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A comment on What Happens When You Drop A Magnet Inside A Copper Tube [video].

Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by IgorPartola on Monday February 03, 2014 @05:01PM (#7173812)
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

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

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by taliesinb on Monday February 03, 2014 @05:37PM (#7174025)
You can also think of it as magnetic flux lines encountering "drag" as they pass through conductors.

The limiting behavior of this view is in type II superconductors, which entirely "pin" flux lines -- they present an effectively infinite resistance to the movement of flux lines. That's why a magnet levitates over such a superconductor. [1]

You can even image these "vortices" directly [2]

[1] http://en.wikipedia.org/wiki/Flux_pinning [2] http://www.youtube.com/watch?v=7_ZgiumS41Q

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by hurowits on Tuesday February 04, 2014 @07:35AM (#7176986)
You might wanna take a look at this video: http://www.youtube.com/watch?v=VyOtIsnG71U
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by exhilaration on Tuesday February 04, 2014 @10:43AM (#7178092)
Incredible. For anyone watching, make sure you at least go past 2:20 - that's when they pull out the magnetic rail.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by IgorPartola on Monday February 03, 2014 @05:48PM (#7174087)
The levitation over a superconductor was the other cool demo that the professor did :).
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by rbanffy on Monday February 03, 2014 @06:22PM (#7174294)
Lucky kids. When I was in school, the only commercial superconductors were liquid Helium cooled Niobium-Titanium alloys, something not many classrooms could handle. ;-)

Luckily, when I got to programming, we already had zeroes and ones. Imagine our predecessors who had to do it with uppercase o's and lowercase l's...

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by mjcohen on Monday February 03, 2014 @06:55PM (#7174474)
Ha! When I was in school, we had to push the electrons around by hand! You kids had it easy!
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by JakeStone on Monday February 03, 2014 @07:16PM (#7174575)
You guys had electrons? We had to use scotch tape on a bunch of quarks and give them a half spin and then push them.

Uphill. Both ways.

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by smoyer on Monday February 03, 2014 @08:28PM (#7174889)
I'm declaring a winner here ... the only possible thing that could top that is "through three-foot deep snow".
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by IgorPartola on Monday February 03, 2014 @08:05PM (#7174787)
I forget what type we used but it was cooled by liquid nitrogen. That prof. used lots of magnets and superconductors in his research, some cooled by Helium-3. Now that stuff is expensive.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by fnordfnordfnord on Monday February 03, 2014 @11:13PM (#7175451)
The high-temperature ones were probably Yttrium-Barium-Copper-Oxides (YBCO) I've seen this demo done with them. The magnet's motion is almost undetectable, they appear suspended in the copper pipe until they start to warm up.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by lotharbot on Tuesday February 04, 2014 @12:03AM (#7175618)
http://search.dilbert.com/comic/We%20Didn%27t%20Have%20Zeros (Sept 8, 1992 Dilbert)
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by Intermernet on Monday February 03, 2014 @06:46PM (#7174419)
Zeroes and Ones? Luxury! In my day we couldn't afford the Ones, we had to make do with just Zeros...
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by dllthomas on Monday February 03, 2014 @06:37PM (#7174379)
lOl
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by huhtenberg on Tuesday February 04, 2014 @07:18AM (#7176926)
> ... nothing special about the tube being made of copper. Any conducting substance will do.

Any non-magnetic conducting substance that is. Otherwise the magnet will just stick to the tube wall.

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by sillysaurus2 on Monday February 03, 2014 @05:54PM (#7174136)
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.

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by whatshisface on Monday February 03, 2014 @06:04PM (#7174189)
The inverse square law of force strength falloff would make it very hard to scale this linearly, that combined with the fact that weight increases with the cube of size means that the magnets involved would have to be incredibly strong.

Not impossibly strong mind you, if magnets can hold a hundred ton train car off the ground I am sure they could slow the fall of an elevator.

I'd not want to be inside that elevator with anything magnetic in my pocket, though.

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by lutusp on Monday February 03, 2014 @07:25PM (#7174620)
> The inverse square law of force strength falloff ...

Because there aren't any magnetic monopoles, I believe in ordinary circumstances the relationship is inverse cube of distance. Reference: http://www.newton.dep.anl.gov/askasci/phy05/phy05629.htm

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by IgorPartola on Monday February 03, 2014 @08:01PM (#7174758)
Yup, magnetic field falls off as 1/(distance3). That's why being 5 feet from an NMR magnet is bad news bears, while being 8 feet away is fine (numbers made up, do not test). I remember seeing signs all over the place at our labs at the uni, saying to not enter if you wear a pacemaker or if you are wearing a wrist watch.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by JackFr on Monday February 03, 2014 @11:49PM (#7175568)
Every elevator you've been in has been held up by a magnet. They're just not obvious inside the motor housing.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by throw_away on Tuesday February 04, 2014 @12:23AM (#7175674)
To be pedantic, they're usually held up by a counterweight or by hydraulics. They generally move by magnets, though.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by Crito on Monday February 03, 2014 @06:02PM (#7174172)
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.

