MadSci Network: Physics

Re: Does static charge or permanant magnet emit radiation

Date: Mon Jan 8 05:28:15 2007
Posted By: Jim Guinn, Staff, Science, Georgia Perimeter College
Area of science: Physics
ID: 1167107703.Ph

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 .

Well, Barun, I hope that answers your question.  Please let us know if you 
would like any more information.

Thank you for your interest.


Jim Guinn
Georgia Perimeter College

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