Hysteresis
the change in magnetism of a body lags behind the changes in the magnetic field.
Many physical systems naturally exhibit hysteresis. Hysteresis phenomena occur in
magnetic materials, ferromagnetic materials and ferroelectric materials, as well as
in the elastic, electric, and magnetic behavior of materials, in which a lag occurs
between the application and the removal of a force or field and its subsequent effect.
Different types of hysterisis are:
Electric hysteresis occurs when applying a varying electric field
elastic hysteresis occurs in response to a varying force.
magnetic hysteresis occurs in response to a varying magnetic force.
Hysteresis in ferromagnetic materials:
When a ferromagnetic material is magnetized in one direction, it will not relax back to
zero magnetization when the imposed magnetizing field is removed. It must be driven
back to zero by a field in the opposite direction. If an alternating magnetic field is applied
to the material, its magnetization will trace out a loop called a hysteresis loop. The lack
of retraceability of the magnetization curve is the property called hysteresis and it is
related to the existence of magnetic domains in the material. Once the magnetic
domains are reoriented, it takes some energy to turn them back again.
Hysteresis Loop
A hysteresis loop shows the relationship between the induced magnetic flux density B
and the magnetizing force H.
The loop is generated by measuring the magnetic flux of a ferromagnetic material while
the magnetizing force is changed. A ferromagnetic material that has never been
previously magnetized or has been thoroughly demagnetized will follow the dashed line
as H is increased. the greater the amount of current applied (H+), the stronger the
magnetic field in the component (B+). At point "a" almost all of the magnetic domains
are aligned and an additional increase in the magnetizing force will produce very little
increase in magnetic flux. The material has reached the point of magnetic saturation.
When H is reduced to zero, the curve will move from point "a" to point "b." At this point,
it can be seen that some magnetic flux remains in the material even though the
magnetizing force is zero. This is referred to as the point of retentivity on the graph and
indicates the remanence or level of residual magnetism in the material. Some of the
magnetic domains remain aligned but some have lost their alignment.
As the magnetizing force is reversed, the curve moves to point "c", where the flux has
been reduced to zero. This is called the point of coercivity on the curve. The reversed
magnetizing force has flipped enough of the domains so that the net flux within the
material is zero. The force required to remove the residual magnetism from the material
is called the coercive force or coercivity of the material.
As the magnetizing force is increased in the negative direction, the material will again
become magnetically saturated but in the opposite direction (point "d"). Reducing H to
zero brings the curve to point "e." It will have a level of residual magnetism equal to that
achieved in the other direction. Increasing H back in the positive direction will return B
to zero. Notice that the curve did not return to the origin of the graph because some force
is required to remove the residual magnetism. The curve will take a different path from
point "f" back to the saturation point where it with complete the loop.
magnetic properties of a material
from hysteresis loop
1. Retentivity it is a material's ability to retain a certain amount of residual
magnetic field when the magnetizing force is removed after achieving
saturation.The value of B at point b on the hysteresis curve.
2 . Residual Magnetism - the magnetic flux density that remains in a material when the
magnetizing force is zero. the residual magnetism and retentivity are the same when the
material has been magnetized to the saturation point.
3. Coercive Force - The amount of reverse magnetic field which must be applied to
a magnetic material to make the magnetic flux return to zero. The value of H at point
c on the hysteresis curve.
4. Permeability, µ - A property of a material that describes the ease with which a
magnetic flux is established in the component.
5 Reluctance - Is the opposition that a ferromagnetic material shows to the
establishment of a magnetic field. Reluctance is analogous to the resistance in an
electrical circuit.
Application of Hysteresis:
A hysteresis is to design different algorithms in all fields i.e in economics,
physics and neuclear physics and in comp. programing hystrisis has many
applications. In our electronics field hysterisis also had many applications in
designing different control systems , Relays , RLC circuits, schmit triggers
circuits and in interfacing.
