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aboard_epsilon
01-06-2010, 08:09 PM
Stupid question ....i could only answer for my self.... "only" ..if i went and bought 100 magnets or more...sadly i only have 10.

I've noticed that, if you stack a load of these pill shaped magnets on top of each other........that for more than 3/4s of the length of them ..you have a magnetic field that is going in one direction.

so the question is ..

if i added more magnets ........and then bent them around in a circle ...
1...would the circle join up happily
2...would the magnetic force continue in the same direction (OR ACT THE SAME AS IT DOES FOR 3/4s OF TEN) all around the circle.

All the best.markj

Evan
01-06-2010, 08:19 PM
Yes, they will join up. It's no different than closing the loop on a horseshoe magnet by placing a keeper bar across the poles. Magentism is a manifestation of the electromagnetic force. It is alwaysbipolar. No matter how you arrange magnets you cannot somehow make on pole go away or hide it. You can very significantly distort the field and you can highly compress the field into a small region but both magnetic poles are always present. If you close the circuit on a string of magnets there will be very little external field since the field always takes the path of least resistance. In effect it short circuits the magnets.

aboard_epsilon
01-06-2010, 08:45 PM
ok thanks Evan

so if you had an overlapped spiral with the ends unjoined ..and the ends fading away- out of the way ..what would you have then.

all the best.markj

Evan
01-06-2010, 09:10 PM
Still a magnet. You can't "fade away" or shield the ends.

darryl
01-06-2010, 09:33 PM
You would have little stray magnetism along the length of the magnet string, but lots between the loose ends. If the loose ends passed through holes in a steel plate thick enough to contain the magnetic flux, you'd have little magnetism to measure on the side where the bulk of the string of magnets is, but certainly a strong pattern on the other side. If the string of magnets is bent into a U shape and stuck to the steel plate, you'd then have little stray magnetism anywhere, except to consider what would happen where the magnets are partially separated from each other, as happens when you bend a stack of them. Wherever you have a gap where magnets aren't in full contact with each other, you have some stray magnetism. If the steel plate isn't thick enough, there will be stray magnetism to find on both sides of the plate.

Black_Moons
01-07-2010, 08:38 AM
evan: Hmm so its magnatism that 'path of least resistance' comes from?

My dad allways tryed to tell me it was electricity, but I told him that was retarded, Electricity follows *all* paths of resistance. Just more of it on the paths of lesser resistance. Else turning on the stove would turn out all your light bulbs (Of witch only one would work anyway!)

Evan
01-07-2010, 08:46 AM
Magnetism is just another manifestion of the electromagnetic force which is responsible for electricity. They are two faces of the same force.

[edit] Even when magnetism has a low resistance path through a highly permeable material there will be some stray field outside that path just like with an electric current. The only exception is the same as with electricity and that is when the path has zero resistance as with a superconductor.

A.K. Boomer
01-07-2010, 09:07 AM
Magentism is a manifestation of the electromagnetic force. It is alwaysbipolar. No matter how you arrange magnets you cannot somehow make on pole go away or hide it. You can very significantly distort the field and you can highly compress the field into a small region but both magnetic poles are always present. If you close the circuit on a string of magnets there will be very little external field since the field always takes the path of least resistance. In effect it short circuits the magnets.



Then whats the Halbach Array? One pole is greatly enhanced and the other rendered inop. That's hardly a short circuit, and its done with simplistic arrangement.... manipulated or not I don't believe the word bipolar fits.

Evan
01-07-2010, 09:12 AM
One pole is greatly enhanced and the other rendered inop.

Nope, that isn't what's happening with a Halbach array at all. The Halbach array strongly compresses the field lines so they all stick out from one side of the stack of magnets, both north and south pole lines.

aboard_epsilon
01-07-2010, 10:38 AM
http://www.youtube.com:80/watch?v=qv-9IAj_YnI&NR=1

bobhdus
01-07-2010, 12:32 PM
Check this out. "Programmable magnets".

http://www.popularmechanics.com/technology/industry/4337757.html

Weston Bye
01-07-2010, 01:21 PM
Check this out. "Programmable magnets".

http://www.popularmechanics.com/technology/industry/4337757.html

Humbug. Reading the article and the company website showed me nothing more than subtle wrinkles and possible applications for pattern-magnetized magnets. They show a couple of dozen patent applications but not a single patent awarded.

