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Q & A: magnets, large and small

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Most recent answer: 12/25/2017
Q:
When I was a child playing with magnets I had an idea to make a type of car with magnets organized so as to lift the car by repelling the earth�s magnetic force. But when I tried putting magnets under the table and putting a magnet on top, the magnet flipped over. I was too young to figure out how to correct that. Several years later Epcot had a �people mover� that used magnetic force to levitate cars. Magnets have always interested me. My question is, why can a small magnet lift a piece of steel from the ground, and cause a compass to point the wrong direction, but when pulled away, the compass returns to pointing north? How is it that the earth�s magnetic field can be overcome by a small magnet but only for a short distance?
- James (age 58)
Florida
A:

The Earth's field isn't very strong at the surface (under 1/2 Gauss) but it has roughly similar strength over most of the surface. A little bar magnet can have much stronger fields (over 1000 Gauss) but those fields fall off quickly as you get farther from the magnet. There are many pictures available showing how the magnetic field gets weaker away from the magnet, e.g. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html . Regions with a lot of field lines in these pictures are regions with strong fields.  The Earth is similar, just much bigger, so you have to go farther away for its field to get much weaker.

Mike W.


(published on 12/25/2017)

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