MadSci Network: Physics
Query:

Re: Demagnetization by AC supply - why should the magnet be removed SLOWLY?

Date: Fri Feb 26 12:59:43 1999
Posted By: Jason Goodman, Graduate Student, Massachusetts Institute of Technology
Area of science: Physics
ID: 917441925.Ph
Message:

An demagnetizer powered by AC will generate a strong oscillating magnetic field, and so will strongly magnetize the target object positively, then negatively, then positively, over and over again 50 or 60 times a second.

If you pull the magnet away from the solenoid very quickly, or switch off the supply, the magnet will feel a very strong + or - field, and then nothing. As a result, it will keep the last magnetization the solenoid gave it. On the other hand, if you gradually move the magnet away from the solenoid, you gradually make the + and - oscillations smaller and smaller until they go to zero when the magnet is far away. As a result the magnet will end up with zero magnetization.

It's a lot like being on a swing on the playground. If you're swinging very fast back and forth and you let go, you'll end up far away from the center of the swing set (and probably hurt yourself). If you gradually slow down by dragging your feet, you'll end up safely at the center.


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