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磁铁的原理 – 译学馆
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磁铁的原理

MAGNETS: How Do They Work?

如果你将一块木头放在另一块木头旁边
If you take a piece of wood and put it next to another piece of wood…
没有现象发生
nothing happens.
如果一块花岗岩放到另一块石头旁边 也没现象
And if you take a piece of granite and put it next to another rock… still nothing.
但是如果你放上两块铁……奇迹出现了
But if you take this piece of iron and put it next to this other piece of iron… magic!
我是说 磁铁
I mean, magnet.
磁性物质可以奇迹般地长距离互相吸引
Magnetic objects are able to magically attract at long distance because they generate magnetic
因为物体之外形成了肉眼难见的磁场
fields that extend invisibly out beyond the object.
但是有个问题:磁场是从哪里来的?
But the mystery is this: where do magnetic fields come from?
Derek:Henry 这简单
Derek: well that’s easy, Henry!
我们很早就知道磁和电本质上是同一事物
We’ve known for a long time that electricity and magnetism are really just two sides of
正如质量与能量 时间与空间的关系
the same coin, kind of like mass and energy or time and space,
它们都能相互转换
and they can be transformed into each other.
实际上 磁场本质上是电场转化来的
In fact, magnetic fields are basically just what electric fields turn into when an electrically
这种现象在带电物体开始运动时产生
charged object starts moving!
Henry:这就解释了在电线中流动的电子流
Henry: That makes sense for explaining why a current of electrons flowing through a wire
为什么会使指南针转动
causes this compass needle to move,
也解释了地球的外地核内的电流如何产生地磁场
or how currents in the earth’s outer core generate the geomagnetic field…
但磁棒或者指南针本身
but a bar magnet or the compass needle itself are just pieces
在其中没有电流通过时 仅仅是一块金属
of metal without any electrical current running through them.
Derek:是这样吗?
Derek: Or are they?
从微观上看 大量电子在组成物质的
At a microscopic level, there are loads of electrons whizzing around in the atoms and
原子及分子周围旋转
molecules that make up any solid.
Henry:没错!
Henry: Right!
这就产生了奇妙的现象 任何常见物体的磁性
This brings up an excellent point – The magnetic behavior of any everyday object is influenced
都受各种效应奇妙组合的影响 这些效应源于
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.
首先 单个粒子
First, individual particles.
不像生活中重力和电力机制 永久磁性
Unlike the everyday workings of gravity and electricity, permanent magnets can only be
只能作为量子力学效应被完全理解
fully understood as a quantum mechanical effect.
像电子和夸克这样的粒子有质量和电荷量
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
另一种固有特性 叫做“微磁”
“tiny magnet”.
只是开玩笑 它其实叫做“内禀磁矩”但实际上
Just kidding, it’s called an “intrinsic magnetic moment,” but really, that’s just technical
有传言说带电荷粒子恰巧也是微磁
mumbo-jumbo saying that particles with electric charge ALSO happen to be tiny magnets.
Derek:若你想了解它们为什么是微磁 你可能也想了解
Derek: If you want to know WHY they’re tiny magnets, well, you might as well ask WHY do
电子为何带电荷 以及有能量和动能的物体
particles have charge in the first place, or why do objects with energy and momentum
为何会由于引力而相互吸引
attract gravitationally?
没有人知道
No one knows…
我们只知道这是宇宙规律
We just know that’s the way the universe works.
Henry:没错 二十世纪二十年代我们就已经
Henry: Exactly, and since the 1920s, we’ve known that each individual electron or proton
知道电子和质子是微磁
is basically a tiny magnet.
这让我们来到了原子层面
Which brings us to the level of atoms.
一个原子就是一群带正电的质子 质子周围有一群
An atom is a bunch of positively charged protons with a bunch of negatively charged electrons
带负电的电子绕其旋转
whizzing around them.
质子的微磁比电子的微磁弱了近千倍
The proton tiny magnets are about 1000 times weaker than the electron ones, so the nucleus
因此原子核对整个原子的磁性几乎没有影响
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
就像电线中的电流 那这种运动应该会产生磁场
moving, like the current in a wire, they would generate magnetic fields from that motion.
确实会产生磁场 叫做“轨道”磁场
Indeed they do, and these are called “orbital” magnetic fields.
Henry:但它们在原子磁场的形成中通常不起作用
Henry: Except, these don’t usually contribute to the magnetic field of an atom.
原因如下:
Here’s why:
量子力学可以准确而复杂地描述原子中的电子
Electrons in atoms are accurately and complicatedly described by quantum mechanics, but the gist
但关键是电子聚集在原子核周围的电子层中
of the story is that electrons congregate in shells around the nucleus.
任何满电子层内的电子均等地向各个方向运动
The electrons in any filled shell zoom equally in all directions and so the currents they
因此它们产生的电流相互抵消 从而不产生磁场
generate cancel out and generate no magnetic field.
这些电子也会成对出现 它们的微磁方向相反
These electrons also come in pairs whose tiny magnets point in opposite directions and also
从而相互抵消
cancel.
