
#55
Nov1609, 07:15 AM

P: 997

Next time you challenge an established law in jest, would you please say so? Claude 



#56
Nov1609, 02:30 PM

P: 79

Claude,
I was entirely serious. Do you have a specific, substantive criticism? Mike 



#57
Nov1709, 07:05 AM

P: 997

Yes I do. There will indeed be an emf/mmf. If the flux in the core is constant/dc, and you unwind the secondary, there will be a flux change and an emf. However considering that v = N*d(phi)/dt, the emf is very small. Unwinding the secondary has a frequency in the subHertz range. While unwinding, you are moving a conductor in the presence of a magnetic field. But the motion involves one turn at a subHertz frequency. Further, what is phi? When wrapped around the toroid, phi can be substantial. But when unwrapped, no longer encircling the core, phi is nanoweber, or less.
The fact that the emf is quite small, and requires good equipment to measure, does not invalidate FL. If phi is 10 nwb, f = 0.10 Hz, N = 1.0 turn, then v = 2*pi*1e8*1e1*1e0 = 2*pi*1e9 = 6.28 nV! These are just off the cuff estimations. A real world scenario may vary by an order of magnitude in either direction. But we are looking at nanovolt levels of emf. Extremely small emf, but still present, is what is going on. Claude 



#58
Nov1709, 10:23 PM

P: 79

Claude,
You are begging the question. You are attempting to use Faraday's Law to validate Faraday's Law. See, this is the crux of the matter. Faraday's Law implies that a flux change due to motion produces an EMF. This is a false principle. The true principles are motional EMF and transformer EMF (given by the one of Maxwell's Laws which is the same as Faraday's Law except in that it uses the partial derivative). One or the other of these true principles covers every case to which Faraday's Law applies. Both of these true principles give an EMF of zero in the case being considered. The partial derivative of the equation for transformer EMF is zero because there is no intrinsic flux change. There is no motional EMF because the wire moves in a direction which results in its not cutting the magnetic flux lines, aside from the fact that the magnetic field is severely reduced external to the primary winding. Faraday's Law specifies an EMF for this case and is thereby proved false. This case is the opposite of the homopolar generator in that in the case of the homopolar generator Faraday's Law gives an EMF of zero where there actually is one, whereas in this case Faraday's Law gives an EMF where there actually is none. Mike 



#59
Nov1809, 07:41 AM

P: 997

You say 0, & FL says 0. You say you're right, while FL is wrong. You have no case at all. As far as my using FL to verify FL, what I'm doing is explaining the action, observing the result, and acknowledging the agreement with FL. All science is based on such methods. We observe, postulate, remeasure, and affirm. It happens that FL agrees with observation, so it is valid. Sure, we made initial assumptions. That in itself does not validate FL, nor invalidate FL. But since observation under all known conditions to date verifies FL, it is considered good law. Claude 



#60
Nov1809, 02:17 PM

P: 79

Claude,
Where do you get the idea that FL requires the cutting of magnetic flux lines? It only addresses the time rate of change of flux linkage. In the case we are discussing, the flux linkage starts out as the number of turns of the secondary winding times the flux in the core. It ends up as zero when that winding is fully unwound, so there is certainly a flux change, and therefore FL specifies an EMF. By the way, there is no frequency involved here. If the secondary is unwound at a constant rate, the time rate of flux change will be constant. Mike 



#61
Nov1809, 08:14 PM

P: 997

This is so well known, I'm amazed you even bring it up. An experiment in the basement will affirm this. When the winding is undone, there is a velocity, and you must consider the magnitude and direction of the flux. I've given the computations above that v is in nanovolts. Depending on dimensions, it could even approach microvolts. You'd need good equipment to measure, but it's there. When you and I are long gone, FL wil be standing like Gibraltar. Claude 



#62
Nov1809, 08:48 PM

P: 79

Claude,
The "u x B" expression is part of the equation for the Lorentz force, gives the motional EMF, and has nothing to do with Faraday's Law, which is EMF = dPhi/dt and involves the change of flux linkage only. The wire of the circuit does not even have to be in the magnetic field as long as it is linked by it. Mike 



#63
Nov1909, 07:28 AM

P: 997

When the conductor moves through a static field, the time rate of change seen by said conductor is u X B. Along a flux line gives 0, since the cross product goes to 0 for 0 angle. Across the flux line gives maximum induction. For an oblique angle, the component of motion in the direction normal to B is used for computation of emf. As far as your statement "The wire of the circuit does not even have to be in the magnetic field as long as it is linked by it." goes, I don't even know where to begin. How can the conductor not be in the field, yet be linked by it? Would you please draw an illustrative diagram? Please clarify your counterexamples to FL. A picture would help immensely. So far you're shooting blanks. Nothing you've stated has a remote chance of invalidating FL. Just curious, how much e/m field theory have you taken? To challenge an established axiom is quite ambitious for just about anyone. Do you have the academic knowledge sufficient for such an ambitious undertaking? Based on what you've stated thus far, I believe that with your current e/m fields skill set, challenging axioms is too ambitious for you. Claude 



