Question

Draw and explain B-H curve.

Answer 1

• The magnetization curve of magnetic material specimen between the flux density B and the field intensity H is known a B-H curve. It is also known as Hysteresis curve. • The B-H curve can be plotted by increasing and then decreasing the field intensity as shown in figure. The flux density B increases, when external field intensity H applied to it is increased. • When the saturation of flux density arises, the increase in flux density ceases even though the field strength is increased. This is shown by OP. If the external field is gradually reduced, then the original curve Op is not retraced, but follows the curve PQ. • When external field is reduced to zero, the magnetic flux does not reduce to zero, i.e. material remains magnetized. • The value of flux density OQ is called remanant flux density Br (or residual magnetism). In order to demagnetize the material completely, the external magnetic field must be reversed and when it reaches the value OR in reverse direction, it is seen that the flux density is zero. • Further increase of field intensity in the reverse direction will now increase the flux density in reverse direction and again at the point S, the saturation occurs. • The residual magnetism in reverse direction is represented by OT and to neutralize it the magnetic field intensity must be increased in positive direction to the value OU. Further increase in field intensity will again magnetize the material and again saturation will occur at P. • When the magnetic material is taken through one complete cycle of magnetization, it traces the loop that is called hysteresis loop. When a material is subjected to cyclic changes of magnetization, the domains change the direction of their orientation in accordance with field intensity. • The work is done in changing the direction of domain which leads to the production of heat within the material. The energy required to take the material through one complete cycle of magnetization is proportional to the area enclosed by the loop.  

Answer 2

Diagram:

Explanation: B-H curve is obtained while plotting the graph between B & H for a magnetic material while magnetized through one complete cycle of magnetization. The value of magnetizing force H can be increased or decreased by increasing or decreasing the current through the coil. Initially when H is increased from zero to maximum, flux density also increases rapidly at first and reaches maximum saturated (region OA). 

Now when H is decreased, B varies less rapidly along OC. At C, when H = 0, flux density retains some value called residual flux density (Br). Br measures the retentively or remanence of the magnetic material. Now if the current is reversed to magnetize the material in the opposite direction, as H is increased Br decreases till the point D where it becomes zero. The magnetizing force required to be applied in the opposite direction to remove the residual flux density is called coercive force (Hc) and is a measure of coercivity of the material. For a further increase in H the material gets magnetized with opposite polarity reaching saturation at the point E. The loop can be traced back to the point A by varying the current. At F, B = -Br for H = 0 & at G, B = 0 for H = Hc. Thus the closed loop OACDEFGA obtained when the magnetic material is taken through one complete cycle of magnetization is called the hysteresis loop. It is a measure of the energy dissipated due to hysteresis, more the area higher is the hysteresis loss.