Electronics, Magnetism, Materials, Mechanics

Hysteresis: what is it?

The hysteresis is very common on materials and systems. It’s the capacity to keep a property, after remove the stimulus that induces this property.

Hysteresis on magnetism

The motion of electric charges produces a magnetic field. Therefore, the movement of electrons around the atom’s nucleus generates a magnetic field.

atom's magnetic field
I is the electric current and vector \vec{m} is magnetic dipole moment. Source: Maestrovirtuale.

Due to crystalline structure, a ferromagnetic material is divided in many magnetic domains, these are regions where atoms are oriented in a direction.

magnetic domains
When a ferromagnetic material is demagnetized, each domain points to a random direction. Source: Bóson treinamentos.

When a ferromagnetic material is submitted to a magnetic field, material’s atoms are aligned with the external magnetic field. Consequently, the material becomes magnetized, with north and south poles.

magnetized ferromagnetic material
Source: eletronPi.

When all magnetic domains are pointed to direction of external magnetic field, the material reaches the saturation. By that, increasing magnetic flux intensity H, won’t increase flux density B. After the removal of external magnetic field, most of domains keep pointed to same direction and the material keeps magnetized.

hysteresis points
The magnetization process goes to point a, the latter is saturation point. Removing the external magnetic field, the material still has part of magnetic flux density (point b). Applying an external field in opposite direction, the material is demagnetized (point c) and the domains are pointed to opposite direction until saturation (point f). After removing the external field, material still keeps part of it (point e). Source: Electrical Academia.

Each material has a unique hysteresis loop. The equation below is the relation between magnetic field and flux density.

B=\mu H

Where \mu is magnetic permeability, which depends on material.

Some hysteresis examples in other areas


The comparator electronic circuit called Schmitt-Trigger, has the function to change the output digital signal (V_{OUT}) from “LOW” level to “HIGH”, when input voltage (V_{IN}) increases to a determined value (V_{TH}). If output signal is on “HIGH” level, it will go to “LOW” level, when input voltage reaches a determined value (V_{TL}).

The Schmitt-Trigger graphic output voltage x input voltage forms a hysteresis. Source: All about circuits.

Elastic materials

When an elastic material receives an external force, the material can return to its original shape if the force stops. However, if the force is high enough, the material deforms and don’t totally recover its original shape.

elastic material hysteresis
The hysteresis of an ideal elastic material. Loading is when force is applied to material and unloading is gradual reduction of force. Source: (Urban, 2010).

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