# Accelerating charged particles and FOR

1. Aug 7, 2004

### raymes_k

if, say, an electron is stationary on earth (ignore the probability or possibility of this for now). it, of course, is not emitting any EM radiation. However, an observer in space (stationary with reference to the sun) is looking at the electron. relative to them, the electron is accelerating (circular motion around the earth). is any EM radiation emitted? where have i gone wrong here?

2. Aug 8, 2004

### pervect

Staff Emeritus
I don't think you've gone wrong at all. As I understand it, photon number is not conserved in arbitrary coordinate transformations, so it's normal to see a different number of photons in accelerated and non-accelerated coordinate systems, including cases where one sees zero number of photons and the other sees non-zero numbers of photons.

3. Aug 8, 2004

### raymes_k

so basically it is possible for the external observer to see photons being emitted. but then theres the question of whether these photons can be used for any constructive purpose. is this violating conservation of energy principles?

4. Aug 8, 2004

### ArmoSkater87

Why do charged particles emit light when accelerated anyways??

5. Aug 8, 2004

### pervect

Staff Emeritus
There are various applets that show what the electric field lines of an accelerating charge looks like. For instance

http://webphysics.davidson.edu/Applets/Retard/Retard_FEL.html

It's fairly easy to see from these that the electric field lines of an accelerating charge get "kinked".

The example in the above URL where you suddenly stop a moving charge exhibits this "kinking" process especially well.

The argument that the kink represents the emission of radiation is harder to follow,IMO, though it is easy to see that such a kink represents a rapidly changing electric field and hence generates a magnetic field. It's probable I'm missing some simple explanation.

The detailed explanation is given by the Lamor formula for an accelerating charge, for instance

http://farside.ph.utexas.edu/~rfitzp/teaching/jk1/lectures/node31.html [Broken]

Last edited by a moderator: May 1, 2017
6. Aug 11, 2004

### ArmoSkater87

Awsome sites, thanks a lot. :)