Faraday's Law: False Claim & Feynman's Critique

In summary, the version of Faraday's Law that includes both motional EMF and transformer EMF is false according to Richard Feynman in his "Lectures on Physics." This so-called "flux rule" does not always work and there are counter examples, such as Faraday's disk dynamo. Despite this, most textbooks and encyclopedias treat it as a true law, leading to confusion and nonsense. However, this is not uncommon in physics as many laws are eventually proven false or incomplete as our understanding evolves.
  • #71
MS La Moreaux said:
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.
 
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  • #72
MS La Moreaux said:
Claude,

FL is not related to u x B and says nothing about time-changing 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 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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  • #73
@Claude http://xkcd.com/386
 
  • #74
anirudh215,

In your reference it states, under "In electromagnetism," "Caution must be taken to avoid cases with moving boundaries: the partial time derivatives are intended to exclude such cases."

Mike
 
  • #75
Claude,

If u x B were included in FL, FL would not fail in the case of the homopolar generator. Your work with transformers is irrelevant because the applicable law was Maxwell's Law for transformer EMF, which is included in FL. That is not the problem with FL. FL both includes too much and not enough, but it does include every case where that one of Maxwell's Laws applies. So one does not have to take post graduate teachings on faith, eh? It seems to me that all of science is based upon faith in natural laws, which cannot be proved. You are not the first post graduate that I have run across that did not know what he was talking about. In regard to FL, the writings of Richard Feynman agree with me. He certainly had more than a mere BS. And he disagreed with the peer-reviewed texts and publications.

Mike
 
  • #76
MS La Moreaux said:
Claude,

If u x B were included in FL, FL would not fail in the case of the homopolar generator. Your work with transformers is irrelevant because the applicable law was Maxwell's Law for transformer EMF, which is included in FL. That is not the problem with FL. FL both includes too much and not enough, but it does include every case where that one of Maxwell's Laws applies. So one does not have to take post graduate teachings on faith, eh? It seems to me that all of science is based upon faith in natural laws, which cannot be proved. You are not the first post graduate that I have run across that did not know what he was talking about. In regard to FL, the writings of Richard Feynman agree with me. He certainly had more than a mere BS. And he disagreed with the peer-reviewed texts and publications.

Mike

But u X B[/b} IS included in FL. I've stated this repeatedly, but you won't listen. One more time.

The eqns. "v = -N*dphi/dt", and " v integral E*dl, where E = u X B", are stating the exact same thing. If a conductor moves within a static mag field, it encounters a spatial flux distribution that varies with position in space, and thus it varies in time. If the conductor is stationary and the magnet moves, then the field encountered by the conductor is time-varying.

The conductor in both cases, encounters a flux, phi, that is varying wrt time. The emf is "-N*dphi/dt", in the reference frame of the conductor. If, however, an observer in the rest frame is viewing the conductor moving relative to the magnet, and wishes to compute the resultant emf, they use E = u X B, then v = integral E*dl, around the path of the loop encircling the flux.

So FL, which states that v = -N*dphi/dt, is complete. As long as it is understood what frame of reference we are using. Motors and generators illustrate my point. Take an ac synchronous motor/generator. One can place the dc field on the stator. The rotor spins through said field, and the rotor emf can be computed. In the rotor's frame of reference, the rotor is at rest, and the stator is spinning the opposite direction. The rotor winding encounters a spatial flux pattern that changes with time. Hence v = N*dphi/dt. The stator's field is static wrt to an observer at rest, but time-changing from the rotor reference frame.

But a stationary observer, or the stator, are both at rest and see the rotor spinning. The emf on the rotor is given by the u X B relation. The stationary observer only sees a static mag field on the stator, and sees the rotor spinning through this static field. The reason for u X B is to compensate for reference frames. Since we seek to know the rotor emf, we must transform from our static frame to that of the spinning rotor. Hence the motional action and the transformer action are essentially one and the same action viewed from 2 different reference frames.

You're making this much harder than it really is. Please show this treatise of mine to your trusted gurus, colleagues, profs, or whomever you deem credible. If I've erred, let them point it out.

We could also place the dc field on the rotor instead, and measure the induced emf on the stator as well. The same principles apply. It's all about relative motion between the 2 reference frames.

Seriously, that's all there is to it. You can make it harder if you wish, but I've given you exactly what peer-reviewed texts, and experts in the field have empirically verified for over a century.

I know that my xfmr design skill is not directly applicable to motional action, but I design, past and present, dc motor drives for actuators, missile guidance, pumps, compressors, etc. Included is regenerative braking (motional emf/mmf). I have substantial experience with motional action as well as static.

Oh yeah, you said "You are not the first post graduate that I have run across that did not know what he was talking about". Well, how much experience/skill do YOU have with motors, xfmrs, generators, induction heating, etc.? Is there any transducer work in your past such as microphones, speakers, etc.? What have you done to be the expert on motional emf? I'm not attacking you, but I'm just curious what makes you think you are head and shoulders above qualified experts. You have a BS, a good achievement. A BS qualifies you to do some good work in science to produce products that benefit mankind.

