Name: How Special Relativity Makes Magnets Work
Uploaded: 2013-11-09T23:23:14.000Z
Duration: 4 min 22 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Only a few elements can be permanent magnets - iron is one. Copper is not. But if you pass

an electric current through any metal it becomes a magnet - an electromagnet. But how does

this work? Well strangely enough, it's a consequence of special relativity.

Special relativity is the fact that in our universe, length and time aren't absolute;

they're perceived differently by observers moving relative to each other (hence, "relativity").

For example, if you measure carefully enough, you'll find that time passes slower for observers

Hey Derek, when did you last shave? Derek1: Six hours ago.

MovingDerek: Actually it was five hours, 59 minutes and 59.99999999999 seconds

And moving objects are also contracted in their direction of motion.

You're looking slim. Only in your frame of reference.

So when an object is moving relative to you, it actually takes up less space than when

it's not moving. And even though this effect is obviously way tinier than we've shown,

length contraction IS what makes an electromagnet work.

Picture a copper wire - it consists of positive metal ions swimming in a sea of free negative

electrons. Now the number of protons is equal to the number of negative electrons so overall

the wire is neutral. So if there were a positive charged, err... positively charged cat nearby,

it would experience no force from the wire on it at all.

And even if there were a current in the wire, the electrons would just be drifting in one

direction, but the density of positive and negative charges would still be the same,

and so the wire would be neutral, so no force on the kitty.

But what if the cat starts moving? Imagine for simplicity that that the cat is moving

in the same direction as the electrons with the same velocity. Well now in my frame of

reference, the wire is still neutral and so there should be no force on the cat, but consider

the same situation in her frame of reference.

In the cat's frame of reference the positive charges in the wire are moving, so according

to special relativity their separation will be ever-so-slightly contracted. Also, from

this perspective the electrons aren't moving so they'll be more spread out than before

- remember, objects take up more space when they're not moving than when they are. These

two changes together mean there's a higher density of positive charges in the wire, so

it's no longer neutral - it's positively charged! Which means that the positively charged cat

will feel a repulsive electric force from the wire.

But in my frame of reference this seems mysterious: there's no force on a stationary charged cat,

but a moving cat is somehow repelled from this neutral wire.

How do you account for this force? Well we say it is the magnetic force, and that's mainly

because a wire with current in it deflects nearby magnets. So really, what this experiment

shows is that a magnetic field is just an electric field viewed from a different frame

In the cat's frame of reference, it is repelled from the wire due to the electric field created

by the excess positive charges produced by the effects of length contraction. In MY frame

of reference, the cat is repelled from a neutral wire due to the magnetic field generated by

So whether you see it as an electric or a magnetic field just depends on your frame

of reference, but in either case the results are the same. So an electromagnet is an everyday

Now that might seem crazy since electrons drift through wires at about .0000000001%

the speed of light - so how can special relativity have anything to do with it? Well the truth

is there are enough electrons in a wire, and the electric interaction is so amazingly strong

that even the minuscule effects of length contraction can produce significant charge

imbalances that produce a noticeable force.

So special relativity explains electromagnets - but what about permanent magnets?

Yeah! I mean there can't be electrical currents flowing around inside lumps of rock, can there?

Click here to go to MinutePhysics where we'll explore magnetite, compasses and all the wizardry