Creating a Poleless Magnet Ring

[stevenb]

The version we have been discussing in this thread is just that one with the partial derivative changed to the total derivative, which is obviously suspect on the face of it. Actually, it makes sense to call that one of Maxwell's Laws Faraday's Law, since the other version is false.... but that Faraday’s Law is an oversimplification for undergraduate students. My view is that if simplification is desired, Faraday’s Law should just be eliminated.

Mike

Unfortunately, what you are putting forth here is complete total and utter misinformation. I don't expect anyone to just take my word for it, but I refer to Jackson section 6.1 for this. In this section Jackson talks about Faraday's Observations, and clearly shows that the mathematical expression of Faraday's Law is with the time derivative on the outside. These are equations 6.1, 6.2 and 6.3. These equations are then restated in summary form as equation 6.4.

Jackson then talks about special relativity (with obvious implied reference to Lorentz invariance). He then goes on to examine the consequences of Galilean invariance. Note that Galilean invariance is an approximation. Maxwell's equations, in correct form, are Lorentz invariant, not Galilean invariant. However, at speeds much less than the speed of light the equations valid under Galilean invariance are useful, even if approximate.

You have spread so much misinformation that I'm forced to post the exact pages I'm referring to. There it is in black and white in a "Graduate Level Text" in the most respected book on electromagnetic fields.

Since, I can only attach 3 pages, I'll follow this post with the last page.

 

[stevenb]

And the final page, with the most important quote "Faraday's law is no approximation".

Study this carefully and get off your kick to try and disprove Faraday's Law. It does nothing but spread misinformation.

 

[MS La Moreaux]

stevenb,

Thank you so much for posting the links to the pages from Jackson. They are perfect for succinctly presenting the mainstream view. You are right on two counts. Jackson backs you up, and I disagree with Jackson.

Let us consider a few claims made by Jackson:

1. On the second page, equation 6.4 refers to circuit C. It is stated that “The circuit C can be though of as any closed geometrical path in space not necessarily coincident with an electric circuit”. This means that if there exists a static magnetic field in space, and one imagines a closed path in the shape of a circle in that field, and one mentally rotates that closed path about one of its diameters, that this mental activity will induce a real emf in that closed path! Pretty neat!

2. In equation 6.4, Faraday’s Law has the total derivative. In equation 6.11 on the fourth page, it has the partial derivative. Thus, Jackson seems to be stating that both forms are equivalent. But the partial derivative eliminates any effect of motion, while the total derivative includes the effects of motion. One includes motional emf while the other does not.

3. There is no electric field associated with v x B. The curl of v x B is therefore always zero. Thus, the second term on the righthand side of the last equation on the third page is always zero, which invalidates equation 6.5.

It seems as though Jackson does not really understand induction. He seems to have a lot of company.

Mike

 

[stevenb]

It seems as though Jackson does not really understand induction. He seems to have a lot of company.

I think induction is a very tricky topic to understand in its deepest levels, but you still have provided no proof that Faraday's Law is invalid. Feynman also provided no proof. All of his objections were answered in the attached well-known paper by Scanlon et. al. (and yes I know you would be counting them with Jackson, since the approach is the same), and there is the much later Munley paper that you also have an objection with.

This forum requires references from established textbooks and refereed journals. You have provided no such references. You keep harping back on the challenges Feynman brought up in the 1960's even though the Scanlon paper addressed those examples, not too long after that. You can offer no proof of your claims in line with the rules of the forum, and you also refuse to do any experiments, or offer any analysis of your own, or try to publish what you claim is your own discovery that invalidates Faraday's Law.

If you are going to accuse Jackson, Scanlon, Henriksen, Allen and Munley of not understanding induction, maybe you should first explain to us how you can have a closed path that encircles a magnetic field line a fractional number of times. Please attach a drawing and educate us.

The bottom line is that your explanations make it clear that you really don't know how to make a proper closed path and surface that is valid for applying Faraday's Law. If you would focus on learning the proper way to do this, you would find that most of your objections go away. Even then, Faraday's Law will be a little confusing to you, and the rest of us, because it is inherently a non-intuitive law that is challenging to think about in some contexts. But, our difficulties should not be taken as evidence that the Law is invalid.

Should you ever find a real violation of Faraday's Law, again I say "by all means publish it". Then it will be a reasonable topic for us to talk about in a forum like this. Why do you spend so much time trying to convince people in forums with unfounded statements, rather than publishing meaningful science in the accepted journals that would lend credence to your claims? This makes no sense to me.

 

[MS La Moreaux]

stevenb,

Thank you for providing the link to the Scanlon paper. His analysis of the homopolar generator suffers from the same flaw as Munley’s. It should be obvious that the magnetic field linking the circuit does not vary with time. It is a steady state case. What happens after a complete revolution? Does the linkage just keep increasing without limit, beyond the total flux of the source? The entire analysis is spurious.

The analysis in the main part of the paper is implicitly restricted to that particular type of case, involving a wire circuit moving through a magnetic field or sitting in a changing magnetic field. Faraday’s Law is not supposed to be limited to such cases.

