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If you have two magnets (not in contact) and then put a non-magnetic object in between the two magnets, does that decrease the attraction/repulsion between the two magnets?

It seems that it wouldn't, because, if it would, then you could allow two magnets to attract each other, then put an object in between before they came in contact, and then be able to pull them apart again without having to put in as much energy as you would have to if the object had not been put in; you would get "free energy" (in the sense of a "free lunch").

Can anyone confirm this?

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4 Answers

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A truly non-magnetic object would be identical to the vacuum, at least for the electromagnetic purposes of the Question, so you wouldn't change anything by exchanging vacuum for vacuum. So the Answer is no.

Insofar as any material other than a vacuum is not non-magnetic, putting something made of it between the two magnets it breaks the premises of your Question.

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Most of the time, by non magnetic one actually means “very weakly magnetic”. Then the answer is yes, it will have an effect, though a very weak one. – Edgar Bonet Jul 26 '11 at 11:03
I agree, @Edgar, that non-magnetic means very weakly magnetic in everyday terms, but whether this is true for a Physicist will depend on the context. "Small enough for a Physicist to ignore it in a given part of a given experiment" can certainly happen, but if one approaches an issue in a conceptual way, which I think this Question does, the usual practice in Physics would be to take smaller to be smaller, not zero. – Peter Morgan Jul 26 '11 at 11:37
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We can decrease or increase the magnetic repulsion between two magnets by placing a ferro magnetic strip or plate in between.

NOTE: There will be no force acting on this ferro magnetic strip by the magnetic field.

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If you put a superconducting plate between the two magnets, you will completely screen their attraction. The superconductor will break the space the magnetic field can go through into two disconnected regions, and there will be no magnetic communication between the two regions.

The reason this isn't a contradiction with conservation of energy along the lines of your thought experiment is because you will need to do work to shove the magnetic field out of the way as you are putting in the screen in, when the magnets are close, and then pulling them apart will be easy, since they will be repelled by the superconductor, since the superconductor does not admit the field.

This is practical, it is a way to levitate magnets, and also a way to shield electromagnetic forces between objects in sensitive experiments that measure force and don't want to deal with Van-der-Waals attraction.

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up vote 4 down vote

Of course it's possible and it's completely the same like with electric materials. Google for terms (electric and magnetic) polarization, permitivity, permeability, susceptibility.

The problem with your perpetum mobile is that you don't account for bringing in the third object at all.

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You could fix this by saying that the object just happened to be flying through space at the time and that you did not have to use any energy to get it to show up and leave. Any issue of unaccounted-for energy being used to move the magnets could also be fixed. – compman Feb 7 '12 at 21:01
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@compman: If the object was whizzing by, you would just be converting it's kinetic energy to do work. – Ron Maimon May 18 '12 at 2:36

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