### 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
Message:
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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
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

might try looking at http://en.wikipedia.org/wiki/Bremsstrahlung .

Sincerely,

Jim Guinn
Georgia Perimeter College

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