Re: Are AM Radio Waves Harmful?

From: Don Klipstein (don_at_manx.misty.com)
Date: 08/24/04 

Date: Tue, 24 Aug 2004 13:45:32 +0000 (UTC)

In article <JadVc.156340$bp1.128947@twister.nyroc.rr.com>, foo@baz.com wrote:
>According to Don Klipstein <don@manx.misty.com>:
>
>> Magnetic fields around cables or wire sets having currents
>> equal and in opposite directions are small when much outside
>> them and decreasing rapidly as you move away. You can get
>> around a milliTesla inside a piece of Romex, but only around
>> a microTesla or two a few inches away and decreasing
>> inverse-square as you move further away.
>
>Unless we are talking about magnetic monopoles, this is a
>dipole, if you are much farther away than the distance between
>the wires, and its field descreases as an inverse-cube.

  This inverse-cube relationship is for a distinct source of overall size
that is small compared to your distance from it. But for wires that are
long compared to your distance from them, it's not quite that good. If
you increase your distance from the wires, you also increase the length of
the portion of the wires that is in your "larger neighborhood".
  Now for an example with numbers:

  Magnetic field produced by two wires is the sum of that of each.

  Put 10 amps through one wire 1 meter away and you get 2 microTesla.
Have a second wire 1 centimeter directly behind it (1.01 meters
from you) carrying the same current in the opposite direction and it
produces where you are 1.9802 microTesla, but at your point exactly
opposing the first 2 microTesla, leaving .0198 microTesla.

  Increase your distance to 2 meters from the first wire and 2.01 meters
from the second. Their fields respectively are 1 and .995025
microTesla, subtracting out to .004975 microTesla.

  The net field amplitude varies inverse-square for straight wires that
are long compared to your distance from them.
   However, once the distance increases to large compared to the length
of the wires, the situation changes and the magnetic field intensity will
vary inverse-cube with distance. Amplitude of a magnetic field produced
by a small source such as a wall wart or an electric alarm clock will also
vary inverse-cube with distance from it as long as your distance from it
is large compared to the size of the motor or transformer or whatever the
source of the magnetic field is.

  The amount of electrical power dissipated in flesh (picowatts per
kilogram or whatever) by an AC magnetic field will vary with the square of
the field amplitude. If you are at a distance from a pair of wires large
compared to the spacing between the wires but small compared to the length
of the wires, this will vary inversely with distance to the 4th power.
For distinct small sources with distance large compared to the size of the
sources, this figure will vary inversely with distance to the 6th power.

 - Don Klipstein (don@misty.com)

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