Rare Earth Neodymium Magnets

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Donald S. Lambert
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Rare Earth Neodymium Magnets

Post by Donald S. Lambert » Fri Feb 07, 2003 7:13 pm

This is a new toy like thing. I saw these listed by Electronic Goldmine and since every course in DC electronics always gets into magnetism I thought I would get some powerful magnetics and play with them.<p>Bet the folks at Electronic Goldmine are laughing at those that buy the magnets. They are powerful and will pich skin off of fingers and if the washer ones can be floated on a dowel with them keeping almost 2 inches above the bottom one. And if the flat ones out of the bracket assembly get together I can't separate them. Too small to grasp and too strong to slide apart or at least for this 77 year old.<p>I was thinking abouit taking a toilet paper core and winding wire on it and dropping them through. Anyone with an idea of how many windings of what gauge wire to use. I was thinking of something to to flash an LED or somesuch.<p>Anybody with ideas of expeiments to do in that line? Greybie

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dacflyer
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Re: Rare Earth Neodymium Magnets

Post by dacflyer » Sat Feb 08, 2003 10:04 pm

why do these magnets loose their power if they get hot,,, when they cool down they they are dead as hell...

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Joseph
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Re: Rare Earth Neodymium Magnets

Post by Joseph » Mon Feb 10, 2003 4:46 am

The reason they would lose their strength is that the heating adds enough energy to the electrons which hold the bond between the atoms to break those bonds. Afterwards, the atoms are no longer aligned. Magnetized items exist when the electrons in the material move in the same direction or plane.<p>[ February 10, 2003: Message edited by: Joseph Meisenhelder ]</p>

Will
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Re: Rare Earth Neodymium Magnets

Post by Will » Sat Feb 15, 2003 1:45 pm

Hi Greybie,
I was just thinking about this very subject the other day and what I am going to do eventually is to make a coil, (The toilet paper former sounds like an excellent idea) fix it somewhere with the axis of the cylinder vertical then make a simple assembly from a fairly weak coil spring which can easily stretch three or four inches, attach a few inches of cord, then one of the neo magnets then another short piece of cord then a suitable weight. Thread the assembly through the coil and then support the spring above the coil so that the neo magnet sits in the middle of the coil. Connect an oscilloscope to the coil then (a) give a gentle tug downward on the weight and the neo magnet will oscillate up and down for a short while and produce a reading on the scope then (b) pull the weight downward as far as is practicable, let it go and observe the magnitude of the pulse it produces on the scope .. . . then . . . (c) With the neo magnet hanging in he middle of the coil apply a DC current pilse to the coil and see what the magnet does - repeat this for the magnet just outside and and (i) above (ii) below the coil and see what the current pulse produces. . . .then. . .(d) with a switch in your current pulse, siwtch it off and on repeatedly and see if you an find the resonance frequency for the magnet moving up and down.
I don't know offhand how to calculate the required number of coil turns. (The formula which connects it all is V (volts) - Blv where B is field strength in Webers/meter^2, 'l' is the length of the conductor (Coil) in metres and 'v' is thr velocity of the magnet in metres/sec^2 - but I can't say how to apply it in practice) - however you can easily get going by starting with a few truns and then increasing them until you get an acceptable reading on the scope. The voltage you observe should be directly proportional to the number of turns.
Let us all know about your results Will
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Will
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Re: Rare Earth Neodymium Magnets

Post by Will » Sat Feb 15, 2003 1:48 pm

The formula in the reply I just posted should read V = Blv - NOT, as I mistakenly posted it as - B - Blv
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dacflyer
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Re: Rare Earth Neodymium Magnets

Post by dacflyer » Tue Feb 18, 2003 3:20 pm

so once these magnets are dead from over heating,,is there no was to revive them ?

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haklesup
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Re: Rare Earth Neodymium Magnets

Post by haklesup » Tue Feb 18, 2003 7:02 pm

First a comment about heating magnets up to kill them. I found this quote in a magnet FAQ at http://www.wondermagnet.com/dev/magfaq.html#q17
Ferromagnetic materials heated above a certain temperature, known as the Curie Temperature or Curie point, completely lose their ferromagnetic behavior. The magnetic domains within the magnet are actually generated when the material cools below the Curie point. The Curie temperature for Neodymium magnets is roughly 590 F.
Actually, the ferromagnetic properties of a material can change for temperatures well below the Curie temperature. Neodymium magnets, for example, begin to lose their magnetization at temperatures as low as 300 F."<p>So the effect is not caused by breaking of bonds (although that may happen in some materials -burning) but by the randomization of the magnetic domains in the material when it is sufficiently hot. I believe that if the material is inside a magnetic field when it is cooled, the magnet domains will be realigned and the magnet will be restored. You can also kill some magnets by hitting them with a hammer just the right way, this has the same effect of reorienting the domains. A weakened magnet has some domains aligned and some random.<p>As for the tiolet paper tube experiment, here are a few comments.<p>Dropping the magnet will result in an accelerating body which will be traveling at different speeds at different points along the coil axis. This will create a ramp shaped pulse if the coil is wound with a constant number of turns per inch. You can reshape the pulse by changing the profile of the windings as you proceed. For a more square wave you need more turns at the top than at the bottom because the voltage produced is proportional to the rate of change of the magnetic field. but that may not be important to your application.<p>The gague of wire is chosen acording to the amount of current you expect to draw. After derermining the voltage and adding in the load to complete the circuit, you can calculate the current and refer to an AWG chart to see what size to use. For an LED, almost any wire will do even 30GA or smaller should be safe for a few mA.<p>In general the voltage will be a function of the number of turns, radius of the coil (assuming it falls through the center), the strenght of the magnet and the speed of the magnet. The duration of the pulse is a function of the length of the coil and the speed of the magnet<p>In any case, calculating all of this (by the way you didn't say how many turns for what purpose)is just a pain in the butt. I would just make a prototype and and hook it up to an o-scope and go from there. Make the measurements with an open load and several different resistive loads for a better understanding.

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