Physics Question #4591

Ashé, a 16 year old female from Taree, Australia asks on March 2, 2009,

Why is there no magnetic field in the centre of a bar magnet?

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The answer

James Livingston answered on March 3, 2009

There is a magnetic field in the center of a bar magnet. In fact, if the bar magnet is fairly long, the interior magnetic field at its center is nearly the same as the field exiting and entering at the poles. However, it is slightly reduced by a reverse "demagnetizing field" resulting from the magnetic poles at the ends of the bar magnet.

This demagnetizing field becomes larger as the bar magnet becomes shorter and the poles become closer to each other. In a thin magnet magnetized through the short direction, the net internal field (the basic magnetic field produced by the atoms of the magnet minus the demagnetizing field from the poles) becomes rather small, approaching zero for a thin magnetic film magnetized perpendicular to the film.

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