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by taliesinb on Tuesday February 04, 2014 @12:34AM (#7175703)
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

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by e12e on Tuesday February 04, 2014 @01:35AM (#7175865)
But it's possible to the same experiment with a battery and an electro-magnet, isn't it? (Not sure how the power-density of lithium-ion batteries would relate to floating 80 kgs down a tube... but seeing as people have demonstrated hover-boards, it should be doable) ? Assuming the purpose is a) float down tube, and b) SCIENCE! -- and not c) with rare earth magnets...
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by sbilstein on Monday February 03, 2014 @11:37PM (#7175530)
Yea my hs physics teacher demoed this and talked to us about possible applications for a while. i don't remember any but thank you wikipedia: http://en.wikipedia.org/wiki/Eddy_current#Applications
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by laureny on Monday February 03, 2014 @06:07PM (#7174208)
> It'd be completely impractical, of course, but a whole lot of fun.

And at the end of the ride, you want to take a selfie and email it to all your friends but you can't because your phone is fried.

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by tall on Monday February 03, 2014 @07:54PM (#7174729)
actually not true, modern phones use SSD, and don't really get affected by magnets in the way you might expect.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by laureny on Monday February 03, 2014 @08:10PM (#7174804)
True, they would only get fried if they used electronic components.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by rpenm on Monday February 03, 2014 @09:49PM (#7175185)
If the elevator carries the magnets, the magnetic field relative to the phone is constant, thus no induced current and no fried components.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by dfox on Tuesday February 04, 2014 @08:23AM (#7177189)
Even hard disks are not affected by external magnetic field in the way most people expect. Strong magnetic fields interfere with the operation of drive (seeking voice coil) but accidental erasure of data on platters by external magnetic field is relatively improbable.

Generally most of the damage caused by strong external magnetic fields to any electronics are caused by second order effects, i.e. something gets confused and causes data-destroying failure.

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by orblivion on Monday February 03, 2014 @06:30PM (#7174335)
Wait so does that mean that, ironically, friction is what it allows magnets to fall through a tube that isn't superconducting?
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by bjornsing on Monday February 03, 2014 @06:32PM (#7174356)
Yes, pretty much, but you usually call it electrical resistance.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by vecter on Monday February 03, 2014 @06:55PM (#7174471)
Technically, this has nothing to do with the Right Hand Rule. That is just a human convention to assign a positive and negative signs to a particular direction. Electromagnetism doesn't care about our silly human notations.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by hnriot on Monday February 03, 2014 @07:25PM (#7174622)
semantics vs syntax - magnets don't have hands obviously, but the metaphor holds just the same.

There is no "technically..." nonsense! It's not a human convention either - it's an anthropomorphic metaphor.

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by robotresearcher on Monday February 03, 2014 @08:29PM (#7174897)
Name another animal that uses that convention.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by nrivadeneira on Monday February 03, 2014 @10:34PM (#7175340)
The point is that it's not a convention - it's a metaphor, as hnriot pointed out.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by robotresearcher on Tuesday February 04, 2014 @06:13PM (#7180884)
It's not a metaphor.

http://en.wikipedia.org/wiki/Metaphor

No one is comparing or likening charge to some other thing. We simply give opposing charge "directions" a name. Everyone uses the name. It's a convention.

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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by nrivadeneira on Wednesday February 12, 2014 @03:47PM (#7227146)
The comparison that is being made is the direction your thumb points when your fingers wrap in a particular direction. It's not merely giving it a name - it's specifically likening the vector to the relative orientation of your fingers. That's why the comparison is memorable. If it were merely a name with nothing that it is compared to, it would not be memorable nor helpful.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by calinet6 on Monday February 03, 2014 @07:18PM (#7174588)
It does, however, exhibit the properties we use language to describe. There's a kernel of truth at the center of it.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by smitec on Monday February 03, 2014 @05:45PM (#7174069)
As long as it isn't a magnetizable substance like iron.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by reddog9287 on Monday February 03, 2014 @11:38PM (#7175535)
Yeah I'm actually pretty surprised that this made it to the front page. Pretty novel and I thought more people would know about it (being a typical physics class demonstration). Cool nonetheless.
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Re:What Happens When You Drop A Magnet Inside A Copper Tube ... by sp332 on Monday February 03, 2014 @06:38PM (#7174385)
I would wrap an electromagnet around the magnet in the tube, then turn on current to create a field in the opposite direction. Hm... If I pulse the field quickly, maybe it would even remove itself :)
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