
#37
Oct1509, 10:03 PM

Sci Advisor
PF Gold
P: 1,721

I gotta 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.




#38
Oct1609, 02:38 PM

P: 79

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 



#39
Oct1709, 08:53 AM

P: 262

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? Look at Fig. 173 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. 



#40
Oct1709, 11:19 AM

P: 4,513





#41
Oct1709, 12:45 PM

P: 262

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. 173 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. 



#42
Oct1709, 08:38 PM

P: 79

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 



#43
Oct1709, 08:59 PM

P: 4,513

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



#44
Oct1709, 09:42 PM

P: 4,513

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.
Flux rule: [tex]emf = wB \frac{dL}{dt} = wBv[/itex] w is the width of a square loop of wire. (The part of the loop in motion is a wire w units long.) L is the length of the loop. B is a magnetic field, uniform in strength perpendicular to the loop v is the velocity of the wire in the L direction. [tex] \nabla \cross \textbf{B}  \frac{\partial \textbf{E}}{\partial t} = \textbf{J}\ \ , where \ \textbf{J}=0[/tex] so of course he doesn't imply it's incorrect. 



#45
Oct1809, 12:09 PM

P: 262

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. 172 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. 



#46
Oct1809, 12:14 PM

P: 262

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? 



#47
Oct1809, 06:58 PM

P: 79

Phrak, Feynman defines the "flux rule" in section 161, 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 



#48
Oct1909, 08:05 AM

P: 262

Mike
I've asked: Can you explain to me what your reply means? 



#49
Oct1909, 08:03 PM

P: 79

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 



#50
Oct2009, 03:50 AM

P: 262

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 172 in case a new edition is published of his lectures on physics. 



#51
Oct2109, 03:09 PM

P: 79

I have to believe that the circuit of many wires will not be equivalent to the rocking plates.
Here is another counter example to Faraday's Law. In a toroidal transformer with the primary winding the inner one, the magnetic field external to the primary winding is severely reduced because geometrical symmetry results in its canceling itself out. Now take a toroidal core with a primary winding and loosely wind a secondary winding with one end going to an external circuit (such as a galvanometer) and the other going to a slipring loosely fitted around the primary winding. The brush contacting the slipring goes to the external circuit. Energize the primary winding with DC current, resulting in a constant magnetic flux in the core. Now gradually unwind the secondary winding. Its flux linkage will steadily decrease, but there will be no EMF in it because there is neither motional nor transformer EMF, thus violating Faraday's Law. Mike 



#52
Oct2109, 05:04 PM

P: 1,011





#53
Oct2109, 05:07 PM

P: 1,011





#54
Oct2109, 06:41 PM

Mentor
P: 28,795

Now, I'm not claiming that the 2nd Law has been shown to be wrong, because this occurs under a very specific condition. But that is the exact point being made to condition being given by the OP. Zz. 


Register to reply 
Related Discussions  
If GUT are false, are stringbased unification also false?  Beyond the Standard Model  1  
true or false/correct if statement is false help  Biology, Chemistry & Other Homework  2  
I can't figure out if this statement is true or false, i said false  Calculus & Beyond Homework  4  
Faraday's Law Help?  Introductory Physics Homework  4  
Faraday's Law  Classical Physics  5 