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A permanent magnet can be used to pick up a string of nails, tacks, or paper clips, even those these are not magnets by themselves. How can this be?

Aprille Ayupan
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1 Answers1

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The phenomenon you're asking about is called "induced magnetism". Iron (nails, tacks, etc) contains domains that can be thought of as tiny bar magnets oriented in random directions. Overall, there is no dominant orientation, so the fields of the domains cancel each other out. However, when an external field penetrates the iron, it causes some of the domains to align themselves with the external field-- more or less the same way that an external magnetic field causes the magnet in a compass to align itself with the field. As long as the external field is present, the iron is a magnet. In some materials, the alignment gets "stuck", so that when the external field is taken away the domains stay aligned the way the external field caused them to be aligned. In others, the alignment quickly becomes randomized after the external field is taken away. But in both cases, the iron at least temporarily becomes a magnet that will be attracted to a permanent magnet. See Ferromagnetism and Soft magnetic materials.

S. McGrew
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  • Under normal laboratory size field intensity individual domains do not and cannot align themselves along the magnetic fields, i.e., they do not change their direction, instead of responding to the magnetic torque by turning they change their relative size by moving the domain boundaries so that the aggregate magnetization is along the field lines. – hyportnex Dec 11 '18 at 14:59
  • True, that's a more accurate way to say it. – S. McGrew Dec 11 '18 at 15:01