| MadSci Network: Physics |
Dear Barun, This is a great question, and your reasoning is excellent, except for one small point. You are absolutely correct in that a moving charge will create a varying electric field and hence a magnetic field, and a moving magnet will create a varying magnetic field and hence an electric field. Both of these occurrences are described by the electromagnetic field equations called Maxwell’s Equations. Your mistake is assuming that having a changing electric and magnetic field necessarily means that there is any radiation. This is rather a subtle point to which I’ll get back, but first let me digress. One important difference between electric and magnetic fields is that as far as we know, there are no magnetic monopoles. That is to say, there are no magnetic “charges”. Magnetic fields are formed from the motion of electric charges; however, electric fields are not formed from the motion of magnetic “charges”. Electric fields are formed by the presence of electric charges, and from the motion of magnets that are formed from the motion of electric charges. Does that make sense? I hope so! Now, back to your initial question. When you look at the equations that describe these changing fields that arise from the constant motion of a charge or a magnet, the rate at which the changing field dies off as you get farther away from it is too great for there to be any radiation. That is to say, the moving object does not change the field enough for it to give off any energy. This is assuming that the motion of the charge or magnet is constant. However, if it is slowing down, then it definitely does give off radiation. This type of radiation is called “braking” or “Bremsstrahlung” radiation. The energy given off in this radiation comes from the decrease in the kinetic energy of the charged particle(s). So, the bottom line answer to your question is that if the charge/magnet is moving at a constant velocity, it does not give off any radiation because the fields don't change enough for that to happen. If the charge/magnet accelerates, that is, if the charge/magnet changes its velocity, then it does produce some radiation, and the type of radiation is called Bremsstrahlung radiation. If you would like some more information on Bremsstrahlung radiation, you might try looking at http://en.wikipedia.org/wiki/Bremsstrahlung . Well, Barun, I hope that answers your question. Please let us know if you would like any more information. Thank you for your interest. Sincerely, Jim Guinn Georgia Perimeter College
Try the links in the MadSci Library for more information on Physics.