http://www.phy.pmf.unizg.hr/~npoljak/files/clanci/guias.pdf
Question:
If we remove the voltmeters and their leads, what is the voltage between A and B?
An even simpler question:
No voltmeters, no leads, no wires. Is the voltage between A and B positive, or negative? <- drama, it's both!
It is not like he taught advanced physics. From what I've seen his lectures were more like physically themed perfo rmances rather than actual lectures. This was fine for the first year EEs that have never seen physics before.
What happens if we remove the voltmeters? How do we measure the voltage without the voltmeters? We will need to take an electric charge and drag it through the circuit from A to B. How much energy do we need to move our charge from A to B?
The answer is: it depends. If we go from A to B through the left side, or through the right side. Not only that the energy is different for each half of the circuit, but it has an opposite sign. No chance to get a zero for the whole loop.
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.
QuoteWhat happens if we remove the voltmeters? How do we measure the voltage without the voltmeters? We will need to take an electric charge and drag it through the circuit from A to B. How much energy do we need to move our charge from A to B?
The answer is: it depends. If we go from A to B through the left side, or through the right side. Not only that the energy is different for each half of the circuit, but it has an opposite sign. No chance to get a zero for the whole loop.
Well, when measuring voltage, why are we "dragging the electron" where it doesn't want to go? Isn't the usual way to do this to let the electron go where it wants? If you have to drag it one way and input energy, and the opposite is true for the other direction, it will take the other way every time. This is why the electrons go in a circle, here, right? From A to B via route 1, and B to A via route 2?
so hmm. it this a close analogy?
A mobius strip doesn't have a front and a back.... So therefore all the dummies using the term front and back are really talking about a special case (which is pretty much everything except a contrived thought experiment). So it is therefore an OUTRAGE for textbooks to be incorrectly stating that things have fronts and backs when it is not universally the case?
[Kirchhoff's Voltages Law (KVL) failure] would be sorta like doing a momentum analysis between two colliding steel balls without figuring the effect of a magnet under the table.
Before taking his degree, Kirchhoff had begun his work in original
research, and published a remarkable paper on electrical conduction in
a thin plate, especially a circular one. His problem was to find the
current in any branch of a network of linear conductors. Starting
from Ohm's familiar law, he derived two results long recognized in
electrical science as Kirchhoff 's laws.
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.
I'll say the professor was correct when he said we can not always apply KVL. Of course, if we first transform the real circuit into a lumped circuit, where we add the externally induced voltages as voltage sources internal to our circuit, then we obtain a new circuit that obeys KVL.
I'll say the professor was correct when he said we can not always apply KVL. Of course, if we first transform the real circuit into a lumped circuit, where we add the externally induced voltages as voltage sources internal to our circuit, then we obtain a new circuit that obeys KVL.
Wow. I was not even aware that there is such a controversy over Kirchhoff's law.
I though Kirchhoff's laws were derivable from Faraday's law and Maxwell equations in general. But quick search shows that it is not very easy.
Dr. Lewin's response to this is: You can't do that! You can't just move the EMF from one side of the equation to the other! That's dead wrong! That's criminal. (https://bit.ly/2qzwkh0)
You can't do that with changing magnetic fields. Curl-E = -dB/dt, so there is no scalar potential that describes electron motion. That is what Walter Lewin was showing. Everything else is just noise.
... is a very big claim that two points on the same circuit measure differently... This is why many people have a big problem with this.