|MadSci Network: Physics|
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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