
What Happens When You Drop A Magnet Inside A Copper Tube [video] - prateekj
http://digg.com/video/what-happens-when-you-drop-a-magnet-inside-a-copper-tube
======
IgorPartola
Yup, it's pretty cool. This was one of the most impressive demo's my physics
professor did in our freshman physics class. The point is that as the magnet
is falling through the copper tube it creates an electric current which then
creates a magnetic field in the opposite direction of the magnet's movement.
In other words, this is the practical application of the Right Hand Rule [1].

Two fun facts about it: first, if you cut a small notch along the length of
the tube, this will not happen as the current cannot go around the tube.

Second, imagine a superconducting tube with an extremely powerful magnet right
in the center of it. Now, try to get the magnet out without cutting the tube.

[1] [http://en.wikipedia.org/wiki/Right-
hand_rule](http://en.wikipedia.org/wiki/Right-hand_rule)

Edit: Of course, there's nothing special about the tube being made of copper.
Any conducting substance will do.

~~~
sillysaurus2
Could this be used as a kind of elevator tech? I'd love to ride one of these
down a gigantic copper tube.

It'd be completely impractical, of course, but a whole lot of fun.

~~~
Crito
You could probably use eddy-current brakes as a safety system for elevators.
Many roller coasters use eddy-current brakes.

It would take a hell of a lot of magnets to strap to a person to allow them to
jump down a copper pipe though.

~~~
taliesinb
Another fun example of braking are the electrodynamic tethers [1], in which
you (a satellite) are the copper tube and the entire earth is the magnet!

[1]
[http://en.wikipedia.org/wiki/Electrodynamic_tether](http://en.wikipedia.org/wiki/Electrodynamic_tether)

------
dublinben
Original link:
[https://www.youtube.com/watch?v=keMpUaoA3Tg](https://www.youtube.com/watch?v=keMpUaoA3Tg)

This digg page is blogspam.

~~~
aunty_helen
The problem with the youtube link however is that any video with 'magnet' in
the title brings you uncomfortably close to thousands of youtube nutters
posting about perpetual motion machines.

~~~
peter_l_downs
s/problem/feature

------
fnordfnordfnord
Lenz's Law in action, lots of good demos online, or take any strong magnet and
move it rapidly while in close proximity to any non-magnetic conductor (brass,
copper, aluminum, etc.) You can feel the force exerted by the generated field.
Automobile speedometers used to operate on this principle.

------
InclinedPlane
Here is a much more informative video:
[http://www.youtube.com/watch?v=NqdOyxJZj0U](http://www.youtube.com/watch?v=NqdOyxJZj0U)

~~~
FollowSteph3
He has a bunch of really good videos

------
baq
be very careful when handling such powerful magnets - if you have something
metalic on you, they might jump off the table and smack you really hard. bones
will be broken.

~~~
IgorPartola
Fingers will also be pinched and possibly broken.

~~~
salgernon
The latch on our clothes dryer broke; the door would no longer snug up and
maintain the seal or necessary pressure for the safety switch. I now have a
2x2x1" rare earth magnet holding the door shut against the dryer frame. Its
strong enough to keep the door snug, but I can still open the dryer door with
a good tug.

I'm not sure why I had a 2x2x1" neodymium magnet laying around, but I'm glad I
did.

~~~
a3n
> I'm not sure why I had a 2x2x1" neodymium magnet laying around, but I'm glad
> I did.

The same reason why there's beer in the fridge, condoms in the night stand and
a guitar pick in your pocket: just in case.

~~~
salgernon
You live a far more exciting life than me. Although mine might be more
surreal.

------
malandrew
I can only begin to imagine how much a piece of copper tubing that thick
costs. That's a _lot_ of copper.

~~~
adsr
A kg is about $7 at the moment it seems, so suppose that's 5 kg then $35.

~~~
malandrew
That's it? I was under the impression that copper was far more expensive and
is the reason why expresso machines are as expensive as they are.

~~~
noir_lord
Copper compared to many metals isn't particular expensive per KG, it is just
_relatively_ expensive in the class of metals that we use a crapload of (for
example aluminimum is around $1.8 per KG) it is still much cheaper than tin at
~$22.

The main reason that copper is so valuable is it has so many uses and is very
easily recycled(I saw an estimate that said 80% of all the copper ever mined
is still in use) so holds it's value (You often get 90-95% of its commodity
value for recycling).

~~~
jonnathanson
This.

Copper is always in demand, has a bajillion uses, and can be scrapped and
reworked very easily. This is why you hear about meth heads stripping copper
wiring out of old homes. It's not that the copper is incredibly precious. It's
that the copper is so easily liquefied into cash at any number of one-stop,
no-questions-asked destinations.

Copper is expensive as a building or product material, not in and of itself,
but in relation to many of the alternatives in any given application. Small
differences in component costs, scaled over large supply chains, add up to
huge differences in margin at the macro level.

------
frankus
Yet another demo:
[https://www.youtube.com/watch?v=9s9I8OI82Ag](https://www.youtube.com/watch?v=9s9I8OI82Ag)

~~~
refurb
The interesting part is that NMR magnets often have a second magnet within
them that creates an opposing force _outside_ of the bore of the NMR (or at
least they used to do it that way).

If you notice on the video the yellow chain which keeps ferrous metals away
from the instrument isn't really that far away. Also, when he drops the
aluminum disc, it actually starts to accelerate after falling only a foot.

Because of the "counter" magnetic field produced, NMRs have a much
small/weaker _external_ magnet field that you'd expect.

------
asteli
This effect gets used in modern trains and roller coasters in the form of
linear eddy current brakes. In practice, this looks like an electromagnet
that's held just over the rail.

More reading:
[http://en.wikipedia.org/wiki/Eddy_current_brake](http://en.wikipedia.org/wiki/Eddy_current_brake)

~~~
dfox
With electromagnet and iron rail the same effect is used for almost an century
on streetcars under the name "electrodynamic brake". Idea is that single
assembly combines this for small braking strengths and friction braking for
complete stop (the electromagnet is on springs and can come into complete
contact with rail, which is used as equivalent of parking brake).