As to fractional turns, consider a linear solenoid. Turns are being removed at one end by unwrapping the wire around the stationary core. As a turn is being removed, the wire crosses through the flux so that the fraction of the flux linked to the circuit is gradually decreased. This, of course, cannot be done with a toroidal electromagnet because the wire has to be threaded through the hole in the torus. With the device of the slip ring, however, the effect is similar. The circuit is completed from the end of the winding through both sides of the slip ring to the brush. As the winding is unwound, the proportion of the flux linked by each side of the slip ring changes gradually until another whole turn is completed and the slip ring momentarily is not part of the circuit.

I did not set out to overthrow Faraday’s Law. I noticed logical errors in my textbooks. After studying the situation long and hard and checking out other sources, I came to the conclusion that Faraday’s Law is false. There are multiple reasons that all lead to this conclusion. No one has been able to refute any of my points, and few have tried. They abuse my motives, denigrate my credentials and repeatedly recite references that they can parrot wonderfully but do not seem to understand. They profess their complete faith in Faraday’s Law and do not seem willing to question it, even in the face of simple, reasonable, straightforward, logical challenges to it.

You claim that motional and transformer emf are just terms used to classify. I believe that each is the result of recognized and independent principles and that it is equally recognized that together they explain every case of induction. You challenge me to perform an experiment that proves my case. My counter example clearly distinguishes between flux change solely due to motion and motional emf. Can you provide references to any experiments that demonstrate that flux change due to motion produces an emf that cannot be attributed to motional emf? It seems to me that Faraday’s Law has not been demonstrated to be true in every type of case. It has not been shown that Faraday’s Law exactly encompasses motional and transformer emf, neither more nor less. It seems that the textbook authors have been copying from one to another for generations without realizing that what they claim has not been and cannot be established. If the reviewers do not understand the subject, what is the use of submitting a paper to be reviewed?

I would really like to find out exactly on what we agree and disagree. If we could establish a common ground from which to argue, I believe that it would be enlightening.

Do you believe that motional emf and transformer emf are independent? I do.

Do you believe that a solely motion-caused change in a flux linking a circuit produces an electric field? I do not.

Do you believe that a solely motion-caused change in a flux linking a circuit produces an emf solely due to the flux change? I do not.

Do you believe that a motion-caused change in a flux linking a nonconductive closed path results in an emf? I do not.

Mike

 

[pallidin]

Well, nothing is "God speak" to be sure.
Have you performed ANY, actual experiment that validates your position?

 

[MS La Moreaux]

pallidin,

No, I have not.

Mike

 

[pallidin]

OK, what I would recommend is that you design and execute an experiment that supports your theory.
Give it much thought... these things are not easy.

 

[MS La Moreaux]

palliidin,

I appreciate the encouragement, but I will not be doing any experiments. I do not see that they are necessary. I believe that analysis of what has already been done and its explanation is all that is required. I know that everything published by "experts" is not correct because they disagree with each other, and other statements just defy logic. I believe that the subject is basically simple, but that mathematics, while necessary, obfuscates the subject. The underlying principles seem to get lost in the exercise. Math is not physics, so that math that is perfectly good in its own right is misapplied to the physical situation. Then, to justify incorrect results, tortured reasoning is used.

Mike

 

[stevenb]

Thank you for providing the link to the Scanlon paper. His analysis of the homopolar generator suffers from the same flaw as Munley’s. It should be obvious that the magnetic field linking the circuit does not vary with time.

You are very welcome. You are partially correct in that his analysis is basically the same. I see no flaw in the analysis myself (I don't see any of the other flaws you mentioned previously either, but I don't see the point of arguing all these points). It should be obvious that his chosen closed path does see a flux change. Your path does not, but you are not using the correct path. He explains this clearly, but you decide to reject it. That is your right, but you are basically choosing to be wrong. This might come across as arrogant, but I mean this from a logical point of view. You chose a method that gives the wrong answer. Everyone else chooses a method that always gives the right answer, based on experimental verification. We call the method that always gives the right answer Faraday's Law. Guess why. Because it mathematically gives the results that agree with Faraday's observations, so we do him the honor. But, you say, "No, don't do it that way. Do it my way. Oh, and when you do it my way, you get the wrong answer, therefore Faraday's Law is wrong". That's basically what you are doing whether you can see it or not. Or, maybe you think you are doing it Feynman's way. Well, his way was wrong. Question, did you ever see Feynman publish a rebuttal paper to the Scanlon paper? Or, was the conspiracy so large as to silence him as well?

What happens after a complete revolution? Does the linkage just keep increasing without limit, beyond the total flux of the source? The entire analysis is spurious.

