Dave, KVL doesn't hold under a varying magnetic field.
Dave, KVL doesn't hold under a varying magnetic field.
Really?
So according to the statement, AC circuits (with transformers and such) can not be solved using KVL!
You better go tell the whole electric power industry that they have been analyzing circuits under the effects of magnetic fields incorrectly for the last 160 years!!!
I think Lewins model on which he applied KVL was incorrect. The lines in a circuit diagram don't interact with a magnetic field.
If you watch the original lecture, the error creeps in when he assumes "A1=A2" and "D1=D2", but this just doesn't hold.
I think Lewins model on which he applied KVL was incorrect. The lines in a circuit diagram don't interact with a magnetic field.
If you watch the original lecture, the error creeps in when he assumes "A1=A2" and "D1=D2", but this just doesn't hold.
Now Lewin doesn't know how to apply Kirchhoff's law to a circuit, and, worse, doesn't know how to draw diagrams.
Precisely. You cannot on one hand assume those "wires" in the circuit be "ideal" and effectively form one "node", and at the same time expect them to span up a area for the magnetic flux to pass through. It doesn't work that way. These circuit lines are dimensionless. They have zero length. The area is zero, therefore, and no magnetic flux passing through them. Besides, KVL is a mere calculation tool for network analysis. If your result doesn't match the measurements, clearly something is wrong with the model. But not with the method as such. That's a bit like blaming spice for not matching reality when you forgot to put an element.
QuoteYou better go tell the whole electric power industry that they have been analyzing circuits under the effects of magnetic fields incorrectly for the last 160 years!!!I don't need to. The whole power electric industry never made the mistake of analyzing a circuit under a varying magnetic field using KVL. In fact the whole power electric industry started out because of Faraday's discoveries which was solely explained by Maxwell's equations. And that's what we use to this very day.
That's precisely your, Mehdi's, Dave's and everybody-else-that-insist-that-KVL-always-hold's mistake.
KVL says that the voltages measured along the path of a mesh add up to zero.
But Lewin showed at least one circuit where the voltages do not add up to zero.
You cannot solve such circuit using KVL. It is impossible. You'll have to resort to the full monty and use Maxwell's equations at least to calculate the EMF produced by the varying magnetic field.
After that, you can of course devise an equivalent circuit where instead of a varying magnetic field producing the extra EMF upon the entire circuit, you have a battery, a generator, a transformer, or any other equivalent lumped (i.e. localized) component to produce the same EMF and get the exact same effect on the other components. In that case, you can solve the equivalent circuit using KVL because you theoretically removed the varying field from the circuit and stashed it away in the equivalent component.
But that is just a theoretical trick that has a lot of caveats.
So how's that possible? How can we have, so to speak, a spooky "component" that produces an EMF in a circuit, but is not present there? This seems to violate the principle of conservation of energy, doesn't it? Those are legit questions. But to answer them you need to abandon KVL, which these people are not prepared to accept. To reconcile their cognitive bias with the real phenomenon that contradicts it, they create all the irrational arguments I listed above and more.
So how's that possible? How can we have, so to speak, a spooky "component" that produces an EMF in a circuit, but is not present there? This seems to violate the principle of conservation of energy, doesn't it? Those are legit questions. But to answer them you need to abandon KVL, which these people are not prepared to accept. To reconcile their cognitive bias with the real phenomenon that contradicts it, they create all the irrational arguments I listed above and more.
That's exactly your problem. There cannot be a "spooky component" that is not present but causes an effect. You have to have it in your model, otherwise all calculations are nonsense. Yes, you cannot calculate current induced by magnetic flux with KVL, yes you need to solve the Maxwell-Faraday equation to calculate the EMF, but after you've done that, you cannot simply forget about it and omit if from your circuit and postulate: "current flows in this circuit but without anything that causes it".
Right there is proof that you have no idea of what you are talking about! If (and that is a big IF) you are an electrical/electronics engineer, you are embarrassing yourself. If your background is in physics, then I know were you are coming from, and your ignorance is understandable.
bsfeechannel understanding of KVL is stuck in 1845.
Right there is proof that you have no idea of what you are talking about! If (and that is a big IF) you are an electrical/electronics engineer, you are embarrassing yourself. If your background is in physics, then I know were you are coming from, and your ignorance is understandable.
Ah the ad hominem argument. Let's add it to the list above.
KVL always holds because in the eyes of EEVBlog forum member jesuscf, EEVBlog member bsfeechannel is perhaps not an engineer, but probably a physicist.
But Lewin showed at least one circuit where the voltages do not add up to zero.Only because he omitted a critical element of the circuit.
His diagram was simply incomplete: he drew a line in the diagram and then promptly neglected that it's a real, physical wire and not an ideal "net" you would find in a schematic.
And that is exactly the error that Dr. Lewin made: He drew up a circuit, but it was not the equivalent of his experiment and then tried to solve that with KVL, and of course it failed.
Solving an equivalent circuit is not a theoretical trick, it is exactly how network analysis works. How do you calculate a circuit that has a BJT in it? With an equivalent circuit using e.g. the Ebers-Moll transistor model.
That's exactly your problem. There cannot be a "spooky component" that is not present but causes an effect.
You have to have it in your model, otherwise all calculations are nonsense.
Yes, you cannot calculate current induced by magnetic flux with KVL, yes you need to solve the Maxwell-Faraday equation to calculate the EMF,
but after you've done that, you cannot simply forget about it and omit if from your circuit and postulate: "current flows in this circuit but without anything that causes it".
Hey bsfeechannel, I am honestly curious about your credentials.
Are you an electrical/electronics engineer with a degree from a university (or similar) or not?
I'll need you to prove it by solving "problem #24" from your hero Dr. Lewin, but as an electrical/electronics engineer will do:
You do not expect to find two diodes and two current sources inside a transistor, do you?
The circuit had just two resistors and nothing else. And his model predicted exactly what happened in practice. KVL failed on the board. And then failed on the bench.
Hey bsfeechannel, I am honestly curious about your credentials.
Thank you for your interest in my credentials.QuoteAre you an electrical/electronics engineer with a degree from a university (or similar) or not?
I think that's irrelevant for the present discussion.QuoteI'll need you to prove it by solving "problem #24" from your hero Dr. Lewin, but as an electrical/electronics engineer will do:
Lewin is not my "hero". I don't subscribe to his channel, nor follow him anywhere on the social networks.
He just happened to show a physical phenomenon whose understanding is very important for electronics engineering.
And for whoever posted the 5-resistor video, maybe I've misunderstood, but I seem to see another glaring error regarding the supposed symmetry of the current between the L-lower and R-upper resistors regardless of the resistance. Try 0R for the right upper one and see if that holds!
As a practical matter, if there are two different voltages between the two points depending on which branch you follow, how do the oscilloscopes 'know' which branch they are measuring?
Is that determined by where they are physically placed?
Are they briefed beforehand?
If you want to debate, correct or wrangle about anything I've said, please answer this question first as I have no desire to argue this issue until that is cleared up.
And as a theoretical matter, the voltage between two points can never be 'path dependent', that's ridiculous. Voltage is at its core an absolute value, we just typically use and measure relative values because, well, circuits and current.
Now as for KVL and magnetic fields, you can all have at it, but there's one thing I haven't seen mentioned, so someone point it out if I've missed it: Change in flux through a loop causes EMF, EMF causes current to flow, that current then causes......counter EMF? No?
Anyway, connecting two test instruments to different points on a wire with current flowing in it and in a changing magnetic field and then claiming they are connected to the 'same' point makes me not want to try to solve the problem.