------
ta_tatata
So it's ok to post links to third party instead of the original content now ?
here's the link to the video instead of this portal profiting from it:
[http://www.youtube.com/watch?v=keMpUaoA3Tg](http://www.youtube.com/watch?v=keMpUaoA3Tg)

------
plq
Another cool physics experiment: What happens when you wring out a wet cloth
in space?

Here's the answer:
[http://youtube.com/watch?v=KFPvdNbftOY](http://youtube.com/watch?v=KFPvdNbftOY)

~~~
chrissyb
As much as i like that video nothing will top his special version of Bowie -
Space Oddity

This might also pique you interest
[http://www.youtube.com/watch?v=_dQJBBklpQQ](http://www.youtube.com/watch?v=_dQJBBklpQQ)

------
noonespecial
He had the large square magnet and the large round one within arms reach of
each other so that he could swap them with one hand without walking away. That
kind of freaks me out a little.

------
BorisMelnik
most impressed by the fact that someone actually got people to click on a digg
link

------
jtreminio
I am impressed that Digg has gone through no less than 2 facelifts since v4.

------
liebfraumilch
What I want to know is where did he get that freaking awesome copper tube??

~~~
JackFr
Agree. I'd love to have one of those.

------
ThePhysicist
If you think that's cool you should have a look at this frog:

[http://www.youtube.com/watch?v=A1vyB-O5i6E&list=PL8E7ED16454...](http://www.youtube.com/watch?v=A1vyB-O5i6E&list=PL8E7ED16454FE1C63)

The physics behind this are explained here:
[http://en.wikipedia.org/wiki/Magnetic_levitation](http://en.wikipedia.org/wiki/Magnetic_levitation)

------
deletes
Where can you buy something like that. A magnet is around 150$, but I have no
idea where to get a copper tube of that thickness.

~~~
IgorPartola
You really don't need anything as fancy. A small magnet and a narrow aluminum
tube will do just fine. It won't be quite as impressive, but drop a pebble
from the same height not through the tube and watch the delay for the magnet
dropping. A four foot length of tube about 1/2 to 3/4 inch in diameter would
do nicely.

Edit: try cast iron if you must have a big pipe. It's likely to be much
cheaper. The thickness shouldn't make much of a difference either, so maybe
even a cooking pot with a cut off bottom would work.

~~~
klodolph
Iron is ferromagnetic, so the magnet will stick to the tube and not fall
through.

~~~
IgorPartola
Ah, but of course you are right. Then aluminum it is!

------
TrainedMonkey
If magnet was not rotated when dropped through the tube, would it simply stick
to one wall in case of slightest imbalance?

~~~
martinvol
I can't clearly remember my psychics class, But I think I'd fall a bit faster,
as the movement through the pipe is considerable as well.

~~~
dfbrown
I don't think the spinning should have any effect on the speed. Only the
vertical movement would create eddy currents. It could be he does it to keep
the magnet stable as it falls (like spin on a football).

~~~
shawabawa3
In the last drop he doesn't spin it and it seems to fall at the same speed, it
also wobbles and hits the side, so I think you're right

------
abcd_f
Eddy currents are also used in exercise equipment with "magnetic resistance".

[http://www.bicycleman.com/recumbent-exercise-
bikes/images/Ma...](http://www.bicycleman.com/recumbent-exercise-
bikes/images/Magnetic-recumbent-bike-ECB.jpg)

------
lostlogin
For a similar effect using an MR scanner see the link below. I have done
similar with an old door stop. [http://www.youtube.com/watch?v=fxC-
AEC0ROk](http://www.youtube.com/watch?v=fxC-AEC0ROk)

------
556790
I wonder, with a tube that is of sufficient thickness and a magnet of
sufficient strength, would it be possible to levitate the magnet inside the
tube?

~~~
omegaham
The best way to do it is with a superconducting magnet.

And no, it doesn't quite levitate, but it takes a very long time for it to
fall through. I think the video we watched in high school physics class had it
take about fifteen seconds to go an inch and a half.

------
mturmon
[http://en.wikipedia.org/wiki/Eddy_current](http://en.wikipedia.org/wiki/Eddy_current)

------
lxmorj
Hey, would it levitate there if the tube was spinning? It falls back and forth
when he rotates it...

~~~
lutusp
> Hey, would it levitate there if the tube was spinning?

If the tube were spinning, the magnet would begin spinning also, but it
wouldn't levitate.

------
orblivion
Is that The Orb playing in the background? EDIT: or maybe Atomic Skunk? It
sounds familiar.

------
cLeEOGPw
Wonder for how long the magnet needs to fall until it is completely
demagnetized.

~~~
ansgri
Why should it. The energy is supplied by gravity.

~~~
cLeEOGPw
As magnet falls, it is slowed down, this drag is created because magnetic
forces are resisting gravity. So after some time, magnet loses that force, I
just wonder how long does it take. It can't fall slowly like that forever.

------
elheffe80
All I could think of was the cost of that copper tube. Wow that is thick.

------
saxaholic
Ooh eddy currents!

------
iterationx
The slinky trick is better
[http://www.youtube.com/watch?v=eCMmmEEyOO0](http://www.youtube.com/watch?v=eCMmmEEyOO0)

------
glial
Spooky.

