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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.

If you take a piece of wood and put it next to another piece of wood... nothing happens.

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