Magnetic materials are substances that exhibit magnetic properties, meaning they can be attracted to or repelled by a magnet.
These materials are characterized by the presence of microscopic magnetic domains, where atomic magnetic moments align in a specific direction. Common examples include iron, cobalt, and nickel.
Introduction of Magnetic Materials- Ferromagnetic & Ferrimagnetic
- Magnetism, the ability to attract or repel some other material, occurs in all materials to some extent.
- The materials which provide a path to the magnetic flux and can be magnetized are called magnetic materials.
- The degree of occurrence can be described in five classes called diamagnetism, paramagnetism, ferrimagnetism, ferromagnetism and anti-ferromagnetism.
- Ferrimagnetism and ferromagnetism are usually strong enough to be perceived as magnets. The other three are weak and detectable with sophisticated scientific instruments.
Ferromagnetic Materials
The materials which possess magnetism in the absence of applied magnetic field is known as Ferromagnetic materials.
E.g. Iron, cobalt, nickel, cadmium
Ferrimagnetic Materials
The materials whose net magnetic moment is not zero because the antiparallel orientation spin of neighboring atom is present even in the absence of external field is known as ferrimagnetic materials.
E.g. Ferrites
B-H Curve (Magnetic Hystseresis Curve)
The graph plotted between values of flux density, (B) against the field strength, (H) produces a set of curves calledMagnetisation Curves,Magnetic Hysteresis Curvesor more commonlyB-H Curves.
TheMagnetic Hysteresisloop above, shows the behavior of a ferromagnetic core graphically as the relationship between B and H is non-linear. Starting with an unmagnetised core both B and H will be at zero, point 0 on the magnetisation curve.
If the magnetisation current, i is increased in a positive direction to some value the magnetic field strength H increases linearly with i and the flux density B will also increase as shown by the curve from point 0 to point a as it heads towards saturation.
Now if the magnetising current in the coil is reduced to zero the magnetic field around the core reduces to zero but the magnetic flux does not reach zero due to the residual magnetism present within the core and this is shown on the curve from point a to point b.
To reduce the flux density at point b to zero we need to reverse the current flowing through the coil. The magnetising force which must be applied to null the residual flux density is called a “Coercive Force”.
This coercive force reverses the magnetic field re-arranging the molecular magnets until the core becomes unmagnetised at point c. An increase in the reverse current causes the core to be magnetised in the opposite direction and increasing this magnetisation current will cause the core to reach saturation but in the opposite direction, point d on the cure which is symmetrical to point b.
If the magnetising current is reduced again to zero the residual magnetism present in the core will be equal to the previous value but in reverse at point e.
Again, reversing the magnetising current flowing through the coil this time into a positive direction will cause the magnetic flux to reach zero, point f on the curve and as before increasing the magnetisation current further in a positive direction will cause the core to reach saturation at point a.
Then the B-H curve follows the path of a-b-c-d-e-f-a as the magnetising current flowing through the coil alternates between a positive and negative value such as the cycle of an AC voltage. This path is called a Magnetic Hysteresis Loop.
Hard & Soft Magnetic Material
- Soft materials exhibit high permeability. They cannot store large amount of magnetic energy. They have negligible coercive force. They have low hysteresis loss and low electrical resistivity.
- Hard magnetic materials possess high value of energy product i.e. BH value They have high retentivity and high coercivity. They exhibit low initial permeability and high hysteresis loss.
S.N. | Hard Magnetic Materials | Soft Magnetic Materials |
1 | Materials which retain their magnetism and are difficult to demagnetize are called hard magnetic materials. | Soft magnetic materials are easy to magnetize and demagnetize. |
2 | They have large hysteresis loss due to large hysteresis loop area. | They have low hysteresis loss due to small hysteresis area. |
3 | Susceptibility and permeability are low. | Susceptibility and permeability are high. |
4 | Coercivity and retentivity values are large. | Coercivity and retentivity values are less. |
5 | Magnetic energy stored is high. | Magnetic energy stored is less. |
6 | They possess high value of BH product. | Since they have low retentivity and coercivity, they are not used for making permanent magnets |
7 | The eddy current loss is high. | The eddy current loss is less because of high resistivity. |
8 | These materials are used for making permanent magnets. | These materials are used for making temporary magnets. |
Some Definitions (Concepts)
- Hysteresis: The phenomenon by virtue of which intensity of magnetisation lags behind the magnetising field, when a magnetic substance is taken through a complete cycle of magnetisation, is called hysteresis.
- Permeability: The measure of degree to which the magnetic lines of force can penetrate into a substance is called permeability of substance. Numerically, it is defined as ratio of magnetic induction B to the magetising field H.
- Coercive force: In order to reduce the residual intensity of magnetisation to zero, a magnetic field has to be applied in the opposite direction. This value of reverse magnetising field is called the coercivity or coercive force for the sample.
- Reluctivity: The reciprocal of permeability is called reluctivity.
Losses in Magnetic Materials
[A] Hysteresis loss
Hysteresis loss occurs due to magnetization and demagnetization. It can be seen in the graph of theBfield versus theHfield for the material, which has the form of a closed loop.
The amount of energy lost in the material in one cycle of the applied field is proportional to the area inside the hysteresis loop. Hysteresis loss increases with higher frequencies as more cycles are undergone per unit time.
[B] Eddy current loss
The induction of eddy currents within the core causes a resistive loss. Higher the resistance of the core material lowers the loss. Lamination of the core material can reduce eddy current loss, as can making the core of a nonconductive magnetic material, like ferrite.
[C] Iron loss
Iron losses are due to vibration of core. Some electrical energy is lost in the form of mechanical energy to produce vibration in the core.
Faradays Law of Electromagnetic Induction
Faraday’s First Law
It states that, Whenever a conductor cuts a magnetic field or viceversa an emf is induced in it and it setsup in such a direction so as to oppose the cause of it.
Faraday’s second law
It states that the magnitude of induced emf is equal to the rate of change of flux linkage.
Mathematically
e = -NdØ/dt
e = induced emf
N= number of turns of coil
dØ/dt = rate of change of flux
The minus sign represents that the induced emf or current sets up in a direction so as to oppose the cause of it.
Self & Mutual Induced emf
- If an emf is induced in a coil due to the current flowing through itself is called self-induced emf. On the other hand, if an emf is induced on another coil due to the current flowing through the previous coil then it is called mutual induced emf.
- Suppose there are two coils A and B. A current is flowing through coil A.
Now if the flux produced due to this current induce an emf on the same coil A, then it is self-inductance, and if it produces emf on B, then it is mutual inductance due to coil A.