But to topple established axioms requires much much much much more. A PhD in phy or EE is still not enough. You'd need a huge lab, budget, and staff with accelerators, scanning microscopes, etc. to advance Maxwell's equations to a new level. Your BS is a useful degree which empowers you to a limited degree.

Even should I get my PhD, and with my 32 yrs. of engr background, I am still NOT qualified to topple Faraday. Here's what you and I have in common - we both have limited knowledge.

Here is where we differ - I am well aware that my knowledge is too limited to go challenging Faraday. Nothing personal, thanks for the interesting chat. Good day.

Claude
 
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  • #77
MS La Moreaux said:
anirudh215,

In your reference it states, under "In electromagnetism," "Caution must be taken to avoid cases with moving boundaries: the partial time derivatives are intended to exclude such cases."

Mike


Please look up how

[tex] \epsilon = -\frac{d \phi}{dt}[/tex]

is derived from Maxwell's equations. I too have the read the Feynman lectures and I strongly feel that was not what Feynman tried to convey. Feynman disagrees that

[tex] \epsilon = -\frac{d \phi}{dt}[/tex]

includes all cases. That is true. It does not include all cases. The most popular example to show that it does not include all cases is the example that you yourself have stated. In such a case, the path of the current is itself not clear and applying the above law does not hold good. The above law is NOT Faraday's law. I have already stated Faraday's law above. When using the law that I have stated, indeed, the integral IS done around a stationary path.

Why don't you take up some standard text like Griffiths' Introduction to Electromagnetism or Schwartz's Principles of Electrodynamics and look this up? It is quite lucidly explained. The vxB part is definitely included in the flux rule which you have stated above.
 
  • #78
anirudh215,

I do not believe that we really have much of a disagreement. You agree that the flux rule does not work in all cases. Therefore it is not a law. You claim that it is not Faraday's Law. Some texts give it as FL and others give one of Maxwell's Law as FL. That Maxwell's Law is true. This entire thread treats the version of FL which is identical to the flux rule. If you insist that FL is not the flux rule, that is a completely separate argument and does not belong in this thread. Personally, I do not see why one law should have two different names.

Mike
 
  • #79
Claude,

If a conductor moves within a uniform magnetic field, it will not encounter a spatial flux distribution that varies with space or time. This is the case with a homopolar generator with a uniform magnetic field penetrating the entire disk. This is a case of pure motional EMF with no time-varying magnetic flux and thus separates the two for analysis. It demonstrates that FL is false.

Do you have any experience with homopolar generators? I imagine that it would be difficult to design one using FL since it does not apply, although it claims to.

It seems to me that FL is very simple. People bring in all sorts of extraneous and incorrect nonsense that might seem to relate to it, but does not really. I am just using logic to analyze FL. I have addressed points which I have never seen addressed by anyone else. I have proved my case.

Mike
 
  • #80
You have never had a case (Just look at the title of this thread).

ZapperZ put the period to this. Your point is unsustainable due to many reasons none of us have time to belabor on.

Why don't you think of publishing your results as a groundbreaking discovery?

Every law has its limits, Feynman NEVER intended to convey what you wish here. I read the relevant chapters more than once. That's not the message. I'd advise you to go back and read it more carefully. He himself would have been furious by the idea of using his name for a baseless claim.

You probably did not even do any of these experiments yourself, and the VERY relevant experimental point (due to Per Oni) of eddy currents are "cleverly" ignored with a comment by you saying: "No, I don't think so". Edit: (Post#51)Theory by itself only goes this far. You start reinventing your own laws.
 
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  • #81
MS La Moreaux said:
anirudh215,

I do not believe that we really have much of a disagreement. You agree that the flux rule does not work in all cases. Therefore it is not a law. You claim that it is not Faraday's Law. Some texts give it as FL and others give one of Maxwell's Law as FL. That Maxwell's Law is true. This entire thread treats the version of FL which is identical to the flux rule. If you insist that FL is not the flux rule, that is a completely separate argument and does not belong in this thread. Personally, I do not see why one law should have two different names.

Mike

Well, why SHOULD it be true; simply because it has the tag name law? For instance, Ohm's LAW is not a law in the sense it is ALWAYS true and it is fairly easy to spot cases where it does not work. As already stated, LAWs have limitations. Their purpose is only suited for certain things. That doesn't exactly make them wrong.

As to the question of why there ought to be two different names, the FL that I stated was for stationary integral paths.

The flux rule, on the other hand, also deals with moving circuits. A popular example for this would be the "rod on rail track" wherein a rod is placed on a rail track perpendicular to which a magnetic field passes through. Given a velocity 'v', the EMF in the loop can be calculated using the flux rule. I am sure you would have seen this example often.
 
