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

[MS La Moreaux]

The version of Faraday's Law which purports to include both motional EMF and transformer EMF for circuits is false. There is no theoretical basis for it. Richard Feynman, in his "Lectures on Physics," pointed out the fact that this so-called law, what he called the "flux rule," does not always work and gave two examples. Every textbook and encyclopedia that I know of treats it as a true law. There is a lot of confusion and nonsense related to it. I believe that it is an indictment of the status quo and a scandal.

 

[CFDFEAGURU]

Could you post this version of Faraday's Law?

Thanks
Matt

 

[MS La Moreaux]

E=-d\Phi/dt

Mike

 

[cabraham]

Every law is "false" to an extent. As measuring equipment eveolves and we can see at a smaller level, FL will be modified. No one ever said that FL is 100% accurate from now until the end of time.

FL is based on observed data. I used it hundreds of times to design transformers, chokes, etc. I've had nothing but success with FL. It isn't perfect, but what is?

Your rant is much ado about nothing. Faraday's law stands tall today and those who wish to knock it down have to produce a better law. Until then, you have no case at all.

Claude

 

[MS La Moreaux]

FL is based on observed data. The problem with it has nothing to do with inability to measure small enough values. There are three problems that come to mind at the moment. 1. There are counter examples where it does not work at all. 2. There is no way to incorporated two independent principles into one term of an equation. 3. There is no principle upon which it is based. It is just an ad hoc formulation, like Bode's Law, which works for admittedly most cases, but is just an accident of geometry and math. It is an engineering convenience but is superfluous as a law. It adds nothing to our understanding as the principles of motional EMF and transformer EMF cover every possible case.

Mike

 

[Gear300]

You might want to look into the quantum description of the phenomenon.

It adds nothing to our understanding as the principles of motional EMF and transformer EMF cover every possible case.

I wouldn't exactly say that.

Newton's laws weren't universal -- Relativity had to modify our outlook...but they still hold famous grounding. For that matter we can't really say any law is universal. It is part of what the institution of science is based on - the ability to build on old knowledge with new knowledge.

In the case of Faraday's law, it did indeed provide us with an understanding in the relation between electricity and magnetism. We were then able to continue on while using this law as a stepping-stone.

 

[Vanadium 50]

1. There are counter examples where it does not work at all.

Same with Ohm's Law. That doesn't work for semiconductors. (And please provide an example where Faraday's doesn't work, so I better understand what you are talking about)

2. There is no way to incorporated two independent principles into one term of an equation.

Same with Ohm's Law, which lumps together resistances caused by electrons and caused by phonons.

3. There is no principle upon which it is based.

At the risk of sounding like a broken record, one can say the same about Ohm's Law.

 

[ZapperZ]

FL is based on observed data. The problem with it has nothing to do with inability to measure small enough values. There are three problems that come to mind at the moment. 1. There are counter examples where it does not work at all. 2. There is no way to incorporated two independent principles into one term of an equation. 3. There is no principle upon which it is based. It is just an ad hoc formulation, like Bode's Law, which works for admittedly most cases, but is just an accident of geometry and math. It is an engineering convenience but is superfluous as a law. It adds nothing to our understanding as the principles of motional EMF and transformer EMF cover every possible case.

Mike

You have not shown any valid references to support your argument. And when I say valid sources, I mean published, peer-reviewed sources, which is the only type we will accept in this forum. Till you can do that, you are violating the speculative post rules from our https://www.physicsforums.com/showthread.php?t=5374" that you had agreed to.

Please post your required sources in the VERY NEXT post, or this discussion will end immediately.

Zz.

 

[Dadface]

I wonder if this is referring to the Faraday paradox which, as I understand it, is only an apparent paradox.I tried a quick google and shall return there later since it looks quite interesting

 

[Peeter]

Every textbook and encyclopedia that I know of treats it as a true law. There is a lot of confusion and nonsense related to it. I believe that it is an indictment of the status quo and a scandal.

You have only to find a textbook that treats all of Maxwell's equations in their entirety (like Feynman's volume II). He's got a nice big table of "true in general" vs. "true only in restricted circumstances".

