Bar magnets

Fig. 14. Two-component magnetic bmsh development showing (a) the magnetic carrier particles (large circles) carrying toner (—), which within the magnetic field of the rotating permanent magnets, behave as individual bar magnets and (b) the production of a developed image. See text.

The dipole-dipole (Keesom) interaetion eomes about from the faet that on the average, two freely rotating dipoles will align themselves so as to result in an attraetive foree, similar to that eommonly observed with bar magnets. In order to ealeulate the net dipole-dipole interaetion, it is neeessary to examine all the possible orientations of the dipoles with respeet to one another. It is also neeessary to determine any jr effeets due to the field assoeiated with a point eharge, in order to determine the net effeet when amorphous solids are plaeed side by side. We also need to eonsider what happens if the dipoles ean reorient in eaeh other s fields. 

An intuitive model of the process can be given. Consider the proton, with / = i then there are two states, characterized by m = +5 and m = —5. In the absence of an applied field, these states are equally populated. The states may be pictured as corresponding to opposite orientations of a tiny bar magnet, which is a crude way of visualizing the magnetic moment vector. Clearly in the absence of an applied field, the orientation of the moment should not affect the energy of the nucleus. 

Now let a steady field be applied. The two nuclear states now correspond to orientation of the bar magnet parallel to the field (i.e., N pole to S pole) or antiparallel to the field (N pole to N pole). There will be an energy difference between these states, the orientation with the field (N to S) being of lower energy than the orientation against the field. 

Stab-kranz, m. (Biol.) corona radiata. -kraut, n, = Eberraute. -magnet, m. bar magnet, -thermometer, n. m. thermometer graduated directly on the stem (instead of having a separate scale). 

In 1821 Michael Faraday sent Ampere details of his memoir on rotary effects, provoking Ampere to consider why linear conductors tended to follow circular paths. Ampere built a device where a conductor rotated around a permanent magnet, and in 1822 used electric currents to make a bar magnet spin. Ampere spent the years from 1821 to 1825 investigating the relationship between the phenomena and devising a mathematical model, publishing his results in 1827. Ampere described the laws of action of electric currents and presented a mathematical formula for the force between two currents. However, not everyone accepted the electrodynamic molecule theory for the electrodynamic molecule. Faraday felt there was no evidence for Ampere s assumptions and even in France the electrodynamic molecule was viewed with skepticism. It was accepted, however, by Wilhelm Weber and became the basis of his theory of electromagnetism. 

If a spinning electron behaved like a spinning ball, the axis of spin could point in any direction. The electron would behave like a bar magnet that could have any orientation relative to the applied magnetic field. In that case, a broad band of silver atoms should appear at the detector, because the field would push the silver atoms by different amounts according to the orientation of the spin. Indeed, that is exactly what Stem and Gerlach observed when they first carried out the experiment. 

Compounds with unpaired electrons are paramagnetic. They tend to move into a magnetic field and can be identified because they seem to weigh more in a Gouy balance when a magnetic field is applied than when it is absent. Paramagnetism arises from the electron spins, which behave like tiny bar magnets that tend to line up with the applied field. The more... 

Many atomic nuclei behave like small bar magnets, with energies that depend on their orientation in a magnetic field. An NMR spectrometer detects transitions between these energy levels. The nucleus most widely used for NMR is the proton, and we shall concentrate on it. Two other very common nuclei, those of carbon-12 and oxygen-16, are nonmagnetic, so they are invisible in NMR. 

B. Bliimich, V. Anferov, S. Anferova, M. Klein, R. Fechete, M. Adams, F. Casanova 2002, (Simple NMR-Mouse with a bar magnet), Concepts Magn. Reson. 15 (4), 255-261. 

Magnetic properties reside in the subatomic particles that make up atoms. Of these, electrons make the biggest contribution, and only these will be considered here. Each electron has a magnetic moment due to the existence of a magnetic dipole, which can be thought of as a minute bar magnet linked to the electron. 

Magnetic materials can be classified in terms of the arrangements of magnetic dipoles in the solid. These dipoles can be thought of, a little imprecisely, as microscopic bar magnets attached to the various atoms present. Materials with no elementary magnetic dipoles at all are diamagnetic. 

Molecules of this type are influenced by an external electric field because they possess a dipole one end of the molecule is electron withdrawing while the other is electron attracting, with the result that one end possesses a higher electron density than the other. As a result, the molecule behaves much like a miniature bar magnet. Applying a voltage between the two... 

M-hexane (>99 %) sodium sulfate (p.a.) one 50 mL vessel with screw cap magnetic stir bar magnetic stirrer. 

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