  • #82
MS La Moreaux said:
Claude,

If u x B were included in FL, FL would not fail in the case of the homopolar generator. Your work with transformers is irrelevant because the applicable law was Maxwell's Law for transformer EMF, which is included in FL. That is not the problem with FL. FL both includes too much and not enough, but it does include every case where that one of Maxwell's Laws applies. So one does not have to take post graduate teachings on faith, eh? It seems to me that all of science is based upon faith in natural laws, which cannot be proved. You are not the first post graduate that I have run across that did not know what he was talking about. In regard to FL, the writings of Richard Feynman agree with me. He certainly had more than a mere BS. And he disagreed with the peer-reviewed texts and publications.

Mike

In the homopolar generator case, HG, it is Hall effect & Lorentz force acting on the free electrons resulting in "induced current". It is not Faraday.

One thing that seems to have escaped all of us, and I'm just as embarassed, is that the induced current in an HG is dc! As in zero frequency. Surely this cannot be per Faraday. Something else is happening outside of FL (Faraday's law).

I design power electronics including switching power supplies & motor drivers. I always need to sense current. I've used low-valued resistors, transformers, & Hall effect sensors to do so. One of the limitations with current xfmrs is the inability to operate to dc (0 freq). A Hall device, OTOH, does not have this limitation.

The output terminals of a Hall device present a voltage which is galvanically isolated from the input terminals, and is a facsimile of the input current. Hence a current through the input terminals induces a voltage at the output terminals galvanically isolated. According to FL, this happens only under ac conditions, never with dc! This does not invalidate FL, it just shows that there is another separate means by which induction takes place.

The HG is such an example. The flux through the disk is dc, as is the induced current. The free electrons incur a Lorentz force since the electrons have a velocity as the disk is spinning, oriented normal to the B field. The force is directed radially outward for positive current, inward for electrons for example, ref magnet polarity & rotational direction. Clearly a dc flux inducing a dc current cannot be per FL.

Faraday published his findings well before Hall discovered the effect in 1879. Lorentz published his force relation just after 1890, I believe. Maxwell finalized his equations, publishing them in 1873, including FL. Hall-Lorentz force acting on free electrons requires no time rate of change, operating down to zero freq.

FL, including motional emf, is still valid, but does not encompass all induction. The HG does not disprove Faraday, but simply demonstrates that Hall-Lorentz force is present as well.

As long as I can remember, "Hall effect devices", and "transformers" are NOT the same thing, although both involve induction, where galvanic isolation occurs. Likewise, motional emf as in generators and motors, per FL, are not the same as Hall induced action.

I don't see any "paradox" whatsoever. The HG operates with dc flux & dc induced current, on the Hall-Lorentz principle. Other generators operate per FL, which only works with ac.

Any comments/feedback are welcome. Cheers.

Claude
 
  • #83
Claude,

It is good to hear from you, again. I am afraid that we still disagree. The operation of the HG has nothing to do with the Hall effect. It is an example of pure motional EMF, which is synonymous with the magnetic part of the Lorentz force. Both motional EMF and the Hall effect are direct applications of the definition of the magnetic field. The Hall effect does not involve motion (excepting the current electrons.) FL supposedly includes motional EMF, something with which you agree. The HG, being an instance of motional EMF, and having no flux change, should, according to FL, produce no EMF, but does, thus showing the failure of FL. FL does not specifically address either ac or dc. It simply purports to explain induction in a circuit solely by means of time rate of magnetic flux change, caused either by an intrinsic change in the magnetic field, or by motion. If the rate of flux change is constant, it predicts dc.

Mike
 
  • #84
But FL does not say that no flux change produces no induction. It says that a flux change produces induction. FL is stated as a positive, and does not mention the negative.

In addition to Lorentz, a current can be "induced" in a galvanically isolated circuit by optical coupling. Have you used optocouplers? The flux due to current in the input LED is static. There is a current in the output transistor due to optical coupling. Optical transfer is yet another way to couple energy.

FL states 2 ways for energy to be transferred between isolated networks, xfmr & motional. Lorentz covers another case extending to dc, whereas FL does not. Optical transfer is another way. I use xfmrs, Hall devices, & optocouplers, all in order to transfer energy and/or info across isolation boundaries.

The flux is static in an HG and yet there is a current (dc). FL says a time changing flux induces a current. FL does not state that a static flux results in no current. That is just a hangup. Obviously there is something besides FL at work, namely Lorentz. Tonight I'll post a treatise on the HG based on special relativity.

Claude
 
  • #85
Claude,

FL is purported to cover both transformer and motional EMF. Motional EMF is mistakenly supposed to be the result solely of flux change. Either it is or it is not. If FL is supposed to cover motional EMF, it would have to do so in every case, including those where the flux change is zero. You see that motional EMF is independent of flux change. It should never be associated with flux change. They are two completely different phenomena. Again, motional EMF and what you are referring to as Lorentz are identical.

There is no magnetic flux involved in optocouplers. They act as switches. They do not really transfer energy between systems so much as transfer a signal. They have nothing to do with electromagnetic induction.