Pattern magnetizing or "programming" if you wish, has been in common daily use for decades. I presented a couple of articles in DM on the subject some time back.

SmoggyTurnip
01-07-2010, 02:50 PM
I told him that was retarded, Electricity follows *all* paths of resistance. Just more of it on the paths of lesser resistance. Else turning on the stove would turn out all your light bulbs (Of witch only one would work anyway!)

*all* paths of resistance is the least path of resistance.

Say you have 100 ohm resistor in parallel with a 50 ohm resistor.

Through the 100 ohm resistor -> 100 ohms
Through the 50 ohm resistor -> 50 ohms
Through both -> 33.3 ohms

..so it goes through both.

Evan
01-07-2010, 03:15 PM
Humbug. Reading the article and the company website showed me nothing more than subtle wrinkles and possible applications for pattern-magnetized magnets. They show a couple of dozen patent applications but not a single patent awarded.

Pattern magnetizing or "programming" if you wish, has been in common daily use for decades. I presented a couple of articles in DM on the subject some time back.


It's deja vu all over again. We just had this thread a short while ago in which I said the same thing. The "programmable" magnet is a marketing pitch designed to attract clueless investors.

darryl
01-07-2010, 04:27 PM
All magnets are programmable. Once the shape is made, the lump is 'programmed', or magnetized. You can change that if you want. Remagnetize in the other polarity, or imprint it any way you can by applying a magnetizing field.

One area which is interesting is anisotropy- I think I have the term right, please correct me if this is wrong- where there's a preferred direction of magnetization in the particular material. Some super magnets can only be magnetized in one direction- if it's through the thickness, it could be polarized either way, but you wouldn't be able to magnetize is across the diameter at all. I discovered this one day while heating a neo magnet stuck to a piece of metal. At a certain temperature, it suddenly fell off the stick. Once cooled, I remagnetized it, but it wouldn't 'take' across the diameter, only through the thickness. Then I read all about it and learned something.

When it comes to a halbach array, if that tight pattern of magnetic field were to be imprinted, the material would have to be oriented in that pattern when cast, then a very strong magnetic field would have to be impressed with that same tight confinement in order to magnetize the material with that pattern. It would seem to be difficult if not impossible to effect the magnetization of the material to the strength of which it is capable, which is why halbach arrays are seen as build-ups instead.

As Evan said, you aren't hiding a pole with a halbach array, you're simply keeping the poles both on one side of the array. You can take a steel tube and stick magnets to it all around, then not find any magnetic field on the inside of the tube- both poles are on the outside- that's similar, with one exception- with a halbach array you're adding magnets which essentially take the place of the steel, but which instead of merely conducting the field lines, also add to the field strength.

And of course, as you assemble the magnets to make the array, you're creating your 'program'.

A.K. Boomer
01-07-2010, 11:07 PM
Yes when I wrote that I thought that it might just be a tighter configuration of something like a horseshoe magnet, but still, all in one stack, that's pretty amazing - I think I remember reading about it being used in some high speed train levitation system so I thought uni-directional, but if the fields themselves were also Halbach in reverse then it would match up as totally opposing forces for levitation...

Evan
01-07-2010, 11:33 PM
When a magnet moves in the vicinity of any conductor it produces a changing magnetic field in that conductor. That produces an electric current. An electric current produces a magnetic field in direct proportion to the strength of the moving magnetic field that induces the current in the first place, minus resistive losses. The induced magnetic field is opposite in polarity and therefore repels the magnet. The faster the magnet is moving in relation to the conductor the stronger the induced current and the resulting repulsion.

What makes a Halbach array special is the extreme flux density of the field because it is so tightly compressed. This makes it possible for a Halbach array to produce a repulsive force when moving over a simple copper or aluminum plate that is capable of supporting hundreds of times the weight of the magnet array. It is also self stabilizing since if it tries to drift to the side it produces an opposing force that keeps it centred on the conductive track.

A maglev railcar using Halbach arrays for levitation needs no power to the magnets and no levitation control system. All it needs is some low speed casters that only need to support the cars below 5 mph. The levitation system cannot fail in operation any more than a dropped rock can fail to fall. The only requirement for propulsion is some method of making the train move. The magnet arrays become more efficient and produce less resistance to motion the faster they move. They have no top speed limit other than some large fraction of the speed of light. It is conceivable that such a maglev train could be powered by a large turbofan jet engine similar to those on large jet aircraft. Top speed could equal that of passenger jets.