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View Full Version : Kirchhoff's voltage law



ashiekh
02-26-2019, 11:59 AM
https://www.youtube.com/watch?v=0TTEFF0D8SA&fbclid=IwAR1ujyCs9vFcRPVsAZIzQf6e_nMEezPxsbZgHe9Q1 4ngSR1mxW-eJsyzbhU

I should add that I think the MIT Prof has it all wrong.

Username is valid
02-26-2019, 03:22 PM
The Kirchhoff rules are universally valid, but they also have limitations.
The rule of mesh to which he refers, applies only in the case that in the considered mesh no change of the magnetic flux takes place, whereby also no energy can be fed in magnetic way or removed.
So Line Integral (E ds) = - d (Phi) / dt = 0
If an electric field flows through the mesh, causing variable flows ((Phi) (t)), the specification d (Phi) / dt = 0 is violated and no longer applies.
In this case, the voltage equation which can be derived from the law of induction applies.
Sum (n, k = 1 [Uk]) = -d (Phi) / (dt). The sense of rotation can no longer be chosen arbitrarily, but must be selected right-screwing to the magnetic flux.
So, no Prof is right, internet opinion is wrong

ashiekh
02-26-2019, 03:38 PM
Dr. Walter Lewin has two different voltage measurements for the same points, and does not understand why.

d Phi/dt gives rise to an induced emf as the guy in the video recognizes; Kirchhoff is not violated, it is like a battery in the circuit. Any Kirchhoff direction is fine as the sum of the voltages is still zero.

The guy in the video has a Masters in Electrical Engineering.

Dr. Lewin forgot that the probe leads were themselves picking up an induced voltage and that this explained the different reading for the same two points.

https://www.bing.com/videos/search?q=walter+lewin&view=detail&mid=16111CF63E965CC4D0FF16111CF63E965CC4D0FF&FORM=VIRE

McSteel
02-27-2019, 09:37 AM
The interesting takeaway of all this is not getting too held up by the actual connections, but going by what participates in the circuit and in what way.

This video (https://www.youtube.com/watch?v=C7tQJ42nGno) is on the popular science level, but does a good job of helping with the perspective change, about as much as Electroboom's video does.

ashiekh
02-27-2019, 10:01 AM
One has to admire Electroboom; he stood up, took a risk, and was right.

Good news is that Kirchhoff's laws are still fine, even if they were figured out before Faraday's law,
and the voltage between two points is unique.

gdjacobs
02-27-2019, 07:15 PM
https://www.youtube.com/watch?v=0TTEFF0D8SA&fbclid=IwAR1ujyCs9vFcRPVsAZIzQf6e_nMEezPxsbZgHe9Q1 4ngSR1mxW-eJsyzbhU

I should add that I think the MIT Prof has it all wrong.

Are you comfortable with the concept that work done by a charge moving through a magnetic field (or any other non-conservative field) is no longer path independent?

ashiekh
02-27-2019, 07:46 PM
Not sure what bearing that has on this particular problem

Are you after justifying how two points on the loop don't have a unique voltage difference? (they do);
the problem was improper probing (as ElectroBoom points out); I think Dr Lewin realized that in his apology.

Now if you are trying to explain why the two scopes gave different readings, I'm for that.

gdjacobs
02-28-2019, 12:57 PM
Not sure what bearing that has on this particular problem

Are you after justifying how two points on the loop don't have a unique voltage difference? (they do);
the problem was improper probing (as ElectroBoom points out); I think Dr Lewin realized that in his apology.

Now if you are trying to explain why the two scopes gave different readings, I'm for that.

It was only improper probing if Dr. Lewin wanted to exclude lead inductance from influencing his measurement (which he could do by twisted pairs or by orienting the loop formed by the leads parallel to the flux vector generated by the drive coil).

There certainly is a tendency in EE to consider conductive links in networks to be purely conductive. It's at the heart of KVL and KCL and therefore the various analysis methods. KCL similarly falls apart if parasitic charge loss (due to corona, for instance) isn't taken into consideration.

ashiekh
02-28-2019, 01:04 PM
It was only improper probing if Dr. Lewin wanted to exclude lead inductance from influencing his measurement (which he could do by twisted pairs or by orienting the loop formed by the leads parallel to the flux vector generated by the drive coil).

There certainly is a tendency in EE to consider conductive links in networks to be purely conductive. It's at the heart of KVL and KCL and therefore the various analysis methods. KCL similarly falls apart if parasitic charge loss (due to corona, for instance) isn't taken into consideration.


I finally agree on both points.

He should have excluded picking up voltage with the probe leads, that was his probe error which I think he now realizes.

So we now seem to be in agreement, and credit needs to go to ElectroBoom who stood his ground.