The objection to the falseness of this particular "law" is curious. So much of how physics is taught is this way. We start with Coulomb's law and find no, that's false (need maxwell's equations) ; pendulum as harmonic oscillator (false: only for small angles) ; Newton's laws ... (false: need relativity) ; classical mechanics (false: need QM) ; QM (need QFT?) ; ...

I view my physics studies as an attempt to learn progressively less false models of the world.

 

[CFDFEAGURU]

Peeter,

I view my physics studies as an attempt to learn progressively less false models of the world.

Very well put.

Thanks
Matt

 

[MS La Moreaux]

I am not speculating. I am repeating what has already been published by a distinguished source. Ironically, Faraday's disk dynamo, or homopolar generator, is a counter example, as pointed out by Richard Feynman in his "Lectures on Physics." I would think that someone of his status would be the equivalent of a peer-reviewed source. By the way, is there a published peer-reviewed source that states or proves that Faraday's Law is based upon an established physical principle?

Mike

 

[cabraham]

I am not speculating. I am repeating what has already been published by a distinguished source. Ironically, Faraday's disk dynamo, or homopolar generator, is a counter example, as pointed out by Richard Feynman in his "Lectures on Physics." I would think that someone of his status would be the equivalent of a peer-reviewed source. By the way, is there a published peer-reviewed source that states or proves that Faraday's Law is based upon an established physical principle?

Mike

Would you please quote exactly what Feynman said about FL? You suggest that Feynman questions the validity of FL. but you must elaborate in detail. FL is pretty simple. v = -N*d(phi)/dt, or in vector form, curl E = -dB/dt. What part is non-valid? Is there another term needed, or factor, or both? If that equation is wrong, please illuminate us with the right one. If you can do that, and have said equation verified through independent testing, then you have something. Otherwise, you're just "talking the talk". I don't think you're as clever as you think you are. You bluff and bluster as if you hold 4 aces, when all you have is a mere pair of deuces, or less.

Claude

 

[ZapperZ]

I am not speculating. I am repeating what has already been published by a distinguished source. Ironically, Faraday's disk dynamo, or homopolar generator, is a counter example, as pointed out by Richard Feynman in his "Lectures on Physics." I would think that someone of his status would be the equivalent of a peer-reviewed source. By the way, is there a published peer-reviewed source that states or proves that Faraday's Law is based upon an established physical principle?

Mike

What "established physical principle" is Coulomb's law is based on? What established physical principle is the symmetry of our universe based on? What established physical principle is the Schrödinger equation based on?

You still haven't given any valid references. What "published by distinguished source" are you talking about? Give exact references the way they do in peer-reviewed sources.

Zz.

 

[cesiumfrog]

[..]any valid references to support your argument.

I think Feynman's lectures are acknowledged as a valid source in the expert literature, wouldn't you agree?

his "Lectures on Physics," pointed out the fact [but nonetheless, other sources] that I know of treats it as a true law. [..] I believe that it is an indictment of the status quo and a scandal.

I think you will find Feynman also points out that Newton's laws are false. (And that Schroedinger's equation is also false.)

And yet, modern textbooks still teach Newton's laws (and Schroedinger's equation). Is that a scandal? An indictment? For example, are all of these authors completely unaware of relativity theory? Or are you just getting unnecessarily over-dramatic about the fact that it is practical to first teach the parts that are simple and widely applicable (and only afterwards to build upon it by teaching the superseding knowledge, which is more abstract, difficult, and not directly relevant to daily experience). Perhaps you're also mistaken about what scientists mean when they use the term "law"? (And as for implying Feynman's course is separate from the status quo...)

v = -N*d(phi)/dt, or in vector form, curl E = -dB/dt. What part is non-valid?

http://en.wikipedia.org/wiki/Faraday_paradox" (linking to a direct quote) the former, not the latter. (In this particular context.)

 

[russ_watters]

I think Feynman's lectures are acknowledged as a valid source in the expert literature, wouldn't you agree? [emphasis added]

Yes, but the OP has not provided a valid reference, which means a direct quote and a link to where it may be read/heard in context. That's the only way for other users to judge the quote for themselves. 'Take my word for it' isn't a reference.

