by MS La Moreaux
P: 963
 Quote by MS La Moreaux I have to believe that the circuit of many wires will not be equivalent to the rocking plates. Here is another counter example to Faraday's Law. In a toroidal transformer with the primary winding the inner one, the magnetic field external to the primary winding is severely reduced because geometrical symmetry results in its canceling itself out. Now take a toroidal core with a primary winding and loosely wind a secondary winding with one end going to an external circuit (such as a galvanometer) and the other going to a slip-ring loosely fitted around the primary winding. The brush contacting the slip-ring goes to the external circuit. Energize the primary winding with DC current, resulting in a constant magnetic flux in the core. Now gradually unwind the secondary winding. Its flux linkage will steadily decrease, but there will be no EMF in it because there is neither motional nor transformer EMF, thus violating Faraday's Law. Mike
Is this a joke? For a while, I thought this post was a legitimate questioning of FL. This "counterexample" is proof that this thread is for entertainment purposes only.

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

Claude
 P: 79 Claude, I was entirely serious. Do you have a specific, substantive criticism? Mike
 P: 963 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 sub-Hertz range. While unwinding, you are moving a conductor in the presence of a magnetic field. But the motion involves one turn at a sub-Hertz 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*1e-8*1e-1*1e0 = 2*pi*1e-9 = 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
 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
P: 963
 Quote by MS La Moreaux 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
Refer to bold quote. This statement actually affirms FL. If the wire moves so as to NOT cut H lines, then FL predicts 0 emf. Since curl E would then be 0. Since E has no curl, the emf is merely the line integral of E around the closed loop, which is of course 0. Hence FL predicts 0 which you insist is the correct answer. Or, if you prefer to look at it from motional quantities, "u X B", is 0, where "u" is velocity. When the motion is along a flux line, not cutting, then the cross product is 0.

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
 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
P: 963
 Quote by MS La Moreaux 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
Where did you get the idea that FL does NOT require the cutting of flux lines. The cutting is spelled out in the vector equation "u X B". The cross product of velocity U and magnetic flux density B is 0 when the velocity is along a flux line. Since the angle is 0 along a flux line, the cross product is also 0. Cutting means 90 degrees, hence the cross product is maximized.

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
 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
P: 963
 Quote by MS La Moreaux 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
Sorry, but it has everything to do with FL. The "dphi/dt" quantity is related to "u X B". In order to obtain non-zero emf/mmf, the flux must be time-changing with respect to the conductor.

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 counter-examples 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
 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)
P: 963
 Quote by chrisphd 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)
You treat magnetic fields as fictituous, pseudo, & derived. Yet Einstein emphasized in his 1905 paper, that elec & mag forces are equally important, and that neither is the "seat". Nobody has successfully refuted this viewpoint.

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
P: 413
 Quote by MS La Moreaux The version of Faraday's Law which purports to include both motional EMF and transformer EMF for circuits is false. There is no theoretical basis for it. Richard Feynman, in his "Lectures on Physics," pointed out the fact that this so-called law, what he called the "flux rule," does not always work and gave two examples. Every textbook and encyclopedia that I know of treats it as a true law. There is a lot of confusion and nonsense related to it. I believe that it is an indictment of the status quo and a scandal.
I am going to reply only to post #1 quoted above. I understand your question MSLM as it is a question that comes up often. Here is the problem:

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 = $$\epsilon$$ = $$\int_{a}^{b} (E + [v \times B]).dl$$

In certain cases, it so happens that the above equation simplifies to
$$\epsilon = - \frac{d \phi}{dt}$$.

In such cases, you will have to revert to the above definition.

$$\nabla$$ x E = $$- \frac{\partial B}{\partial t}$$

or in integral form can be written as

$$\int E.dl = - \frac{d \phi}{dt}$$ (This is only for stationary integral paths)

and is not to be confused with

$$\epsilon = - \frac{d \phi}{dt}$$

which does not work all the time. For instance, in your example above, the whole equation is ill-suited.
P: 963
Wll then, it looks like we agree after all. My understanding of relativity has been that just as an H field can be regarded as a relativistic view of an E field, so can an E field be viewed as a relativistic view of an H field. We do agree. Einstein emphasized this and regarded neither as the "seat".

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 di-polar, not monopolar. H lines are closed loops, or "solenoidal" in nature. Solenoidal flux lines indicates di-pole and NOT monopole since monopoles have a start and an end. Also, discrete charge E fields are conservative, whereas induction/solenoidal E fields are non-conservative.

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 time-varying 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 non-existance of H monopoles as a basis for treating E as more basic than H. Also, conservative E fields do not transform to non-conservative 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
 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.
 P: 79 Claude, FL is not related to u x B and says nothing about time-changing flux with respect to a conductor. EMF = - d$$\Phi$$/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 time-varying 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 11-09, 11:53 PM and my post of Oct 15-09, 08:14 PM. Mike
P: 413
 Quote by MS La Moreaux 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.
You can convert the partial derivative to the ordinary derivative using Stokes' theorem.

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.
P: 963
 Quote by MS La Moreaux Claude, FL is not related to u x B and says nothing about time-changing flux with respect to a conductor. EMF = - d$$\Phi$$/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 time-varying 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 11-09, 11:53 PM and my post of Oct 15-09, 08:14 PM. Mike
I already explained that u X B is related to -N*dphi/dt, if you consider the relative motion between the conductor and static magnetic field. With a stationary conductor, the field must change in order to obtain induction. FL covers this. But if the field is static with the conductor moving, consider the scenario from the conductor's moving reference frame. The conductor "sees" a time-varying magnetic field. When I have time, I'll compute the math based on relative motion, and the u X B in the moving frame should map into -N*dphi/dt in the rest frame. They are equivalent.

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 super-human, which I don't believe you are.

My understanding agrees with the peer-reviewed 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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