
Dropping a Magnet Through a Copper Pipe - jashmenn
http://blog.makezine.com/archive/2011/12/dropping-a-magnet-through-a-copper-pipe.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+makezineonline+%28MAKE%29&utm_content=Google+Reader
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jacquesm
I've played around with some pretty bad ass magnets during the time that I was
building wind turbines and one of the more interesting effects was that if you
dropped one near anything made of steel you were actually in danger of getting
shrapnel embedded in your body.

They move so fast it is scary, sometimes they explode on impact. This makes
you pretty nervous about dropping them.

Then, one day one got dropped over a chunk of solid aluminum. It floated
gently to the metal landing with a soft 'click'. Besides the initial surprise
(I realized the eddy currents induced a magnet field of opposing polarity in
the aluminum) what struck me most was the force of that opposing magnet. If
you tried to force the magnet close to the aluminum at speed it would resist
so strongly that you never managed to smash it into it with any kind of
effectiveness. Always just that soft 'click'.

I still have a bunch of 3"x2"x1" neos waiting for some project, and whenever
someone visits that's interested in technology I show them what those things
can do, if you have tried to pry one of those from a chunk of solid steel (or
if you're unlucky, another magnet) you know what I mean when I say I have a
lot of respect for those little golden blocks.

~~~
artursapek
There's a story floating around my school of a student a few years ago who
wanted to make a sort of hovercraft by fixing in position two opposing large
supermagnets (I go to an art school). He couldn't build it when his
supermagnets arrived in the mail because it was deemed too dangerous,
supposedly they could have snapped or crushed someone's arm/leg easily had
they come undone, and that was assuming he even survived the construction
himself.

~~~
jacquesm
Let me give you one example of just how dangerous these things are from my own
experience.

I was assembling the rotor/stator assembly of my windmill, to be safe I'd
bolted the stator to the welding table and was lowering the the rotor around
it using a chainfall.

Suddenly and to my complete surprise the welding table _leapt up_ to the rotor
just to hang there suspended by the magnetic field.

The table surface was a 3/8" thick 8x4 sheet with a fairly sturdy steel frame
under it. You'd be hard pressed to move it, let alone lift it.

Fortunately I didn't have any fingers in the airgap or I would probably be
typing this a whole lot slower.

~~~
po
See also: The gallery of flying objects sent in by people who work with MRI
machines:

<http://www.simplyphysics.com/flying_objects.html>

I worked with an NMR machine in college and would always have to empty my
pockets before entering the room. I'm not sure I really understood the danger.
I heard stories about people having piercings ripped out though.

~~~
jarek
A child was killed in 2001 when an oxygen tank got pulled into an MRI machine
he was in: [http://www.nytimes.com/2001/07/31/nyregion/boy-6-dies-of-
sku...](http://www.nytimes.com/2001/07/31/nyregion/boy-6-dies-of-skull-injury-
during-mri.html)

Some system or another was running low on oxygen, someone panicked and grabbed
a tank carelessly, next thing you know a tank is free where it shouldn't be.

~~~
lostlogin
Projectile injuries are one issue. Heating, induced current, hearing loss are
others. FDA is investigating setting up a registry for safety problems to help
reduce the frequency of accidents.
[http://www.auntminnie.com/index.aspx?sec=sup_n&sub=bai&#...</a>. There are
some shockers out there. Metal detectors on doors etc. Stopping the guy with a
pacemaker is important too.

------
swombat
Most of the other "zen magnet" videos linked as "related videos" are pretty
damn awesome. I strongly recommend watching them all, right now. Really,
there's nothing more important for you to do at this point in time.

Exploding zen magnet:
[http://www.youtube.com/watch?feature=player_embedded&v=q...](http://www.youtube.com/watch?feature=player_embedded&v=q7bFLchsPyM)

140-sided zen magnet:
[http://www.youtube.com/watch?v=eRiMexbocBI&feature=relmf...](http://www.youtube.com/watch?v=eRiMexbocBI&feature=relmfu)

Interlaced dodecahedron:
[http://www.youtube.com/watch?v=P2qfCn3gclQ&feature=relmf...](http://www.youtube.com/watch?v=P2qfCn3gclQ&feature=relmfu)

"Hell's Diamond":
[http://www.youtube.com/watch?v=OF2i8eG7KhA&feature=relmf...](http://www.youtube.com/watch?v=OF2i8eG7KhA&feature=relmfu)

------
presidentender
It is not often that something runs so strongly against my intuition. Today, I
am reminded just how little I know.

