Another fun use for rare earth magnets which gets a rise out of people
who haven't seen it before is this recipe:
1 cylindrical rare earth magnet about 1/2" diameter by 1/2" long.
1 piece of 1/2" copper water pipe about a foot long.
Holding the pipe vertical, drop the magnet into it and watch how long it
takes it to ooze all the way down through it.
The hard part is finding a "simple" way to explain the effect to
Mine takes about 5 seconds to drop through about 10 inches of pipe.
BTW, the priciple which makes for the slow decent is that when the
magnet moves with respect to the pipe it's magnetic field induces a dc
electric current flowing in a circle in the metal of the pipe. That
current creates its own magnetic field which exerts a opposing force on
Since the induced current and the opposing force on the magnet only
occur when the magnet is moving, a balance of forces establishes the
steady speed of magnet's decent.
It's akin to the "regenerative braking" used on hybrid cars, except the
car's electric motor windings are a lot more complex that the single
shorted turn created by the pipe.
Much of the current created when running the car's electric motors as
generators when braking gets put back into its battery, while with the
magnet and pipe, the current is all wasted in imperceptible resistance
heating of the pipe. <G>
At least that's the way I understand it...
Another way to put this is that as magnet induces current in the pipe,
the current wastes energy by heating the pipe slightly. That energy
has to come from somewhere, which if from the mechanical energy of the
magnet. That's why it slows down -- the mechanical energy is converted
Yeah, that's good. But technically, it's backwards.
The loss due to resistance is why it falls at all, not why it slows down.
If you do the same experiment without resistance, the magnet hovers without
When you add resistance, it causes the energy holding the magnet to slowly
be converted to heat which allows the magnet to fall. So the heat loss is
why it falls, not why it it slows down.
By talking about the heat loss, you have still failed to explain why the
magnet is coupled to the pipe in the first place since the copper pipe is
non magnetic or why a magnet will hover above a superconductor without
falling at all. I don't know of any easy ways to explain the effect to
someone that doesn't already understand electromagnetism.
That's very profound.
I have one more question. How could I make the magnet fall even more
slowly. Specifically, if I try to get a pipe with diameter very close
to that of the magnet, would that slow down its fall even further?
Of course. What I explained is why the force from the pipe that acts
on the magnet, is opposed to its movement. It could not possibly be
aiding the movement.
I think so, but I'm not sure.
I made them fall slower simply by putting 4 together (4 was all I had) and
dropping them through the pipe. Can't say I really understand why they
fall slower when together. The combined magnetic field would be larger but
so would the combined weight so I don't know why the two effects don't just
cancel each other out and end up with the magnets falling at the same rate.
Maybe the field from one magnet tends to make the field from the others
stronger so the combined field is more than the sum of the parts???
I think if you were to replace the copper pipe with a wire coil and then
connect the ends of the coil together it might make it fall slower. That
is, the magnets would be inside the wire coil. Or maybe just use many
small copper wire rings???
Yeah, but your answer I think is the easiest way to make someone grasp what
is happening even if the truth is more complex. I'll be using that answer
the next time I have to explain it to someone... :)
On 2007-04-15 21:51:34 -0700, email@example.com (Curt Welch) said:
That is the case. Less air space is better. The pipe and the magnet
will be in closer proximity to the magnetic fields. The field weakens
through space. Making the walls of the pipe thicker, or using a
material that is a better conductor will also make the magnet fall
That is not really true. Consider a permanent magnet generator. The
magnet is spinning within what? A coil of copper wire. Magnets affect
more than just ferrous materials.
A. First, you drop the magnet in close proximity to a conductor. From
playing with generators, we've all learned that when we move a magnetic
field in proximity to a conductor, we induce electrical current.
B. Now we have current flowing through the pipe.
C. We've learned that when we run electrical current through a
conductor, we induce a magnetic field. The current running through the
pipe is inducing a magnetic field.
D. The magnetic field is repelling the magnet and opposing the force
Now go google or wiki on Lenz' Law.
The magnet, in this case, is affecting electrons within the
metal. Anything conductive will respond to magnet movement. The key is
movement: the field has to be cutting through a conductor to generate
electron flow, and whether it's the field that's moving through the
conductor (as in an alternator) or the conductor moving through the
field (as in a generator) the result is the same. No movement equals
no electron flow generated.
The magnet hovering over a superconductor assumes that a
current was stimulated within the superconductor in the first place.
The hovering magnet cannot do that, since there is no field movement.
Nothing is free.
It can be done by placing the magnet on the superconductor before chilling
it. No (apparent) motion is required.
Actually, even the tiniest vibrations are enough to set up currents that
disturb the stability of the setup enough to wiggle it a bit more, causing
more current, wiggling it more... ad nauseum, until the thing levitates, and
On Sun, 15 Apr 2007 16:57:36 -0500, Ignoramus18842
Not much, untill it fits so closely it starts acting like a pneumatic
damper. Retarding force is proportional to flux density thru the
pipe and pipe wall conductivity. Flux density varies inversly with
path length. Making a tighter fit will have a fairly small effect
because the gap from magnet to pipe ID is a small percentage of the
pathlength from pole to pole of the magnet.
Increasing pipe wall thickness will increase conductivity and thus
induced current, thus making it fall slower.
By the way. These rare earth magnets are very useful for hanging
tools, car keys, as well as papers on the refrigerator. Be careful
around small children. If a child swallows two magnets, they can pinch
their intestines. Toy manufacturers mostly stopped making toys with
magnets that can be swallowed, at least that's what I heard.
On Apr 13, 2:00 am, firstname.lastname@example.org wrote:
Computer hard drives have them. Find some old drives, take
them apart, and look for them in the mechanism that swings the
recording/reading head across the disc. I have one large magnet from a
big old drive that is arc-shaped and has a steel flux plate across it.
Getting that plate off is fun, and putting it back on can pinch your
fingers hard enough to raise blisters. I use it to demonstrate the
magnetic drag phenomenon to my Aircraft Systems class by running a
strip of aluminum through it. They need to understand how a magnetic-
drag tachometer works (magnet spinning inside an aluminum cup; the
magnet is rotated by the tach cable and the cup drives the needle).
Newer drives have smaller magnets that are still really handy.
The effect is even more pronounced if the "pipe" is a coil of heavy wire,
and shorted. More EMF can be generated by the multiple turns than by just
We used to do this trick with the really heavy head-positioner mechanisms
from old SMD removable disk drives.
The magnet weighed about twenty pounds. The coil of the positioner was
around 100 turns of about - oh - ten gauge square magnet wire. If you
shorted the coil, the slider (weighing 'bout another pound) would take about
a half-minute to fall six inches. It was truly eerie, looking almost like
it was viscous-damped.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.