Unless the reference is posted so the others here can know what the heck the OP is talking about, this thread will have to be locked.

 

[cesiumfrog]

Yes [that source is valid], but the OP has not provided a valid reference, which means a direct quote and a link to where it may be read/heard in context. That's the only way for other users to judge the quote for themselves. 'Take my word for it' isn't a reference. Unless the reference is posted so the others here can know what the heck the OP is talking about, this thread will have to be locked.

I've put a link in above, but seriously, how will an exact page citation help? Anyone who has the volumes need only look up the homopolar generator in their index anyway, anyone else is still stuck either way.

 

[ZapperZ]

I think Feynman's lectures are acknowledged as a valid source in the expert literature, wouldn't you agree?

What the OP did was similar to saying that he found it PRL. PRL is a respectable journal, but the WAY it is cited is not a VALID reference. Besides, is this what the OP meant as one of the "distinguished source"? If you read the post, it doesn't sound like it.

Zz.

 

[Phrak]

The version of Faraday's Law which purports to include both motional EMF and transformer EMF for circuits is false. There is no theoretical basis for it. Richard Feynman, in his "Lectures on Physics," pointed out the fact that this so-called law, what he called the "flux rule," does not always work and gave two examples.

If this is what you gotten out of reading Feynman's lecture notes 17-1 and 17-2, you've misunderstood.

 

[MS La Moreaux]

I have no access to Feynman's "Lectures on Physics" at this time. There is a link to the relevant pages. It is link No.1 of Steve's 20:39 15 September 2009 comment in item No.48 of the discussion page to the article "Faraday's law of induction" at Wikipedia. The URL that came up is http://student.fizika.org/~jsisko/Knjige/Opca%20Fizika/Feynman%20Lectures%20on%20%20Laws%20of%20Induction.pdf. [When I preview this post I find that this URL is shortened for some reason.]

As to what part of a the equation (or equations) is false, the answer is that the equation is simply false. It cannot be broken down into parts. If it had the partial derivative instead of the ordinary one, it would be one of Maxwell's Laws, the one responsible for transformer EMF. What it lacks is a part for motional EMF. Transformer EMF and motional EMF are examples of the kind of principles to which I was referring.

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

 

[cesiumfrog]

[..]the answer is that the equation is simply false. [..] The homopolar generator is a counter example to Faraday's Law.

Mike, what is your point? (Every equation of physics currently is already known to have a counter example. That hasn't prevented such laws being very valuable in appropriate contexts.)

 

[atyy]

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.

Faraday's law gives the emf around any closed path in space, whether or not a conducting wire lies in the path. If a conducting wire lies in the path, then a current will flow in the wire. In the example you give, (some of) the conducting wire(s) is (are) moving in time, hence (some of) the closed path(s) is (are) moving in time, and the rate of change of flux through those paths is non-zero.

Edit: This is wrong. See the next two posts.

 

[vanesch]

I think that Feynman explains it very well.

Faraday's law is a law about the loop integral of the electric field (in a given reference frame) and how it is related to the change of the flux of the magnetic induction (in that same reference frame).

Now, if in that frame, you have a closed conductor circuit, then the "electromotive force" in that circuit (which will determine the current, "as if it were a battery") will be given by this integral of the electric field (which would normally only be obtainable by a battery).

That's the "transformer" part of what you mentioned earlier.

However, and by "sheer luck" in some cases, you can even sloppily use an "extension" of Faraday's law when the circuit is actually moving in a "fixed" magnetic field. In some cases, by pure coincidence, one could almost say, the overall electromotive force also equals the change in magnetic flux "as seen by the circuit".

But it doesn't work always. It is not because Faraday's law is "wrong", it is because we have used the symbols in Faraday's law in cases where it doesn't apply. Strictly speaking, Faraday's law has only something to do with loop integrals of electric fields in a given reference frame. And then it is always 'right' (apart from limits to the applicability of Maxwell's equations themselves).

