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Q & A: combined bar magnets

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Most recent answer: 07/27/2015
Q:
This is a question i always had in my mind since the age of 8. I know that a bar magnet will show the south and north. And also i know that the south pole of a magnet will attract to the north pole of a magnet. Now my question is.... if we take 2 bar magnets with the same length, weight, mass and all the other things(exactly same magnets) and make them attracted to the opposite poles. Now the north pole of one magnet is attracted to the south pole in both way. Now these two magnets are considered as one magnet. So which side will be the north pole and which side will be the south pole...
- Martin P Joseph (age 26)
Kerala, India
A:

Your suspicion that the combination won't have a North or South pole is correct. The symmetry tells you that, since each end is an equal combination of North and South.

You started out with two dipole magnets and combined them so that their dipoles canceled. Now what you're left with is a quadrupole. Say that each magnet was, for simplicity, a 2x1 rectangle so that the combined magnet is a square. Although either magnet by itself changed the signs of its fields when rotated 180°, for the combination rotating 180° just brings it back to an equivalent position. Rotating by 90° approximately just changes the sign of the fields. (For a pure quadrupole, that would be exact, but in this case there are octupole etc. moments too.) 

There are other types of magnets that have no North or South pole. For example, a toroidal magnet has none.

Mike W.


(published on 07/27/2015)

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