Name: 磁石。它們是如何工作的？ (MAGNETS: How Do They Work?)
Uploaded: 2021-01-14T04:40:06.000Z
Duration: 6 min 26 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

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!

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

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

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

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

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

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,

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

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

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.