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Q & A: Breaking a Magnet

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Most recent answer: 07/13/2012
Q:
What happens when you break a magnetised piece of steel rod in half?
- jennie (age 13)
clacton, england
A:
Jennie,

If you break a magnetized piece of steel (bar magnet) in half, you get two smaller bar magnets. The North and South arrange themselves in the following way:

Before: N======S

After: N===S N===S


Mats

(published on 10/22/2007)

Follow-Up #1: Breaking circular magnets into two pieces

Q:
If the magnet is of circular shape and I break it into two pieces then show pictorially where the newly developed N and South poles will be?
- keshav (age 30)
india
A:
If the magnet is initially magnetized perpendicular to the flat surfaces, such as in a refrigerator door magnet, the N and S poles are on the flat surfaces pointing outward. If you slice it so as to obtain two half-moon shapes the N and S poles don't change directions.  Likewise if you you slice it like an Oreo cookie into two disks.

If the magnet is in the shape of a doughnut and is magnetized toroidally there is really no N or S pole direction to be defined.   If you slice it so as to have two C-shaped pieces then each piece will have a N and a S pole coming out of the sliced end parts of the C.  An 'Oreo' type slice will get you two separate toroidal magnets, again with no particular N or S pole.  If you cut a pie-shaped notch out of the doughnut there will be an N and a S pole on the faces of the notch.

LeeH


(published on 10/22/2007)

Follow-Up #2: holding magnets together

Q:
If you break a magnet length wise, they will not fit back together because the like poles repel. I was wondering why a magnet doesn't just fly apart from all those like poles next to each other?
- mike (age 31)
napavine wa
A:
Great question. The magnetic forces are pushing the parts of the magnet apart, just as you say. It doesn't fly apart only because the short-range bonds between the atoms are strong enough to keep it together.

That raises a related question. Why doesn't the magnetism on one side just flip directions, so that it sticks well with the magnetism on the other side? In a plain piece of iron, that's pretty much just what happens. Unless the iron is a very thin needle, the magnetism breaks up into domains pointing different directions, so that they don't repel their neighbors.  There's an art to making permanent magnets, finding materials in which the domains are strongly magnetized  yet have trouble flipping around.

Mike W.


(published on 02/16/2010)

Follow-Up #3: Why don't magnetic domains flip?

Q:
Why would it be difficult for a material to have trouble flipping around its domains? Can you specify what a domain is?
- Lily (age 21)
Seattle, WA
A:
First, a ferromagnetic domain is a region in which the magnetism of the individual particles mostly lines up in the same direction.

The reason that the domains can stay stuck for a long time is that certain directions are easier (lower energy)  than others. It's always equally easy to point one direction or its opposite, but the intermediate directions can require more energy. It can take a very rare event for a domain in a "hard" magnet to get enough energy to flip through that intermediate high-energy state. In a "soft" magnet (like pure iron) the energy required can be quite low, so the domains don't stay stuck.

The name of this effect (different energies along different magnetic directions) is "magnetic anisotropy". It can come either from the basic properties of the crystal or from the uneven distribution of defects in the crystal. Most good permanent magnets use alloys, in which the random locations of the different atoms makes a good deal of local anisotropy.

Mike W.

(published on 07/13/2012)

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