This is a perfect example of one of your many misconceptions. No offense intended here, I mean this as constructive criticism. Yes, the flux does keep increasing without limit for the chosen path. The point you are missing is that these chosen paths, and their associated fluxes, are arbitrary. The flux change, and not the flux value is the physically meaningful entity. Remember that Faraday's law works for any and all closed paths (i.e. as prescribed). You are free to choose, but some are easier to analyze than others, so pick the easy one. However, and this is the important point, once you choose, you can't abandon that contour and choose another. Stick to the chosen contour (by the given rules which are required by basic physical principles), no matter what it does.

You make the same mistake in your toroid thought experiment. You assume because you can consider the unwound coil as having no wraps, while the starting arrangement has (let's say) 100 wraps, that flux must change. But I can choose a contour for the unwound coil that has 1 million encirclements by going 1 million times around your slip ring. Once I choose this contour, I can reverse to rotation and rewind the physical coil with 100 wire wraps, but my chosen contour still has 1 million wraps. That's what Faraday's law demands. If you don't do this then you are not applying Faraday's law. The number of coil wraps is not generally the same as the number of encirclements used in Faraday's Law. In simple cases it often is, but there is no requirement in general. It's that simple.

As to fractional turns, consider a linear solenoid. Turns are being removed at one end by unwrapping the wire around the stationary core. As a turn is being removed, the wire crosses through the flux so that the fraction of the flux linked to the circuit is gradually decreased.

Correct!

This, of course, cannot be done with a toroidal electromagnet because the wire has to be threaded through the hole in the torus.

Correct!

With the device of the slip ring, however, the effect is similar. The circuit is completed from the end of the winding through both sides of the slip ring to the brush. As the winding is unwound, the proportion of the flux linked by each side of the slip ring changes gradually until another whole turn is completed and the slip ring momentarily is not part of the circuit.

Sorry friend, but you are completely wrong here. Remember I asked you to attach a drawing? If you draw it out, you will see your error. What you are describing is not possible. If you do not cross through the flux by cutting the toroid, you can not decrease the number of turns fractionally. Draw it out please. This is too trivial a thing to even be discussing.

If you can't get past this simple part, there is no point for me to go on. If you can at least acknowledge this fact, then I would be willing to go on to your other questions and carry the discussion further. I don't mean that in a rude way, but honestly, if we can't agree on this simple topological point, there is no chance for us to find common ground. If you can't acknowledge this point, then I'm perfectly happy to leave the thread as is because there is no chance for any serious person to be misinformed by your ideas. They will see the unwillingness to face simple logic right here. And, if there are any that don't see it, well they don't have much chance of learning this anyway, and no harm is done.

Bottom line, ... if you wrap a closed contour path around a closed magnetic flux line, you must do this an integer number of times. Like I said, draw me a counter example. Draw an example with 2.25 turns, just to keep it simple. Maybe if I see how you are visualizing it, we can get to the bottom of the confusion.

By the way, you go into many questions on what you and i believe. What we believe is irrelevant. Science has a process and the process has been followed and continues to be followed. I personally see no flaws in the current acceptance of Faraday's Law, but I see a great number of flaws in your ideas. Obviously, you will say just the reverse, so I guess I can't blame you for sticking to your guns. However, I do blame you for not following the scientific method here, but I've already said enough on that point. You are free to either accept this criticism or not.

and repeatedly recite references that they can parrot wonderfully but do not seem to understand. They profess their complete faith in Faraday’s Law and do not seem willing to question it, even in the face of simple, reasonable, straightforward, logical challenges to it.

Don't think that I have not given this topic considerable thought. I've studied EM theory for 25 years, taken 2 undergrad classes, 2 grad level classes, about 10 other grad level classes that use EM theory. Then after school, studied it as a hobby and use it my profession as an electrical engineer. I did an experiment on Faraday's Law as part of engineering research that was published just last month. I still consider myself an amateur when it comes to EM fields, and still continue to study and apply it, but I know enough to follow any of the papers and texts that have come up here. They are logically sound and consistent from both a mathematical and experimental point of view. This doesn't mean that Faraday's Law is not open to question, but it means that it takes more than your words, without experiment and analysis or solid logical foundation, to question it.

I recall reading your words at another forum, that you don't even understand special relativity. Have you considered the possibility that instead of the entire world being wrong, that maybe you are the one with the improper foundation to make full sense of this subject?

 

[stevenb]

Like I said, draw me a counter example. Draw an example with 2.25 turns, just to keep it simple. Maybe if I see how you are visualizing it, we can get to the bottom of the confusion.

So, I thought that I should post a drawing if I'm going to ask you to post a drawing. In my attached example, I show a closed contour (dotted red line) which will have an attached surface that must cut through the toroid's internal field 2 times. The drawing shows 4 views with one quarter turns of the slip ring. A voltmeter is shown (note that the voltmeter's resistance closes the path, you can implement the voltmeter by any means, or just short the path if it bothers you) with an attached conductive wire (solid black line) that attaches to a conductive brush. The brush always contacts the conductive slip ring which is attached (at the black dot) to a wire which initially wraps the toroid twice. As the wire is pulled, the slip ring turns, and the wire unwinds. The red dotted line shows a contour path that can be used with Faraday's law. I could choose another contour, but I decide on this one because I find it simplest to think about it.

