Weigthing Magnetic Interacion

electromagnetism

Weighting magnetic interactions
The use of toys in physics teaching is common practice [1–3] because a lot of physics may be demonstrated with them, if properly employed. In particular, there is a well known toy that provides many insights into magnetic-field properties: the Geomag magnetic building kit. This kit consists of a number of strong cylindrical bar magnets (6 mmin diameter and 25 mm in length, completely covered in hard plastic except at their two ends) and of a number of ferromagnetic steel balls (12.7 mm in diameter). Geomag kits have been used in the past for educational purposes either to elucidate qualitatively some magnetic force/torque [4] and field [5] issues, or as a tool for the practical illustration of the well-known ‘method of images’ inelectrostatics by establishing analogies between electric and magnetic phenomena [6]. Here, we illustrate an original use of Geomags to perform quantitative experiments on the interactions between magnets, as well as among magnets
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inner small magnets steel ball

inner steel bar

unbroken Geomag bar

Figure 1. Experimental parts: removing the plastic case allowsus to extract the two small strong neodymium magnets. and ferromagnetic materials. The proposed experiment, realizable by using easily found and low-cost materials, allows us to explore quantitatively— among other things—a ‘Geomag paradox’ [5]: the
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spacer screw

screw-mounted small magnet

steel ball little magnet

Figure 2. Left: the precision electronicscale (Laica BX9310 model). Right: the screw support. The vertical transparent part is a plexiglass slab stuck on a wood/aluminium base. attraction of two homologous magnetic poles when properly faced through an interposed ferromagnetic object. Experimental set-up Some components that are needed to prepare the experiment are obtained from a Geomag kit (shown in figure 1). A Geomag magnetic bar(completely embedded in a plastic case) is constituted by two small strong neodymium magnets placed at the ends of a high-permeability steel bar. By removing the case, the small magnets can be readily extracted. For our experiment, we have stuck two of them on wooden supports with south or north poles facing up. The fastened magnets constitute an easy-tohandle component, which can be put on the plateof electronic scales (figure 3). This solution has the additional advantage of outdistancing the magnet from the scales’ electronics, to reduce possible magnetic interference on electronic circuits. Another magnet is stuck on the end of a diamagNovember 2009

precision electronic scale

Figure 3. Experimental device used to quantitatively determine the interaction strength between magnetsand/or ferromagnetic objects in various configurations. In particular, the figure shows the case of two magnets facing like poles through a ferromagnetic sphere. netic screw, approximately 15 cm long. This screw is held above precision electronic scales by a support (figures 2 and 3). We used a balance (Laica BX9310 model, 50 mg sensitivity) found in hardware stores for about t20. One small magnetis fixed on the screw tip, while the other, stuck on the block, is placed on the scales pan; care should be taken to align it exactly in the screw-axial direction. By simply measuring the weight variation with respect to the case when the magnets are suffiP h ysic s E ducat ion

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Frontline ciently far away, this experimental device allows us to directly obtain the magnetic interactionstrength (either attractive or repulsive) between magnets when they are a given distance apart. Repeating measurements while the screw is properly and progressively turned allows us to obtain the interaction force as a function of distance. Results and perspectives Using the device described we have performed several distance-dependent measurements, by using various arrangements of magnets and...