~~~
vacri
You know plenty; there is simply even more to know.

------
jtchang
Imagine an amusement park ride where they put you in a suit full of magnets
and then drop you down a copper tube.

That would be one hell of an experience.

~~~
noonespecial
The braking phase of this ride is mostly that. This way there is no active
component of the brake that can fail. Its just a big magnet and a big hunk of
aluminum.

<http://en.wikipedia.org/wiki/Drop_tower_%28ride%29>

~~~
jtchang
Imagine certain parts of the shaft are copper. It would feel crazy to be free
falling, then abruptly slow then, then speed up again all the way to the
bottom.

Okay. No more nightmare fuel.

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tzs
"I could do this all day. It's so cool".

Am I correct in assuming that if he did it all day, it would not actually be
so cool, as the copper tube would be heated up by the induced currents?

~~~
digitalsushi
Without using any math, I believe it would be a safe assumption that between
the pause of reloading the magnet manually and the heat his hand would absorb
by grabbing the pipe, heat would never accumulate beyond the ambient
temperature in the pipe. However, it would be delightful to hear why I am
wrong, since it would be very interesting.

~~~
CognitiveLens
You are correct - there's really not that much energy being converted to
electric current, much less being converted from current into heat through the
resistance of copper.

I would be interested in hearing what would happen if the pipe was a
superconductor, though - would the induced current be so strong that the
magnet wouldn't drop at all?

~~~
alan
Yes. It would hang there. That was one of the demonstrations of why
superconductors are so cool back in the day. I remember seeing those
nitrogen(I think) liquid cooled super conductors holding a magnet above them.

~~~
billswift
I don't think so. If I remember the explanation, superconductors "block"
magnetism, rather like having another magnet with the same pole opposed. In
that case, the magnet would probably just fall as though the tube was made of
plastic or other non-conductor. (Just speculation, it has been several years
since I read about superconductivity).

~~~
dedward
They are diamagnetic i believe (so is water' very weakly so. - google for
levitating frogs for a laugh.)

Diamagnetic materials create an equal field to what they are exposed to...
Causing repulsion. In the case of superconducting, the field generated is very
strong and why we can levitate any old magnet over a superconductor.

Levitating a frog (mostly water right?) takes some incredibly strong
fields..... Liquid cooled bitter magnets etc....

Side note - rare earth supermagets are toxic. When they shatter, you dont want
to ingest any of the bits/dust

------
CognitiveLens
I was fascinated by this demonstration in my high school physics class. The
teacher went further and dropped the same magnet through another copper pipe
of the same diameter that had a slit cut along its length - the magnet dropped
straight through the slit pipe without slowing down. This provided an
important "counterexample" demonstrating that the induced currents were
circular around the circumference of the solid pipe - breaking the circle
eliminated the braking force.

~~~
jules
That's not true. A slit will cause it to fall faster, but not as fast as with
a non conductive pipe. I did this experiment a couple of years ago to answer
that question. Unfortunately I can't find the exact timing data. If you do the
thinking and figure out how the currents should flow due to the moving magnet,
this makes sense.

~~~
CognitiveLens
I do remember the demonstration clearly, and I have done the thinking to
figure out how the currents should flow, but I would be interested in your
explanation of why there would still be significant braking force in a slit
cylinder.

Essentially, the eddy currents have to be moving perpendicular to the
direction of the magnet's travel, using the right hand rule as mentioned in
other comments. At any point along the surface of the cylinder, the current
moves along the circumference, creating a magnetic field that pushes upward.
Currents in any other direction would spin the magnet, which doesn't happen,
so the slit effectively limits the eddy currents, except for some negligible,
localized loops along the length of the cylinder.