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
外层电子的固有磁性构成了大部分的原子磁场
in the outer shell that gives an atom the majority of its magnetic field.
因此 元素周期表主族元素和过渡元素区的边缘附近
So atoms near the side of any of the major blocks of the periodic table, which have full
有全满(或几乎全满)的外电子层的原子 没什么磁性
(or nearly full) outer electron shells, aren’t very magnetic.
各分区中部位置的原子有半满的外电子层 它们有磁性
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:等等 铬没有磁性!
Derek: Wait, but chromium isn’t magnetic!
Henry:啊 但一个原子有磁性并不意味着
Henry: Ah, but just because an atom is magnetic doesn’t mean that a material made up of lots
许多该种原子组成的物质也有磁性
of that atom will be magnetic.
这让我们来到了晶体层面
Which brings us to the level of crystals.
当一群磁性原子组成固体时 通常有两种情况
When a bunch of magnetic atoms get together to make a solid, they generally have two options.
一是所有原子将其磁场同方向地排列成一行
One is for all of the atoms to align their magnetic fields with each other, or they can
二是将磁场交替排列成一行 使之相互抵消
align the magnetic fields in an alternating fashion so that they all cancel out.
原子会选择二者中耗能少的方式
The atoms will do whichever one requires less energy.
Derek:这就是为什么铬原子有磁性 而固体铬无磁性
Derek: That’s why chromium, for example, is a very magnetic atom but a very un-magnetic
因为铬是已知的最反铁磁性的物质
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,
铁是有铁磁性的
ferromagnetic.
或者用通常说法:有磁性的
Or, in usual parlance: magnetic.
Henry:有时却会出现这种现象
Henry: Sometimes.
磁性的最后一个层面是场域
The last and final level of magnetism is that of domains.
本质上 即使在原子磁场排成一列的磁性物质中
Essentially, even in a magnetic material where the magnetic fields of atoms line up together,
也可能出现一部分该物质的原子排列指向一个方向
it’s possible that one chunk of the material will have all its atoms lined up pointing
另一部分的原子都指向另一方向 等等
one way, and another chunk will have all its atoms pointing another way, and so on.
Derek:如果所有的“域”大小相近
Derek: If all of these “Domains” are of approximately similar size, none may be strong enough to
没有一个域强到能迫使其他域与自己排成一列
force the others to align with it, and so a piece of iron, for example, might have no
那么一块铁因为内部互相冲突的磁域而没有磁场
magnetic field because of all of the warring magnetic kingdoms within it.
Henry:但如果从物质外施加足够强的磁场或力或压强
Henry: However, if you apply a strong enough magnetic field/force/pressure from outside
你就能增强一个磁域 使其控制相邻磁域
the material, you can help favor one domain/help one domain expand its control over its neighbors,
以此类推 直到所有磁域合而为一
and so on until all of the domains have been unified into one kingdom, all pointing in
都指向同一方向
the same direction.
Derek:现在你终于可以用铁拳统治了
Derek : And now, finally, you can rule with an iron fist…
我是说 磁铁
I mean, magnet.
它有磁性 因为它是铁磁性的且所有磁域排成一列
It’s magnetic because it’s ferromagnetic and all of the domains are aligned.
Henry:没错
Henry: Exactly!
值得注意的是 磁性是基本量子特性
What’s remarkable is that magnetism is a fundamentally quantum property amplified to the size of
放大到常见物体上 每个永久磁铁都暗示我们
everyday objects: every permanent magnet is a reminder that quantum mechanics underlies
量子力学是我们宇宙的基础 为了使物质有磁性
our universe – in order for any object to be magnetic, it has to have a unified kingdom
它必须有统一的磁域 而每个磁域由无数个磁性原子组成
of magnetic domains, each made up of bajillions of magnetic atoms which also need to be aligned
磁性原子需要排成一列 而每个原子有磁性的前提是
with each other, each of which can only be magnetic in the first place if it has an approximately
原子有大约半满的外电子层 从而使其固有磁场
half-filled outer shell of electrons so their intrinsic magnetic fields can align and not
能排成一列而不相互抵消
cancel each other out.
显然 这些标准很难达到
Not surprisingly, these criteria are pretty difficult to fulfill, which is why there are
这也是为什么只有有限的适宜用来做磁铁的材料
only a limited number of suitable materials you can use when you’re building a magnet.
Derek:或者你可以给任何一个导电体通电
Derek: OR you could just run a current through any electrical conductor and generate a magnetic
来产生磁场
field that way.
Henry:但是等下……
Henry: But hey…
首先 为什么这种现象存在
Why does that work in the first place?
点击这里进入网站“真理元素” 你就能发现
Click here to go to over to Veritasium and we’ll find out what special relativity and
狭义相对论和光速与电磁铁有什么关系
the speed of light have to do with electromagnets.
我想弄明白
I want to find out.

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视频概述

本视频介绍了物质为何会有磁性以及磁性产生的条件和原理。

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视频来源

https://www.youtube.com/watch?v=hFAOXdXZ5TM

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