MadSci Network: Physics


Date: Fri Sep 22 09:39:13 2000
Posted By: Yaxun Liu, Grad student, Electrical and Computer Engineering, University of Waterloo
Area of science: Physics
ID: 969479345.Ph

Hi, Dianne,
  I think we may have different understanding
for "the polarity of a permanent magnet". If
your permanent magnet is a bar magnet with
two poles, and by "blocking the polarity of
a permanent magnet" you mean decreasing
the strength of the magnetic field near one pole
while keep another pole intact, you really can 
do that. The simplest way, as you have said,
is to attach a larger iron block to one end
of the bar magnet.
  As I have said, a permanent magnet can be
regarded as a group of numerous small magnets,
or magnetic dipoles. Each magnetic dipole 
generates magnetic field, and the magnetic field 
of the whole permanent magnet is a collective 
effect of the magnetic dipoles. If the orientation
of all the magnetic dipoles are the same, the total
magnetic field is strong. If the orientation
of these magnetic dipoles are random, the total
magnetic field is weak. In a permanent magnet
the magnetic dipoles are oriented in regular
ways so that the permanent magnet generates
magnetic field of specific patterns. For a
bar magnet, its magnetic dipoles are oriented
along the length of the bar, therefore its magnetic
field is like that of a magnetic dipole, i.e.
having two poles.
  When we attach an iron block to one end of
the bar magnet, the iron block is magnitized. It
will generate magnetic field in a similar way
as the bar magnet. We can regard it as a magnet
also. Its one end which is attached to the bar 
magnet will have opposite polarity to that of 
the bar magnet which is near to it, therefore 
it will weaken the original magnetic field. Its 
other end will have the same polarity as that 
of the bar magnet which is near to it, however, 
since it has a larger cross-section, the magnetic 
field is more dispersed, therefore weaker than the
magnetic field of the bar magnet without iron
block attached.
  Hope this may help.

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