~~~
jules
Alternatively, in numbers. I'm ignoring constant factors and only doing
proportional calculations. Where you see an = sign you should think
"proportional to".

Suppose we have a magnet at the origin with magnetic moment 1 in the z
direction: m = [0,0,1]. The magnet is moving in vertically (in the z
direction). Further we have the pipe vertically with radius 1. The problem now
is to determine the current through the pipe. Since the problem is symmetrical
around the z-axis we can consider just the current though the points r =
[1,0,z].

The magnetic field due to the dipole is B(r) = 3r(m dot r)/r^5 - m/r^3. The
flux through the pipe at r = [1,0,z] is just the x component of the magnetic
field, since the pipe is vertical. Plugging in and simplifying we get Phi =
3z/sqrt(1+z^2)^5. Since the magnet is moving in the z direction, lets say z =
t, to compute the change in Phi we need to compute dPhi/dz. If we put that
into Wolfram Alpha we get a picture like this:

[http://www.wolframalpha.com/input/?i=d%2Fdx+x%2Fsqrt%281%2Bx...](http://www.wolframalpha.com/input/?i=d%2Fdx+x%2Fsqrt%281%2Bx%5E2%29%29%5E5%2C+x%3D-3..3%2Cy%3D-1..2)

As you can see you get a strong current in one direction around the pipe at
the middle of the magnets, and two weaker currents in the opposite direction
above and below the magnet.

This doesn't yet say anything about the case of the slit, but it does at least
show that the currents are indeed moving like I sketched for the no slit case
in the picture of my other response.

~~~
CognitiveLens
Okay I see how the currents move with the slit - thank you for the visuals! I
just found a video using a solid vs. slit pipe that has a good example of the
timing difference - not nearly as dramatic as my memory of the demonstration,
but still informative!

<http://www.youtube.com/watch?v=-X5CDjHFruk>

------
digitalsushi
Admit you're doing the right hand rule right now. And that you just googled
what was going on, and for 2 minutes you felt like the young geek that forged
your path here, whatever the discipline. (Ok, the physicists are not doing the
right hand rule, they are rolling their eyes)

------
michaelf
This video made me curious about what a magnet factory would look like and I
found a really great video that goes step by step through the process at a
neodymium magnet manufacturer in Shanghai:

<http://www.youtube.com/watch?v=BHuWloNGo6c>

I was sort of hoping that the factory would look like that plexiglass prison
in the X-Men movie that was designed to prevent Magneto from using his powers.

Alas, it's nothing like that, but it's an interesting video nonetheless.

~~~
robododo
Hrm, I think I have to refuse to watch the video then.

The idea of a plexiglass factory is enticing enough that I choose the fantasy
over reality.

------
nyellin
Back in high school, I lost 1st place in the Weizmann Safe Cracking Physics
Contest over this. You had to build a safe that anyone with a sufficiently
advanced knowledge of physics could open. The winning team had placed a
magnetic switch in the middle of a long tube inside their safe. You could
toggle the switch if a magnet fell down the tube slowly enough, but it never
worked no matter how you dropped it. The solution was to first insert a roll
of tinfoil inside the tube.

They also used a gauss cannon: <http://en.wikipedia.org/wiki/Coilgun>

<http://wis-wander.weizmann.ac.il/14th-safe-cracking-contest>

<http://www.weizmann.ac.il/zemed/upload/13(3).pdf>

------
mrb
I accidentally discovered eddy currents not too long ago, while mindlessly
playing with a copper CPU heatsink and an HDD magnet (geeky, right?) I
expected copper to be ferromagnetic, but was surprised when I felt this
dragging force that neither repulsed nor attracted the magnet. Curious about
this discovery, it took me a few minutes of googling to eventually learn about
eddy currents... I am surprised I never learned about them at school.

~~~
acgourley
should have been in any "AP Physics" highschool class or college level
classical physics class.

~~~
georgieporgie
It wasn't mentioned in my non-AP HS physics class, nor in my normal college
physics class. :-(

------
jtreminio
This is amazing to me as I've never seen or heard of it before.