#64
Nov1909, 09:25 AM

P: 59

When one asks what is the physical principle behind a law, one must determine whether mathematical consequences of the law are most fundamental or if the law itself is most fundamental. For example, Maxwell's equations lead to Einstein's postulate of relativity that the speed of light (laws of physics) is (are) the same for all inertial observers. One might then make the claim that Einstein's postulate is in fact more fundamental than Maxwell's Equations.
When one takes this view, assuming only the postulates of relativity and assuming Electric fields exists due to charged sources, one can easily deduce the appearance of the presence of a force in certain reference frames with properties that exactly match that of the so called "magnetic field". That is, the magnetic force in this view can be regarded as pseudo force directly derivable from more fundamental postulates. (ie, relativity and electric field) In other words, this means that the behavior of charged particles can be exactly predicted merely by assuming Einstein's postulates in relativity and that charged particles produce an electric field thus removing the necessity of the magnetic field (whereas without Einstein's postulates, the behavior of charges couldn't be explained without the presence of magnetic field). In order to get back Maxwell's equations, you examine how the equations from the above analysis transform if one were to neglect the postulate of relativity. By doing this it is then possible to derive Maxwell's equations including Faraday's Law. In this sense, one may then say the physical basis for Faraday's Law is the postulates of relativity. (Note: Faraday's law did come first, but was purely empirical. It was then able to be used to drive Einstein to think of more fundamental postulates. These fundamental postulates are then the physics behind our empirically observed Faraday's Law) 



#65
Nov1909, 10:20 AM

P: 997

So in a nutshell, the OP claimed that FL is false. What are you saying? Is FL true or false? Please answer. You gave your treatise but never answered the original question explicitly. Thanks in advance. Claude 



#66
Nov1909, 10:22 AM

P: 413

Maxwell's equations themselves don't define EMF. EMF is defined as the line integral of the force per unit charge, the line integral done along the line you wish to calculate the EMF across. EMF = [tex] \epsilon [/tex] = [tex]\int_{a}^{b} (E + [v \times B]).dl [/tex] In certain cases, it so happens that the above equation simplifies to [tex] \epsilon =  \frac{d \phi}{dt} [/tex]. In such cases, you will have to revert to the above definition. Faraday's law is [tex] \nabla [/tex] x E = [tex]  \frac{\partial B}{\partial t} [/tex] or in integral form can be written as [tex] \int E.dl =  \frac{d \phi}{dt} [/tex] (This is only for stationary integral paths) and is not to be confused with [tex] \epsilon =  \frac{d \phi}{dt} [/tex] which does not work all the time. For instance, in your example above, the whole equation is illsuited. 



#67
Nov1909, 10:44 AM

P: 59

To Claude:
Just as magnetic fields can be derived from Einstein's postulates and the Electric field, one can also use gravitational fields and Einstein's postulates to derive a psuedo gravitational field that is analogous to the magnetic field in electromagnetism. However this pseudo gravitational force is always regarded as fictitious. One may ask, why is the pseudo gravitational force fictitious but magnetism isn't? The answer is that magnetism was empirically observed first, and those who observed it thus believed it was real as they had no grounds to claim it was fictitious at the time. Had humans been the size of planets, we would have noticed the pseudo gravitational fields first and called these real and magnetic fields fictitious. Basically you can see that neither the pseudo gravitational force or the pseudo electric force should have any fundamental physical priority over the other. However, in saying that, these ideas of what forces are derived and what are real is purely a matter of "interpretation", analogous to the various interpretations of quantum mechanics. They are different ways of thinking about theories that lead to the same physical conclusions. For example, one could claim only magnetic forces are real and use Einstein's postulates to derive the electric force. This is what Einstein means when he says that neither can take the "seat". Physically my intuition prefers to accept that electric forces are real because magnetic forces have no monopoles, where as electric forces do. It is thus easier to think of these monopoles as the sources of the charge, and the magnetic force being pseudo. This is the same situation for gravitational fields. With regards to the openers question: The opener believed there was no theoretical/intuitive basis for the existence of Faraday's law. I disagree and a valid physical basis is Einstein's postulates and the electric field. From this basis, Faraday's law can be derived. This means that since Faraday's law is not itself fundamental (ie, based on the fundamental postulates listed), deviations observed from Faraday's law can be attributed to slight errors in the underlying fundamental postulates. For example, if only general relativity applies to a particular problem, then a new version of Faraday's law may be derivable using Einstein's principle of general relativity as opposed to the special theory. The point being however, is that one may know when Faraday's law is applicable, by seeing if its fundamental basis is also applicable to the situation you wish to model. 



