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Faraday's magnetic induction 
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#1
Jan2410, 10:43 PM

P: 58

Hi everyone...
While going through some texts.. At one place i found that in faraday's law of magnetic induction, >a time varying magnetic field induces an electric field.. And, >a spatially varying electric field induces a magnetic field.. Which one of these is correct? I know of former, but by any chance, second one can also be correct?? 


#2
Jan2410, 10:58 PM

P: 4,512

A time varying magnetic field results in a spatially varying electric field and visa versa.
Now, as long as the electric field varies spatially, then it must be nonzero somewhere, therefore we obtain your first statement. Thinking that one field causes the other field is incorrect. That is, there is no cause and effect pairing. The word 'induce' is basically a grammatical error with historical roots. There is one field, called the electromagnetic field tensor, that has various components. These components are the electric and magnetic field strengths. 


#3
Jan2410, 11:14 PM

P: 58

Ok..
Suppose i have an electric and a magnetic field (constant).. [faraday law also shows that a spatially non varying E leads to a temporally non varying B].. If i vary B in time.. This will lead to a spatially varying E.. So will this E add to new originally existing E field... Or, the it is the originally existing field that will start varying... ? 


#4
Jan2610, 03:22 AM

Sci Advisor
PF Gold
P: 1,776

Faraday's magnetic induction



#5
Jan2610, 03:34 AM

P: 58

Am i correct? 


#6
Jan2610, 06:04 AM

Sci Advisor
PF Gold
P: 1,776

That would depend but on the whole I would say no. First, the orientation of the magnetic field that is varying will dictate the orientation of the varying electric field. This electric field could be oriented in such a way that it does not impact the flow of current in your wire.
Second, since we now have timevarying fields, we can only induce timevarying currents. The resulting fields are electromagnetic waves, and they will induce currents on the surface of the wire (assuming a perfect conductor). These currents will simply add to whatever current is already impressed upon the wire. If you can somehow induce a wave that is of the same frequency and will induce currents that are perfectly out of phase with the applied currents, then you could cancel out the applied currents. However, this really isn't feasible, and so you can filter out whatever induced signals are on the wire and regain your original signal should you wish. 


#7
Jan2610, 06:20 AM

P: 58

ok.. i get sm idea..
thx.. this may be a little off the topic, but do you know if a plasma, in stable state, has any net electric field? 


#8
Jan2610, 08:29 AM

Sci Advisor
PF Gold
P: 1,776




#9
Jan2610, 08:32 AM

P: 58

ok.. thanx for the help..



#10
Jan2610, 10:12 AM

P: 250




#11
Jan2610, 10:47 AM

P: 370

My point is that any statement like this should make sense to a very educated person who understands the tensor/relativity aspects of field theory. But, it should also make sense to an undergraduate student in a first semester of EM field theory. That statement will confuse the heck out of the uninitiated student. 


#12
Jan2710, 08:51 AM

P: 58




#13
Jan2710, 10:13 PM

P: 4,512

It turns out to be nonzero curl. Grad x E + dB/dt = 0, in natural units. This is the MaxwellFaraday equation. So where ever you can factorout a nonzero curl(E) from the field, there will be an associated changing B field. The corollary is that wherever one finds a time changing B field, there is a circuital electric field. The electric field doesn't terminate on chargeor, at least not all of it, but wraps back on itself. 


#14
Jan2810, 09:32 AM

P: 166

I think the earlier part of this is getting close to some questions I've had. In Feynman's lectures  vol. 2 either chap 19, 21 or 23  I think 23 (I'm at work and don't have the lectures here), Feynman shows in a capacitor that a changing EField induces a Bfield, then he shows that the Bfield induces a new EField (he calculates the Efield) and then adds the 2 Efields together. Well he says that "new" Efield creates a new Bfield  he calculates that new B and adds the two B's together and so on. He does 3 or so to see the pattern. He ends up showing the Efield and Bfield is this complicated expression involving a Bessel function. I had undergrad electromagnetics  junior and senior level and this was never mentioned. This Feynman lecture and results does not seem obvious to me  Is this thread getting close to this topic of Feynman's lecture? Can you elaborate further on this? Why was it not mentioned in electrodynamics (undergrad level at least)? It seems so interestesting but I never would have known to go there, I would have stopped at the first calculation. Thanks Sparky 


#15
Jan3010, 10:56 AM

P: 58




#16
Jan3010, 01:14 PM

P: 250




#17
Jan3010, 09:04 PM

P: 58

But since, the magnetic field in an EM wave is not the same as that due to a current carrying wire, I can't say the same thing for this field.. Right?? 


#18
Jan3110, 09:42 AM

P: 250




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