I would like to know what happens in the following situation. (I am not planning on actually doing this - I just want to understand).

Suppose I have a permanent magnet contained in a plastic capsule, where the magnet has enough space to move and rotate. The capsule is placed near a mass of ferrous metal, and the magnet inside is attracted to the metal. This ferrous metal object is then magnetized, and has magnetic field lines moving through it originating from the magnet inside the capsule.

Another, stronger, magnet is moved near the capsule. The north pole of the larger magnet is near the north pole of the magnet in the capsule, and causes the magnet inside the capsule to flip around. The larger magnet is then moved away, and again the capsule magnet is attracted to the ferrous metal nearby.

I am interested to know what difference would be made to this operation if the capsule is made of copper instead of plastic. The flipping of the magnet inside the capsule causes a changing magnetic field which induces an electric field inside the conductive copper for a brief period. How would this influence the extent to which the nearby ferrous metal object is magnetized at that moment the magnet is moving? If we compare the magnetic field lines in the ferrous metal in the two situations - one with the plastic capsule and one with the copper capsule - how would they differ?

  • $\begingroup$ the intent of the stronger magnet was just to cause the inner magnet to move right?Its influence on the ferrous material can be ignored? For all intents and purposes, is you question equivalent to asking what would happen if the magnet inside had suddenly started vibrating or doing some other random stuff? $\endgroup$
    – lineage
    Oct 19, 2019 at 1:47

1 Answer 1


With the copper capsule, the chages in the magnetic filed induces an electric filed in the copper, which drives a current in the copper such that the current induces a magnetic filed opposing the original changes to the magnetic filed. This makes the magnet inside the capsule move a bit slower. Wit the plastic capsule, no current is induced (even though there will still be an induced electric field) and there is noting to oppose the change in magnetic field.

The reason why the copper only causes the magnetic filed changes to happen slower is because resistivity in the metal dissipates the induced current. In the end the induced current is not quite enough to fully oppose all changes to magnetic fields. However, in the case of a superconductor, there is no current dissipation, and the magnetic field does not penetrate through the superconductor. (For the picky, a zero resistivity material would be able to have a frozen in magnetic field which will not change. A superconductor, however, have other quanrum mechanical reasons for not even allowing a static magnetic field to penetrate it.)

  • $\begingroup$ Andréas Sundström thanks for your answer. Can I just confirm/clarify? The net strength of the magnetization of the ferrous metal object is the same in the two cases - just slower with the copper? $\endgroup$
    – EddieP
    Oct 19, 2019 at 16:04
  • 1
    $\begingroup$ @EddieP As long as the only difference is the material of the smaller magnet's casing, the end magnetization of the ferrous object should be the same. $\endgroup$ Oct 20, 2019 at 7:37
  • $\begingroup$ Thanks for your additional comment. I apologise if I am asking excessive questions but - would a capsule made from a higher resistivity material than copper mean the field changes would happen slower, or faster than with the copper capsule? $\endgroup$
    – EddieP
    Oct 22, 2019 at 16:24
  • $\begingroup$ @EddieP If you use a higher resistivity material, that would mean a faster change in magnetic field. The higher the resistivity, the faster the induced currents dissipates, thus leading to faster change in the magnetic field. $\endgroup$ Oct 22, 2019 at 18:13
  • $\begingroup$ Thanks again for your answer. I have just ONE more question - please excuse me if this is too much! If this operation were repeated, where the stronger magnet was moved back and forth, each time with a different pole closer to the magnet in the capsule, so the capsule magnet was flipped many times - would a lower frequency be better if one wanted to avoid the induced current in the copper capsule delaying the magnetization of the ferrous metal object? $\endgroup$
    – EddieP
    Oct 25, 2019 at 14:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.