
#91
Mar510, 04:48 AM

P: 133

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
Mar510, 02:36 PM

P: 79

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
Mar510, 04:34 PM

Mentor
P: 28,841

If you guys do not stop snipping at each other and do not go back to strictly discussing ontopic subjects, this thread will be closed!
Zz. 



#94
Mar510, 04:43 PM

P: 370





#95
Mar510, 04:53 PM

Mentor
P: 28,841

Zz. 



#96
Mar510, 06:20 PM

P: 185

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
Mar610, 01:18 AM

P: 133

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 lets put the disk in one frame and the magnet in another. For drill lets 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
Mar610, 03:42 AM

Mentor
P: 15,627

That is completely wrong. An electric field can be created by a changing magnetic field.




#99
Mar610, 04:02 AM

P: 262

Disconnecting the meter temporarily, while you remove the magnet will prove your point as well? 



#100
Mar610, 11:12 AM

P: 370

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] 



#101
Mar610, 01:31 PM

P: 79

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 currentcarrying 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
Mar610, 01:43 PM

P: 79

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
Mar610, 02:32 PM

P: 370

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
Mar610, 02:50 PM

P: 79

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 



#105
Mar610, 03:35 PM

PF Gold
P: 1,370





#106
Mar610, 04:04 PM

P: 185




#107
Mar610, 04:07 PM

P: 370

First, I was asking you to provide a version of equations that you feel is correct, but you provided the one which is under discussion that you feel is wrong. Am I correct about that? If so, do you have an equation that you feel is correct, or are you just saying the given one is wrong? As far as the equation you gave, it looks like you are saying E is emf, and B is total flux. One confusing thing about this is that I normally use E for electric field and B for magnetic fields, so I want to make sure I'm understanding this. As far as I can tell, the main difference between the equation you quoted and the one I wrote is the location of the time derivative. I wrote the time derivative inside the integral, while you seem to be writing it outside the integral. It occurs to me that we need to be careful about the location because this form is supposed to be valid for time dependent surfaces. There is a difference between the equations, and perhaps the one I gave is wrong. Again, I will look at this more carefully. What this discussion is telling me is that we need to be careful about distinguishing between the fundamental form of Faraday's Law and the correct integral formulation under a given set of assumptions. We don't want to say Faraday's Law is wrong just because we decide to formulate an improper mathematical statement, or to make an approximate statement for practical applications. I'll think about this more, and will have some comments later. Getting back to some of your other comments. 



#108
Mar610, 04:38 PM

PF Gold
P: 1,370

Perhaps it is more obvious in Faraday's original: http://en.wikipedia.org/wiki/File:Fa..._generator.jpg 


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