Question: If this same thing were to be done in a circular copper pipe that
feeds into itself (ie a hoola hoop made of copper), and that hoop were rotated
at the correct speed, would the magnet in effect never actually move and just
hover in mid air?

~~~
gus_massa
Yes, the force between the magnet and the cooper pipe is proportional to the
relative speed, so you can leave the magnet still and move the pipe and get a
force that cancels the gravity.

But in a real experiment, I think that it will be very difficult to keep the
magnet in the right position, it will rotate and bounce against the walls, and
the force will vary, so the average force has to be equal to the gravity. So
your experiment is ideally possible, but I think that it will be very
difficult to implement.

(In the original experiment, after a few moments, the magnet falls at an
almost constant speed. This is the same speed that the pipe should have in
alternative experiment.)

~~~
khafra
How 'bout with 3 rotating rings below it, orthogonally oriented, with a PID
controller or something to change their speeds as appropriate?

------
felipemnoa
So in a nutshell what seems to be happening is that the moving magnetic field
is causing the electrons in the copper to move, this electrons then give rise
to a magnetic field which repulses the original magnetic field which is why
the magnet slows down.

~~~
jacquesm
What about the part of the tube where the magnet has just past half-way (the
magnet length, not the tube), wouldn't you get an opposite effect right past
that point where the magnet is slowed down due to an attraction from above?

After all, a magnet has two poles, the leading end I'd imagine would be
repelled while the trailing part would be attracted.

If that were not the case not then the forces would cancel out and the magnet
would fall at its normal speed. (I'm assuming the magnet has 'top' and
'bottom' as the poles).

~~~
mpyne
No, it's still be going slow. The very short story is "Lenz's Law" (IIRC), but
basically this of it like this:

    
    
        top
        |
        |N
        |S
        |
    

As the south pole falls down, it induces currents in the part of the pipe
below it which tend to form a south pole to inhibit the movement (a north pole
is also formed, _above_ the south pole which also adds to the effect)

As the north pole falls down, it also induces currents, in the part of the
pipe _above_ , which tends to form a south pole above it and a north pole
below it, and both of _those_ effects act to retard the acceleration of the
magnet.

Perhaps easiest to visualize as:

    
    
        s   s
          N
        n | n
        n | n
          S
        s   s
    

where the lowercase letters are the poles in the piping formed from the eddy
currents as the magnet falls.

~~~
jacquesm
> As the north pole falls down, it also induces currents, in the part of the
> pipe above, which tends to form a south pole above it and a north pole below
> it, and both of those effects act to retard the acceleration of the magnet.

Yes, that was exactly my point.

So it is not just a 'repelling' force, there is attraction involved as well.
The 'root' comment just mentioned 'repels' as if that was the whole story, but
it clearly isn't.

------
Jun8
Couldn't resist this _Hitchhiker's Guide_ quote on the subject of eddies:

    
    
      "I have detected," he said, "disturbances in the wash." …
      "The wash?" said Arthur.
      "The space-time wash," said Ford. …
      Arthur nodded, and then cleared his throat. 
      "Are we talking about," he asked cautiously, "some sort of Vogon laundromat, 
      or what are we talking about?"
      "Eddies," said Ford, "in the space-time continuum."
      "Ah," nodded Arthur, "is he? Is he?" 
      He pushed his hands into the pocket of his dressing gown 
      and looked knowledgeably into the distance.
      "What?" said Ford.
      "Er, who," said Arthur, "is Eddy, then, exactly, then?"

------
MaggieL
At the Franklin Institute Science Museum years ago, they had a huge (2 foot
diameter? More?) copper disk attached to a crank suspended in the gap of a
big-ass electromagnet. The idea was you spun the crank, and then operated a
foot pedal that applied current to the electromagnet, which braked the wheel
with eddy currents.

The disk had slotted sectors, so you could tell that the braking effect was
less when those sectors were in the gap.

I don't know if the exhibit survived the themparkification of fi.edu... I hope
so but somehow I doubt it.

