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About accelerating charges... 
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#1
May813, 06:59 AM

P: 17

So we know that an accellerating charge will emit EM waves.
Consider putting a voltage through a circular wire. Electrons are travelling in the wire with constant speed but their velocity is changing constantly as it is changing direction due to the shape of the wire. Now does that mean the electron will emit EM waves? Consider another scenario, we now have a charged metal sphere. Does it emit EM wave if we let it drop to the ground? 


#2
May813, 09:37 AM

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P: 11,629

A ring with a constant current does not emit EM waves  the contributions from all electrons together cancel each other.



#3
May813, 08:37 PM

P: 17

how about a straight wire and you turn up the voltage making electrons accelerate?



#4
May913, 05:45 AM

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P: 11,629

About accelerating charges...
You need some timedependence in the current, otherwise it will not work.



#5
May913, 05:49 AM

P: 17

So as you increase voltage constantly,
your current increases comstantly, and therefore the electrons are accelerating in the wire with constant acceleration (dV/dt = c>0 dI/dt > 0 dv/dt = a >0). So what kind of EM wave do the electrons emit? 


#6
May913, 05:59 AM

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P: 11,629

That should give some radiation. What do you mean with "what kind of EM wave"? The frequency will depend on the setup, and I doubt that it is possible to give an answer without a detailed analysis (and probably a numerical simulation).



#7
May913, 06:21 AM

P: 17

Oh because I can't image how radiation will be given off by a piece of wire. Say we increase the voltage from 1V to 12V in 12seconds, so that dV/dt = 1, and we connect a copper wire to the adjustable power source, where the copper wire has a radius of 5mm. What radiation will the electrons from the wire give off?



#8
May913, 06:32 AM

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dV/dt = 1 V/s
Actually, 11/12 and not 1. It is impossible to answer this without knowledge of the cable shape, and probably numerical simulations (and nobody wants to do them unless it has some real application, I think). The circuit will emit a tiny amount of extremelylowfrequency radiation. 


#9
May913, 07:00 AM

P: 17

So it actually emits radiation, and I think every electrical appliances we use will emit very low frequency radiation too? Also, how about if we accelerate a piece of wire horizontally (say we hold the wire and run across the room). Can we treat this as acceleration of a charge? If we can, and another person is observing this, will he see the radiation but I will not because I am in the same frame of reference as the wire?



#10
May913, 09:27 AM

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The existence of radiation is frameindependent. You are not in an inertial frame if you accelerate. 


#11
May913, 09:48 AM

P: 445




#12
May913, 09:51 AM

P: 17

Mmm.. interesting! 5060Hz is in the radio wave range so we should get a small interference when we turn on the radio with a lot of electrical appliances operating around us!
I am just wondering, what is the difference between putting an increasing current through a wire and accelerating the whole wire horizontally as when you consider electrons in the wire they are accelerating in space in both cases? Good point about frameindependent:D Yeah I forgot that EM waves travel at the speed of light in all frame of reference! 


#13
May913, 10:04 AM

P: 17

1) electrons in circuits (in fact positive holes in metals as well) 2) Electrons orbiting a positive nucleus (we know that they are not really orbiting and it is all about probability when you get into college physics/chemistry but there is still a chance for the electrons to accelerate) 3) In fact all matter contains electrons and protons, meaning everything emits EM waves then? If it is true well we know that the radiation is very small that it is hard to have an effect. However in theory radiation should exist for any charged particles (as they all have some probability to accelerate). 


#14
May913, 06:19 PM

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P: 11,629

Textbooks about electromagnetism should cover this in some way. 


#15
May913, 06:30 PM

P: 292




#16
May913, 06:44 PM

C. Spirit
Sci Advisor
Thanks
P: 5,443

If the issue is taking radiation due to uniform acceleration of a charge (assuming you can somehow maintain uniform acceleration by applying an external force, because the charge solely on its own will undergo back reaction from the radiation due to the AbrahamLorentz self force and will cease to undergo uniform acceleration) and then applying the equivalence principle to get what is ostensibly a nonsensical answer, note that you cannot apply the equivalence principle in such a case because the massenergy distribution is not localized (the electromagnetic field carried by the charge extends out to infinity).



#17
May913, 07:24 PM

PF Gold
P: 1,150

You can learn this from a nice book : Mark A. Heald, Jerry B. Marion: Classical Electromagnetic Radiation. 


#18
May913, 10:43 PM

P: 445




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