Register to reply 
Can magnetic field expand faster than light? 
Share this thread: 
#1
Jan2409, 01:58 PM

P: 77

When I was reading abuth EM oscilators, a question suddenly ran through my mind:
“Is there a frequency, when the speed of a colapsing/forming magnetic field would exceed the speed of light?“ Einstain said that nothing can go faster than light. Well, ok, but if the magnetic field (generated by EM oscilator, with AC current) was for example 1km vast (from it`s source), with frequency higher than 75 kHz, then the magnetic field would have to travel the trajectory of 4km in one period (it would have to expand to 1km distance and diminish back, then reverse polarity and to 1km and back) faster than 300000km per s. As long as this is, according to Einstein, impossible, could it be a reason why EM waves are generated at higher frequencies? 


#2
Jan2409, 02:33 PM

P: 15,319




#3
Jan2409, 04:19 PM

P: 77

So does it mean, that even if I turn the polarity of the field almost instantly, for a moment, in some distant point, the field would still have the same polariy and magnetic induction as before? What would happen to magnetic field of a toroidal permanent magnet rotating at a speed near the speed of light? To not exceed the speed of light the field lines would have to curve, how?



#4
Jan2409, 11:47 PM

P: 15,319

Can magnetic field expand faster than light?



#5
Jan2509, 12:38 AM

P: 4,512

Yes. And they do curve. From a rotating magnet, with the poles not aligned with the axis of rotation, as you imply, there are spiraling magnetic fields alternating in direction propagating outward at the speed of light (in vacuum conditions). The guys proficient in Cosmology can tell you about this. 


#6
Jan2509, 04:44 AM

P: 77

Thanks for your answers



#7
Jan2509, 09:00 PM

P: 22

There are some misconceptions that you seem to have.
It will benefit you if you can stop thinking of electric and magnetic fields as isolated entities. While it is educationally pragmatic to isolate them, it is not always appropriate to do so. Particularly for considerations of 'light'. Maxwell's laws tell us that a timevarying Efield will generate a BField and timevarying Bfield will generate an Efield. Magnetic fields do not "attract". They exert a force on charges moving through them. So yes, if the field at a point has not yet "realized" the generator is off, then it will still exert a force on a charge moving through it. Rotating Permanent Magnets do emit electromagnetic waves. So does accelerating anything that contains charge. The challenge is to accelerate them so intensely and coherently that the EM waves will have an amplitude observable above any background radiation. The EM waves generated by rotating a magnet would be "normal"; however, the analysis necessary to predict the frequency and amplitude of the waves would be different. 


#8
Jan2509, 10:49 PM

P: 4,512




#9
Jan2509, 11:33 PM

Mentor
P: 11,787




#10
Jan2609, 01:13 PM

P: 77

Thanks for replyes



#11
Jan2609, 03:58 PM

P: 4,512

A magnetic field, in a region of space free of charge, and changing over distance will result in an electric field. As far as propagating magnetic fields go, they will always be associated with propagating electric fields. In the case of a spinning magnet, it is an induced electric field. But propagating fields are not the only fields allowed in free space, The magnetic field about a stationary magnet is one example, or course. Upclose to the spinning magnet, the fields don't look like propagating fields at all. 


#12
Jan2709, 03:47 PM

P: 77

As it is mentioned above, I asked about speed of a change in realy vast magnetic field. DaveC said that



#13
Jan2709, 05:37 PM

P: 22

I am going to restate my understanding of your question:
If the magnetic field pushes on the charge moving through it, then the charge must push back on something because "every action has an equal and opposite reaction". How is the "push back" manifested? I am not confident in my answer, and I should probably let someone else answer, but I believe the "push back" manifests itself as an alteration in the field itself. 


#14
Jan2809, 04:32 PM

P: 7

so in principle. if I had two magnets in a void, X distance away from each other so the fields don't overlap...
if one of those magnets were spinning the field would eventually become an elliptical pattern and push the 2nd magnet away? or did I misread something...? 


#15
Jan2809, 04:46 PM

P: 15,319




#16
Jan2809, 05:28 PM

P: 22

Moonknight, What is it you read?



#17
Jan2909, 04:53 AM

P: 4,512

This same sort of fuzzy business comes up with say, a discrete capacitor, where the near electric fields are calculated in one way but the transmitted radiation another. How it all meshes together is a mess involving intractable and mysterious Bessel functions of some sort. Concerning the capacitor, Feynman takes so steps toward it in his "Lecture on Physics". 


#18
Jan2909, 05:34 AM

P: 3

From what I understand the only way to move faster than the speed of light is to literally stretch spacetime itself. And when I say stretch I mean the sort of stretching we might have seen with accelerating expansion.
Since a magnetic field would exists within this structured, yet distorted structure  my answer would be "no". Its top speed would always be relative (and smaller) than the paradigm it exists within. Taylor 


Register to reply 
Related Discussions  
Light in changing magnetic field  Classical Physics  13  
Speed of light Schmeed of light: things can travel faster  Special & General Relativity  13  
Rotation of light utilizing a magnetic field  Classical Physics  1  
Does Light have an Magnetic Field in a Vacuum?  Classical Physics  13  
Since the Universe is expand we rule out galaxies whose light will never reach us?  Cosmology  9 