Shear (magnet)

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As a magnetic shear is called the self- demagnetization of a permanent magnet when removed from the closed magnetic circuit , which was used for the magnetization.

When a permanent magnet is magnetized, it is brought into a closed ring of flux guide pieces ( soft iron parts) in which a strong magnetic field is generated by magnetizing coils , the magnetizing field . The magnetic flux is concentrated by the flux guide pieces and passed through the magnet, which magnetizes it itself.

As a result of the removal from the closed magnetic circuit, the magnetic flux can no longer seamlessly exit the contact surfaces of the magnet into the flux guide pieces or enter the magnet again from the guide pieces. Rather, the magnetic moments now present in the magnet must build up a new magnetic field in the surrounding air space, which until now was only penetrated by the flow of a slight stray field. Two poles form on the contact surfaces, from which the field lines or the magnetic flux emerge, run around the magnet and re-enter. The energy for this comes from the magnetization of the magnet, which is thus reduced. This process is called shearing or self-demagnetization and means that no permanent magnet (removed from the magnetic circuit) actually has the remanent flux density specified for its material .

The strength of the shear depends

  • of the magnet material, in particular the shape of its demagnetization or hysteresis curve and its remanence : high- retentive materials such as AlNiCo are sheared the most
  • on the shape of the magnet: the shear decreases with increasing ratio of length (in the direction of magnetization) to diameter of the magnet (a spherical magnet has the lowest shear).

Therefore, high-retentive magnets have to be dimensioned more cylindrically, whereas low-retentive magnets can also be made very flat without significant shear losses.

Individual evidence

  1. ↑ K.-H. Schroeter: Magnet dimensioning with the help of the demagnetization characteristic. (php) June 11, 2008, accessed on November 27, 2010 (German).

swell

  • Karl Schüler, Kurt Brinkmann: Permanent magnets. Materials and Applications . Springer, Berlin 1970, ISBN 3-540-05245-3 .