#68
Nov1909, 01:51 PM

P: 997

Regarding monopoles, many prefer to start with E, and then view H as a relativistic manifestation of E. Their reasoning is along your lines, that monopoles exist for E, but not for H. Although you have the correct viewpoint that either can be the relativistic view of the other, some firmly, and wrongly, insist that E is the seat since H has no monopoles, while E does. But if we examine FL, the OP original question, there is a marked difference between E fields due to discrete charged particles, i.e. monopoles, vs. E fields due to induction/Faraday. With monopoles, the E lines have a source and a sink (start and end), whereas H fields do not since H is dipolar, not monopolar. H lines are closed loops, or "solenoidal" in nature. Solenoidal flux lines indicates dipole and NOT monopole since monopoles have a start and an end. Also, discrete charge E fields are conservative, whereas induction/solenoidal E fields are nonconservative. But the E fields induced per FL are solenoidal in nature. They have no start or end. They look like H loops. Although E monopoles do exist, that is not what happens when E fields are induced due to timevarying H fields. These E lines do not have a monopolar like appearance. Correct me if I'm wrong, but E lines with start and end points, DO NOT relativistically transform into solenoidal closed loops in a moving reference frame. What is happening here does not involve monopoles. Thus I cannot accept the existance of E monopoles and the nonexistance of H monopoles as a basis for treating E as more basic than H. Also, conservative E fields do not transform to nonconservative under relativistic transformations. We agree that neither is the seat, and we also agree with Einstein, so I think we hold a safe position. Thanks for your input. Claude 



#69
Nov1909, 03:17 PM

P: 328

Nowhere in his lectures does Feynman question the validity of Faraday's Law.
He merely adressed the issue of scale, which puts limits on the applications of every law. General Relativity breaks down on a small scale, and quantum mechanics takes over, but neither is more or less "valid". Since we haven't discovered the infamous "Theory of Everything", we have to break physics into separate models with particular jurisdictions. 



#70
Nov1909, 08:41 PM

P: 79

Claude,
FL is not related to u x B and says nothing about timechanging flux with respect to a conductor. EMF =  d[tex]\Phi[/tex]/dt is the whole of FL with the only restriction being that it only applies to circuits. A simple case of a flux linkage without the conductor being in the magnetic field is a magnetic flux confined to a tube threading a circuit without touching the wire of the circuit. Transformer EMF does not even require a circuit, just a closed path. An intrinsically timevarying magnetic field produces an electric field. Any closed path within this electric field which has a magnetic flux linkage will have an EMF. I do not know at this point how to get a drawing into my reply. I will try to find out later. The homopolar generator is clearly a case of steady state operation. There is no flux change. To a first approximation, the magnetic flux lines are parallel to the plane of the circuit, so there is no flux linkage. FL gives an EMF of zero because of the zero flux change, which is completely wrong. In the case of the modified toroidal transformer, there is a constant flux in the core. The secondary winding is gradually unwound, obviously eventually eliminating its flux linkage to the core flux. The slip ring and brush combination is just to allow the winding to be unwound without breaking the circuit. There is obviously a change in the flux linkage, and so FL predicts an EMF, which could be substantial if the core flux is big and the unwinding is fast. There is no intrinsic change of flux, so Maxwell's Law for transformer EMF does not apply. For a theoretical example I see no reason not to assume no magnetic field leakage from the core. Therefore there is no motional EMF. Since between them transformer EMF and motional EMF cover all cases, there is no EMF I have a bachelor of science in engineering degree in electrical engineering from the University of Michigan. It seems that, unlike others, I actually understand Faraday's Law, and was able to spot the inconsistencies and pure nonsense surrounding it in the textbooks. It seems from your comments that your understanding of the subject is between minimal and nonexistent. chrisphd, The version of FL we have been discussing cannot be derived. anirudh215, You are confusing the FL we have been discussing with one of Maxwell's Laws. The version of relevance utilizes the ordinary derivative, not the partial derivative. Archosaur, When you say that nowhere in his lectures does Feynman question the validity of FL, you are wrong. See these previous posts in this thread: Phrak Oct 1109, 11:53 PM and my post of Oct 1509, 08:14 PM. Mike 



#71
Nov2009, 12:31 AM

P: 413

http://en.wikipedia.org/wiki/Stokes'_theorem What I stated above was Faraday's law in differential form; below that I also stated it in integral form. 



#72
Nov2009, 08:07 AM

P: 997

As far as my understanding of FL being "between minimal and nonexistent", here is my reply. That is a pretty bold statement from someone with a mere BS. I have about a year or less left on the Ph.D. (EE). I worked in the magnetic components design group for 4 years in the 1980's for a fortune 50 company famous for transformers. I used FL on a daily basis. Since then, I've designed some custom transformers for switching power converters. You don't know as much as you think. At the BS level, one can only accept e/m field teachings on faith. It takes post graduate study and/or working in the field to really digest it. If you "saw its inconsistencies" in undergrad, you are superhuman, which I don't believe you are. My understanding agrees with the peerreviewed texts and publications. Who are you to say otherwise? I'll try to make time this weekend to compute the equivalence between "N*dphi/dt" & "u X B". Claude 


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