Mike
 
  • #86
MS La Moreaux said:
Claude,

FL is purported to cover both transformer and motional EMF. Motional EMF is mistakenly supposed to be the result solely of flux change. Either it is or it is not. If FL is supposed to cover motional EMF, it would have to do so in every case, including those where the flux change is zero. You see that motional EMF is independent of flux change. It should never be associated with flux change. They are two completely different phenomena. Again, motional EMF and what you are referring to as Lorentz are identical.

There is no magnetic flux involved in optocouplers. They act as switches. They do not really transfer energy between systems so much as transfer a signal. They have nothing to do with electromagnetic induction.

Mike

Please review the attached, as it explains the HG using SR. In a nutshell, the Lorentz force moves electrons towards the center of the disk and holes (+ve charge) towards the rim. This charge separation results in a static E field and potential. Separation of charges explains the static nature of the E field. The current density J is related per J=sigma*E, which is Ohm's law.

Regarding FL, you firmly state that FL is valid for xfmr induction, i.e. time-varying flux. You state that it must be true always, and that with static flux, no inducton can take place. Yet a Hall effect current sensor produces induction with static flux. A xfmr cannot do this. So there is something else going on here. In a HG, the flux & induced current/voltage are static. FL relates the induced I/V to the change rate of the flux. Motional emf/mmf is due to the flux spatially varying so that the actual flux encountered by the circuit is time changing. Moving through a spatial flux change results in a time varying flux in the circuit reference frame.

Thus if phi varies with spatial position r, and velocity u is dr/dt, then d(phi)/dr * dr/dt = d(phi)/dt, by the chain rule. So FL covers this aspect of induction. Moving through a flux that is static wrt time, but varying wrt position, is equivalent to sitting still with the flux time-varying.

The HG is best explained by Lorentz force & relativity. Remember in a HG, we set the electrons in motion by expending energy in rotating the disk. Then the magnetic field acts upon the moving electrons per Lorentz. Charges are separated and a static E field is formed, with a dc voltage & current.

E fields induced per FL are solenoidal. No sources or sinks exist, they have rotation, & are non-conservative. E = -dA/dt, where A = magnetic vector potential, B = curl A.

E fields in the HG have sources and sinks due to charge separation, have no rotation, are conservative, and E = -grad V, where V is scalar electric potential.

Clearly the E fields in the HG are markedly different than those in FL type generators. This is a different mechanism, likely related via relativity.

Optical is energy. Have you seen what happens when sunlight is focused through a magnifying glass into a point and focused on a piece of paper? No energy?! Indeed!

That's all for now. Let's review the paper and discuss further later. BR.

Claude
 

Attachments

  • q218unipolar.pdf
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  • #87
One more thing should be mentioned. According to Maxwell's published law of Faraday (1873):

curl E = -dB/dt.

What is he saying here? If the flux time derivative is zero, is E zero? Of course not. He's saying that the curl of E is zero. In the HG case, that is exactly what is measured. The Lorentz force due to electron motion relative to the static field acts radially so that plus & minus charges are separated, center & periphery, resulting in an E field radially. But this E field is due to charged particles. It has no curl.

Maxwell/Faraday predicts just that. A non-rotational E field. The HG perfectly affirms and vindicates FL per Maxwell. Transformers & motors/generators OTOH, per Faraday induction, exhibit E fields with curl. So there is no paradox after all. Maxwell & Faraday got it right all along.

I'm glad that's settled. Good day to all and feedback is welcome.

Claude
 
  • #88
I'm not sure if this is helps or not, but for what it's worth, in Daniel Fleish's "A student's guide to Maxwell's Equations" he distinguishes b/w the convenient Flux Rule, which is a handy formula, and a modified version of Faraday's Law, which is more subtle and rigorous, and, he believes, correct. I'll post the quote and formula if I can find it.

Regarding Feynmann's quote, I found the likely source in the Wikedia article http://en.wikipedia.org/wiki/Faraday_paradox" [Broken]

The "flux rule" does not work in this case. It must be applied to circuits in which the material of the circuit remains the same. When the material of the circuit is changing, we must return to the basic laws. The correct physics is always given by the two basic laws
  • 30e07241f7dce068047cbe7fb1ca21b2.png

    http://upload.wikimedia.org/math/1/8/f/18ff316d4977d68c2cd17db2fd64ec0b.png [Broken]
-- – Richard Feynman  The Feynman Lectures on Physics

Just after the Feynman quote, the argument is made that once SR is taken into account, Faraday's law holds true. I don't know enough to pass any judgment, but at first pass it seems Feynman is arguing more against the universality of the handy flux rule, but not necessary the fully expressed Faraday's law w/ perhaps SR taken into account.
 
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  • #89
Claude,

I did not say that with static flux, no induction could take place; I said that that is what FL implies. That is one reason why FL is false. Flux change due to motion is not physically (in the sense of physics) the same as time-varying flux without motion. That is the whole crux of the matter. The two have completely different physical results. The former leads to motional EMF and the latter results in transformer EMF in accordance with one of Maxwell's four classic laws. When you say that E fields induced per FL are solenoidal, you are mistaken. That is only true when there is an intrinsic change in the flux. It is not true when there is motional EMF. FL does not distinguish between the two, which is why it is false.

