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.
I think you can wear a steel watch around a strong magnet, there will be little magnetic filed penetrating the outer body. AFAIK, gold watches are not recommended.
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.
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.
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.
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.
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&#.... There are some shockers out there.
Metal detectors on doors etc. Stopping the guy with a pacemaker is important too.
And those, i think, are just from the fixed magnets (which may be a type of superconducmting electromagnet thats just normally on.). During a scan, dont mangetic fields go way way up? The fixed ones are just there for stability and whatnot.
They are superconducting magnets kept that way through cryogenic helium. They can lose their magnetism through a process called quenching. If you search on youtube for "MRI quench" there are several videos of people messing around with MRI's before they get decommissioned. Not exactly safe.
Wow, I have experience welding and am trying to imagine this happening to the table I used. Sounds like that could have been a lot worse.
I don't really know much about this sort of thing short of seeing a huge generator on a field trip to a dam in sixth grade. The stator spins around the rotor which contains magnets, and these magnets have to be constant, permanent ones. Is that correct? I assume it's not viable to use electromagnets for this purpose since they use electricity. I bet it would be nice to be able to turn them off.
The reason why you don't use electromagnets on a windmill this size is that the additional complexity isn't worth it. Though it would make assembly and dis-assembly a lot easier / safer ;)
It definitely could have been a lot worse, I recall having a tea break after it happened, once I stopped shaking enough to hold a glass without spilling.
Here is a picture of what the thing looked like in that stage of the final assembly:
Hehe, yes. And on the table is my Texan friend Ron B. who is pretending someone stabbed him with a very large chisel.
Ron is a God when it comes to woodworking and helped out with the making of the blades when I got stuck, he traveled all the way from Texas to Northern Ontario for that. Amazing guy, really.
Actually large generators normally do use electromagnets today. They need an external source of power when starting, but drain off a very small part of the generated power to keep running. As jacques wrote, that isn't done with small generators because the added complexity usually isn't worth it.
I took plenty of pictures (see link above and other pics in that album) but not of that particular mishap, it was decidedly unsafe to leave it all hanging suspended like that so we lowered the table/windmill combo to the ground using the chainfall.
I'd hate to have had the whole thing give and land on someone's foot.
The rotor was only press-fit into two bearings, no point in trying to find out if it would hold or not, it was meant to be pushed on, not pulled on by a welding table.
I believe it's related to this. Try leaning into the bore fast then pulling your head out, fast. Many people lose their balance temporarily. Something to do with inducing currents in the fluid in the inner ear I believe. A smaller version of the effect causes the sensation that you are moving when you lie still in the bore.
Edit: can't get the link easily as am on iPad, but google "MRI eddy currents" and watch a slab of aluminium fall
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.
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.
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.
We don't see the tube, but assume that it is 1m long. The experimenter picks drops the 100g magnet into the tube once per second and that it has zero velocity at the tube exit. i.e. initial and final kinetic energy is zero, but the experimenter keeps inputting gravitational potential energy, all of which gets converted to heat.
Power = m * g * h / t = 0.1 * 10 * 1 / 1 = 1 W.
So, dropping a magnet every second generates 1 W of heat, which is virtually nothing.
This result is logical. The only energy input is the experimenter repeatedly moving the magnet from the bottom to the top of the pipe, and this simple movement probably won't tire him out. He could, indeed, do it all day.
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.
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?
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.
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).
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
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.
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.
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.
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:
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.
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!
I was thinking that the currents are like this: http://dl.dropbox.com/u/388822/magnet%2Bpipe.png (apologies for the crappy picture). The green lines are the magnetic field lines. These lines go through the copper pipe at the top, and at the bottom in opposite directions. Since the magnet is falling, just above the point where a line penetrates, the flux is decreasing. Just below, the flux is increasing. Hence there should be two opposite currents. When there is a slit, the currents get rerouted like in the right picture. Another way to think about it is that every field line creates a circular current around it. When you connect all these circular currents you get what's in the picture (if I did it correctly).
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)
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:
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.
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.
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.
They told us about it when mentioning why cores of inductors are made of layered steel and not a single piece. That apparently keeps the eddy currents at bay.
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?
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.)
No, what you get (assuming a constant diameter of the tube without dents or shape changes) is that the magnet will induce a stronger magnetic field in the wall that is closer which will push the magnet towards the middle.
It will overshoot a bit (but not as much as the original deviation) and so on until it goes smoothly down the center of the tube surrounding it without touching the walls.
Likely there will always be a slight wobble but eventually that should become too small to detect.
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.
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).
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.
> 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.
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?"
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.
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!
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.
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.
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.
Magnetic visualizer sprays are also great fun -- assuming that you can still buy any of them. You can actually see the magnetically recorded data on floppies, the magnetic stripe on ATM cards, hotel card keys, driver's licenses, and analog data on audio tapes. In the case of digital data, it will look like bars and blanks. If you know the encoding format, you can visually figure out what's encoded there.
Some commercial products were Magview, Magcheck, Ferro-see, and Sprague-Mag. I don't know if any of these are still available.
The spray is iron powder suspended in a fluid (like trichlorotrifluoroethane) that evaporates rapidly and leaves magnetic particles oriented according to the magnetic field. The pattern can be made permanent by spraying with a fixative or hairspray, or lifted with cellophane tape.
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.
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."
"I have always wanted to show that there are many different "types" of "Magnetic fields" as the standard science says that there is only one "type"... THIS IS JUST VERY WRONG !!"
"Magnetism is the primery force and electricity is the product of that effect, science says that is not so.. THEY ARE WRONG again.."
"When a potential is applied a vortex is formed around the magnet and a spin of water takes place show through the gas bubbles.. This is very IMPORTANT as this will allow scientist to develop better, faster and more powerful machines to help the human condition.."
Uhm... clicks button to collapse video information again - well, I have to admit that the video is nice. Without sound. And text.
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.