Hysteresis

What is 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.

Definition: The meaning of hysteresis is”lagging”.  Hysteresis is characterized as a lag of magnetization intensity (B) behind the magnetic field intensity (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.

There are two types of hysteresis;

  • Rate-dependent hysteresis
  • Rate-Independent hysteresis

Hysteresis Loop

The hysteresis loop shows the relationship between the intensity of magnetization and the magnetizing field. 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 intensity of the magnetism (B)  is increased when the magnetic field (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 loop hysteresis 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.

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 spent, 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 = 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

  • 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 case of soft iron is less than the area of the loop in steel.
  • Due to 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 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.