Light is certainly energy. But energy transfer in an optocoupler, though necessary, is incidental to the transfer of a signal, which is the purpose of an optocoupler.

Mike
 
  • #90
diagopod,

The flux rule discussed by Feynman is exactly the same as what we are referring to as Faraday's Law in this thread. Relativity is really irrelevant, as it came along long after FL. FL is very simple and simply false, as I have demonstrated earlier in this thread.

Mike
 
  • #91
cabraham said:
But u X B[/b} IS included in FL. I've stated this repeatedly, but you won't listen. One more time.

Oh yeah, you said "You are not the first post graduate that I have run across that did not know what he was talking about". Well, how much experience/skill do YOU have with motors, xfmrs, generators, induction heating, etc.? Is there any transducer work in your past such as microphones, speakers, etc.? What have you done to be the expert on motional emf? I'm not attacking you, but I'm just curious what makes you think you are head and shoulders above qualified experts. You have a BS, a good achievement. A BS qualifies you to do some good work in science to produce products that benefit mankind.

But to topple established axioms requires much much much much more. A PhD in phy or EE is still not enough. You'd need a huge lab, budget, and staff with accelerators, scanning microscopes, etc. to advance Maxwell's equations to a new level. Your BS is a useful degree which empowers you to a limited degree.

Even should I get my PhD, and with my 32 yrs. of engr background, I am still NOT qualified to topple Faraday. Here's what you and I have in common - we both have limited knowledge.

Here is where we differ - I am well aware that my knowledge is too limited to go challenging Faraday. Nothing personal, thanks for the interesting chat. Good day.

Claude


Hey, Claude, don't say dumb things and then run off! Where did you ever get the idea that somehow "credentials" are needed to topple Faraday's law? And where did you ever get the idea that real science is only done by spending vast sums of taxpayer money? Are you getting your paycheck from the government or something? And why wouldn't 32 years of engineering background qualify you to challenge Faraday? Hey, you live in the 21st century and Faraday, though smart, didn't know squat compared to you!

The truth is (and I sure hope you are not spreading your errors among the young) that science is done with the MIND! It all starts between the ears. I don't care how much money you spend, if you can't think it won't be science!

And the truth is that Faraday's law as typically stated IS wrong (but interestingly NOT wrong according to how Faraday stated it!). Does anyone here understand how the rocking plates work? Why doesn't the changing flux give a voltage? Here, I'll explain it to you guys. Here's the equivalent idea and one more case where Faraday's supposed law is invalid.

Imagine a large rectangular loop of wire with a meter in the circuit. Imagine a magnet putting a local flux through an area in the end of the loop near the meter. Imagine a wire and a switch connecting the sides of this loop that when thrown cuts it into two loops. Now move the magnet to the other end of the large loop (nothing happens as flux enclosed in loop has not changed). Now close the shorting switch. Remove the magnet. Open the switch. Voila. The flux has gone from max to zero and the meter does not move! Faraday is invalid!

But the error that makes Faraday's law misinterpreted is that one assumes that a changing magnetic field (flux) causes an induced E field. (An E field in a conductor creates a current) Sorry, the equation Curl E = -db/dt or as often stated EMF = -dB/dt are TRUE relations but they are not CAUSAL relations!

You need to understand what that means. It means that while the value of an induced E and a magnetic field are RELATED they DO NOT cause each other. Hence a voltage is NOT repeat NOT created by a changing magnetic field! If one examines the causality of Maxwell's equations one finds that BOTH magnetic and electric fields are BOTH created by ONLY by charges and their motions (currents). Hence an induced EMF is created by a current somewhere as it's source. And that current ALSO creates a magnetic field. BOTH are related (as they come from the same source) by Faraday's law in many cases, but the changing magnetic field is NOT causing the EMF! Indeed even in the case of moving magnets one can show that the EMF is created by the moving atomic CURRENTS that create the fields of the permanent magnets.

Hence, as Feynman clearly states, Faraday's law is NOT valid for all cases. In cases for example where the configuration of our setup is changing (our switching example) it simply does not work. It didn't work in Faraday's time either as proved by the generator bearing his name! Which is why Faraday NEVER said that a changing magnetic field induces an EMF. He said that a changing CURRENT can induce another current nearby! Obviously even though Faraday didn't have much of a clue, he still knew more about the subject than all the "modern" physicists with their PhDs, money, accelerators and scanning microscopes! OK?
 
  • #92
bjacoby,

Well, at last a posting that supports me! Believe it or not, yours is the first since I initiated this thread. Thanks.

Mike
 
  • #93
If you guys do not stop snipping at each other and do not go back to strictly discussing on-topic subjects, this thread will be closed!

Zz.
 
  • #94
ZapperZ said:
If you guys do not stop snipping at each other and do not go back to strictly discussing on-topic subjects, this thread will be closed!

Zz.

I'm amazed that a thread like this is even allowed to exist in this forum. I wasn't aware of it till now. I did a one minute scan and found many statements of misinformation. For example it was claimed that the Faraday disk violates Faraday's Law. Nonsense, it does not and the principle of operation of the Faraday disk is not even based on Faraday's Law.
 
