Hysteresis refers to the lagging of the magnetization of a ferromagnetic material like iron. If we place the ferromagnetic materials within a coil of wire carrying an electric current, then the magnetic field which is created by the current causes all of the atomic magnets in the material to align with the field. As a result of this alignment, the total magnetic field increases. This alignment does not occur instantaneously, but in fact lags behind it.
The word hysteresis is also used to describe rate-independent state. It implies that if a certain set of inputs x(t) produce an output Y(t), then the inputs X(αt) produce output Y(αt) for any α > 0.
For ferromagnetic materials, by removing external magnetic field i.e. H = 0, the magnetic moment of some domains remain aligned in the applied direction of previous magnetizing field which results into a residual magnetism. The lack of retracibility as shown in figure is called hysteresis and the curve is known as hysteresis loop.
So, the process of demagnetising a material completely by applying magnetising field in a negative direction is defined as Coercivity. Coercivity assesses the softness or hardness of a magnetic material. Coercivity signifies magnetic hardness or softness of substance:
Magnetic hard substance (steel) ——> High coercvity
Magnetic soft substance (soft iron) ——> Low coercivity
Thus complete cycle of magnetisation and demagnetisation is represented by BCDEFGB
We now discuss the hysteresis loop in detail:
The figure depicts the hysteresis loop. The loop is created by measuring the magnetic flux of a ferromagnetic material while the magnetizing force is changed. If there is a ferromagnetic material which is completely demagnetized or has never been magnetized will follow the dotted line as H is increased. The line clearly shows that greater the amount of current applied (H+), the stronger is the magnetic field in the component (B+). Most of the magnetic forces are aligned at point ‘a’ and any increase in the magnetizing force will result in a slight increase in magnetic flux. This is the stage when the level of magnetic saturation has been attained by the material.
- The curve moves from point ‘a’ to point ‘b’ when H is reduced to zero. At this point, one can notice that even though the magnetic force has become zero, some quantity of magnetic flux still remains in the material. This is referred to as the point of retentivity on the graph and indicates the remanence or level of residual magnetism in the material.
- Point ‘c’ on the curve denotes the state where the flux has been reduced to zero and the curve reaches this point on reversal of the magnetizing force. This point is termed as the point of coercivity on the curve.
Remark: Coercive material is the force required to remove the residual magnetism from the material.
- Now we come at point‘d’. When the magnetizing force is increased in the negative direction, the material will again reach the stage of saturation but in the opposite direction.
- Reducing H to zero brings the curve to point ‘e’. The level of residual magnetism at this stage is equal to the amount attained in the other direction. If H is again increased in the positive direction, B will be returned to zero.
- It is important to note here that the curve did not return to the origin of the graph because some kind of force is required to remove the residual magnetism. The curve now assumes a different path from point ‘f’ back to the saturation point where the loop is completed.
We can derive various magnetic properties of a material from the hysteresis loop. Some prime ones include:
(i) Retentivity: The ability of a material to retain some quantity of residual magnetic field when on achieving the stage of saturation, the magnetizing force is removed is called retentivity of the material. This is depicted by the value Bat point b on the curve.
(ii) Residual Magnetism or Residual Flux: It is the amount of magnetic flux density that remains in the material when the magnetizing force is reduced to zero. At saturation level, both retentivity and residual magnetism are the same but before that residual magnetism may be lower than the retentivity.
(iii) Coercive force: In order to make magnetic flux equal to zero, the amount of reverse magnetic field which is required to be applied to the magnetic material is called the coercive force.
(iv) Permeability: The property that explains the effortlessness with which a magnetic flux is established in the component.
(v) 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.
Click here for more on hysteresis loop.
The hysteresis loop is an important topic of the IIT JEE Physics syllabus. Though it is quite confusing but, it is important to attain expertise in this topic as questions are often framed on this topic in the JEE. Askiitians offers extensive study material on the topic which can help you in grasping the concepts easily.