MadSci Network: Physics

Re: Why is there a space in the magnetic field around a bar magnet?

Date: Thu Oct 25 12:04:23 2001
Posted By: Dr. Fred Jeffers, Staff, Magnetic Recording Research, Iomega corp.
Area of science: Physics
ID: 1003976568.Ph

     I assume that you mean that the filings were attracted to the ends of 
the bar magnet and not to the sides.  The simple answer is that the field 
is much stronger at the ends of the magnet than it is at the sides.  A 
more complete answer is that the magnetic force on an iron particle is 
proportional to the field times the rate at which the field is changing as 
a function of distance.  The latter is called the "field gradient".  At 
the ends of the magnet the field and the field gradient are both large and 
the forces is very strong.  At the sides of the magnet, the field is 
smaller and the field gradient very small.  Hence the force there is 
small.  The ends of the magnet attract most of the particles, leaving very 
few at the sides.   

[note added by MadSci Admin:  There are also some previous answers
in our archives which discuss bar magnets.  They can be found by
searching with our search engine which is at]

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