Magnetic
levitation is a concept which still sounds like magic
to most people, despite its introduction in infrastructure
and gadgets around the world - the most popular being
the maglev train.
Maglev
trains utilize a strong magnetic field in the rail to
suspend a train using the forces of repulsion, and to
propel the train using both repulsion and attraction.
High speed maglev train
(Image courtesy of FYS)
Bismuth
- a diamagnetic material
While maglev
trains operate using electromagnets, the same effects
can be demonstrated using static magnets, which is will
be demonstrated here.
Some metals
are attracted by both poles of a magnet, such as iron.
These metals are known as ferrous metals.
However,
there exists another group of metals which have a property
called diamagnetism. Materials such as bismuth, carbon,
graphite and aluminium possess this weird property.
Diamagnetic materials are seen to slightly repel both
poles of a magnet.
The idea
behind diamagnetic levitation in the lab is to levitate
a small, but very powerful neodymium magnet between
two layers of diamagnetic material. This is done by
attracting the small magnet using a larger one above
the top piece of diamagnetic material.
As the
big magnet pulls the small one up, it arrives at an
equilibrium state whereby the small magnet has enough
force to overcome gravity, but not enough to overcome
the additional repelling forces posed by the top piece
of diamagnetic material. Thus it appears to hover
in mid air.
Location
of neodymium magnets inside a hard disk
(Image courtesy of Renewable Energy
UK)
Powerful
rare-earth neodymium magnets are found in hard disks,
CD-rom drives and speakers.
They'll be pretty easy to find in an old hard disk (may
need a hammer to bust it open first though), but in
a CD-drive you will have to completely dismantle the
optical mechanisms PEACEFULLY (avoid the hammer this
time). They can be harder to get out of speakers.
As
with any salvage job, patience is a virtue.
Although
bismuth is by far the best material for this demonstration
and provides the best results, it is relatively expensive
and hard to come-by. In the mean time... aluminium will
have to do. It sucks, and it's tricky to set up, but
it works.
Here's
the setup in real life. Compare it to the above diagram
to see the similarities.
And....
results! Hovering a mere fraction of a millimeter above
and below the aluminium disks.
Now
here is possibly the easiest, fastest, and cheapest
form of magnetic "levitation" ever!
The
ring magnets in the photo are magnets from a dismantled
magnetron - the heating element in microwave ovens.
If you do go salvaging these, they can be found in-between
the magnetron cooling fins. A hammer is useful for
doing the hard work, provided you don't go smash your
way through the magnets themselves.
These
magnetron magnets have a lower power density relative
to neodymium magnets, but because they are quite large,
still pack a lot of punch.
To
put this quick and easy demonstration together, a PVC
pipe of suitable diameter is simply placed into the
rings of a few magnets which serve as a base.
Another
magnet is placed through the PVC pipe in the opposite
way so that it repels the 'base' magnets.
Note
that if the PVC pipe wasn't there, the top magnet would
just flip over and snap straight onto the base magnets,
and you would probably end up with shards of magnet
all over the floor. Though I would like to see this
done with transparent PVC tube!
Or...
using only a couple of magnets as a base, the rest of
them can be suspended in a S-(NS)-(SN)-(NS)... configuration,
so that all magnets repel each other.
The
width of the gaps between the magnets seem to follow
some sort of natural growth trend, although there is
a clearly a limiting factor as the top magnet cannot
be suspended anymore than its repelling power allows
it.
The
gaps are evidently smaller towards the base, as the
lowest magnet has to carry the weight of all the magnets
above it.
