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 Schrodinger 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.

 

[Dadface]

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.

I don't think the general opinion here is that they are false,far from it.I think each one has a domain of applicability.Newtonian mechanics domain,for example, is extremely broad but you wouldn't use it to find the KE of particles moving at relativistic speeds.

 

[cesiumfrog]

To get to the integral form of Faraday's law from curl E we have to use Stokes' theorem, which doesn't involve time varying closed paths.

This seems to be in the domain that classical EM is valid, and I don't see a mathematical reason to disallow taking the integral form of any of Maxwell's equations (whether time-varying or steady-state), hence we have that the electric field (in the lab frame of reference) around the circuit sums to zero (regardless of whether the plate turns).

Is the mistake (the paradox) to assume that the total electromotive force is always proportional to merely the electric part (and forgetting the second part of the Lorentz force, namely that the fixed magnetic field constitutes an extra electric force component in the reference frame of the current, and that this becomes unbalanced by different relative motion of the current in the rotor/stator)? (Still, a complete description would presumably use more of Maxwell's equations to obtain the flow of current over the entire disc, rather than just treating the dragging qualitatively.)

 

[atyy]

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.

The mistake here is that I exchanged the order of integration and differentiation where I shouldn't have. So even with a time varying path, the line integral of E should be zero.

This seems to be in the domain that classical EM is valid, and I don't see a mathematical reason to disallow taking the integral form of any of Maxwell's equations (whether time-varying or steady-state), hence we have that the electric field (in the lab frame of reference) around the circuit sums to zero (regardless of whether the plate turns).

Is the mistake (the paradox) to assume that the total electromotive force is always proportional to merely the electric part (and forgetting the second part of the Lorentz force, namely that the fixed magnetic field constitutes an extra electric force component in the reference frame of the current, and that this becomes unbalanced by different relative motion of the current in the rotor/stator)? (Still, a complete description would presumably use more of Maxwell's equations to obtain the flow of current over the entire disc, rather than just treating the dragging qualitatively.)

Yes, I guess there are two things called "Faraday's law". There's the thing that is one of Maxwell equations (differential and integral forms), and these are never wrong, and emf for a moving conductor actually comes from the Lorentz force law.

Then there's the "sheer luck" form, which has limited applicability and sometimes can be used for moving conductors. Very interesting!

 

[vanesch]

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.

Of course, but here the thing that is at work is more trivial. It are not the "boundaries of applicability of a certain paradigm" in science which is at work, but simply a mis-use of a formula, which, unfortunately perhaps, still gives the "right" result in certain cases, but not all.

 

[lugita15]

I don't have it with me at the moment, but if memory serves me right Griffith explains the real reason behind this apparent "coincidence" in his Introduction to Electrodynamics. Basically, it boils down to the fact that the observed emf is given by the flux rule, regardless of whether it is generated by an induced electric field or an electric force, because the two situations are really the same situation, except that they occur in different inertial reference frames. So special relativity makes the answer the same in both cases.

I read Griffith a couple years ago, so don't quote me on this. BTW, the one thing I was never able to fully understand about this explanation is that it doesn't cover all the cases in which the flux rule applies, so it doesn't fully explain away the "coincidence." For instance, what if you have a coil which is expanding or contracting in area. Or consider a fixed loop through which a magnetic field is directed perpendicular to the plane of the loop, but it is varying in time. Both of these problems obviously obey the flux rule, but can the *deeper* reason for this phenomenon be found using a relativistic analysis?

Another explanation for why the apparent coincidence is given in http://www.iop.org/EJ/article/0295-5075/81/6/60002/epl_81_6_60002.html" .

My high school physics teacher used to say that the reason Faraday's Law yields all kinds of confusing and counter-intuitive results is that it is based in relativity. If you take all the other Maxwell equations, they do not contradict Newtonian mechanics. It is only after you add Faraday's Law that it can be shown that Electric and Magnetic fields do not obey Galilean transformations, but obey Lorentz transformations instead.

