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Given two objects, one that is a permanent magnet and a "non-magnet" that is simply magnetizable, how would we determined which is which? Can this be done just by observing the motion they induce in one another, i.e. without using any external fields, magnets, or magnetizable objects?

The key distinction between the two objects is that the poles of the permanent magnet are fixed, but for the non-magnet the locations of its poles depends on the orientation of the two objects. However I can't seem to turn this fact into a procedure for identifying the magnet.

The question comes from the first section on the chapter on magnetism from a college physics text, so the answer should be expressible in very basic terms.

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4 Answers 4

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I have a spherical neodymium magnet and a Mond process ball that I stick to my fridge. The way I usually have people find out which is which is by dropping the down a piece of copper pipe. I guess that doesn't count because the pipe is being magnetized by Faraday's law as the magnet drops. But a test I just tried is to stick them together and try turning one of them. The Mond process ball, having no preferred orientation, is free to turn, but the permanent magnet wants one of its poles to be in contact with its partner, so when you try to turn it, it snaps back into position.

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  • $\begingroup$ I like the second solution! Do you think it generalizes to arbitrary geometries, not just spheres? $\endgroup$
    – creillyucla
    Commented Mar 26, 2020 at 3:20
  • $\begingroup$ Spheres are great because a uniformly magnetized sphere has exactly a dipole field. I had hoped that I could sense the change in force as I rotated the magnet between my fingers because it should be something like $4\times$ as great when the pole is pointing at the other object as when it is pointing at right angles to the object. Hmm... doing it again just now I can tell the difference, but it's kinda subtle. An asymmetric object has a preferred orientation in a magnetic field. If you need instrumentation to tell them apart, you might just as well hang them by thread and see which is compass. $\endgroup$
    – user5713492
    Commented Mar 26, 2020 at 3:50
  • $\begingroup$ I do not understand, nickel is ferromagnetic anyway. "Nickel is one of four elements (the others are iron, cobalt, and gadolinium)[7] that are ferromagnetic at approximately room temperature." en.wikipedia.org/wiki/Nickel $\endgroup$
    – anna v
    Commented Mar 26, 2020 at 4:49
  • $\begingroup$ @user5713492 that would be using an external field though! $\endgroup$
    – creillyucla
    Commented Mar 26, 2020 at 4:58
  • $\begingroup$ @annav Yeah, I know and have all $4$ elements. But the rules specified "magnetizable" so I think that includes non-permanent ferromagnets, not forcing you to go to dysprosium (which I have) or terbium (I wish it weren't so expensive) for the second object. Take a piece of gadolinium out of the freezer and touch a Mond process ball to it and there is no force detected. $\endgroup$
    – user5713492
    Commented Mar 26, 2020 at 5:33
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Assume you had two bars, one a magnet, the other a magnetic material. If you also had superhuman strength, you could twist them individually into a horseshoe shape. The ends of the magnet would then attract each other, the magnetic material would feel no such force.

An alternative, though I haven't tried it out, could be as follows. Hold one bar (say Y) fixed on the y-axis, a little asymmetrically about the origin. Place the other bar (say X) along the x-axis, and hold it fixed. Now, holding the bar X fixed, let Y move. If Y is attracted end-first, Y is the magnet. If it is attracted middle first, then X is the magnet.

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  • $\begingroup$ Wouldn't the non-magnet also rotate when released, since doing so would increase its magnetic moment? $\endgroup$
    – creillyucla
    Commented Mar 26, 2020 at 3:09
  • $\begingroup$ I am really not sure about this. The induced pole would be at the closest point at equilibrium, but beyond that, an experiment would help here. $\endgroup$
    – NewUser
    Commented Mar 26, 2020 at 3:14
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If you only have these two objects, then take two bars . The permanent magnet one will have field lines . The magnetizable one none, or only acquired ones from the field of the permanent.

magnet

The permanent magnet one will stick much harder to the "non permanent" if the two bars are brought together in a T shape at their center, because the true magnet has strong lines at the pole, but the pole induced to the "non permanent" will be much weaker, due to the few lines.