As I asked, can you draw a similar example with the contour encircling the toroid 2.25 times (or choose any fractional value you like) at any point in time?

 

[MS La Moreaux]

stevenb,

Wow! What spectacular drawings. I have no idea how to do that. I certainly do appreciate the trouble you have gone to in providing them.

Congratulations! You got me! (Appropriate head slap here.) Before seeing your images, after further reflection, I came to the conclusion that my counter example is flawed, but for a different reason than you give. In my analysis the flux decreases to zero in steps, one for each turn of the winding. But in between, the circuit would split into two paths around the slip ring and then reconnect, providing a smooth decrease in the flux. The two paths, of course, are not addressed by Faraday’s Law. I would post a correction to my original thread, but it is locked.

I must say that I do not understand the use of the contour. I would think that it would have to be the circuit. If it is not the circuit, then how is it related to reality? What is the meaning of the emf if the emf is not that of the circuit? This seems to be one of those many areas of confusion surrounding Faraday’s Law. Is there supposed to be an emf around the contour in a case of pure motional emf where the contour is not a circuit?

I apologize for my slow responses, which may get even slower. I have a busy schedule ahead.

Mike

 

[MS La Moreaux]

The idea of the contour circling the slip ring one or more times makes no sense. The slip ring is a single-turn winding, and traversing it one complete time or more does not constitute a closed path in the sense of Faraday's Law.

Returning to Scanlon's analysis of Faraday's disk generator, he assumes a uniform magnetic field over the disk. In the version Richard Feynman addressed, this was not the case. A bar magnet was positioned near one portion of the disk. Thus, according to Scanlon's analysis, there would be a pulsing emf as the contour passed by the magnet. This is not what actually happens; the emf is constant.

To make my objection to Faraday's Law clear, let us examine a typical wire-wound generator. The emf is motional. The wire of the armature winding cuts the magnetic field lines, resulting in magnetic forces on the free electrons in the wire, constituting an emf. This emf is numerically equal to minus the time rate of change of the magnetic flux linking the winding, but it is not the result of that changing flux. The motional emf and the flux change are mathematically dependent, but physically independent. Therefore, the changing flux linkage has no effect. Faraday's Law does not explicitly state, but implies, and it seems to be widely believed, that the flux change produces the emf. The mathematical equality between the flux change and the emf in this example was derived only for this type of case and only holds for this type of case, not for every case.

Mike

 

[stevenb]

The idea of the contour circling the slip ring one or more times makes no sense. The slip ring is a single-turn winding, and traversing it one complete time or more does not constitute a closed path in the sense of Faraday's Law.

I believe it does make sense and it does constitute a closed path as allowed by Faraday's Law. Again, you are not free to make up your own rules. If you make your own rules, all you do is disprove Moreaux's Law and not Faraday's Law. You will get no argument from me that Moreaux's law, and his general approach to science, are flawed.

However, there is no requirement to use the multiple encirclements contour, if it bothers you. You are free to create a mathematical contour that instantaneously changes the number of loops by integer steps. However, applying Faraday's law at the time that you switch contours becomes tricky mathematically. If you instantaneously change the contour abruptly, then the integrated flux changes instantaneously and the time derivative goes to infinity. Thus, Faraday's law shows no EMF as the ring spins, and at the instant of switching contours, the calculation is undefined. So, you can't say what the EMF is at that instant.

But, now someone else comes along and uses the same approach, but changes the path just slightly so that his contour switch happens 1 picosecond later. Now he finds the EMF is zero for all times, except he doesn't know the answer for the (different) time of his contour switch. Then you both compare notes and realize that EMF must be zero always.

Returning to Scanlon's analysis of Faraday's disk generator, he assumes a uniform magnetic field over the disk. In the version Richard Feynman addressed, this was not the case. A bar magnet was positioned near one portion of the disk. Thus, according to Scanlon's analysis, there would be a pulsing emf as the contour passed by the magnet. This is not what actually happens; the emf is constant.

You amaze me with the words "according to Scanlon's analysis". You are describing a different problem which they didn't solve, so you can't say what their analysis would be for your example. I'm also amazed that you have not presented your own analysis, yet you claim to know the answer that Faraday's Law will give. This is not a trivial problem to solve, and Munley discusses some of the issues here such as the non-conservative nature of the field which results in EMF that is path dependent. So it's not clear how Faraday's Law relates to a meter measurement. I'd have to think on this long and hard before I could make any definitive statements here. I'd love to see your solution, then I could make a judgement. Until then, your objection is just a wild claim with no support. Indeed, you have claimed both the mathematical solution and the experimental result with no support.

If you want to overturn Faraday's Law, the burden in on you to prove both statements.

1. Faraday's Law predicts pulsing EMF to an observer in a particular reference frame.

2. Experiment shows constant EMF for the same observer and reference frame.

One or both of your statements may, in fact, be true, but just stating them doesn't make it so, and you can't expect the rest of the world to accept your statements on faith. You have been given considerable license by the moderator here to present your views. You say you don't want to publish because the "establishment" will reject your ideas, but you don't even take advantage of this venue. Where is your proof? Let's see it.

