Physics Question #3809

M Harish, a 14 year old male from Mumbai asks on March 1, 2007,

Why does a bar magnet repel when it breaks?    n====|====s

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The answer

James Livingston answered on March 5, 2007

If a bar magnet breaks in two at its center, it creates new North and South poles at the two new ends. If the break is smooth and perpendicular to the length, the two new poles can be encouraged to rejoin. However, the magnetic energy of the system can be lowered if one or both of the halves rotate until the two halves are side-by-side with adjacent N and S poles. In other words, as:


With this side-by-side arrangement, there is much less external magnetic field, and therefore much less magnetic energy, than with the two halves rejoined to form the original long magnet. External magnetic fields extend from north to south poles and have much shorter external paths to travel when the two halves are side-by-side than if they are end-to-end. Since magnetic forces always are in the direction to lower magnetic energy, there will be magnetic forces between the two halves that tend to rotate them into the side-by-side position.

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