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HYSTERISIS CURVE - ( B- H CURVE)

  HYSTERESIS CURVE ( I – H CURVE)
or B-H CURVE

Hysteresis is the phenomenon of lagging intensity of magnetization with external field during the process of magnetization of a Ferro specimen.

The graph which is drawn between intensity of magnetization ( I ) and the external field H during the process of magnetization of a ferromagnetic specimen is known as HYSTERESIS CURVE OR I-H CURVE.
We can draw the graph for magnetic flux density (B) developed inside the Ferro specimen instead of I.
Hence it is also called B-H curve.
In this process a ferromagnetic specimen is placed in an external magnetic field. The field strength H can be varied. The intensity of magnetization ( I ) or the magnetic flux density ( B )developed with in the Ferro specimen is observed by varying the external field H.

 1. In this process the external field is first increased it is observed that the intensity of magnetization of Ferro specimen is also increased. 
At a point of external field H, the value of I gets saturated.
2. Then the external field H is decreased gradually and it is observed that magnetization intensity I also decreases.

 3. Then the filed ‘H’ is reverse and the process is continued. The intensity of magnetization ‘I’ is observed at all points.
Graph
A graph is plotted between external field H (on X-axis)

And intensity of magnetization I or flux density B (on y-axis)



Analysis of B-H graph:

1.It can be observed from the graph that the value of magnetic flux (or intensity of magnetization) increases with external field H, (The path OA in the graph).

2.At the point A the value of B (or I) gets saturated.

 3. Then the external field is reversed and observed that magnetic flux inside the Ferro specimen is decreased but not in the direction of OA. 
The magnetic flux B (or I) decreases along the path AB.

 4.At a point (point ‘B’) when the external filed becomes zero, the Ferro magnetic specimen has an amount of magnetic flux inside it.
The intensity of magnetization I or the magnetic flux density B inside the Ferro specimen
Which remains when the external field is zero is defined as ‘RETENTIVITY’.

 5.Now the external field is reversed and increased (the path BC)

 6. It is observed that the magnetic flux becomes zero at a value of external field(point‘C’)

 7. The reverse external field where the intensity of magnetization (or magnetic flux) of Ferro specimen is destroyed is defined as ‘COERCIVITY’.
The process is completed by increasing the filed until the specimen reaches to its initial stage (path DEFA).
The loss of energy during Hysteresis:

When a Ferro magnetic substance is magnetized in a cyclic process, there is a loss of energy. The reason is the Ferro specimen consists random alignment of dipoles in the domains.
 When it is placed in an external field and magnetized, the magnetic dipoles try to align along the direction. In this process an amount of energy is utilized. 
When the external filed id removed the energy initially used to magnetize the Ferro specimen is not recover completely. 
The reason is an amount of magnetic flux is retained with in the Ferro specimen that we call it as Retentivity . 
Therefore there is always a loss in energy during Hysteresis cycle.
Applications hysteresis curve:

Hysteresis curve is used to select a suitable material to prepare prepare permanent magnet.
 Permanent magnets are used in the devices like ammeter, voltmeter and galvanometer.

 2. Permanent magnets are also used in loud speakers, telephones and micro phones.

 3. A material used to prepare electromagnetic cores must have strong flux density with low external field. These are used in transformers, dynamos, chokes etc.
With hysteresis curve we can select the material by observing the values of Retentivity and Coercivity.
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