  • #95
elect_eng said:
I'm amazed that a thread like this is even allowed to exist in this forum. I wasn't aware of it till now. I did a one minute scan and found many statements of misinformation. For example it was claimed that the Faraday disk violates Faraday's Law. Nonsense, it does not and the principle of operation of the Faraday disk is not even based on Faraday's Law.

This thread IS being reviewed right now. In the meantime, please feel free either to respond, or to report the offending posts.

Zz.
 
  • #96
cabraham said:
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

Can you explain this a little more than you have. It seems that a person could clearly isolate charges and then measure their relative velocities. And when we measure stuff, at rest, they have a force on each other. We can attribute this to the E field. I'll let you comment on that if you want. And I'll express my concerns a little bit below.

So, my misunderstanding is based on the fact that you 'can' find the relative velocities, and then measure forces. How is there a symmetry here? We can grab a physical thing that has charge and test it with another charged thing, and this force is observed when there is no motion. How do you account for the 'static' electric field? If you want to explian it with symmetry, it seems that the E fields would have to be due to some sort of moving magnetic charges, but since the E field is localized (not moving) and falls off as 1/r^2 (not a dipole field or something), then I don't see it. Thanks!
 
  • #97
elect_eng said:
I'm amazed that a thread like this is even allowed to exist in this forum. I wasn't aware of it till now. I did a one minute scan and found many statements of misinformation. For example it was claimed that the Faraday disk violates Faraday's Law. Nonsense, it does not and the principle of operation of the Faraday disk is not even based on Faraday's Law.

I'm not amazed that as our discussion homes in on a careful evaluation of some basic physics dogma that some are ready to call for censorship. Any discussion that questions tradition is bound to get a bit messy! Let us take the above statement that a Faraday disk does not violate Faraday's Law. We all agree that a potential is induced in the disk. And that means an E field is induced in the disk. The standard argument is that an induced E field is created by either a changing magnetic flux or by "flux cutting". Obviously in a Faraday generator the magnet is fixed and the flux is not changing through the disk. So one assumes therefore it must be a case of "flux cutting". In the usual "flux cutting" case as the flux is "cut" the area of the flux changes and hence the there is ALSO a dB/dt. In the Faraday generator the area does not change. Even worse, the disk is continuous so there is no obvious "flux cutting" as might be the case if the disk were slotted. We know the disk nuclei are fixed and rotate with the disk but they can't move sideways to create a potential either! What the free electrons do is rather up for grabs. It's hard to make a case that they also are dragged around the disk as it turns. Hence both flux changing and flux cutting fail.

Well what does succeed? First let me note again that the proper statement of Faraday's law is that a current creates another current. There is no magnetic field involved at all! This is proper causality! So let us start with the magnet. How does it work? Well, it is believed that electrons circulating around the atoms create a current loop that creates a permanent magnetic field. One can show that for homogeneous materials this is equivalent to a larger current circulating about the outside of the magnet. cylindrical magnets and thin solenoids make nearly identical fields.

Now near that circulating current we have a rotating disk. So let's put the disk in one frame and the magnet in another. For drill let's rotate the magnet rather than the disk. So now we wish to calculate the electric field induced about the magnet. Well, if the magnet is still, there is none as the magnet material is electrically neutral. But what happens when the magnet rotates? Here is the crux. Remember that only CHARGE and CURRENTS can be sources of E fields! Hence the magnetic field does not matter!

So if one examines a neutral current moving with constant velocity in the direction it is flowing one finds that two E fields are induced One is an ordinary electrostatic E field due to the motion and the other is an Induced electrokinetic E field also due to the motion. It can be shown that in the case of a current loop rotating about its axis so the current has a constant velocity in the direction it is flowing an E field is induced equal to -V x B. But make no mistake here. This field is NOT caused by B! Just it's VALUE can be calculated using B! The induction is from a current loop DIRECTLY to the E field producing the generator potential!

Of course this is EXACTLY how Faraday stated his observations: "When an electric current is passed through one of two parallel wires it causes at first a current in the same direction through the other, but this current does not last a moment notwithstanding the inducting current (from a voltaic battery) is continued..."

Here we have the equivalent currents in the magnet creating a current (E field) in the surrounding space by virtue of their relative motion. If the magnet and disk are in the same frame, then there is no E field observed and no induced potential.

Hence Faraday's law as stated by Faraday is correct, but the traditional versions that ascribe the creation of an E field as being created by a B field are simply incorrect. The magnetic field is just something that is ALSO there. It has the same current source as the
induction and therefore is RELATED to it, but most certainly does not CAUSE the generator action.

It is the hard fact that ALL induced E fields are created by charges and currents and NOT by magnetic fields that means that Faraday's Law as usually interpreted is false.
 
  • #98
That is completely wrong. An electric field can be created by a changing magnetic field.
 
