Magnetics Business & Technology - Winter 2017 - 10

FEATURE ARTICLE

The Eighteenth Mistake
By Stan Trout

Not long after I completed my series on the seventeen mistakes
people make with permanent magnets, I received a suggestion for
an eighteenth. This mistake involves confusing the polarity of magnets, a topic I mention early in my bootcamps. But I never made the
leap to connect this topic to my list of mistakes. So, I appreciate the
recommendation. Here is my description of the 18th mistake.
Two things we know about magnets are: they have both north
and south poles, and that opposite poles attract. A compass is a
nice example of this concept, since the needle is free to pivot and
turn in reaction to a magnetic field. The arrow of the compass we
are told, points north. It does so in response to the magnetic field
originating from the magnetic pole located near the Earth's geographic north pole, at least when in the northern hemisphere. Since
the Earth's magnetic pole attracts the arrow of the compass, either
the Earth's pole or the arrow of the compass is a north pole and the
other is a south pole. The question is which is which? It turns out
that there are three conflicting, yet equally valid ways of looking at
this situation.
One approach is to say that the tip of the compass arrow is a
north pole, since it points toward the geographic north. That means
the magnetic pole nearest the Earth's geographic north pole must
be a south pole, since opposites attract.
The second approach is to say that the magnetic pole nearest the
Earth's north pole is a north pole. That means the tip of the arrow
must be a south pole.
Which is correct? It turns out that the answer is not as important
as you might imagine. The critical concern is consistency. In other

words, first define the terms north pole and south pole, and then
rigidly hold to that definition.
My approach is to use the first definition. In this case, we are really saying the north pole of a magnet is a north seeking pole, meaning that it is the pole which turns toward the geographic north pole
when the magnet is allowed to rotate freely. In my seminars I show a
picture of the Earth with a bar magnet superimposed on it, which is
shown below. At first look the bar magnet might appear upside down,
since its south
pole is up, and
the north pole
is down, in contradiction to the
geographic poles.
But it must be
this way to be
consistent with
our definition of
north and south
poles.
What happens
if we use the
other definition?
The
magnetic
pole nearest the
Earth's geographic pole is now a Source: James S. Walker, Physics, 4th Edition,
Addison-Wesley 2010

From Earth

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