If H is
now decreased gradually, flux density B will not decrease along AO, as might be
excepted, but will decrease less rapidly along AB. When H = 0, B is not but has
a finite value Br = OB. It means that on
removing the magnetizing force H, the material is not completely demagnetize. Br = OB is the measure of retentivity or
remanence of the material. This is called residual magnetism.
When H is reversed, then B is reduced to zero
at point C where H = OC. This value of H required to wipe off residual
magnetism is known as coercive force and is a measure of the coercivity
of the material.
After the magnetization has been
reduced to zero, value of H is further increased in the reverse direction, the
material again reaches a state of magnetic saturation, represented by point L.
By taking back from its value corresponding to negative saturation (= OL) to
its value for positive saturation (= OM), a similar curve DEFA is obtained.
The lagging of B behind H is given
by the name ‘hysteresis’, which literally means ‘to lag behind’. The
closed loop ABCDEFA is called hysteresis loop.
Let us consider there is a unit
volume of specimen with N elementary magnets. M is magnetic moment of each
elementary magnet and the axis of molecular magnet be inclined at an angle q with the
magnetizing field H.
The component of total magnetic
moment per unit volume, parallel to field = SMcosq = I, and the component perpendicular to
field = SMsinq = 0. If I is
increased by an amount dI, then magnetic moment is increased by dSMcosq=dI
Þ SM(-sinq)dq = dI ……….(1)
The work
done on all molecular magnets
dW = - m0SMHsinqdq
Þ dW = m0HdI (from
eq.(1))
work done
in a complete cycle
Þ 
Hence, the work done per unit volume of the material per
cycle is equal to m0 times the area of I-H loop and is dissipated
in the form of heat.
Q.7 Show that hysteresis loss is equal to the
area of its loop. (AKTU. 2005 - 06)
Related
Questions -
Q. Show that the area of this curve is
equal to the hysteresis loss in each cycle. (AKTU. 2010 - 11)