  • #99
bjacoby said:
Now close the shorting switch. Remove the magnet. Open the switch. Voila. The flux has gone from max to zero and the meter does not move!
It depends on the size of your short circuit and the sensitivity of your meter whether there’s a deflection. According to Ohms law if your short circuit has a resistance (it has) and a current flows through the short (it does) there will be a voltage across the meter. By the way the current in the short is normally called an eddy current.

Disconnecting the meter temporarily, while you remove the magnet will prove your point as well?
 
  • #100
bjacoby said:
... Let us take the above statement that a Faraday disk does not violate Faraday's Law. We all agree that a potential is induced in the disk. And that means an E field is induced in the disk. The standard argument is that an induced E field is created by either a changing magnetic flux or by "flux cutting". Obviously in a Faraday generator the magnet is fixed and the flux is not changing through the disk. So one assumes therefore it must be a case of "flux cutting". In the usual "flux cutting" case as the flux is "cut" the area of the flux changes and hence the there is ALSO a dB/dt. In the Faraday generator the area does not change. Even worse, the disk is continuous so there is no obvious "flux cutting" as might be the case if the disk were slotted. We know the disk nuclei are fixed and rotate with the disk but they can't move sideways to create a potential either! What the free electrons do is rather up for grabs. It's hard to make a case that they also are dragged around the disk as it turns. Hence both flux changing and flux cutting fail.

Well what does succeed? First let me note again that the proper statement of Faraday's law is that a current creates another current. There is no magnetic field involved at all! This is proper causality! So let us start with the magnet. How does it work? Well, it is believed that electrons circulating around the atoms create a current loop that creates a permanent magnetic field. One can show that for homogeneous materials this is equivalent to a larger current circulating about the outside of the magnet. cylindrical magnets and thin solenoids make nearly identical fields.

Now near that circulating current we have a rotating disk. So let's put the disk in one frame and the magnet in another. For drill let's rotate the magnet rather than the disk. So now we wish to calculate the electric field induced about the magnet. Well, if the magnet is still, there is none as the magnet material is electrically neutral. But what happens when the magnet rotates? Here is the crux. Remember that only CHARGE and CURRENTS can be sources of E fields! Hence the magnetic field does not matter!

So if one examines a neutral current moving with constant velocity in the direction it is flowing one finds that two E fields are induced One is an ordinary electrostatic E field due to the motion and the other is an Induced electrokinetic E field also due to the motion. It can be shown that in the case of a current loop rotating about its axis so the current has a constant velocity in the direction it is flowing an E field is induced equal to -V x B. But make no mistake here. This field is NOT caused by B! Just it's VALUE can be calculated using B! The induction is from a current loop DIRECTLY to the E field producing the generator potential!

Of course this is EXACTLY how Faraday stated his observations: "When an electric current is passed through one of two parallel wires it causes at first a current in the same direction through the other, but this current does not last a moment notwithstanding the inducting current (from a voltaic battery) is continued..."

Here we have the equivalent currents in the magnet creating a current (E field) in the surrounding space by virtue of their relative motion. If the magnet and disk are in the same frame, then there is no E field observed and no induced potential.

Hence Faraday's law as stated by Faraday is correct, but the traditional versions that ascribe the creation of an E field as being created by a B field are simply incorrect. The magnetic field is just something that is ALSO there. It has the same current source as the
induction and therefore is RELATED to it, but most certainly does not CAUSE the generator action.

It is the hard fact that ALL induced E fields are created by charges and currents and NOT by magnetic fields that means that Faraday's Law as usually interpreted is false.

As far as I can tell, your long explanation is basically saying that the electric field (force driving the electrons across the disk) is a VxB type interaction. Well yes, we know that. That is simply the well-known Lorentz force, which is the basic underlying mechanism that explains the principle of operation of the Faraday disk. The Lorentz force pushes the electrons either to the center, or out to the perimeter of the disk (depending on the field direction and rotation direction). The charge imbalance implies a potential difference. By tapping the voltage across the center and the perimeter, one is able to drive high currents, all due to Lorentz force. This is different than the typical generator which is usually explained by Faraday's Law in terms of changing magnetic fields cutting loops. Note that the Lorentz force is basically a defining equation for the magnetic field. This does not invalidate Faraday's Law. Faraday's Law is still obeyed always within the assumptions of classical physics. It has never been disproved; - not ever, and certainly not by your feeble logic.

The very title of this thread is an affront to the scientific method and seeks to do nothing but mislead the novice.

Note that it is important to use a proper form of Faraday's Law based on the situation. This can be done with the Tensor formulation of Maxwell's equations, or a proper general vector form of Faraday's law as follows:

[tex]\int (E+v \times B) \cdot dl=-\int {{dB}\over{dt}}\cdot ds[/tex]
 
Last edited:
  • #101
bjacoby,

You said that with the Faraday disk it is hard to make a case that the [free] electrons are dragged around with the disk as it turns. It is not hard at all, actually. In a typical current-carrying wire, the free electrons composing the drift current travel at a net speed of about a micron a second. If they did not travel around with the disk, they would constitute a tremendous current that would vaporized the disk with instantaneous heating.

