If you take a piece of wood and put it next to another piece of wood…
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.
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
粒子 原子 原子团 以及原子团的集合
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
等基本特性 与之类似 大多数微粒还有
mass and electrical charge, most particles ALSO have another intrinsic property, called
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
No one knows…
We just know that’s the way 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：你可能会想 尽管不是全部 但许多电子也在运动
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.
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
然而 在半满的电子层里 所有电子都不配对
However, in a half-filled shell, all of the electrons are unpaired and their tiny magnets
它们的微磁场方向相同 并相互叠加 意味着
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.
比如 镍 钴 铁 锰 铬 等等
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.
另一方面 铁和铁磁性是同义词 所以意料中地
Iron, on the other hand, is the name-sake of ferromagnetism, so it is, unsurprisingly,
Or, in usual parlance: magnetic.
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.
It’s magnetic because it’s ferromagnetic and all of the domains are aligned.
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.
I want to find out.