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Q & A: slow magnetization

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Most recent answer: 03/02/2012
Q:
It seems that if a magnet is left in place for an extended time it holds with greater force than when first applied. Do magnets create a temporary residual magnetic field on a ferrous surface?
- Rez (age 39)
pcola, fl, usa
A:
great question. The effect is real but I'm a little surprised it was easily noticeable.

Iron typically starts off magnetized in small domains, but with the domain magnetizations pointing all different directions, leaving no net magnetization. When a permanent magnet is brought nearby, the domains re-align so that they tend to be attracted to the magnet. After the magnet is removed, you can notice that the iron is still a little magnetized, because the domains don't all lose that alignment.  That same sluggish response of the alignment that leaves the noticeable residual magnetization causes the iron's magnetization to gradually creep up after contacting the magnet. I'm a little surprised you noticed that creep because it occurs against a fairly large background of more rapid response.

Mike W.

(published on 03/02/2012)

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