To make my objection to Faraday's Law clear, let us examine a typical wire-wound generator. The emf is motional. The wire of the armature winding cuts the magnetic field lines, resulting in magnetic forces on the free electrons in the wire, constituting an emf. This emf is numerically equal to minus the time rate of change of the magnetic flux linking the winding, but it is not the result of that changing flux. The motional emf and the flux change are mathematically dependent, but physically independent. Therefore, the changing flux linkage has no effect.

I'm not sure what you are getting at here. I think I agree so far (remember that what you and I agree on is irrelevant because we could both be wrong), but so what? Maxwell's equations are not really cause and effect relationships because electric field and magnetic field are just components of the electromagnetic field tensor. They are related, but it is incorrect to say that changing magnetic field causes electric field, or that changing electric field causes magnetic fields. This is another reason why your lack of training in special relativity is a big problem. Without an understanding of the covariant nature of the electromagnetic field tensor in vacuum, as well as an appreciation of the reference frame issues that come up with fields in media, you are at a serious disadvantage. Scanlon et. al. discuss this issue and point out that a circuit with moving media is akin to a fluid, and this brings in reference frame complications. This is tricky stuff to to get right, but again I point out that our personal difficulties are not evidence that the law has exceptions. One must think carefully and apply all relevant known scientific principles. Some of these principles were not fully understood until long after Faraday's and Maxwell's publications. Still, Faraday is not the one who tried to formulate a mathematical formula, and indeed he was highly suspect of attempts to do this. Faraday's observations and intuitive description of the Law still hold up today.

Faraday's Law does not explicitly state, but implies, and it seems to be widely believed, that the flux change produces the emf.

You are correct that the law does not STATE that. As to what it IMPLIES, that is more debatable, but I'm willing to agree (for the sake of argument) that the implication is there in the vector form of Maxwell's equations. A proper tensorial representation would not have this implication. Still, this is all irrelevant to the validity of Faraday's Law. It either predicts correctly, or it does not. So far, you have not provided convincing evidence that the Law fails to predict. I'm open to consider your evidence, but so far I've seen nothing that looks like a serious analysis or any attempt whatsoever at an experiment.

The mathematical equality between the flux change and the emf in this example was derived only for this type of case and only holds for this type of case, not for every case.

This is an unfounded statement. I believe it is incorrect, but I'm open to see the evidence you have as to it's correctness. However, the first part of this is still irrelevant to the validity of the law. Even a law that is a wild guess might ultimately hold up to scientific scrutiny.

Again, you have not provided anything that even hints at proof that Faraday's Law is invalid. So far, it is alive and well, ... retired and taking a snooze at a beach in Florida. I have yet to meet an immortal law of nature, so perhaps its days are numbered and perhaps you are its "Grim Reaper". Do you have an axe? Do you have the strength to wield it?

 

[pallidin]

Again, you have not provided anything that even hints at proof that Faraday's Law is invalid. So far, it is alive and well, ... retired and taking a snooze at a beach in Florida. I have yet to meet an immortal law of nature, so perhaps its days are numbered and perhaps you are its "Grim Reaper". Do you have an axe? Do you have the strength to wield it?

Ah... I like Florida.
Still, the question stands intoned by steven; what EVIDENCE disputes Faraday's laws?
Not meaning YOUR evidence, rather, evidence that is acceptably replaceable.

 

[mrspeedybob]

All magnetic field lines of a permanent magnet must pass through the magnetic material, since the iron atoms are the source of the field lines, and therefore the lines must pass through the iron atoms. Any external field lines must therefore pass through the surface of the torus. There is no way for this to happen without violating symmetry. Thus, there can be no external field.

Mike

I think there is a serious flaw in your reasoning here. It is true that all field lines must pass through the material from which they originate but magnetic field lines add like vectors so two fields of opposite direction can pass through the boundary of the material at the same point and cancel each other out resulting in the illusion of a field that remains entirely outside the material. I made a picture to illustrate.

In this picture, if you were to measure magnetic flux at the surface of the bar you would read zero because all flux lines are canceled out by opposing flux lines. They have no effect, that doesn't mean they don't exist. There existence is demonstrated by the existence of the external field which doesn't appear to enter the material, but actually does.

 

[MS La Moreaux]

stevenb,

Regarding the bar magnet version of the disk generator, the output would have to be constant by symmetry.