 

[MS La Moreaux]

Here is a link to the relevant pages of The Feynman Lectures on Physics, The Definitive Edition, Volume II, sections 17-1 and 17-2, pages 17-1 to 17-3: "[URL

Feynman states, "We have also given the "flux rule," which says that the emf is equal to the rate at which the magnetic flux through such a conducting circuit is changing...First we'll consider a case in which the flux changes because a circuit is moved in a steady field." This indicates that by "flux rule' he means what we are calling Faraday's Law, even though he uses the latter term for the one of Maxwell's Laws responsible for transformer EMF.

He goes on to describe the homopolar generator and states, "Clearly, here is a case where the v x B force in the moving disc gives rise to an emf which cannot be equated to a change of flux."

He gives another counter example to Faraday's Law and then states, "The correct physics is always given by the two basic laws..."[for the Lorentz force and for transformer EMF]. This implies that Faraday's Law is incorrect physics.

Mike

 

[Born2bwire]

I got to echo ZapperZ's sentiments here, what's the point of this thread? The reasons why Faraday's Law does not correctly apply to this situation have already been discussed.

 

[MS La Moreaux]

The point is that virtually all the textbooks seem to treat Faraday's Law as describing a physical principle, namely that a flux change due to an intrinsic change in the flux, or due to motion, induces an EMF in a circuit. This is false. In every instance where the flux changes due to motion, the EMF is actually motional (v x B). The associated flux change is just along for the ride. It is like guilt by association. The change in flux happens to accompany most cases of motional EMF, even though they are independent. In the case of the homopolar generator, the two are separated, in effect, because there is no flux change, just pure motional EMF, and Faraday's Law fails completely. This is not to disparage Faraday. When he formulated Faraday's Law it was an entirely reasonable thing to do. Later on, however, the subject became better understood with Maxwell's Laws and the expression for the Lorentz force. Faraday's Law at that point was obsolete except as an engineering convenience, yet modern physics does not seem to recognize the fact; Feynman seems to have been ignored.

Mike

 

[Per Oni]

The point is that virtually all the textbooks seem to treat Faraday's Law as describing a physical principle, namely that a flux change due to an intrinsic change in the flux, or due to motion, induces an EMF in a circuit. This is false.

This is not false.

In every instance where the flux changes due to motion, the EMF is actually motional (v x B). The associated flux change is just along for the ride. It is like guilt by association. The change in flux happens to accompany most cases of motional EMF, even though they are independent. In the case of the homopolar generator, the two are separated, in effect, because there is no flux change, just pure motional EMF,

How do you think this EMF is generated? (in fact EMF is not the same as v x B).
Do you think there’s a difference in principle when a conductor cuts a homogenous magnetic field, or as in the case of a homopolar generator, the homogenous magnetic field cuts a conductor?

Feynman seems to have been ignored.

And just as well.
Look at Fig. 17-3 of your quoted book.
Underneath he states: “When the plates are rocked in a uniform magnetic field, there can be a large change in the flux linkage without the generation of an emf. “
This is plain nonsense.
Here in the copper plates, eddy currents exist, caused by the EMF’s generated by the change in flux linkages. However there’s no way the galvanometer can measure these EMF’s because of the internal short circuits in the copper plates.

Sometimes a flea bites an elephant.

 

[Phrak]

And just as well.
Look at Fig. 17-3 of your quoted book.
Underneath he states: “When the plates are rocked in a uniform magnetic field, there can be a large change in the flux linkage without the generation of an emf. “
This is plain nonsense.

This is not nonsense. Re-establishing a current requires no eddy currents and does not produce an emf. This can be clearly seen in an arrangment of a series of parallel wires. As the wires are sequencially connected to a galvenometer, so as to change the amount of enclosed flux, Maxwell's equations predict no emf.

