Work and World

MAGNETS: How Do They Work?

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.
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