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I heard that when you take two magnets and get them closer together so they reject each other (north pole to north pole or south pole to south pole) they weakens. Does anybody knows how to calculate how much it weakens depending on their distance?

And does it work the other way around? If you take two magnets the attract each other (north pole to south pole) will they get stronger? If so, how to calculate how stronger they'll get depending on their distance

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It is possible to partially demagnetise a permanent magnet by exposing it to an opposing field. Definitely true of iron-based and early (AlNiCo?) alloys. For modern rare earth and ferrite magnets it is much less likely (though if you are also applying heat, it's possible). You won't find a general formula to calculate how much.: it depends on too many variables. However you may find guidelines or approximate formulas for specific materials; try the magnet manufacturers for datasheets. –  Brian Drummond Feb 25 '13 at 11:45
A couple of examples ... asahi-kasei.co.jp/ake/en/technology/magnet/magnet.html and google.com/… –  Brian Drummond Feb 25 '13 at 13:41
Brain's comment is correct. To expand a little, you'll want to look at the 2nd quandrant of the BH curve for the magnet. This is often called the demagnetization curve. If there is a "knee" in that curve, then it's possible you could demagnetize if the other magnet's field causes the operating point to drop below the knee. –  Brad Feb 25 '13 at 14:53
does demagnetization means cancellinthe magnet's magnetizm? or weaken it? –  Someonation Feb 26 '13 at 12:41
Sorry, I should have been more clear. I come from the world of motors where "demagnetization" generally means a weakening of the magnetic field. –  Brad Feb 26 '13 at 13:16
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