~~~
wgrover
There's a similar demo at the Exploratorium in San Francisco - you can drop
disks of different materials (and with different cutouts) through the
electromagnet and see the different rates at which they drop. I recall it also
had a thoughtful warning sign instructing folks to keep their (magnetic
stripe) BART cards away from the electromagnet!

------
jamgraham
I need a magnet suit and a big pipe

------
zafka
This is pretty funny. I just did this demo two weeks ago at work. We had two
motors that were neck and neck for a project, then when we put them both in am
aluminum housing, one drew 50% more current at full speed idle. I found both
this demo, and the demo of dropping a big magnet on to an aluminum sheet. I
for sure need to play more with magnets, In fact I am lusting for the big
magnet that jacquesm talked about.

------
jurre
So tempted to go and buy a magnet and copper pipe now!

~~~
VMG
I have a bunch of strong magnets from old hard drives and I am thinking hard
how this could finally be a good use of them. But the tube is opaque, so I
can't see how I can make a toy out of it.

I guess I have to buy a copper tube to find out.

~~~
DanBC
Put strong magnet in small tupperware box. Put box on beach, collect fines of
magnetic material. Transfer many fines to a clear pot, mix with mineral oil.
Tape strong magnet to outside of clear pot, see magnetic field in 3D.

~~~
ramidarigaz
That sounds ridiculously fun. I may have to go do that. I just need a beach...

~~~
HeyLaughingBoy
You can just buy iron filings cheap from these guys:
[http://www.scientificsonline.com/catalogsearch/result/?q=iro...](http://www.scientificsonline.com/catalogsearch/result/?q=iron+filings)

------
ineedtogroove
Have you guys ever seen a magnetic vortex?

<http://www.youtube.com/watch?v=SAl1LVPbYhY>
<http://www.youtube.com/watch?v=rLlm8UWDNe4>

Please explain to me how this works, I will buy you a donut

~~~
noonespecial
Those are both examples of homopolar motors.

<http://en.wikipedia.org/wiki/Homopolar_motor>

In the first example, the water is acting as the "wire" as the electrolysis
current flows through it. The water rotates just like the wire and drags the
generated browns gas along with it.

~~~
ineedtogroove
OK, so how is a non-ferrous material (water) being moved by a magnet?

Clearly in the video of the water vortex there is a proof showing that the
water does not move with an iron slug, but does move with the magnet.

I might buy you a donut

~~~
noonespecial
The catch is that anything you run current through _becomes_ magnetic, even
though its not ferrous. Copper wire, water, ionized air, whatever.

Take this unexpected effect, sprinkle in a little crazy and you've got your
"new theory of physics that will give us all free energy if the government
doesn't suppress it yada-yada."

------
joejohnson
Here is a really easy to understand explanation of eddy currents and Lenz's
Law
[http://www.youtube.com/watch?NR=1&feature=endscreen&...](http://www.youtube.com/watch?NR=1&feature=endscreen&v=kU6NSh7hr7Q)

~~~
po
What is the "special green paper" that shows the magnetic field they are
using?

~~~
carey
Searching for "magnetic field paper" leads me to [http://www.grand-
illusions.com/acatalog/Magnetic_Field_Paper...](http://www.grand-
illusions.com/acatalog/Magnetic_Field_Paper.html) .

------
petercooper
At the time of writing, the linked site is down, but I found what I suspect is
a similar demonstration at
<http://www.bbc.co.uk/bang/handson/magneticcopper.shtml>

~~~
marquis
Not available to viewers outside of the UK, it seems. Here's another demo on
Youtube: <http://www.youtube.com/watch?v=30oPZO_z7-4>

------
untog
I cannot shake my perception that this video was, in fact, created and
narrated by Zack Galifianakis.

"Just weird stuff. Eddy currents."

------
wicknicks
I've read about eddy currents in textbooks. But seeing them work is simply
fascinating.

Maybe one day space rockets can land on planets using this principle.

------
drstrangevibes
i wonder what would happen if you had strips of steel interlaced with copper ,
youd create a push-me pull-me effect i would expect.

