Why are the poles of a magnet stronger?

[stmartin]

Are the force around the poles compressed, so it makes the poles of the magnet stronger than the other parts? Thanks.

 

[genneth]

There is no "force" around a magnet. There is a field. The field affects things with magnetic charge -- in the case of magnetisms, magnetic dipoles. The field looks roughly like that pattern you get when you sprinkle iron filings about the magnet. The force exerted on a dipole due to a field is proportional to the strength of the dipole (obviously -- we actually define strength of dipoles this way) and the rate at which the field falls off. Note that all dipoles will experience a torque that tries to align them to the field (which is why the field is shown by iron filings -- they're miniature dipoles), but only a non-uniform field causes a linear force, displacing the dipoles. If you study the field lines about a bar magnet, you will notice that the field lines "come together" to end at the pole -- this signals the fact that the field is strongly varying there, because you go from very few lines crossing per area to many lines crossing per area. Thus a dipole placed there feels a much greater force.

 

[stmartin]

Ok, thank you very much.