Magnetism Wearing Off
Most recent answer: 10/22/2007
- Courtland (age 15)
Lawrenceville, GA, USA
All the little domains in a permanent magnet feel the forces of their neighbors and they would like to alternate. Random thermal fluctuations allow them to do this, but it is unlikely that enough atoms in a single domain will all fluctuate together enough to flip the domain around so its poles point the other way, but it does happen at a small rate. This rate depends on how easy it is to magnetize the material in the first place (how strong those interactions are between the electrons of neighboring atoms). Most permanent magnets will retain their magnetism for many many years, although the decay process can be sped up by heating the magnet or by exposing the magnet to external magnetic fields which oppose its field. Magnetized rock deposits (iron compounds like magnetite) have been found that record the Earth’s magnetic field’s history for at least a hundred million years, so this process is very slow.
2) Magnetic recording media, including cassette tapes, videotapes, and floppy disks. These items’ magnetism most certainly wears off, all by itself, over time, and the process can be accelerated by the handling. Tapes and floppy disks are made of a plastic material that iron or another magnetizable powder is attached to. The grains of powder are magnetized when a magnetic field is applied from the read/write head of the tape player of floppy disk drive. The pattern on these consists of tightly spaced North and South poles. Because of all of the poles so closely packed together, some of them repel others, and gradually over time they will weaken like the permanent magnets mentioned above when placed next to permanent magnets which repel them. Thin tape also has the problem of being wound around spools in the cassette -- the next turn of the tape’s poles affect the tape beneath it. I remember recording a piece of music on a tape -- it was silent at the very beginning and then a very sudden, loud part arrives. After a few years, a faint copy of the loud part could be heard in the silent section one tape-turn earlier, because the poles on the tape in the loud part had magnetized the tape they were next to (sort of backwards of "wearing off", but the net effect is to smear out all the music on my tape over time). Floppy disks also are less reliable years after they are written. Furthermore, the more a tape or a floppy is read, the more it wears out.
(published on 10/22/2007)
Follow-Up #1: magnets not sticking any more
- James (age 60)
I can think of a couple of possibilities. One would be that something (soap film, etc.) built up on the wall so the magnets aren't getting quite close enough to the steel anymore. That could be fixed by scrubbing. But that doesn't sound likely. More likely, the magnets weren't stable in the heat of the shower. The domain alignment gradually got lost. You probably have to replace them.
posted without vetting until Lee get's back
(published on 01/05/2014)