I already provided my position here in this thread. That's why I just refer to article I agree to and do not repeat what is already said. Don't blame me if you did not read thread or do not remember what I did say or whatever.
Those who do not agree (to "KVL for the birds") say that Kirchhoff’s loop equations apply to Lewin’s circuit.
You better behave
Kirchhoff died absolutely ignorant of the confirmation of Maxwell's theory so that's why his theory doesn't account for that. Period.
Here's a dollar, kid. Go get yourself a better education.
Here's a dollar, kid. Go get yourself a better education.
So, you criticize Lewin because someone else wrote an article criticizing him? Not because you yourself master the fundamentals and is in a position to confront him? What kind of engineers do we want to be? Just a bunch of dilettantes ranting at random in forums? Let's shut up and do our homework. Thank you.
No they don't. Suppose that Kirchhoff didn't know about Faraday's law of induction. I show a loop of wire with two resistors and hide the solenoid under the table. I ask Kirchhoff to measure the voltages with a voltmeter. Kirchhoff wouldn't know how to explain how a loop of wire with two resistors and no voltage source has some voltage on them. Worse, he wouldn't know how to explain why the voltmeter shows diferent voltages depending on the position of the voltmeter.
Kirchhoff wouldn't know how to explain how two pieces of wire hanging out in the breeze (that today we call dipole antenna) suddenly have voltages and currents without no visible voltage source connected to them.
Although Faraday demonstrated the phenomenon of induction in 1831, he couldn't find a mathematical formulation and therefore his theories were rejected by the scientists of the time. Meanwhile in 1845 Kirchhoff came up with his law that do not account for any kind of varying field for that matter. In 1865, Maxwell could finally produce his now famous equations that gave a mathematical formulation to Faraday's law.
Kirchhoff died in october 1887. Hertz published his first paper confirming Maxwell's equations in november 1887.
So there is no paradox in Lewin's explanations. Kirchhoff died absolutely ignorant of the confirmation of Maxwell's theory so that's why his theory doesn't account for that. Period.
So who told you that the loop is a secondary of a transformer? Certainly not Kirchhoff. Because when that theory could finally be confirmed, he was DEAD. End of story.
Don't you see how detrimental this attitude is to science and understanding?
IMHO, this is also the actual mistake Lewin is making.
Honest questioning and discussion should be valued.
Not everyone is as strong as a person as you or me or ogden. Not everyone wants to fight. Some may actually get upset about they way Lewin arrogantly attacks their "qualifications" when they just asked honest questions, especially if you are in the role of a student. They won't ask more questions, but instead, do as you teach them to do: close up your desire for understanding, just shut up and "learn". But, just reading isn't the way you learn science. You need to really understand, and for that, asking questions, yes, even questioning your professor - would be the right thing to do.
But I guess we'll disagree on this. And, I think you are genuinely a bad person for not even trying to think about this side of the coin - and the life goes on .
It just seams to me that Dr. Lewin has a different idea of what KVL actually is, this is what is leading him to the conclusion of KVL being just a special case of Faradays law with no magnetic field (Its not, you can't even stick an electric field into KVL).
Kirchhoff (KVL) IS a special case of Faraday.
What does Maxwell say? That the EMF along a closed line in space is a function of how the surface integral of a magnetic field across an arbitrary surface bounded by the closed line varies in relation to time.
The magnetic field can vary in intensity and direction. The surface can vary in size, direction and/or shape (including the closed line).
When that surfce integral does not vary with time, EMF is zero. And that coincides exactly with what Kirchhoff said: that the sum of the voltages around a mesh (a closed line) is zero.
In what circumstances does the surface integral equal zero? When the magnetic field AND the surface are constant, for example, among many other cases.
So Kirchhoff ended up being a subset Faraday. I even drew it as a Venn diagram many messages ago in this thread.
I did some experimentation with a stepped pulse instead of the nice Gaussian, but still working on that.
I really don't see how Kirchhoff's law could possibly be used to analyze something like this (and truly be successful).
So you need to apply a similar voltage step response across your solenoid coil (Not just a pulse).
I think is pretty standard for excitation pulses to be 1 Volt in these sims.
Yes this is what i find to be the issue. Faraday and Kirchhoff are put into the same basket while they describe different things.
Here is a definition of Faradays law:
"The electromotive force induced in a circuit by variation of the magnetic flux through the circuit is proportional to the negative of the time rate of change of the magnetic linkage"
Here is a definition of Kirchhoff voltage law:
"The algebraic sum of all the voltages around any closed loop in a circuit is equal to zero"
Kirchhoff on the other hand does talk about the voltages summing to zero. Notice that it says "all the voltages" so this implies induced EMF too as that's a voltage inside the loop.
In a circuit mesh schematic a wire has zero length, zero resistance and zero reactance. These wires are immune to all field effects. Building the two resistor circuit with such wires in the real world results in a circuit that must have a circumference of zero, this means it also must have a loop area of zero, zero area means no induced EMF and KVL works.
This however has a problem because Dr. Lewins circuit clearly has a loop area larger than zero so it makes the circuit mesh model act different(You get 0V on both resistors). We fix this by replacing out ideal wires with inductors.
This plugs in Faradays law and tells it about how big the loop area is.
YES I KNOW that Kirchhoffs voltage law does indeed break in certain special cases, i'm not saying it always works.
Can you explain in what way is it a special case of Faradays law, if the two laws describe two separate concepts?
That diagram i fully agree with. Kircchhoffs laws are an abstracted application of Maxwells equations and are meant to be only used on lumped circuit meshes.
Since the mid-20th century, it has been understood that Maxwell's equations are not exact, but a classical limit of the fundamental theory of quantum electrodynamics.
When Kirchhoff says all the voltages, he is not considering any kind of EMF.
