
How do magnets work? - ColinWright
http://www.coolmagnetman.com/maghow.htm
======
lucaswoj
Lets start with a simple case: Imagine there are two parallel wires with
current flowing in opposite directions. There are roughly the same number of
electrons and protons in each wire but the electrons, unlike the protons, are
in motion. Now, imagine you ARE an electron whizzing down one of those wires.
If you look around, you'll see all the other electrons in your wire are (more
or less) stationary relative to you but the protons are moving backwards at a
great speed. Now, you look over to the other parallel wire. Over there, the
protons are moving at the same speed and in the same direction as the protons
in your wire but the electrons in the other wire appear to be moving backwards
twice as fast! In any interval of time, you pass twice as many electrons as
you do protons. Because opposite charges attract, this produces an attractive
force that we label "magnetism."

Disclaimer: This may be utterly wrong. This is my understanding from the
internet and a first-year engineering electromagnetism course.

~~~
pak
I would refine your explanation a bit--you were 90% there but you got the
direction of the magnetic force wrong, and the way I learned it involved
special relativity. So when you are sitting on an electron in one wire and you
see the protons zipping backwards in the other wire and the electrons twice as
fast as that, the phenomenon of "length contraction"
(<http://en.wikipedia.org/wiki/Length_contraction>) occurs because all these
particles are moving close to the speed of light. Since from your reference
frame the electrons are moving _faster_ than the protons, they are length
contracted to a greater degree, so they appear to have a higher charge density
than the protons. Therefore the repulsive electric force between your electron
and the charge density of the electrons is greater than the attractive force
between your electron and the charge density of the protons, so your electron
is repelled (and we observe this as the magnetic force).

Laughable ASCII diagram (just note that within each wire, the opposite charges
are intermingled, not separated like shown):

    
    
              Lab reference frame     Reference frame of e- on wire 1           
                                                   V
                  - - - - - - - ==>          -  -  -  -  - 
      wire 1      +  +  +  +  +          <== + + + + + + +
    
              <== - - - - - - -        <==== -------------
      wire 2      +  +  +  +  +          <== + + + + + + +
    
                                      charge density of e-'s in other 
                                      wire is greater than that of p+'s
    

So, two wires with current in _opposite_ directions _repel_ each other, not
attract. You can do the above exercise with the currents in the _same_
direction to see that relativity would then postulate that the protons in the
other wire have greater apparent charge density than the electrons, causing a
net attractive electric force observed as magnetism.

~~~
zbyszek
The free electrons in a conductor move at nothing near the speed of light,
though.

~~~
Zaak
Even though the speed of the electrons is nowhere near the speed of light,
electromagnetism is so strong that the tiny length contraction at those speeds
is enough to produce the magnetic force.

~~~
ScottBurson
Yes, this is the correct answer. It's a relativistic effect that is
nonetheless observable at velocities not usually considered to be
relativistic.

I think this is extremely cool, BTW: any ordinary refrigerator magnet serves
to demonstrate the truth of special relativity. Indeed, since the propagation
of a photon involves oscillating electric and magnetic fields, the very
existence of light itself depends on relativity.

So in a strange and unexpected way, one could say that when Michelson and
Morley went searching for the ether, they actually found it. The fact that the
speed of light is the same in every inertial reference frame is precisely what
allows it to propagate at all.

------
sambeau
I prefer this explanation by Richard Feynman:

<http://www.youtube.com/watch?v=MO0r930Sn_8>

~~~
ColinWright
That's not really an explanation, though. It's really saying - you don't know
enough (yet) for me to explain what I know, although I don't know everything.

~~~
sambeau
It was still the _best_ explanation he could give.

------
mmcconnell1618
I think this summarizes the article: "Why? We don't really know, but we can
observe some consistent rules."

------
fraserharris
Comments tl;dr: Electric fields + special relativity = magnetic fields

This is 'relatively' unknown triumph of special relativity ;-)

"What led me more or less directly to the special theory of relativity was the
conviction that the electromotive force acting on a body in motion in a
magnetic field was nothing else but an electric field." - Albert Einstein

------
splat
Here's a question for you. Take two bar magnets. Put one on a table. Hold the
other above it. The magnet on the table will be attracted to the one you're
holding and will jump up and stick to it. The magnet on the table has just
done some work. But magnetic fields do no work. What just happened?

~~~
bdonlan
You did the work by moving the magnets away from each other. This converts
some of the kinetic energy you created by moving them into potential energy.
As you moved the magnets back closer to each other, some of this potential
energy converted back to kinetic energy, until they were close enough that the
force exerted by the magnetic field exceeded that of gravity, at which point
there was a large conversion of magnetic potential energy into kinetic, and
then gravitational potential energy.

Further, note that since a force was exerted over a distance , work was
indisputably done. Exactly what did that work is a matter of opinion, but work
was certainly done.

------
heyrhett
I don't see any relativity in the actual post, but I'm happy to see it here in
the comments. For more info, of course, the real source is Einstein's "On the
Electrodynamics of Moving Bodies"

------
eperoumal
reminds me of that : <http://www.youtube.com/watch?v=OvmvxAcT_Yc>

