Hysteresis
Consider an iron bar being
magnetised slowly by a magnetising field H whose strength can be changed. It is
found that the magnetic induction B inside the material increases with the
strength of the magnetising field and then attains a saturated level. This is
depicted by the path OP in the Fig.
If the magnetising field is now
decreased slowly, then magnetic induction also decreases but it does not follow the path PO. Instead,
when H = 0, B has non zero value equal to OQ. This implies that some magnetism
is left in the specimen. The value of
magnetic induction of a substance,
when the magnetising field is reduced to zero, is called remanance or residual
magnetic induction of the material. OQ represents the residual magnetism of the material. Now, if we apply the
magnetising field in the reverse direction, the magnetic induction decreases
along QR till it becomes zero at R. Thus to reduce the residual magnetism
(remanent magnetism) to zero, we have to apply a magnetising field OR in the
opposite direction.
The value of the magnetising field H which has to be
applied to the magnetic material in the reverse direction so as to reduce its
residual magnetism to zero is called its coercivity.
When the strength of the
magnetising field H is further increased in the reverse direction, the magnetic
induction increases along RS till it acquires saturation at a point S (points P
and S are symmetrical). If we now again change the direction of the field, the
magnetic induction follows the path STUP. This
closed curve PQRSTUP is called the ?hysteresis loop? and it represents a cycle of magnetisation. The word
?hysteresis? literally means lagging
behind. We have seen that magnetic induction B lags behind the magnetising
field H in a cycle of magnetisation. This
phenomenon of lagging of magnetic
induction behind the magnetising field is called hysteresis.
Hysteresis loss
In the process of magnetisation
of a ferromagnetic substance through a cycle, there is expenditure of energy.
The energy spent in magnetising a specimen is not recoverable and there occurs
a loss of energy in the form of heat. This is so because, during a cycle of
magnetisation, the molecular magnets in the specimen are oriented and
reoriented a number of times. This molecular motion results in the production
of heat. It has been found that loss of
heat energy per unit volume of the
specimen in each cycle of magnetisation is equal to the area of the hysteresis
loop.
The shape and size of the hysteresis loop is characteristic of
each material because of the differences in their retentivity, coercivity,
permeability, susceptibility and energy losses etc. By studying hysteresis
loops of various materials, one can select suitable materials for different
purposes.
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