Electricity & Megnetism

# Magnetic hysteresis

Magnetic hysteresis is the name given to the demagnetization of a material that has been magnetized after being subjected to a changing magnetic field.

Hysteresis is the property of a material or physical system to maintain its properties even without the continuity of the stimuli that provoked them. The word hysteresis derives from a Greek term meaning delay.

magnetic hysteresis

Ferromagnetic materials are easily magnetized, a process by which they are exposed to a magnetic field and become magnets. Imagine a ferromagnetic material placed inside a solenoid, through which electric current will be conducted . The flow of charges through the solenoid generates a magnetic field , which magnetizes the ferromagnetic core. Magnetic hysteresis is perceived in the behavior of the magnetic fields generated by the solenoid and the ferromagnetic core when there is a decrease in the intensity of the electric current flowing through the wire winding.

The following graph, known as the hysteresis loop , has the strength of the magnetic field generated by the solenoid (B S ) and the field created by the magnetization of the ferromagnetic core (B M ).

Excerpt Oa: the curve established in this section indicates an almost linear increase in the magnetization of the ferromagnetic core compared to the field created by the solenoid. The increase in the electric current flowing through the solenoid’s wire winding creates ever-increasing magnetic fields, which, in turn, magnetize the ferromagnetic core, which then generates ever-increasing magnetic fields.

Excerpt ab: by decreasing the intensity of the electric current flowing through the solenoid, the magnetic field of this element becomes smaller and smaller. By generating such a decrease, the intensity of the magnetic field generated by the ferromagnetic core decreases, but the delay of this field does not occur through the initial path.

bc stretch: in order to reset the magnetic field generated by the ferromagnetic core, a current in the opposite direction to the initial one is applied to the solenoid in order to create a magnetic field opposite to that of the ferromagnetic material, eliminating its magnetization.

Section cd: after the cancellation of the field generated by the ferromagnetic core, the current in the opposite direction to the initial one continues to travel through the solenoid. This magnetizes the ferromagnetic material, leading to point d, where the value of the magnetic field B M is opposite to the value previously existing at point a.

Defa section: if there is a new reversal in the direction of the solenoid current and it is gradually increased, the magnetic field generated by the wire winding will cause the ferromagnetic material to return to the initial magnetization point.

applications

The high or low hysteresis of the materials determines their use in different technologies. Hardened steel is used in the manufacture of permanent artificial magnets because it has very high hysteresis, that is, the material cannot be easily demagnetized after being magnetized. So-called sweet iron has low hysteresis, so its demagnetization is easy. This material is used in the composition of electromagnets , which must be completely demagnetized when they are turned off.