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Suppose a magnet is placed on the left of an unmagnetized ferromagnetic object (imagine it as an infinite plate so no magnetic field from the magnet directly "leaks" to the other side).

Does the space to the right of the object experience no magnetic field because the ferromagnetic object acted as a magnetic shield, or does it experience a magnetic field because the ferromagnetic object became an induced magnet? Why?

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2 Answers 2

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If it were a perfect ferromagnetic object (that is with infinite magnetic permeability $\mu$) than it would act as a perfect screen.

In nature there is no material with infinite magnetic permeability, therefore the ferromagnetic object would act as an imperfect screen allowing some magnetic field on its right side.

Think of the ferromagnetic object as a "conductor" of magnetic field. The higher the $\mu$ the better the "magnetic conductor".

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  • $\begingroup$ Thank you for your explanation. Assuming perfect ferromagnetic object, could you shed light on why it wouldn't act as an induced magnet instead? My very shallow understanding of magnetism tells me that the magnetic domains in the ferromagnet can align with the magnetic field to become an induced magnet; why does it not happen in this case? $\endgroup$
    – Tham
    Dec 4, 2021 at 12:04
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There will always be some field from the magnet on the far side.
A simple way to think about magnetic shielding is as "compressed space" for B lines.
For 1 mm of a material with relative permeability of 1000 a very approximate calculation of the far side field would be to treat the shield as having added 1 m of distance to the magnet.

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