Origin of diamagnetism
In 1905 Langevin explained the origin of diamagnetism. Consider first a model of an atom with a single electron of charge e and mass m travelling with speed v in a circular orbit of radius r. this “current” produces a magnetic moment m equal to the product of the current and the area of the orbit,
When an induction B is applied, the electrons experiences a force -e(v × B) at right angles to its direction of motion. With r constant, the effect of applying a magnetic field is to increase or decrease the angular speed of the electron, depending on its direction of circulation. This increase or decrease its direction of circulation. This increase or decrease its orbital magnetic moment. Now consider an atom with two or more electrons, with the orbits so oriented that there is no net magnetic moments. Such atoms will show diamagnetic behavior. For when an external field is applied, the orbital magnetic moments of the electrons circulating in opposite directions in each atom will no longer cancel. A magnetic moment will be induced whose direction is opposite to B. a rod of such material when placed in the nonuniform field between the poles of a strong electromagnet will also itself at right angle to the field.
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