When a magnetizable material is magnetized by applying an external magnetic field, the magnetic flux density inside the material increases non-linearly. The shape of the curve is as shown in the figure. This curve showing the variation of developed magnetic flux density (B) vs the applied magnetizing field (H) is called hysteresis loop.
As the applied magnetic field (H) is increased, the magnetic flux density (B) in the material increase from o to b along a and attains saturation at b. On decreasing H to zero, it is observed that B decreases along the path bc and not bo. Thus the material retains some magnetic properties even in the absence of H, this value of B is called retentivity. On further decreasing the value of H, B becomes zero at a particular negative value of H. This value of H (od in the curve) is called coercivity of the material. Further decrease in H saturates the value of B at point e. If the magnetizing field (H) is now increased, it follows the path efgb, thus forming a loop curve. This loop is called hysteresis loop.
Retentivity: The magnetism retained by the magnetic material even when the magnetizing field is reduced to zero is called the retentivity of the material.
Coercivity: The amount of reverse magnetic field required to reduce the magnetic flux density in a magnetic material to zero is called coercivity.
- Properties of permanent magnets:
- High magnetic permeability
- High coercivity
- High retentivity
- Soft iron core is used as electromagnets because it has high magnetic permeability and low retentivity.