MadSci Network: Physics

Re: Where does the Magnetic field go once it enters a metal? Does it come out?

Date: Wed Aug 1 14:39:40 2001
Posted By: John Balbach, Physicist
Area of science: Physics
ID: 991394015.Ph

This largely depends on the metal in question. Most metals will not affect a magnetic field (unless they are moving or carry electric currents). The question you want to ask is, "What happens when the magnetic field enters a ferromagnetic object?"

Ferromagnetic materials will distort any magnetic field that is nearby. This is used to good effect in large electromagnets. In an iron-core electromagnet, the field is trapped in the iron "yoke" of the magnet. This yoke will typically look something like this:

||        ||
||        ||
||        ||
||===  ===||
<-- the coils of the electromagnet are
||        || mounted around the crossbar, the high
||        || field is accessible in the space
||        ||

If you consider the fields of a bar magnet and a horseshoe magnet, you can see how the magnetic field is affected by the shape of the magnet. The magnetic field can be trapped entirely inside a ferromagnetic loop, and this technique is often used with transformers.

More often, the field will exit out the other side, but you should carefully examine the shape of the object and the nature of the magnetic field before deciding where the "other side" is located.

This topic is often not addressed in introductory texts of physics, so you might have to go looking in a text devoted to electricity and magnetism.

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