 

[Per Oni]

The eddy currents in your parallel wires have already run during the time when you established the magnetic field. Since the energy is now already dissipated no further emf will be observed.
Purely theoretically :If you had 100% ideal diodes in the middle of your wires you would still see a deflection on a 100% efficient galvanometer because during the stet up of this experiment the wires become slightly charged.
To go back to Fig. 17-3 it would be relatively easy to prove that eddy currents are running because of the Joule heating in the plates. Care must be taken not to include heating by friction of the 2 plates against each other. If the plates are heating up it must be that emf’s are generated, in contradiction to what RF states.

 

[MS La Moreaux]

We all agree on the existence of the two principles of transformer EMF (given by the appropriate Maxwell's equation) and motional EMF (related to part of the Lorentz force expression). Per Oni seems to believe in a third principle, that an EMF is induced in a circuit whose flux linkage is changing solely due to the circuit's motion relative to a magnetic field, said EMF being entirely due to the flux change and not due to motional EMF. Since there is motional EMF in such a case, how could it not be the entire EMF?

In the counter example of the rocking plates, the point is that the physical motion is so slight that any motional EMF is insignificant, while the flux change is relatively large. There will not be any significant eddy currents, unless, of course you believe in the third principle.

Mike

 

[Phrak]

The eddy currents in your parallel wires have already run during the time when you established the magnetic field. Since the energy is now already dissipated no further emf will be observed.
Purely theoretically :If you had 100% ideal diodes in the middle of your wires you would still see a deflection on a 100% efficient galvanometer because during the stet up of this experiment the wires become slightly charged.
To go back to Fig. 17-3 it would be relatively easy to prove that eddy currents are running because of the Joule heating in the plates. Care must be taken not to include heating by friction of the 2 plates against each other. If the plates are heating up it must be that emf’s are generated, in contradiction to what RF states.

I don't know where you come up with this idea. Making and breaking contacts will not generate an emf. No emf; no eddy currents. If there is any heating of the plates the generating current will show up on the ammeter.

If you wish to pursue this, show me, using Maxwell's equations where making or breaking a circuit in a static magnetic field will produce current flow.

 

[Phrak]

I'm going to backup a little Mike, and address one of your previous threads since I've only been half paying attention to this thread.

Feynman states, "We have also given the "flux rule," which says that the emf is equal to the rate at which the magnetic flux through such a conducting circuit is changing...First we'll consider a case in which the flux changes because a circuit is moved in a steady field." This indicates that by "flux rule' he means what we are calling Faraday's Law, even though he uses the latter term for the one of Maxwell's Laws responsible for transformer EMF.

The 'flux rule' given by Feynman is derrived from Faraday's Law.

Flux rule: emf = wB \frac{dL}{dt} = wBv[/itex]<br /> <br /> w is the width of a square loop of wire. (The part of the loop in motion is a wire w units long.)<br /> L is the length of the loop.<br /> B is a magnetic field, uniform in strength perpendicular to the loop<br /> v is the velocity of the wire in the L direction.<br /> <br /> <blockquote data-attributes="" data-quote="" data-source="" class="bbCodeBlock bbCodeBlock--expandable bbCodeBlock--quote js-expandWatch"> <div class="bbCodeBlock-content"> <div class="bbCodeBlock-expandContent js-expandContent "> He goes on to describe the homopolar generator and states, &quot;Clearly, here is a case where the <i>v</i> x <b>B</b> force in the moving disc gives rise to an emf which cannot be equated to a change of flux.&quot;<br /> <br /> He gives another counter example to Faraday&#039;s Law and then states, &quot;The <i>correct</i> physics is always given by the two basic laws...&quot;[for the Lorentz force and for transformer EMF]. This implies that Faraday&#039;s Law is <i>incorrect</i> physics.<br /> <br /> Mike </div> </div> </blockquote><br /> One of the two basic laws Feynman refers to <i>is</i> Fraday&#039;s Law, <br /> <br /> <br /> \nabla \cross \textbf{B} - \frac{\partial \textbf{E}}{\partial t} = \textbf{J}\ \ , where \ \textbf{J}=0<br /> <br /> so of course he doesn&#039;t imply it&#039;s incorrect.

