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I have a question about magnetic repulsion. Suppose I have a permanent magnet (axially magnetized) placed on top of the iron core of an electromagnet. I want to switch on the elctromagnet to repel the permanent magnet. (Please assume the permanent magnet is contained in some kind of tube that allows up and down movement but prevents it from spinning around. Please also assume that the electromagnet always produces a field strong enough to repel the permanent magnet - whatever that may be)

If the permanent magnet's magnetic field is very weak, will the electromagnet have trouble repelling it?

I know that if the electromagnet were attracting the permanent magnet, the only thing that would matter would be the strength of the electromagnet. What about repelling the permanent magnet? Assuming the electromagnet is always strong enough, does the magnetic field of the permanent magnet play any role? Is there a chance that an extremely weak magnet would simply act as a piece of ferrous metal and not be repelled by the electromagnet at all?

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  • $\begingroup$ "strong enough" is really a statement about the product of the two magnetic moments, so it's hard to conceptualize one thing being strong enough, but the other one being arbitrarily weak. Put another way, for some fixed nonzero magnetic moment of the permanent magnet, there will always be some current you can push through the electomagnet that will enable levitation. $\endgroup$ – Jerry Schirmer Mar 27 '15 at 15:11
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Take it to the limits to examine the problem. Imagine that your weak permanent magnet is really incredibly weak, weaker than the Earth's field, while it's mass is in the kg. When the electromagnet is turned on is the repulsion of that tiny field enough to overcome the attraction of a few kg of "spare" ferromagnetic material to the electromagnet? Well, from experiments I did when I was a kid, a really powerful magnet will "overpower" a much weaker one no matter what it's orientation.

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    $\begingroup$ I guess I am trying to determine a rule of thumb for the optimum strength of the permanent magnet, with the goal of the electromagnet repelling the perm magnet using the weakest magnetic field possible. My thinking was the weakest possible permanent magnet would be best, but then I thought there could be a possibility the permanent magnet could be so weak the electromagnet would need to produce a stronger field to repel it. To minimize the field strength the electromagnet needs produce to repel, is a permanent magnet that is "not to strong, not too weak" the best, or is the weaker the better? $\endgroup$ – EddieP Mar 29 '15 at 3:08
  • $\begingroup$ This is a guess, but a rule of thumb might be that the electromagnet will repel the permanent when the latter has more than half its atoms aligned to create its own field. If so, that may well be the point where the actual field strength is greater than half the theoretical maximum field $\endgroup$ – user56903 Mar 29 '15 at 8:35
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The critical concept here is magnetic saturation of your ferromagnet material.

In the ideal case, if your weak PM magnet is saturated (completely magnetized,) then the electromagnet can only repel, not attract. Of course this would only occur if you'd designed a ferromagnet material which gives the desired value of weak field when fully magnetized.

On the other hand, if you weakly magnetized a hunk of steel, the material will be far from saturation. In that case your electromagnet may attract the steel itself more strongly than it repels the alike-pole of the permanent magnet.

Thought experiment: Make a PM magnet ring by placing two steel horse shoe magnets together. Fully magnetize this object by using a high-current pulser coil (with windings passing through the hole in the donut.) In the ideal case, the permeability will fall to zero, and the fully saturated ring will act like empty space. It's made of steel, yet it won't be attracted by external magnets. For real-world materials the BH curve isn't a perfect square, so an outside magnet would attract such a ring only weakly. Perhaps eliminate this small attraction by winding a toroid coil around your ring magnet and applying a large direct current to guarantee that the material is completely saturated. (A similar trick is used in the sensor-ring of a Flux-Gate Compass.)

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  • $\begingroup$ The critical concept here is coercivity. $\endgroup$ – Pieter Dec 23 '16 at 20:36

protected by ACuriousMind Apr 1 '17 at 18:48

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