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Q & A: magnets in ac fields

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Most recent answer: 10/22/2007
Q:
Hi physics guys. I’m using a "pad heater" (flat 4x4 inch device with heating element wires inside) to warm the the oil in my motor oil pan so it will start in the mornings. I want to use magnets placed one the back side of the pad to secure it to the oil pan (rather than glue it), so I can remove it when not in use. Will the electric current (220v AC, 100w) harm the strength of the magnet? Will it just fall off the oil pan one night? Thanks a bunch,
- Josh (age 29)
Ulaanbaatar, Mongolia (really)
A:
Josh- Nice question. In principle, putting a magnet near a large ac current (which makes a large ac magnetic field) can demagnetize the magnet. That's how magnetic tapes are erased. In this case, with only 0.5 A in the pad, which is very unlikely to be in the form of a coil wound to produce a big magnetic field, I wouldn't worry about it.

Mike W.

There may be a safety issue here, tough. The bottom of the oil pan could have a sharp corner on it which can poke through the heating device if it isn't very tough. There is a potential for a short circuit if you're not careful.

Magnets will also demagnetize when they are hot. Hopefully your heating pad won't get that hot (there'd be a fire hazard first, especially if there is a film of oil all over everything). Some magnets, like refrigerator magnets, are made out of a magnetic material suspended in rubber or plastic, and this can melt into a sticky mess at a lower temperature -- so don't use that kind.

Tom

(published on 10/22/2007)

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