Mike
 
  • #102
elect_eng,

You state that the operation of the Faraday disk is not based on Faraday's Law. That means that the basis of the operation of the Faraday disk is motional EMF and that motional EMF and Faraday's Law are independent. If they are independent, Faraday's Law cannot include motional EMF, which is what I have been maintaining all along.

The equation you posted is exactly equivalent to what we have been referring to as Faraday's Law in this thread and is certainly false, as I have conclusively demonstrated throughout this thread.

Mike
 
  • #103
MS La Moreaux said:
The equation you posted is exactly equivalent to what we have been referring to as Faraday's Law in this thread and is certainly false, as I have conclusively demonstrated throughout this thread.

Mike

OK, before I try to comment on this, can you please express what you believe to be the correct formulas. I've read through many of the previous posts, but it is hard to absorb everything, and explanations in words can be misinterpreted.

If you write the equations I'll be able to clearly understand what you are saying.

At the same time, I'll double check the one I posted. It's a version I pulled from memory and I dont' use it in practice. I'd like to go back and review the assumptions implied in that version.
 
  • #104
elect_eng,

If the righthand side of your equation is replaced by the symbol for EMF, the equation will be Lorentz's, which is correct. The version of Faraday's Law which is the subject of this thread is E = -d(phi)/dt, where the left side is EMF and phi is the magnetic flux.

Mike
 
Last edited:
  • #105
MS La Moreaux said:
The homopolar generator is a counter example to Faraday's Law. It is obviously an example of a steady-state situation when it is running at a constant speed. There is no time variation in the magnetic flux linking the circuit. In fact, to at least a first approximation, the magnetic flux lines are parallel to the plane of the circuit. Faraday's Law gives a value of zero for the EMF, but homopolar generators do work.

Mike

Can you show me a picture of a homopolar generator where the magnetic lines of flux are parallel to the plane of the circuit. I have always pictured them as being perpendicular to the current flow.
 
<h2>1. What is Faraday's Law and why is it important?</h2><p>Faraday's Law, also known as Faraday's electromagnetic induction law, states that a changing magnetic field can induce an electric current in a conductor. This law is important because it explains how electricity can be generated through the use of generators and transformers, and it is the basis for many modern technologies such as electric motors and power plants.</p><h2>2. What is the false claim regarding Faraday's Law?</h2><p>The false claim regarding Faraday's Law is that it is a violation of the law of conservation of energy. This claim suggests that the energy produced by the induced current is greater than the energy put into creating the changing magnetic field, which would violate the principle of energy conservation.</p><h2>3. Who made the false claim and what was Feynman's critique of it?</h2><p>The false claim was made by a group of physicists in the 19th century, including Hermann von Helmholtz. Richard Feynman, a renowned physicist, criticized this claim in his lectures on physics, stating that it was based on a misunderstanding of Faraday's Law and did not take into account the energy required to maintain the changing magnetic field.</p><h2>4. How did Feynman's critique impact the understanding of Faraday's Law?</h2><p>Feynman's critique helped to dispel the false claim and reaffirm the validity of Faraday's Law. His explanation of the energy conservation principle and the role of the changing magnetic field in inducing the current helped to clarify any misconceptions and solidify the understanding of Faraday's Law.</p><h2>5. What are some real-world applications of Faraday's Law?</h2><p>Faraday's Law has numerous real-world applications, including the generation of electricity in power plants, the functioning of electric motors and generators, and the operation of transformers in electrical systems. It is also used in technologies such as magnetic levitation trains and induction cooktops.</p>

1. What is Faraday's Law and why is it important?

Faraday's Law, also known as Faraday's electromagnetic induction law, states that a changing magnetic field can induce an electric current in a conductor. This law is important because it explains how electricity can be generated through the use of generators and transformers, and it is the basis for many modern technologies such as electric motors and power plants.

2. What is the false claim regarding Faraday's Law?

The false claim regarding Faraday's Law is that it is a violation of the law of conservation of energy. This claim suggests that the energy produced by the induced current is greater than the energy put into creating the changing magnetic field, which would violate the principle of energy conservation.

3. Who made the false claim and what was Feynman's critique of it?

The false claim was made by a group of physicists in the 19th century, including Hermann von Helmholtz. Richard Feynman, a renowned physicist, criticized this claim in his lectures on physics, stating that it was based on a misunderstanding of Faraday's Law and did not take into account the energy required to maintain the changing magnetic field.

4. How did Feynman's critique impact the understanding of Faraday's Law?

Feynman's critique helped to dispel the false claim and reaffirm the validity of Faraday's Law. His explanation of the energy conservation principle and the role of the changing magnetic field in inducing the current helped to clarify any misconceptions and solidify the understanding of Faraday's Law.

5. What are some real-world applications of Faraday's Law?

Faraday's Law has numerous real-world applications, including the generation of electricity in power plants, the functioning of electric motors and generators, and the operation of transformers in electrical systems. It is also used in technologies such as magnetic levitation trains and induction cooktops.

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