Hysteresis

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What is Hysteresis?

Hysteresis happens in a system that involves a magnetic field. It is also the common property of ferromagnetic substances. In general, when the magnetization of ferromagnetic materials lags behind the magnetic field, this effect can be referred to as the hysteresis effect.

The terms “hysteresis” means ”lagging.”  The term, Hysteresis, is characterized as a lag of magnetization intensity (B) behind the intensity of the magnetic field (H).

All the ferromagnetic materials exhibit hysteresis phenomena. To understand the concept in a better way, let us take an instance where a ferromagnetic substance is kept inside a current-carrying coil. Due to the magnetic field, which is present, the substance gets magnetized. If we reverse the direction of the current flow, the substance gets demagnetized, and this process is called Hysteresis.

There are two types of Hysteresis, which can be given as,

  • Rate-dependent hysteresis

  • Rate-Independent hysteresis


Hysteresis Loop

The hysteresis loop indicates the relationship between the intensity of magnetization and the magnetizing field. This loop is generated by measuring the magnetic flux that is coming out from the ferromagnetic substance while changing the external magnetizing field.

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Pointing to the graph, if B is measured for different values of H and if we plot the results in graphic forms, the graph will indicate a hysteresis loop.

  • The intensity of the magnetism (B) increases when the magnetic field (H) is increased from 0 (zero).

  • With the increased magnetic field, there is an increase in the magnetism value and finally reaches to point A which is known as a saturation point where B is constant.

  • With the decrease in the value of the magnetic field, there is a decrease in the magnetism value. But, at point B and H are equal to zero, material or substance retains a few amounts of magnetism, called residual or retentivity magnetism.

  • When there occurs a decrease in the magnetic field towards the negative side, there happens a decrease in magnetism. At point C, the substance is entirely demagnetized.

  • The force that is required to remove the retentivity of the material is called Coercive force (C).

  • The cycle is continued in the opposite direction, where the retentivity point is E, the saturation point is D, and the coercive force is F.

  • Due to the opposite and forward direction process, the cycle is complete, and this cycle is known as the hysteresis loop.


Advantages of Hysteresis Loop

A smaller region of loop hysteresis is a sign of less loss of Hysteresis. The hysteresis loop gives a substance with the importance of coercivity and retentivity. Therefore, selecting the way of the right material to make a permanent magnet is made simpler by the heart of machines.

Residual magnetism can be calculated by the B-H graph, therefore, it is simple to choose material for electromagnets.


Energy Loss Due To Hysteresis

The best example of knowing energy loss due to Hysteresis is a transformer, and as we know that during the magnetization and demagnetization process, the energy is required.

During the magnetization and demagnetisation cycle of magnetic substances, some energy will be spent. This spent energy appears in the form of heat, and this heat loss is known as hysteresis loss.

Energy loss per unit volume of the substance = Area of the hysteresis curve

In Transformers, the energy is lost continuously in the form of heat because of the continuous magnetization and demagnetisation process. Due to this, the energy loss efficiency of the transformer gets reduced.

To stop this energy loss, a soft iron core is used in the Transformers because the hysteresis loss or the energy loss in the case of soft iron is much smaller than other materials.


Difference Between Soft And Hard Magnetic Materials

Soft Magnet

Hard Magnet

Magnetization and demagnetization process is easy.

Magnetization and demagnetization process is difficult.

A soft magnet can be formed by heating and then cooling gradually.

A hard magnet can be formed by heating and then cooling suddenly.

The hysteresis loop area is small, and the retentivity and coercivity are also small.

The hysteresis loop area is large, whereas the retentivity and coercivity are also high.

These are temporary magnets.

These are permanent magnets.

Examples are Ferrites Garnet, Ferrous-nickel alloy.

Examples are tungsten, Steel, carbon steel, and chromium steel.


Soft Iron vs. Steel

  • Soft iron can magnetize and demagnetize easily compared to Steel

  • Retentivity of soft iron is more to that of the retentivity of Steel

  • The coercivity of steel is more compared to the coercivity of soft iron

  • Because of the small area, the energy loss in soft iron is less compared to the energy loss in steel

  • The area of the loop in case of soft iron is less than that of steel

  • Both I and χ are high in soft iron, whereas both are low in steel

  • Soft irons are used in electromagnetic tapes, transformers, tape recorders, and many more

  • Magnetic permeability is high in soft iron to that of steel

FAQ (Frequently Asked Questions)

1. Explain Magnetization and Demagnetization?

Ans: The method of developing the magnetic properties inside a magnetic substance is called magnetization; any of the magnetic substances can be magnetized using an electric current or by touching with a strong magnet.


In simple words, if we put any magnetic substance in the external magnetizing field, then the corresponding material gets magnetized, and if we reverse the direction of the external magnetizing field, then the material or substance gets demagnetized.


If ferromagnetic materials are located inside a current-carrying coil, the magnetizing field H is caused by the current forces, a few or all the atomic magnetic dipoles present in the material to align with the external magnetizing field. In this way, the material gets magnetized.

2. Explain Retentivity and Coercivity?

Ans: If a ferromagnetic material gets magnetized by applying the external magnetizing field, if we remove the external magnetizing field after magnetization, the material will not relax back to its zero magnetization position.


  • Retentivity

The magnetization amount available when the external magnetizing field is removed is called retentivity.

A material is able to retain some amount of magnetic property while an external magnetizing field is removed.

The value of B at point b present in the hysteresis loop.


  • Coercivity

The reverse (-ve H) external magnetizing field amount required to completely demagnetize the substance is called coercivity of substance.

The value of H at point c present in the hysteresis loop.