Imagine there is some material that I have 2 chunks of and I have them arranged like the image below. Then I center a south-facing magnet over the 2 depicted by the "S" in the image. Is there some material that when I do this, the 2 chunks repel each other but then if I take the magnet away, they stop repelling?

enter image description here


1 Answer 1


The fact that materials adjust their magnetic polarity to an external field ("outside magnet") is the norm rather than an exception. There are several possibilities for what you specifically desire:

  1. Using material diamagnets is out of the game, because diamagnetism is extremely weak (bismuth is a stronger diamagnet, but still pretty weak)
  2. Using ferromagnets might naively seem feasible, because ferromagnetism is pretty strong for some materials (iron, cobalt, rare-earth compounds), but then again, although the induced north poles next to "S" would mutually repel each other, they would both be attracted much more strongly by "S", resulting in a net attraction effect
  3. Using paramagnets... is almost the same as ferromagnets, just, again, weaker
  4. Using superconductors would be ideal, because a superconductor behaves like an "ideal diamagnet", which means there would be two induced south poles next to "S" (and of course also two north poles further away from "S"), with the maximum strength possible, and which would all three repel each other; but superconduction requires a lot of experimental effort (you need to have at least access to liquid nitrogen, for high-TC superconductors), and the result depends on when superconduction is turned on (the magnetic flux "gets frozen" by the SC); it is also the principle behind superconductive levitation (when directed against gravitation)
  5. Realizing "S" as an electromagnet with alternating current (that is, it's not really a south pole, but continuously alternating between north and south) and the chunks of material being ordinary good conductors (aluminum, copper,... maybe silver if you are willing to invest more ;-) ); this is kind of the poor man's version of superconduction in that it mimics high diamagnetism (although not as strong as for real superconduction, and only for alternating current electromagnetism); it is the principle behind oldschool magnetic levitation (see. e.g. the videos by Eric Laithwaite on Youtube), or the common asynchronous induction motor; see also the "jumping ring experiment", although this involves only one "chunk of material", ie. the ring, and the "alternating current" only exists when current is turned on for the solenoid; however, this all comes at a price: although the conductors are good, they show losses; especially if nothing moves, but only stays in place (as in pure levitation), the efficiency becomes very poor; it can be imagined somehow similar to an induction motor that stalls due to high application torque; but maybe this is not so important for the application you envision...
  • $\begingroup$ But wouldn't ferromagnets attract instead of repel? $\endgroup$ Jul 3, 2021 at 0:31
  • $\begingroup$ Yes, that's what I wrote. The induced magnetic moments would be attracted to the source magnet. Hence my note about "naive" feasibility... $\endgroup$
    – oliver
    Jul 3, 2021 at 7:38

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