In general, we seem to be coming from different directions and talking past each other. I realize that this frustrates you. It does me, too, and I am sorry. You have more education in the subject and bring in references to things that I have not even heard of. I, of course, cannot respond in kind, much as that would be desirable. I am just going by what is in my freshman physics and undergraduate electromagnetics textbooks. I have not done any experiments, to speak of. Basically, I only know what I read and have been taught. I rely upon what is in those textbooks for the most part, although I find inconsistencies in them. I take it that you disagree with my acquaintance, Professor Riles, who said that Faraday’s Law is an oversimplification for undergraduate students. If you do not disagree with him, then I cannot hope to argue with you at the higher level because of your superior knowledge and would have to defer to your way of thinking. If you do disagree with him, then I have to assume that my textbooks are correct, for the most part. My physics textbook states that Faraday’s Law only applies to circuits. The general closed path, whether conductive or not, is reserved for Maxwell’s Law for transformer emf. This would seem to obviate your elaborate arbitrary contours. In the case of the disk generator with a uniform magnetic field across the disk (the field lines orthogonal to the disk), consider making the disk nonconductive. You seem to be saying that there would still be the same emf according to Faraday’s Law. Is this so?

Mike

 

[MS La Moreaux]

mrspeedybob,

Oh, dear, I thought that this had been settled. Your reasoning is very clever and insightful, and I certainly did not consider the resultant vectors. The field lines of the iron atoms seem analogous to those around the turns of a toroidal electromagnet. In that case, the external magnetic field is severely attenuated by symmetry. I cannot find a flaw in your analysis, probably because I cannot visualize the field sufficiently, but since I know that the effect in the toroidal electromagnet is real, I must conclude that somehow your analysis is incorrect. Perhaps someone else can shed more light on this matter.

Mike

 

[stevenb]

Regarding the bar magnet version of the disk generator, the output would have to be constant by symmetry.

I agree with you. I would expect an experiment to give constant output. An experiment is always a good idea, especially in a case like this where it is so simple, but I would be surprised to see a different result.

Again, I would have to put more thought into this example to be sure of how to analyze this using Faraday's Law. One thing is clear. The answer you get with Faraday's Law (or by any other method, such as using Lorentz force directly) is path dependent. So there is no clearly defined EMF through Faraday's law, unless you clearly identify the path a charge would take. So one needs to think carefully about what it means to measure a voltage with meter, and define a "measured EMF" that would consider all paths via some formalism. I'm confident that the proper analysis over all paths will yield the constant value you are seeking. This is not a trivial problem to solve, but it is an interesting one. I plan to spend some time thinking about this. But, I must stress that this example does not invalidate Faraday's law, even if it does cause a person to pause and think, or even if it stumps a person completely. This issue is discussed by both Munley and Scanlon et al.

In general, we seem to be coming from different directions and talking past each other. I realize that this frustrates you. It does me, too, and I am sorry. You have more education in the subject and bring in references to things that I have not even heard of. I, of course, cannot respond in kind, much as that would be desirable. I am just going by what is in my freshman physics and undergraduate electromagnetics textbooks. I have not done any experiments, to speak of. Basically, I only know what I read and have been taught. I rely upon what is in those textbooks for the most part, although I find inconsistencies in them.

I find inconsistencies in textbooks also, and Scanlon et al, discuss this at great length. This just tells you that the subject is not simple. I can't stress enough how unintuitive Faraday's Law is, particularly when you deal with nonconservative fields. If you want to say that teachers need to do a better job, or that textbooks need to be improved, then I agree. The references I provided are well respected in the scientific community, so they would be a good place to base your understanding off of. That doesn't mean that they are necessarily unquestionable, and nothing in science is unquestionable anyway. However, I do disagree with prematurely forming the conclusion that accepted scientific laws (such as Faraday's) have exceptions, without adequate scientific proof using accepted scientific methods.

I take it that you disagree with my acquaintance, Professor Riles, who said that Faraday’s Law is an oversimplification for undergraduate students. If you do not disagree with him, then I cannot hope to argue with you at the higher level because of your superior knowledge and would have to defer to your way of thinking. If you do disagree with him, then I have to assume that my textbooks are correct, for the most part.

I don't disagree with anything that you quoted from Prof. Riles. I don't see anywhere in his quote where he say "Faraday's Law is an oversimplification for undergraduate students". I doubt very much that he would say this. I also did a search on him and read some of his comments at his website. I don't recall reading anything I would necessarily disagree with (at least not strongly). A couple of his comments are ambiguous to me (which may be my failing, not his), so I can't say whether I agree or disagree on those, but there is nothing in his statements that can be compared with your claims about Faraday's Law being false or having exceptions. Also, I noted at his website that he mention's Jackson as a respected graduate level text.

So, I don't think it's fair to lump Prof. Riles in with your ideas, based on hearsay. You can contact him and have him give a direct opinion here, but without that, I will assume his views are not as extreme as yours.

My physics textbook states that Faraday’s Law only applies to circuits. The general closed path, whether conductive or not, is reserved for Maxwell’s Law for transformer emf. This would seem to obviate your elaborate arbitrary contours. In the case of the disk generator with a uniform magnetic field across the disk (the field lines orthogonal to the disk), consider making the disk nonconductive. You seem to be saying that there would still be the same emf according to Faraday’s Law. Is this so?

To me, that group of sentences is just misconception layered on top of misinformation.

1. Faraday's law applied to circuits is called Kirchoff's Voltage Law (which is also mistreated in many texts, but let's not go there). Please provide the reference to your physics book, so we can identify a book that should be avoided.

