MadSci Network: Physics

Re: What happens to a magnet when it is frozen?

Date: Tue Mar 6 01:45:02 2001
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Physics
ID: 983406437.Ph

Greetings, Em:

As you probably know, if you saw a magnet in two, each piece is
still a magnet.  If you could grind it into powder (carefully, so
as not to overheat it), each speck of magnet-dust will still be a
magnet.  The fundamental source of magnetism in solid substances
is an unbalanced pair of electrons in orbit about some atom or
molecule.  When such atoms or molecules are piled together, there
is a tendency for their magnetic fields to cause the particles to
pair up in such a way that the 'north' end of each is as near as
possible to the 'south' end of the other.  Thus a pile of magnet-
powder is not likely to exhibit very much overall magnetism;
attractions and repulsions basically cancel each other out.

In a way, that situation is not much different from a magnet that
HAS been heated.  Heat is always associated with random motion of
molecules -- and random motion results in random orientations of
the particles being heated.  It should be obvious that in a pile
of randomly oriented magnet-particles, there are equal numbers
in pointing all directions, and again there is no overall
magnetism.  Any magnet at room temperature can be heated until
it beomes a non-magnet, due to the increased random motion of
its molecules.  For each magnetic substance, there is a
particular temperature at which its magnetic ability suddenly
drops to zero.  That temperature is known as the "Curie Point"
for the substance (named after the physicist who first studied
the effect).

To manufacture a magnet, it is necessary to force some overall
orientation upon the individual particles in a magnetic substance.
An external force of some sort is always necessary.  Planet Earth
is conveniently equipped with electric currents flowing in its
iron core (their cause has been debated for many decades), and the
whole world is a giant magnet as a result.  With respect to any
flow of molten rock, the Earth's magnetic field is an "external
force" that permeates it, and encourages magnetic molecules in
the molten rock to become aligned.  Naturally occuring magnets
are known as "lodestones".  They are relatively feeble, compared
to the magnets that we can manufacture today (using very powerful
electromagnetic fields), but lodestones were quite strong enough
to mystify people for thousands of years.

In some materials, simply striking it sharply can cause some of
their magnetic molecules to become aligned!  Those unbalanced
pairs of electrons are somehow literally associated with a kind
of off-center heaviness.  A mechanical shock can cause the
molecules to become jostled, and since this is a DIRECTIONAL
mechanical shock, there is a tendency for all the 'heavy' sides
of the particles to become a little more aligned in a particular
direction.  That tiny change from randomness is enough to yield
a noticeable magnetic field.  Several shocks, in the same
direction of course, can enhance that resultant field -- and as
you might expect, shocks in other directions can diminish it.

And now, on to the effects of cooling.  In this situation you
must first realize that your magnet is NOT a perfectly-aligned
collection of particles.  There is, at the very least, plain
old room temperature working to jostle molecules.  Since you do
have a solid substance, the molecules don't jostle very much,
but it is always enough to make the magnet less perfect than
it otherwise might be.  (There are some magnetic materials that
have a Curie Point BELOW room temperature!  So we never notice
their magnetism, until after they are cooled.)

Within your cooling magnet, the molecules begin to jostle a
little less vigorously.  Now think about the fact that the
magnet is permeated quite thoroughly by its own magnetic field.
Its own field helps the lesser-jostling molecules to come into
greater magnetic alignment!  And, as has been pointed out, the
more the particles are aligned, the stronger the magnet.  That
fact should qualify as the Answer to your Question....

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2001. All rights reserved.