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What are coercivity and retentivity?


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Solution

Hysteresis loop:

  1. The magnetic flux density and magnetizing field strength are represented by the hysteresis loop. This loop is typically created by monitoring the magnetic flux emitted by the ferromagnetic material while the outside magnetizing field is changed.
  2. The meaning of hysteresis is ”lagging”. It is characterized as a lag of magnetic flux density (B) behind the magnetic field strength (H).
  3. 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.

Coercivity:

  1. The smallest amount of magnetizing strength is necessary to return the component to its former condition.
  2. Coercivity, also called magnetic coercivity, coercive field, or coercive force is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized.
  3. Coercivity is usually measured in oersted or ampere/meter units and is denoted HC.

Retentivity:

  1. The capacity of a magnetic field to stay in a substance after the external source has been removed is called retentivity. It provides information on the magnetic strength of such a substance.
  2. Retentivity is the residual magnetization displayed by materials whenever the applied magnetic field is reduced to zero after reaching the saturating level of magnetization.

Consider the graph above, 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.

  1. The magnetic flux density (B) is increased when the magnetic field strength (H) is increased from 0 (zero).
  2. With increasing the magnetic field there is an increase in the value of magnetism and finally reaches point A which is called the saturation point where B is constant.
  3. 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.
  4. When there is a decrease in the magnetic field towards the negative side, magnetism also decreases. At point C the substance is completely demagnetized.
  5. The force required to remove the retentivity of the material is known as Coercive force (C).
  6. In the opposite direction, the cycle is continued where the saturation point is D, retentivity point is E and coercive force is F.
  7. Due to the forward and opposite direction process, the cycle is complete and this cycle is called the hysteresis loop.

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