2. Your statement about what general closed paths are "reserved" for is completely unfounded. What is this based on? You seem to just pull these rules out of the air.

3. What does it mean to say the disk is non-conductive? Where in your analysis does the conductivity of the disk come into play? Did you assume it is a perfect conductor? Did you assume that the conductivity was that of copper at room temperature? Or, did you assume it was steel, lead, tin, aluminum? Does non-conductive mean just a good insulator, or a perfect insulator? How do you then measure the voltage? Do you use a real volt meter, or a hypothetical perfect voltmeter with infinite internal resistance? How do you calculate the voltage from a voltage source with infinite source resistance using a meter with infinite load resistance? What if I take a real copper disk and short it out with a superconductor? What then is the output voltage?

Ultimately, what do you care what I think anyway? I must stress that all this sidesteps the big issue here. What I think or say, or what a textbook author thinks or says, or what you come up with in thought experiments is not the sum total of what the current status is of the scientific validity of Faraday's Law. There is a mountain of scientific evidence out there that takes priority over all of this. So, ask questions, raise objections, devise thought experiments, do real experiments and talk about this subject all you want, but hold back on stating "Faraday's Law is false", until you have a scientifically credible basis to state it. Neither one of us is in a position to make such a statement. However, I am in a position to state I am not aware of any known scientific evidence that contradicts Faraday's law. Even in studying it in the context of General Relativity, it is found to be correct. The one area where we might be able to question it is in the context of quantum field theory. This is an area that I've been trying to learn on my own, but it is most challenging and I can't say how Faraday's law holds up in this domain. This is a question I'm curious about myself and hope to answer (or have answered for me), someday.

 

[MagnetDave]

Mike,

I'll resist the urge to comment on your quixotic mission to knock over Faraday's Law - which is how I originally found the forum. I found this post because, well, it touches on my expertise, so I'll confine this to the original question:

Yes, you can make a toroidal magnet where the field is very nearly completely internal. If you envision slicing the ring into a succession of wedges, and magnetizing each wedge in the tangential direction, and then assembling them, this will produce your desired ring.

However - when you say "a magnet with no poles" you are revealing a fair amount of misunderstanding of magnets. This is more analogous to a snake eating its tail than a magnet with no poles. (In fact, we do this routinely to ship magnets safely, and the practice is called snaking.)

I strongly recommend you try your concept out experimentally. However, on my limited understanding of it, your zero result seems to be exactly what Faraday's Law would predict, so I'm not sure how it disproves it.

 

[MS La Moreaux]

stevenb,

My physics textbook is University Physics, Third Edition, by Francis Weston Sears and Mark W. Zemansky, Addison-Wesley Publishing Company, Inc., Copyright 1964, Second Printing - March 1965. It states, “Hence we can say: The induced electromotive force in a circuit equals the negative of the time rate of change of magnetic flux through the area bounded by the circuit. The preceding statement is known as the Faraday law.”

When Faraday’s Law is restricted to circuits it is bad enough, but when that restriction is removed, as you seem to imply that it is, it is even worse. It would seem that you would say that if the disk of the disk generator were removed, you could still use the same contour of Scanlon’s analysis and rotate it at the same rotational speed and get the same emf. If this is the case, then in contrast to what you said previously, Faraday’s Law does seem to state that the flux change produces the emf. (I am not quibbling over cause and effect but merely using language like “responsible for” and “produces” as shorthand for convenience.) I have to go back to the basics, again. Motional emf requires a conductor. Maxwell’s Law for transformer emf does not. Your version of Faraday’s Law seems to be a generalization of that Maxwell’s Law extended to closed paths with motion. But Maxwell’s Law requires an intrinsic flux change. The extended version does not. If there is no transformer emf and no motional emf, what is responsible for the emf? Transformer emf is a function of an electric field associated with the changing magnetic field. Motional emf and Scanlon’s contour are not associated with an electric field. The emf in motional emf is the result of magnetic force on the electrons in the conductor. What is the supposed emf of Scanlon’s contour due to? The question of measurement is valid, but only from a practical standpoint. Conceptually, an emf is real whether or not it can be measured practically. A stationary electron situated somewhere along the contour at some instant should experience a force. What kind of force would it be, electric or magnetic? If either, what would be the mechanism? Scientists do prefer mechanisms, where possible, rather than just results. Quantum mechanics is a notable exception. Often results are questioned when there is no apparent mechanism to produce them.

Mike

 

[MS La Moreaux]

MagnetDave,

I have already gone over how to make the magnet. That is not the problem.

I do not see a problem with the magnet having no poles. Poles involve a concentration of field lines emerging from the surface, and symmetry prevents this.

As I have already posted, I now realize that Faraday’s Law does not apply to my example because of the slip ring.

Mike

 

[stevenb]

My physics textbook states that Faraday’s Law only applies to circuits. The general closed path, whether conductive or not, is reserved for Maxwell’s Law for transformer emf.

