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Hysteresis

Hysteresis occurs in a system that involves a magnetic field. Hysteresis is the common property of ferromagnetic substances. Generally, when the magnetization of ferromagnetic materials lags behind the magnetic field this effect can be described as the hysteresis effect.

What is Hysteresis?

Definition: The meaning of hysteresis is”lagging”.  Hysteresis is characterized as a lag of magnetic flux density (B) behind the magnetic field strength (H). 

 All ferromagnetic materials exhibit the phenomena of hysteresis. To give you a better understanding of the concept, we will take an instance where a ferromagnetic substance is placed inside a current-carrying coil. Due to the magnetic field that is present the substance gets magnetized. If we reverse the direction of current then the substance gets demagnetized, this process is known as hysteresis.

Systems that show hysteresis are usually nonlinear. So this can be mathematically challenging to some hysteretic models such as the Preisach model and the Bouc–Wen model. Additionally, there are models such as phenomenological models for specific phenomena such as the Jiles–Atherton model that is used to describe ferromagnetism.

Types of Hysteresis

There are two types of hysteresis;

  • Rate-dependent hysteresis: In this types of hysteresis there is a lag between input and output. We can take the example of a sinusoidal input X(t) resulting in a sinusoidal output Y(t), there is a phase lag φ:
    \(\begin{array}{l}{\displaystyle {\begin{aligned}X(t)&=X_{0}\sin \omega t\\Y(t)&=Y_{0}\sin \left(\omega t-\varphi \right).\end{aligned}}}\end{array} \)
  • Rate-independent hysteresis: This hysteresis found in systems tend to have a persistent memory of the past that still remains even after the transients have disappeared.

Hysteresis Loop

The hysteresis loop shows the relationship between the magnetic flux density and the magnetizing field strength. The loop is generated by measuring the magnetic flux coming out from the ferromagnetic substance while changing the external magnetizing field.

Hysteresis Loop

 

Looking at the graph, if B is measured for various values of H and if the results are plotted in graphic forms then the graph will show a hysteresis loop.

  • The magnetic flux density (B)  is increased when the magnetic field strength(H) is increased from 0 (zero).
  • With increasing the magnetic field there is an increase in the value of magnetism and finally reaches point A which is called saturation point where B is constant.
  • With a decrease in the value of the magnetic field, there is a decrease in the value of magnetism. But at B and H are equal to zero, substance or material retains some amount of magnetism is called retentivity or residual magnetism.
  • When there is a decrease in the magnetic field towards the negative side, magnetism also decreases. At point C the substance is completely demagnetized.
  • The force required to remove the retentivity of the material is known as Coercive force (C).
  • In the opposite direction, the cycle is continued where the saturation point is D, retentivity point is E and coercive force is F.
  • Due to the forward and opposite direction process, the cycle is complete and this cycle is called the hysteresis loop.

Advantages of Hysteresis Loop

1. A smaller region of the hysteresis loop is indicative of less loss of hysteresis. 

2. Hysteresis loop provides substance with the importance of retentivity and coercivity. Therefore the way to select the right material to make permanent magnet is made simpler by the heart of machines.

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

Retentivity and Coercivity

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

Retentivity

The amount of magnetization present when the external magnetizing field is removed is known as retentivity.

  • It is a material’s ability to retain a certain amount of magnetic property while an external magnetizing field is removed.
  • The value of B at point b in the hysteresis loop.

Coercivity

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

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

Hysteresis Applications

Hysteresis can be mostly found in Chemistry, Physics, Engineering, Economics and Biology. Common examples further include magnetic hysteresis, ferroelectric hysteresis, superconducting hysteresis,  mechanical hysteresis, optical hysteresis, electron beam hysteresis, adsorption hysteresis, economic hysteresis, etc. In any case, we will look at some of the important uses of hysteresis.