 

[Per Oni]

In the counter example of the rocking plates, the point is that the physical motion is so slight that any motional EMF is insignificant, while the flux change is relatively large. There will not be any significant eddy currents, unless, of course you believe in the third principle.
Mike

Instead of the rocking plates think of a flexible wire between the dots where the galvanometer is connected, in the middle sagging through to point P. Now move the middle of this wire from point P to P’. I hope you will agree that an emf will be displayed on the meter due to a change of flux in this circuit.
The rocking plates can be seen as a load of parallel wires. Whether the physical motion is slight or not doesn’t make any difference to the application of Faradays law.

In a similar way one could say that the disk drawn in Fig. 17-2 consist in reality of a huge number of parallel wires. When the disk is rotated each wire will experience a change of enclosed flux, and therefore generate an emf. Again the emf will not be fully displayed due to eddy currents in short circuits.
The point here is that RF writes: "the moving disk gives rise to an emf which cannot be equated to a change in flux." Again I disagree.

 

[Per Oni]

I don't know where you come up with this idea.

During the setup of your experiment you switched on the power supply to the magnet or you moved the wires under the magnet, in both cases emf’s are generated in the wires.

Perhaps more to the point: when connecting different wires to a meter can we still speak of the same circuit, or are there now in fact a load of parallel circuits?

 

[MS La Moreaux]

Phrak, Feynman defines the "flux rule" in section 16-1, and it is the same as what we are calling Faraday's Law. Neither of the two basic laws he refers to is what we are calling Faraday's Law.

Per Oni, flux change due to motion has no effect in itself. It is the motion which has the effect, namely motional EMF. You seem to be using Faraday's Law to argue for the validity of Faraday's Law.

Mike

 

[Per Oni]

Mike

I've asked:

I hope you will agree that an emf will be displayed on the meter due to a change of flux in this circuit.

Can I deduce from your (at least for me) somewhat strange reply that your answer is yes?

Can you explain to me what your reply means?

 

[MS La Moreaux]

Yes, if an actual wire moves in that way there will be an EMF due to motional EMF. Induced EMF is only due to either motional EMF or transformer EMF. Faraday's Law purports to include both of those, but only for circuits. It actually, improperly, includes more, namely the case of flux changing solely due to motion when there is no motional EMF. This is the problem with the rocking plates. One cannot properly approach motional EMF through flux change. Just because it happens to work in most cases, it does not mean that there is a physical basis for it, just a geometric and mathematical basis. Math is essential for physics, but it is not the same thing as physics. If one is not careful to apply or interpret the math according to the underlying physical concepts, one can easily be led astray. For instance, moment is measured in pound feet and work is measured in foot pounds. These are identical mathematically, but completely different physically. Motional EMF requires a conductor, does not require a closed path, involves a magnetic force on the charged particles and requires motion. Transformer EMF does not require a conductor, does require a closed path, involves an electric force on charged particles (if present), and does not involve motion. These two principles are physical principles and, between them, cover all the cases. Faraday's Law is not based upon a physical principle, it just seems that way. It tries to squeeze the two very different and independent principles into a single term of an equation, which, I believe, is impossible.

Mike

 

[Per Oni]

Yes, if an actual wire moves in that way there will be an EMF due to motional EMF. ... This is the problem with the rocking plates. One cannot properly approach motional EMF through flux change.
Mike

You agree that when the rocking plates are replaced with one wire in the manner I described before that an emf is generated. I can describe to you a circuit involving many wires which will replace the rocking plates and each wire in turn will generate an emf caused by a changing magnetic flux through its particular circuit. But it’s a bit boring and I’ve got more to do.

I will end my contribution to this thread by saying sorry if I appeared a bit disrespectful to Mr. Feynman. Personally I think he was a great guy and scientist.
I can only hope that a footnote will be added to 17-2 in case a new edition is published of his lectures on physics.