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I understand the fact that a static charge produces an electric field around it.

I accept the fact(without questioning) that a moving charge produces a magnetic field.

Question 1 : If a moving charge produces both electric and magnetic fields why can we not say that it produces an EMW?

Question 2 : How does an accelerated charge produce EMW?

Nabarun Kashyap
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  • Not an accelerated but a charge under acceleration emit EM radiation (in the form of photons). – HolgerFiedler Jan 07 '16 at 08:19
  • Moving (accelerated) charges are producing electromagnetic waves. You shouldn't take anything for granted without questioning, by the way... all of these things are experimentally tested and you can repeat all these experiments at home, if you like. They are all pretty easy. – CuriousOne Jan 07 '16 at 08:21
  • In positive acceleration particles gain more photons as loosing, in negative accelerations like break or curved movement do to an applied magnetic field the particle loose photons – HolgerFiedler Jan 07 '16 at 08:24
  • @HolgerFiedler: What happens when they are running out of photons? ;-) – CuriousOne Jan 07 '16 at 08:57
  • &CuriousOne They have lost their kinetic energy and are in rest. – HolgerFiedler Jan 07 '16 at 09:21
  • @HolgerFiedler: I don't know why... but this sounds a little like a Halloween special. :-) – CuriousOne Jan 07 '16 at 09:44
  • @CuriousOne This is the answer of a physicist, a teacher or a joker? Why not have a deeper discussion in chat room? For example about this answer http://physics.stackexchange.com/questions/227933/how-can-interference-occur-within-diffraction/228012#228012 – HolgerFiedler Jan 07 '16 at 09:55
  • You can find the answers in section 11.2 in $\textit{Introduction to Electrodynamics}$ by David J.Griffiths – Prof Shonku Jan 07 '16 at 12:52
  • @CuriousOne Today, what do you think about this topic? – HolgerFiedler Jun 11 '21 at 04:37

1 Answers1

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Question 1:

One way to understand it to rely on the principle of relativity stating that the physics is the same in all inertial frames of reference (galilean frames of reference). In particular, any frame moving at constant velocity with respect to a known inertial frame is inertial too.

In the case of a particle moving at constant velocity in an internal frame (like a laboratory for instance), it does indeed create both an electric field and a magnetic field. However, if we consider an observer moving at the same constant velocity as the charged particle, the latter will be at rest with respect to this frame and there won't be any magnetic field anymore, just an electrostatic field. Since there exists a frame in which no EMW exist, it has to be true in all inertial frames as well.

Question 2:

The previous argument breaks down if the charged particle is seen to be accelerated from an inertial frame of reference since accelerated frames are not inertial and not subject to any simple (i.e. not requiring general relativity) formulation of the principle of relativity. So from that only we can conclude that emitting EMW is possible for an accelerated charged particle. To see that it is inevitable is more involved but basically it has to do with the fact that if a charge is accelerated then, in all inertial frames there is a varying electric field that is generated and such a field generates in turn a varying magnetic field which in turns generates a varying electric field and so on and so forth thus enabling actual propagation of an EMW.

gatsu
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