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Question:
Does the strength of a 'permanent' magnetic field decrease with the square of the distance or just as the distance increases? thanks atp



Replies:
Actually, I think for a permanent dipole magnet the field decreases as the fourth power of distance (I could be wrong though). This is different from electric and gravitational fields in the sense that there is no such thing as an isolated magnetic charge - the field from an isolated electric charge does decrease as the square of distance, and similarly for the gravitational field from a mass. But you cannot have an isolated north pole of a magnet - it must always be tied to a south pole nearby, producing a dipole field pattern that (at longer distances) decays faster than the simple inverse square law because the two nearby poles almost cancel out.

Actually, that should be inverse third power, not fourth, for the decay of a magnetic dipole field at long distances.



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