Does Kirchhoff's Law Hold? Disagreeing Walter Lewin demonstrates moment 2 days ago   13:21

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Dr. Walter Lewin’s videos on Kirchhoff’s Voltage Law:
https://up-tube.com/upvideo/aTDoN2wjxJV
https://up-tube.com/upvideo/YmG_LM0kPSL
https://up-tube.com/upvideo/jm_TdB-Hex4
https://up-tube.com/upvideo/s3-FsmrchCf

By: Mehdi Sadaghdar

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#Kirchhoff #KVL #KCL #ElectroBOOM

Comments 3818 Comments

Scientific Lee
If the em field sent out a double helix of energy and one went clockwise (+.9v ) and one went ccw (-0.1v)... that would be the up and down...I call it a Halowave.
David Schwartz
Mehdi is wrong. I'll try to explain why as clearly as possible:

Voltage between two points is defined as the work done to move a unit of charge between those two points. Kirchoff's voltage law says that if those two points are the same point, the work done is zero. For the the electron to do work in a light bulb, it must have work done to it by, say, a battery.

Now imagine Kirchoff's law holds. Imagine any arbitrary path between two points, P1 and P2. We can form a closed loop with a direct path from P2 back to P1. If Kirchoff's voltage law holds, then the voltage from P1 to P2 along the arbitrary path must equal the negative of the voltage from P2 to P1 along the direct path. Since the voltage from P2 to P1 along the direct path is a constant, the voltage from P1 to P2 along the arbitrary path must be a constant as well. In other words, Kirchoff's voltage law is precisely equivalent to saying the electric field is path independent.

Kirchoff's voltage law is true if, and only if, the electric field is path independent because it says the voltage difference between two points along any path must equal the negative of the voltage difference between those two points on a direct path.

Unfortunately, it now gets complicated. But I will point you to two very clear sources that explain that the electric field is path independent if, and only if, the magnetic field is constant and that motors and generators work precisely because a changing magnetic field makes the electric field path dependent:
https://en.wikipedia.org/wiki/Electric_potential#Generalization_to_electrodynamics
https://en.wikipedia.org/wiki/Faraday%27s_law_of_induction
"The most widespread version of Faraday's law states: The electromotive force around a closed path is equal to the negative of the time rate of change of the magnetic flux enclosed by the path."

In other words, Kirchoff's voltage law says that the EMF around a closed path is zero and Faraday's law -- the way induction works -- says that it is non-zero. They cannot both be right.

To agree with Mehdi, you would also have to disagree with this statement, "The Maxwell–Faraday equation states that a time-varying magnetic field will always accompany a spatially varying, non-conservative electric field, and vice versa." A path-independent field is conservative, and the Maxwell-Faraday equation states that a time-varying magnetic field makes the electric field non-conservative and thus path-dependent. As shown above, a path-dependent electric field violates Kirchoff's voltage law. So you cannot have both the Maxwell-Faraday equation true and Kirchoff's voltage law true if the magnetic field is time varying.
Synonymium One
Omg what a good Video, i hope Dr. Lewin will answer, i'm really looking forward to this discussion!
simran kamboj
I m with proff lewin
motbus
It calls for a epic rap battles of history
Martin lewis versus Electroboom
Simone Mastroianni
I don't understand some things but you are very clear with your explanation and I liked it! :)
YingTao Lai
physiker doesn't like or agree this kind of modeling but engineer need a model it as simple as possible.
Maybe there is no absolutely right or wrong at here.But the divergence between scientist and engineer.
JoeAceJR
You should make a solar backpack.
Protophanes
Lewin espouses his unquestionable belief in bigbangtheology because it's wikipedia's indubitable first-effect of an idiotic first-cause -> it's a dogmatic singularity, stupid ! ... ugh ...

but on the less delusional side -> Blackbody Radiation, Planck's Law, and Kirchhoff's [fallacy]:
https://up-tube.com/channel/HPY7DVBMgrJCcOJOBy8v0r-t/cynlyvfgf

your Mum doesn't understand you either !
: )
Tapas Banerjee
See when you have an static electric field . Then it is perfectly right to talk about voltage because the field is conservation . The lineintegral of the electric field arount any closed loop is O and the line integral of electric field gives the same value for any path you choose . But the problem comes when the field becomes nonconservative . Now if you do the line integrals from one point to another it depends on the path you choose . They give 5 for one path and 8 for another between the same two points. You cannot define potential in this case . You have to talk the whole thing in terms of the line integral concept . It you talk about voltage then you are a bloody fool in the earthwho don't understand electromagnetism . So my suggestion is leave your physics degree and go to your high school again . How dare you call Walter Lewin wrong . Think on your own then comment on such things . Cheater !
Muck2014
My Mom thinks I am mostly ok to. https://up-tube.com/upvideo/jBs0lXtFe14
Berry Wang
All in all, KVL and KCL are just approximation theories. You cannot analyse properties of such circuits by just using KVL and KCL .
Pogi Mijares
Hey Mehdi can you make a vidio and make multiple step up transformer.
Pls
matt frank
i have a question, i am trying to boost the power of a hand held bug zapper. if i increase the capacitor value that is running parallel to the output would that increase the current at the output? or would that just increase the time it supplies the current? it says the output is 2,000 volts but i have no way to test it.
Jeremy Christstardy
Dr walter lewin has posted a new video on kvl. You should see it
Vaibhav Bhasin
So electroboom , you are saying we must find a " wireless measuremnt " system ?
kelzang Tobgyel
Kirchoffs laws/rules are often thought of as true rules or laws of nature when in fact it's only a nifty tool we derived from Faraday's laws. Ie. If the rate of change of flux in the loop happens to be zero (which is true for most cases) then the sum of all potential drops/rises in a loop taken in one direction is zero.

I believe the professor was simply highlighting this misunderstanding. By showing us a circuit where the change in flux of the loop is not zero. So he's not showing us that Kirchoffs rules are wrong per se. He's merely highlighting that condition that most of us tend to forget since it's pretty rare to come across it in real life.

Edit.
You must remember that even if a number of different possible solutions exist where Kirchoffs rule is applicable. (Such as your examples in the latter half of the video) Just one where it is not is sufficient to prove it's not an absolute law. I don't believe there were any real errors in your circuiting the first time right ? If that's true then I believe you successfully proved yourself that Kirchoffs rules do not always hold !
MrMatled
Esse canal é incrível! Claro que eu venho pelas informações úteis de aprendizado, mas.......cadê você quase morrendo ou se queimando com potência dissipada em forma de calor?
Ramiro Serra
Have you noticed how close are KVL "supporters" to flat-earthers? Argumentatively speaking...
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Walter Lewin demonstrates moment Does Kirchhoff's Law Hold? Disagreeing 2 days ago   13:58

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