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Re: Earth's Magnetic Field



Jim Green wrote:

[quoting MVB:]
|The whole business is easy if you recognize that the magnetic pole of
|the Earth which is located in the northern part of the planet is a
|*south*-seeking pole. It therefore attracts the opposite poles of
|compasses, the north-seeking poles. This way the north poles of
|compasses point roughly north and navigators go in the right direction.

Wait a minute here, Folks, let us be clear with our language: A
"'south'-seeking pole" is a NORTH pole -- the confusion comes from the
modifier "seeking":


No, not at all. The point of adding the "seeking" is to make it
explicit that you are dealing with a pole that will, when the magnet is
suspended, attempt to move toward the south. Thus a south-seeking pole
is the south pole of the magnet which is a physical south pole.

What you are describing is a "geomagnetic" north pole, which is a
physical south pole. So the terminology is

north-seeking pole = compass north pole = physical north pole
geomagnetic north pole = compass south pole = physical south pole

Now it makes sense that the mobile, compass pole would get the
"-seeking" name rather than the immobile, geomagnetic pole, but of
course definitions are not always rational. My recollection alone could
be wrong, albeit logical. So I checked some older physics texts to find
one that uses the terminology. In the Project Physics Course text,
which I believe to have been carefully put together, in section 14.11:

"We define the direction of the magnetic field vector B at each point to
be the same as the direction of the force on the *north-seeking* [my
emphasis] pole of a compass needle placed at that point. The force on
the south-seeking pole will be in a direction exactly opposite.... A
compass will respond to the opposite forces on its ends by turning until
it points as closely as possible in the direction of the field."
Miller's College Physics [5th edition] is even more direct: "The 'north'
pole of a bar magnet is the 'north-seeking' pole, which points northward
in the Earth's magnetic field."

If I remember correctly, the situation is made worse by some or most
planetary astronomers defining the planetary magnetic field as pointing
away from the geomagnetic north pole (or equivalents for other bodies).
Thus their magnetic-field direction is opposite to the physical one, and
the magnetic force on a charge obeys a *left-hand* rule. [If I am wrong
on this one, I'm sure I will be corrected quickly.]

--
Maurice Barnhill, mvb@udel.edu
http://www.physics.udel.edu/~barnhill/
Physics Dept., University of Delaware, Newark, DE 19716