  • Several applications of hysteresis are found in ferromagnets. It is mostly used to retain a memory, for example, hard disks, magnetic tape, and credit cards.
  • Hysteresis is applied in many artificial systems such as in thermostats and Schmitt triggers which are designed to prevent unwanted frequent or unwanted rapid switching.
  • A hysteresis is sometimes intentionally made part to computer algorithms.
  • Hysteresis can be observed when decreasing the angle of attack of a wing after stall, regarding the lift and drag coefficients.
  • The existence of the bubble shape hysteresis has important consequences in interfacial rheology experiments involving bubbles.
  • In Biology, it is found in Cell biology and genetics, Immunology, Neuroscience, Respiratory physiology, Voice and speech physiology, Ecology and Epidemiology.

In essence, hysteresis is encountered in many different areas of science and has a lot of uses.

Energy Loss due to Hysteresis

  • A transformer is the best example of studying energy loss due to hysteresis, as we know that during the process of magnetization and demagnetization energy is required.
  • During the cycle of magnetization and demagnetization of magnetic substances, energy is spent and this is spent energy appears in the form of heat. This heat loss is known as hysteresis loss.
  • The loss of energy per unit volume of the substance is equal to the area of the hysteresis curve
  • In transformers due to the continuous process of magnetization and demagnetization energy is lost in the form of heat continuously, due to this energy loss efficiency of the transformer gets reduced.
  • To stop this energy loss soft iron core is used in transformers because the energy loss or hysteresis loss in the case of soft iron is much smaller than other materials.

Difference between the soft magnet and hard magnet

Soft Magnet

Hard Magnet
Magnetization and demagnetization is easy Magnetization and demagnetization is difficult
A soft magnet can be produced by heating and gradual cooling A hard magnet can be produced by heating and sudden cooling
The hysteresis loop area is small, retentivity and coercivity is also small The hysteresis loop area is large, retentivity and coercivity is also high
Soft magnets are temporary magnets Hard magnets are permanent magnets
Examples: Ferrous-nickel alloy, Ferrites Garnets Examples: Steel, carbon steel, chromium steel, tungsten

Soft Iron vs Steel

  • The retentivity of soft iron is more than the retentivity of Steel.
  • Soft iron can easily magnetize and demagnetize compared to Steel.
  • The coercivity of steel is more than the coercivity of soft iron.
  • Area of the loop in the case of soft iron is less than the area of the loop in steel.
  • Due to the small area energy loss in soft iron is less than energy loss in steel.
  • I and χ both are high in soft iron whereas in steel both are low.
  • Magnetic permeability is high in soft iron compared to steel.
  • Soft irons are used in transformers, electromagnetic tapes and tape recorders, etc..
  • The steel is used to make permanent magnets.

Magnetization and Demagnetization

The method of developing magnetic properties inside a magnetic substance is known as magnetization. Any magnetic substance can be magnetized with the help of an electric current or by touching with a strong magnet.

  • In simple language, if we put any magnetic substance in the external magnetizing field then the material gets magnetized and if we reverse the direction of the external magnetizing field then the material gets demagnetized.
  • When ferromagnetic materials are placed inside a current-carrying coil the magnetizing field H caused by the current forces some or all the atomic magnetic dipoles in the material to align with the external magnetizing field, in this way material gets magnetized.

Frequently Asked Questions on Hysteresis

In magnetic materials, what is hysteresis?

When an external magnetic field is introduced to a ferromagnet substance like iron, the atomic dipoles align themselves with it, this is known as magnetic hysteresis. Part of the alignment will be kept even if the field is removed; the substance has become magnetic. The magnet will remain magnetised indefinitely once it has been magnetised.

How can hysteresis be reduced?

Hysteresis losses can be reduced by selecting materials with the smallest hysteresis loop area. Materials like silicon steel or high-grade steel are used with a small hysteresis loop area is used to make the core of the electrical machines.

What is retentivity?

Retentivity is the property of a magnetic substance to retain magnetism in the absence of a magnetising field.

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