What Happens When You Drop A Magnet Inside A Copper Tube [video] | Hacker News Reader

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

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246 pointsprateekj123 comments

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.

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

The levitation over a superconductor was the other cool demo that the professor did :).

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...

lOl

Zeroes and Ones? Luxury! In my day we couldn't afford the Ones, we had to make do with just Zeros...

Ha! When I was in school, we had to push the electrons around by hand! You kids had it easy!

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.

I'm declaring a winner here ... the only possible thing that could top that is "through three-foot deep snow".

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.

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.

http://search.dilbert.com/comic/We%20Didn%27t%20Have%20Zeros (Sept 8, 1992 Dilbert)

You might wanna take a look at this video: http://www.youtube.com/watch?v=VyOtIsnG71U

Incredible. For anyone watching, make sure you at least go past 2:20 - that's when they pull out the magnetic rail.

As long as it isn't a magnetizable substance like iron.

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.

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.

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

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

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...

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.

> 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

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.

Every elevator you've been in has been held up by a magnet. They're just not obvious inside the motor housing.

To be pedantic, they're usually held up by a counterweight or by hydraulics. They generally move by magnets, though.

> 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.

actually not true, modern phones use SSD, and don't really get affected by magnets in the way you might expect.

True, they would only get fried if they used electronic components.

If the elevator carries the magnets, the magnetic field relative to the phone is constant, thus no induced current and no fried components.

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.

Wait so does that mean that, ironically, friction is what it allows magnets to fall through a tube that isn't superconducting?

Yes, pretty much, but you usually call it electrical resistance.

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 :)

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.

It does, however, exhibit the properties we use language to describe. There's a kernel of truth at the center of it.

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.

Name another animal that uses that convention.

The point is that it's not a convention - it's a metaphor, as hnriot pointed out.

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.

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.

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.

> ... 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.

Original link: https://www.youtube.com/watch?v=keMpUaoA3Tg

This digg page is blogspam.

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.

s/problem/feature

Fantastic point.

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.

Here is a much more informative video: http://www.youtube.com/watch?v=NqdOyxJZj0U

He has a bunch of really good videos

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.

Fingers will also be pinched and possibly broken.

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.

> 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.

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

Ha. I did exactly the same thing with a dryer years ago but with a neodymium magnet from a hard drive head actuator.

It is interesting that with the neodymium magnets being rare earth materials and apparent limits being placed on their mining/import/export etc., the price of them has sky-rocketed. There was a time when all the bass cabinets for bass players were getting smaller and smaller and lighter and lighter thanks to these magnets because it meant you could have a smaller speaker driver (hence less weight) but now they're having to resort to the heavier speakers again.

A shame really! I never did buy any light cabinets but always wish I had! At least Class D amplifiers are doing the rounds and so massive wattage amplifiers are now tiny and lightweight.

And if you have little kids treat them like guns. Lock them up when you aren't around.

Always good to lock up your kids when you aren't around. ;)

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

The effect works just fine with thinner tubes and smaller magnets - I've done it using 15mm 'plumbing' copper pipes.

Should work with a paper tube and copper wire as well? [edit: not saying that would be cheaper and/or easier, just that it should be possible...]

[edit2: A stripped wire, that is - not one with insulation -- which would be more suitable for making a spool]

If you close the wire (join the 2 points) you should get something, but certainly not as strong.

You can try that with aluminium if cooper is too hard to get. You'll probably get better results.

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

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.

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).

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.

Here in the UK there is constant copper theft going on. The church up the road here had their roof pinched 3 times in 6 months. But it is good that if you have new heating (with a combi boiler) and your condenser boiler taken out, that massive water tank you had in the loft to hold water now suddenly turns into money!

Copper and aluminum are two commonly used metals that are manufactured by relatively costly process that at the same time allows essentially complete recycling.

On the other hand there are not that many uses for pure copper (most of them are electricity related) as there are many other metals or copper alloys that are significantly cheaper.

A quick browse over at onlinemetals leads me to believe that you're probably off by a factor of 10x-20x.

http://www.onlinemetals.com/merchant.cfm?id=1288&step=2&top_...

That may be, but most of that markup is probably in the part it self, if you compare to a steel tube of the same dimension you'll see an indication of that. I was going by the current price the metal trades for, that is, a ballpark figure as something to "begin to imagine" what it could cost based on the material alone.

This piece is close: http://www.amazonsupply.com/dp/B003JU1FBG/ref=sp_dp_g2c_asin

6.5 inch outer, 5.5 inner, 12 inches tall (twice what the video shows) for only $700. So I'd say $300 would be a good estimate of what you can get that piece of copper for.

That magnet is also probably pretty expensive, that's a big one.

Yet another demo: https://www.youtube.com/watch?v=9s9I8OI82Ag

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.

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

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).

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

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

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

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.

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

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

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

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

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

http://www.youtube.com/watch?v=A1vyB-O5i6E&list=PL8E7ED16454...

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

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.

Not sure where to get one at that thickness or diameter, but a specialized plumbing supply company can get you most standard size and diameter of copper piping.

That one is huge, though. In theory, this should work with smaller, but powerful magnets, and thinner tubes, correct?

I don't think you can make a permanent magnet any stronger as it is. There is a certain limit. I think the size matters in this case.

I can only find thin pipes for plumbing online, but I would really like the think one.

You can't make a permanent magnet stronger, I believe. I worded that weird. I meant a smaller magnet, that is powerful, and a smaller tube. My brain doesn't work so well sometimes.

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.

it's a less impressive demo if the pipe is made out of something ferrous, like cast iron.

But it would be amusing!

"Watch this!" ....CLINK

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

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

It also works with a small neodymium magnet which can be bought for less than $1 and a small copper plumbing tube which costs about the same.

Of course, but the size makes it fun.

With thin tube the effect is not noticeable on the outer surface. Essentially, thicker tube means more pronounced effect.

Would a thinner wall tube work just as well? It seems the effect depends on electrical conduction which should be sufficient even with much less copper. Or does it depend on how many flux lines are hitting the copper cross section?

I found a kit (magnet + copper tube) here: http://www.teachersource.com/product/645/electricity-magneti...

Not on the same scale, but appears to be designed to demonstrate the same concept.

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

Copper isn't ferrous, magnets can't stick to it.

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.

They used crystal balls instead of magnets in my psychics class :)

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).

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

No. Because copper isn't ferromagnetic, it will not be magnetized by the magnet, and consequently the magnet and the copper will not be attracted to each other. (In fact copper is diamagnetic, so it will actually slightly repel.)

Source: I am a magnet scientist.

> I am a magnet scientist.

Judging entirely by the name, that sounds awesome :D What do you do all day?

A lot of reading, plumbing, and electrical work. But when things are running smoothly, we grow complex oxide crystals using pulsed laser deposition and then measure their magnetism with SQUIDs (superconducting quantum interference devices).

So what would happen, minus the spin - straight through at roughly G?

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

http://www.bicycleman.com/recumbent-exercise-bikes/images/Ma...

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

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?

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.

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

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

> 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.

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

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

Why should it. The energy is supplied by gravity.

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.

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

Ooh eddy currents!

The slinky trick is better http://www.youtube.com/watch?v=eCMmmEEyOO0

Spooky.

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