## Hysteresis

When there is a change in the magnetic field in a system then Hysteresis occurs. Hysteresis is a property of a ferromagnetic material by which the magnetism in the material lags behind the magnetic field.

## Hysteresis Loop

The Hysteresis curve or B-H curve shows the relationship between the intensity of magnetism(B) and magnetic field (H). The relation between B and H in ferromagnetic materials is complex. It is often not linear and it depends on the magnetic history of the sample. To find the B-H curve of the ferromagnetic material we should take the demagnetised form of the material, in the shape of the rod. We should place it in a solenoid and increase the current through the solenoid.

The magnetism(B) in the material rises when the magnetic field(H) is increased from zero and reaches the saturation point. Now if the magnetic field is reduced to zero, the curve does not retrace the path. It is pointless to increase the current (and hence the magnetic intensity H) beyond this. Next, we decrease H and reduce it to zero. At H = 0, B ≠ 0 the substance retains some of the value of magnetism. The value of B at H = 0 is called retentivity or remanence. The domains are not completely randomised even though the external driving field has been removed.

Next, the current in the solenoid is reversed and slowly increased. The value of magnetism decreases and becomes zero at a certain value of H, which is called the coercivity of the material. If the value of the magnetic field (H) is further increased the material gets magnetised in the opposite direction and gets saturated. As the reversed current is increased in magnitude, we once again obtain saturation. If the current is increased further the hysteresis curve completes the cycle. The hysteresis curve is used to study the behaviour of the materials and its magnetic properties.

## Importance of Hysteresis loop

1. If the area of the hysteresis loop is small it means that the hysteresis loss is small. The hysteresis area of silicon steel is very small, for this reason it has been used in the making of transformer core and rotating machines.
2. Hysteresis loop provides the information about the retentivity and coercivity of the material. Therefore the materials with high value of retentivity and coercivity are used in the making permanent magnet.

## Hysteresis loss

Energy is given to the ferromagnetic material during magnetisation, but during demagnetisation the material will not release all the energy it received.Some of the energy is lost.This loss of energy per cycle is called hysteresis loss. This energy gets converted into heat energy.