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If you take a piece of wood and put it next
to another piece of wood... nothing happens.
And if you take a piece of granite and put
it next to another rock... still nothing.
But if you take this piece of iron and put
it next to this other piece of iron... magic!
I mean, magnet.
Magnetic objects are able to magically attract
at long distance because they generate magnetic
fields that extend invisibly out beyond the
object. But the mystery is this: where do
magnetic fields come from?
Derek: well that's easy, Henry! We've known
for a long time that electricity and magnetism
are really just two sides of the same coin,
kind of like mass and energy or time and space,
and they can be transformed into each other.
In fact, magnetic fields are basically just
what electric fields turn into when an electrically
charged object starts moving!
Henry: That makes sense for explaining why
a current of electrons flowing through a wire
causes this compass needle to move, or how
currents in the earth's outer core generate
the geomagnetic field... but a bar magnet
or the compass needle itself are just pieces
of metal without any electrical current running
through them.
Derek: Or are they? At a microscopic level,
there are loads of electrons whizzing around
in the atoms and molecules that make up any
solid.
Henry: Right! This brings up an excellent
point - The magnetic behavior of any everyday
object is influenced by a fascinating combination
of effects ranging from the level of particles
to atoms, collections of atoms, and collections
of collections of atoms. First, individual
particles.
Unlike the everyday workings of gravity and
electricity, permanent magnets can only be
fully understood as a quantum mechanical effect.
In much the same way that particles like electrons
and quarks have fundamental properties called
mass and electrical charge, most particles
ALSO have another intrinsic property, called
"tiny magnet". Just kidding, it's called an
"intrinsic magnetic moment," but really, that's
just technical mumbo-jumbo saying that particles
with electric charge ALSO happen to be tiny
magnets.
Derek: If you want to know WHY they're tiny
magnets, well, you might as well ask WHY do
particles have charge in the first place,
or why do objects with energy and momentum
attract gravitationally? No one knows... We
just know these things are true/that is how
the universe works.
Henry: Exactly, and since the 1920s, we've
known that each individual electron or proton
is basically a tiny magnet. Which brings us
to the level of atoms.
An atom is a bunch of positively charged protons
with a bunch of negatively charged electrons
whizzing around them. The proton tiny magnets
are about 1000 times weaker than the electron
ones, so the nucleus of the atom has almost
no effect on the magnetism of the atom as
a whole.
Derek: And you might think that since many
(though not all) of the electrons are also
moving, like the current in a wire, they would
generate magnetic fields from that motion.
Indeed they do, and these are called "orbital"
magnetic fields.
Henry: Except, these don't usually contribute
to the magnetic field of an atom. Here's why:
Electrons in atoms are accurately and complicatedly
described by quantum mechanics, but the gist
of the story is that electrons congregate
in shells around the nucleus. The electrons
in any filled shell zoom equally in all directions
and so the currents they generate cancel out
and generate no magnetic field. These electrons
also come in pairs whose tiny magnets point
in opposite directions and also cancel.
However, in a half-filled shell, all of the
electrons are unpaired and their tiny magnets
point in the same direction and add up, meaning
that it's the intrinsic magnetism of the electrons
in the outer shell that gives an atom the
majority of its magnetic field.
So atoms near the side of any of the major
blocks of the periodic table, which have full
(or nearly full) outer electron shells, aren't
very magnetic. And atoms in the MIDDLE of
the blocks have half-full outer electron shells
and are magnetic. For example, Nickel, Cobalt,
Iron, Manganese, Chromium, etc.
Derek: Wait, but chromium isn't magnetic!
Henry: Ah, but just because an atom is magnetic
doesn't mean that a material made up of lots
of that atom will be magnetic. Which brings
us to the level of crystals.
When a bunch of magnetic atoms get together
to make a solid, they generally have two options.
One is for all of the atoms to align their
magnetic fields with each other, or they can
align the magnetic fields in an alternating
fashion so that they all cancel out. The atoms
will do whichever one requires less energy.
Derek: That's why chromium, for example, is
a very magnetic atom but a very un-magnetic
solid - because it's one of the most anti-ferromagnetic
materials we know. Iron, on the other hand,
is the name-sake of ferromagnetism, so it
is, unsurprisingly, ferromagnetic. Or, in
usual parlance: magnetic.
Henry: Sometimes.
The last and final level of magnetism is that
of domains. Essentially, even in a magnetic
material where the magnetic fields of atoms
line up together, it's possible that one chunk
of the material will have all its atoms lined
up pointing one way, and another chunk will
have all its atoms pointing another way, and
so on.
Derek: If all of these "Domains" are of approximately
similar size, none may be strong enough to
force the others to align with it, and so
a piece of iron, for example, might have no
magnetic field because of all of the warring
magnetic kingdoms within it.
Henry: However, if you apply a strong enough
magnetic field/force/pressure from outside
the material, you can help favor one domain/help
one domain expand its control over its neighbors,
and so on until all of the domains have been
unified into one kingdom, all pointing in
the same direction.
Derek : And now, finally, you can rule with
an iron fist... I mean, magnet.
Henry: Exactly! What's remarkable is that
magnetism is a fundamentally quantum property
amplified to the size of everyday objects:
every permanent magnet is a reminder that
quantum mechanics underlies our universe - in
order for any object to be magnetic, it has
to have a unified kingdom of magnetic domains,
each made up of bajillions of magnetic atoms
which also need to be aligned with each other,
each of which can only be magnetic in the
first place if it has an approximately half-filled
outer shell of electrons so their intrinsic
magnetic fields can align and not cancel each
other out. Not surprisingly, these criteria
are pretty difficult to fulfill, which is
why there are only a limited number of suitable
materials you can use when you're building
a magnet.
Derek: OR you could just run a current through
any electrical conductor and generate a magnetic
field that way.
Henry: But hey... Why does that work in the
first place? Click here to go to over to Veritasium
and we'll find out what special relativity
and the speed of light have to do with electromagnets.