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  • $\begingroup$ good suggestion but I think (not certain) that the rules are no other magnetizable objects are allowed $\endgroup$
    – creillyucla
    Commented Mar 26, 2020 at 4:55
  • $\begingroup$ @creillyucla thanks,I edited , I took away the third party suggestion. $\endgroup$
    – anna v
    Commented Mar 26, 2020 at 6:23
  • $\begingroup$ I tried this with a nail and four fridge magnet stuck end-to-end and it seems to agree with your procedure. That is, the nail will "snap" to the poles when moved around the magnet, but the magnet will stick anywhere on the nail. How do I know this is not an accident of the particular geometry of the nail and magnet? The induced dipole moment will be strong for different nail orientations, so how do we know that a geometry geometry will not lead to "snapping" of the magnet to the locations which optimize the induced dipole moment in the non-magnet? $\endgroup$
    – creillyucla
    Commented Mar 26, 2020 at 15:01
  • $\begingroup$ @creillyucla it is the density of field lines , optically, could be done mathematicaly too, weaker fields. Induced quantities cannot be as strong as the original ones supplying the energy. $\endgroup$
    – anna v
    Commented Mar 27, 2020 at 8:40
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10 ways to Distinguish a permanent magnet from a non-magnet

Distinguishing between a permanent magnet and a non-magnet involves a careful analysis of various characteristics and behaviors. Permanent magnets, especially those incorporating rare earth materials like neodymium magnets, possess distinct properties that set them apart from non-magnetic materials. Here are ten comprehensive ways to differentiate between the two:

  1. Shape and Appearance: A significant identifier is the shape of the magnet. Permanent magnets often exhibit specific shapes like rings, blocks, horseshoes, or disks. These shapes are intentionally designed to optimize their magnetic properties. In contrast, non-magnetic materials may assume diverse shapes and lack the specific design associated with magnetic functionality.

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  1. Magnetic Attraction: One of the fundamental attributes of permanent rare earth magnets is their ability to attract certain materials, particularly metals like iron and steel. Using small metal objects provides a simple yet effective way to determine if the material in question is attracted to the permanent magnet.

  2. Material Composition: Permanent magnets, especially Rare Earth Permanent Magnets like neodymium magnets, are crafted from ferromagnetic materials such as iron, nickel, or cobalt. These materials imbue the magnets with their magnetic properties, a characteristic absent in non-magnetic materials.

  3. Polarity Testing: Permanent magnets are characterized by having distinct north and south poles. Employing a small compass facilitates the determination of the magnet’s polarity. The compass needle aligns with the magnetic field, indicating the direction towards the north pole of the magnet.

  4. Hysteresis Loop Test: The hysteresis loop test involves a graphical representation of a material’s magnetic properties. Permanent magnets exhibit a closed loop in the hysteresis curve, symbolizing their ability to retain magnetization. In contrast, non-magnetic materials display an open loop or no loop at all.

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  1. Retentivity Test: Retentivity, or the ability to retain magnetization, is a key characteristic of permanent magnets. These strong holding force magnets, such as neodymium magnets, showcase high retentivity. Non-magnetic materials lack this property and do not retain magnetization.

  2. Magnetic Susceptibility Test: By inducing a magnetic response in nearby materials, permanent magnets demonstrate their magnetic susceptibility. In contrast, non-magnetic materials do not exhibit any significant magnetic response when subjected to the influence of a permanent magnet.

  3. Temperature Dependence: Permanent magnets maintain their magnetic properties over a broad range of temperatures. This temperature stability is a distinctive feature. Conversely, non-magnetic materials typically do not display temperature-dependent magnetic behavior.

  4. Magnetization Behavior: A defining characteristic of permanent magnets is their ability to be magnetized and retain magnetization even after the external magnetic field is removed. Non-magnetic materials lack this magnetization behavior.

  5. Use of Magnetometer: Magnetometers, devices capable of measuring magnetic field strength, can be employed to further confirm the magnetism of an object. Permanent magnets, including neodymium magnets, produce a measurable magnetic field, whereas non-magnetic materials do not exhibit such characteristics.

Distinguishing between permanent NdFeB magnets, particularly those incorporating rare earth materials like neodymium magnets, and non-magnetic materials involves a comprehensive analysis of various characteristics and behaviors. The material composition, distinctive shapes, magnetic attraction, polarity testing, hysteresis loop, retentivity, magnetic susceptibility, temperature dependence, magnetization behavior, and the use of magnetometers collectively offer a robust toolkit for differentiation.

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