 

[atyy]

However, and by "sheer luck" in some cases, you can even sloppily use an "extension" of Faraday's law when the circuit is actually moving in a "fixed" magnetic field. In some cases, by pure coincidence, one could almost say, the overall electromotive force also equals the change in magnetic flux "as seen by the circuit".

Hmmm, that was the other explanation I considered. Does Faraday's law not apply to closed paths which are changing in time?

 

[Phrak]

Yes, but the OP has not provided a valid reference, which means a direct quote and a link to where it may be read/heard in context. That's the only way for other users to judge the quote for themselves. 'Take my word for it' isn't a reference.

Unless the reference is posted so the others here can know what the heck the OP is talking about, this thread will have to be locked.

No need to get up in arms about it. Don't you have a copy? So here's a reference and quote.

The Feynman Lectures on Physics, Feynman, Leighton, and Sands, 1964

Volume II, section 17-2. Exceptions to the "flux rule"

"We now give some examples, due in part to Faraday, which show the importance of keeping clearly in mind the distinction between the two effects responsible for emf's. Our examples involve situations to which the "flux rule" cannot be applied--either because there is no wire at all or because the path taken by induced currents moves about within an extended volume of a conductor."

These exceptions consist of a 1) homoplanar motor and 2) two conducting plates with arced surfaces that have a contact point, that when rocked together change the amount of enclosed flux but produce no change in emf around the loop. Each plate is connected to a wire to a galvenometer to form a complete circuit. These are given in figures 17-2 and 17-3.

In closing to 17-2,

"When the material of the curcuit is changing, we must return to the basic laws. The correct physics is always given by the two basic laws

F = q(E + v \cross B)[/itex]<br /> <br /> \nabla \cross E = -\frac{\partial B}{\partial t}\ .[/itex]&amp;quot;&lt;br /&gt; &lt;br /&gt; The flux rule is given in section 17-1 by&lt;br /&gt; &lt;br /&gt; &amp;quot;emf = wB \frac{dL}{dt} = wBv \ .[/itex]&amp;amp;quot;

 

[atyy]

The correct physics is always given by the two basic laws

F = q(E + v \cross B)[/itex]<br /> <br /> \nabla \cross E = -\frac{\partial B}{\partial t}\ .[/itex]&amp;quot;&lt;br /&gt;

&lt;br /&gt; &lt;br /&gt; OK, my explanation in post #22 is wrong. To get to the integral form of Faraday&amp;#039;s law from curl E we have to use Stokes&amp;#039; theorem, which doesn&amp;#039;t involve time varying closed paths.

 

[atyy]

Sorry, I am confused again. One can define a time-varying flux by means of a time varying boundary. So if we do surface integrals on both sides of curlE=-dB/dt, then take Stokes law on the LHS to get emf, and changing flux on the RHS due to the time varying boundary, won't it be non-zero?

Maybe for the RHS we can use the "flux transport theorem" given in Chapter 12 of "Advanced Engineering Mathematics" by Jeffrey, after taking dB/dt=0 and divB=0, I seem to get:

d(B-flux)/dt=line-integral-around-time-varying-boundary-of-(B cross v)

where v is the velocity of the boundary, which seems like the Lorentz force law.

 

[Dadface]

What is important is that we become aware of those circumstances within which we can accurately and usefully apply a law.I think that having an awareness that according to present knowledge certain laws have their limitations makes the subject more interesting.Perhaps authors should publicise this more,it can encourage some students to investigate further.

 

[vanesch]

Hmmm, that was the other explanation I considered. Does Faraday's law not apply to closed paths which are changing in time?

Sometimes ! However, not always. That's the point Feynman is making in his lectures (which I don't have handy right now).

 

[ZapperZ]

I really don't understand this thread.

I can find situation where the Photoelectric effect doesn't work, Ohm's law doesn't work, 2nd Law of thermodynamics doesn't work, etc.. .etc. In fact, one can say the same about the whole of Newtonian mechanics.

Does that mean that each one of them is "false"? This is absurd! If you truly believe that, get out of your house immediately!

Zz.