It states, “Hence we can say: The induced electromotive force in a circuit equals the negative of the time rate of change of magnetic flux through the area bounded by the circuit. The preceding statement is known as the Faraday law.”

The above are your two statements about your physics book. The second statement indicates that your first statement is a misinterpretation (yet another among the many others). The book is not stating that Faraday's Law ONLY applies to (conductive) circuits. It is saying that Faraday's law CAN be applied to conductive circuits.

So, not only do you spread misinformation about Faraday's law, but you also misquote your teachers and your textbooks. Very sad indeed.

As I have already posted, I now realize that Faraday’s Law does not apply to my example because of the slip ring.

Yet again, complete nonsense is stated. First, your example proves Faraday's Law is false, then Faraday's law does not apply to your example at all. What sense can anyone make of your message? Hey, here's a thought (one that you are sure to ignore), maybe Faraday's law is just fine and does apply to your example, hence providing one more drop of water to the ocean that is the scientific evidence supporting Faraday's law.

Your other comments are so full of additional misinterpretations and misrepresentations and misinformation, that I can't even comment anymore, as we seem to be moving backwards.

I think MagnetDave has found the perfect adjective when he describes your quest as quixotic. I'm now realizing that my attempt to help you is equally quixotic. Hence, I will now bow out of this thread and give poor Rocinante a much needed rest.

 

[MS La Moreaux]

stevenb,

I believe it is you that is misinterpreting the textbooks, as well as my posts. I agree that we are not getting anywhere, so farewell.

Mike

 

[stevenb]

stevenb,

I believe it is you that is misinterpreting the textbooks, as well as my posts.

No, the situation is crystal clear to me.

But, thank you for grouping me on the side of Faraday, Jackson, Munley, Scanlon et al. Those are good terms to leave on. I'm also happy to be not too far from the viewpoints of Riles, Sears and Zamansky as clear from your quotes of them, as well as your obvious misinterpretations of those quotes.

Any statement pronouncing to the casual reader that I'm not your Sancho Panza is most welcome.

 

[mrspeedybob]

mrspeedybob,

Oh, dear, I thought that this had been settled. Your reasoning is very clever and insightful, and I certainly did not consider the resultant vectors. The field lines of the iron atoms seem analogous to those around the turns of a toroidal electromagnet. In that case, the external magnetic field is severely attenuated by symmetry. I cannot find a flaw in your analysis, probably because I cannot visualize the field sufficiently, but since I know that the effect in the toroidal electromagnet is real, I must conclude that somehow your analysis is incorrect. Perhaps someone else can shed more light on this matter.

Mike

The reason the field is significantly stronger within the material is that the fields produced from opposite sides of the material (or from opposite sides of the coil) add together at points in between them and partially cancel for points not in between them. They do not however completely cancel because one side of the coil or material is always closer then the other side. If you actually construct this magnet you will find that it does indeed have an external field,

 

[MS La Moreaux]

In analyzing Faraday's disk dynamo (the case with a uniform magnetic field through the disk), I have come up with the following. The force on a free electron in the disk is radial. Its path in the disk is therefore along a radius, relative to the disk. The drift speed of an electron is on the order of microns per second. Therefore, it will be dwarfed by the rotation speed of the disk and will be dragged along with the disk as the disk rotates. If we consider the drift path of a single electron as the circuit, it will be a very tight spiral of millions of turns between the axle and the rim, as viewed in space rather than relative to the disk. This spiral will be stationary in space, not rotating with the disk. Thus, there will be no change with time of the flux linking the circuit.

Mike

 

[MS La Moreaux]

mrspeedybob,

It has occurred to me that the external magnetic field of a ring magnet would only be due to the atoms at the surface and would thus be negligible. In the interior, the atoms would be like little bar magnets lined up head to tail. Thus, the flux would flow from one to another all the way around the ring, keeping their flux entirely confined to the material of the ring.

My physics textbook states that an infinitely long electromagnet would have no external magnetic field. If a finite electromagnet is bent around so that its ends are joined seamlessly, it would also have no external field.

Mike

 

[thehacker3]

mrspeedybob,

It has occurred to me that the external magnetic field of a ring magnet would only be due to the atoms at the surface and would thus be negligible. In the interior, the atoms would be like little bar magnets lined up head to tail. Thus, the flux would flow from one to another all the way around the ring, keeping their flux entirely confined to the material of the ring.

My physics textbook states that an infinitely long electromagnet would have no external magnetic field. If a finite electromagnet is bent around so that its ends are joined seamlessly, it would also have no external field.

Mike

But ring magnets DO have a decently strong magnetic field - so what gives?

 

[MS La Moreaux]

thehacker3,

Are you referring to the internal or external field?

Mike

 

[thehacker3]

thehacker3,

Are you referring to the internal or external field?

Mike

External

 

[MS La Moreaux]

thehacker3,

Where do you get the idea that ring magnets have a decently strong external field? Do you understand that the ring magnet is magnetized circularly so that there are no poles? Toroidal transformers are used because they have severely reduced external fields.

Mike