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Question about E fieldby cragar
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#1
Dec2512, 09:47 AM

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Lets say I have an infinite uniformly charged rod, and this charged rod is rotating like a barber pole. Lets say I am standing outside the rod, will I be able to tell if the source is rotating or will the E field be constant. Lets say the charged rod is wrapped with a dielectric
if the rod is rotating fast enough could there be Cerenkov radiation 


#2
Dec2512, 05:35 PM

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Loosely: for something like, say, the classical verylong uniformly charged rod rotating in vaccuum, I'd expect a varying Efield for most places you could stand, you'd be able to tell it was rotating (in your reference frame) because you'd be able to look at it and see, but I would not expect cherenkov radiation unless the whole setup was immersed in a dielectric medium. I would not expect E to be constant even in the static (nonrotating) case  I'd expect the direction to be radial and the magnitude to decrease with radial distance. Hence the magnitude and direction will depend on where you measure it. Where were you going with this? 


#3
Dec2512, 11:35 PM

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I guess what I meant by constant E field is that at a point in space it is not varying with time. The rod does not need to infinite it can be finite. How would you be able to tell
that the cylinder was rotating with out looking at it. What experiment would you be able to do locally to tell that. Part of the reason I wanted it to be infinite is so there is no B field outside the rod. Maybe we should just have two charged concentric charged spheres and then have the inside sphere turning. What do mean when you say it needs to be immersed in a dielectric medium, why cant it just be surrounded by one. 


#4
Dec2612, 12:35 AM

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Question about E field
Oh you mean rotating about it's axis?
Somehow I thought you meant rotating endoverend. Why not do the math? 


#5
Dec2612, 02:31 AM

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use gauss's law or do a field transformation on the E field.



#6
Dec2612, 03:40 AM

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You wanted it to be infinite so that there is no B field outside the rod? Why does this imply zero B field? If you think about some general point, the moving currents on one side of the centre of the pole will be closer than on the other side, so the B field won't get cancelled out. I know how to calculate the (constant with time) parts of the E and B field (i.e. ignoring the fact that moving charges would cause radiation). But I am not familiar with calculating radiation given off... 


#7
Dec2612, 04:34 AM

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The reason the B field is zero outside is because its an infinite solenoid and the current enclosed is zero in my amperian loop.
Ok yes sometimes accelerating charges radiate but is this true for a solenoid. And is this true for an infinite uniformly charged rod. 


#8
Dec2612, 08:17 AM

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#9
Dec2612, 10:41 AM

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ok this still doesn't address my original question about
Cerenkov radiation plus I still don't think current loops radiate maybe look at this forum. well they radiate if we consider points charges instead of a continuous charge distribution, but any way. http://www.physicsforums.com/showthread.php?t=561208 


#10
Dec2612, 10:56 AM

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Right, so if we're going to assume that there is zero magnetic field outside, then there won't be any energy emitted in the form of electromagnetic waves. Because there must be a nonzero magnetic field to allow energy to be propagated (simple use of Poynting vector). So if you want to find something which is going to emit EM energy, then its going to have to be not strictly a solenoid in this sense. edit: out of curiosity, is this machine going to be used for some science fiction writing? or is it just general curiosity? 


#11
Dec2612, 11:00 AM

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Ok my question isn't about the source radiating. Its about if the source causes Cerenkov radiation in the dielectric.
actually the point you make about the poynting vector might solve this. 


#12
Dec2612, 11:08 AM

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I'm still not sure what you mean by Cerenkov radiation. Isn't that usually generated when a charged particle is moving through a medium at a speed faster than the phase speed of light?
So in this situation, you've got a pole which is made of some uniformly charged medium... I don't see how there are charged particles moving through some medium with relative velocity... 


#13
Dec2712, 02:56 AM

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the acceleration field vary as 1/r,it is necessary otherwise there will not be radiation detected at long distances.1/r^{2} variation vanishes rather rapidly to give any contribution.The electric field which contains 1/r contribution contains acceleration of particles.For a charge particle moving through dielectric can radiate under condition of it's velocity surpassing the velocity of light in that medium evaluated by some refractive index formalism.That's all.if there is a solenoid in which current is time varying then there is a theoretical framework for calculating radiation from it.it is called multipole radiation.



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