If, however, you have a constant magnetic flux, Kirchhoff holds. So Lewin has no problem with Kirchhoff.
QuoteSince the mid-20th century, it has been understood that Maxwell's equations are not exact, but a classical limit of the fundamental theory of quantum electrodynamics.
from https://en.wikipedia.org/wiki/Maxwell%27s_equations#Limitations_for_a_theory_of_electromagnetism
you might have to fix your Venn diagram.
Does that mean Maxwell "is for the birds"?
That depends on the symmetry of the induced field. Let me give you my take on that paper (who appears to be a draft considering there is at least a minor error and a (?) meaning what, exactly? Do you know if this was ever published and where is the definitive version?)
Well, it's interesting but it is nonetheless amazing that to compute the emf on an open path, the area always pops out.
What happens if your EMF is ZERO? Doesn't that ring you a bell? Where did you see ZERO before?
Ahh! KIRCHHOFF! He says that "The algebraic sum of all the voltages around any closed loop in a circuit is equal to ZERO"!!!!!!!QuoteKirchhoff on the other hand does talk about the voltages summing to zero. Notice that it says "all the voltages" so this implies induced EMF too as that's a voltage inside the loop.
Here is where your problem lies. When Kirchhoff says all the voltages, he is not considering any kind of EMF. In Kirchhoff's wonderful world there is absolutely NO EMF whatso-fluxing-ever!QuoteIn a circuit mesh schematic a wire has zero length, zero resistance and zero reactance. These wires are immune to all field effects. Building the two resistor circuit with such wires in the real world results in a circuit that must have a circumference of zero, this means it also must have a loop area of zero, zero area means no induced EMF and KVL works.
Philosophical question: if Kirchhoff only applies to circuits that cannot exist, why do we need his stupid theory?Quote
That diagram i fully agree with. Kircchhoffs laws are an abstracted application of Maxwells equations and are meant to be only used on lumped circuit meshes.
You do not only have a problem with Kirchhoff, Maxwell and Lewin. You also have a problem with Georg Cantor.
Between you and me. Forget all you know about circuits. Follow the guidelines I published in a previous message. Study calculus, vector analysis and a good book on electromagnetism. You'd be better off than struggling with theories you do not master.
Physics has no use for Kirchhoffs law since it doesn't deal with anything physical.
The problem is that calculating all of this for a real physical circuit involving only a few components would already result in a LOT of math.
So KVL only interacts with Faradays law when circuit mesh modeling deems it necessary.
Physics has no use for Kirchhoffs law since it doesn't deal with anything physical.
Kirchhoff, one of the most important physicists of the 19th century, must be rolling over in his grave.
QuoteThe problem is that calculating all of this for a real physical circuit involving only a few components would already result in a LOT of math.
You don't get it. You have an impressive equipment on your bench all of it making extensive use of Maxwell and yet you have no clue about the theory used to design it.
In Kirchhoff's world you wouldn't have coaxial cables, ground planes, canned circuits, EMI certification, impedance matching, delay lines, coupling, decoupling, transformers, inductors, motors, generators, radiation, etc., etc., etc.
Kirchhoff is from a time when the only source of electrical energy widely available were batteries. There were no changing fields. No transients. No kind of interference. Only well-behaved DC circuits. Today, even toys using batteries have SMPSs with inductors and transformers, these specifically making use of Faraday's law.
So you can't escape Maxwell. It's everywhere these days. When we say Kirchhoff, we are in reality saying Maxwell, or Faraday in case of induction, under certain VERY special conditions. But it is still Maxwell.
Don't fool yourself or you'll end up like Cyriel Mabilde: an advocate of ignorance.
QuoteSo KVL only interacts with Faradays law when circuit mesh modeling deems it necessary.
So you're saying that on your planet the laws of Nature obey the desires of the engineer? I'm moving there right now.
KVL applies to any lumped electric circuit; it does not matter whether the circuit elements are linear, nonlinear, active, passive, time-varying, time-invariant, etc. In other words, KVL is independent of the nature of the elements.
so physics doesn't really need KVL and KCL.
Why doesn't everyone write software in a hex editor? Its the most fundamental way of doing it after all. Its just simply more practical to work on a higher level with a compiler. Yes you are missing out on some fine details with all these high level language abstractions, but in 99% of cases it doesn't matter and it does get the job done significantly faster.
I was making an example of why circuit analysis is useful in order to justify why one might want to use KVL rather than the more the accurate alternatives (the answer is deadlines).
But at least the bent laws help things calculate faster in my real world, the trick is bending them just right so that it acts almost the same as in the real world while only taking 1% of the math to get there.
So, never try anymore to hide your lack of knowledge behind excuses like that.
Abstraction and neglect of the fundamentals are orthogonal things.
Abstraction means to take away from the designer tasks and routines that are necessary but will not have any kind of influence on the result. So, if you have a menial task to do, you hand it down to a computer, or another person, or company, or the compiler, etc., so that you can concentrate on the specifics of your project. However this doesn't remove from the designer the responsibility to have full mastery of the fundamental concepts of his field.
If abstraction were the same as neglect of the fundamentals, any computer science or software engineering course will be just a class on some stupid "high level" language and nothing more.
So. You have, say, a radar to design. You have a tight deadline. You turn to your colleague and say "Bugger that! Let's use Kirchhoff and go home."
The math required for the work with Kirchhoff requires a decade or so of learning. You do not learn Kirchhoff in the first grade, do you? If the math required for Kirchhoff were so simple, you wouldn't have spice programs out there to solve them for you.
The basic math required to work with Maxwell requires less than 100 hours of study. And if the equations get really complex, you also have proper software to deal with them.
So, never try anymore to hide your lack of knowledge behind excuses like that. Convince yourself and others of the need to be ready to learn something new every day.