CLASSIFICATION OF MAGNETIC MATERIALS | Physics class 12 notes

  

What is magnetic materials and its types? magnetic properties of materials class 12 physics is most important topic of NCERT  Magnetic effect of electric current chapter  in Class 12.  Questions are frequently asked in the CBSE  board ,   ICSE Board and other competitive exam ( IIT JEE, NEET,  AIIMS, State Engineering exam )  from  Physics classification of magnetic materials  .

“magnetic materials class 12 notes “ will be very beneficial for the students who are engaged in the preparation of  upcoming board exam.

In this  topic, the following terms will be illustrated.

Contents :

* 1. IMPORTANT TERMS USED IN MAGNETISM : Magnetic flux density (B), Absolute Magnetic permeability ( μ ), Relative permeability, Magnetizing force or Magnetic intensity (H), Intensity of magnetisation (I),

* 2. Magnetic susceptibility ( Xm ) :  Relation Between  μr  and  Xm

* 3. CLASSIFICATION OF MAGNETIC MATERIALS

* 4. DIAMAGNETIC SUBSTANCES : Properties of diamagnetic substances , Cause of diamagnetism

* 5. PARAMAGNETIC SUBSTANCES: Properties of paramagnetic substances , Cause of paramagnetism

* 6. Ferromagnetic Substances : Properties of ferromagnetic substances, Cause of ferromagnetism,

* 7. Curie temperature

* 8. Magnetic Hysteresis ,  B-H curve, hysteresis loss, importance of hysteresis loop, applications of ferromagnetic materials

 

IMPORTANT TERMS USED IN MAGNETISM

Magnetic flux density (B)

*  Number of magnetic field lines passing per unit area of the material is known as Magnetic flux density in a material.

* The greater the number of magnetic field lines passing per unit area of the material, the greater is the magnetic flux density in the material and vice versa.

* The SI unit of magnetic flux density is Tesla (T)   or Wb/m² .

* It is a  vector quantity.

The quantity B has several names viz magnetic induction, magnetic field strength and magnetic flux density.

 

Absolute Magnetic permeability ( μ )

* The magnetic permeability of a material is a measure of its conductivity for magnetic field lines.

* The greater the permeability of a material, the greater is its conductivity for magnetic field lines and vice-versa.

Relative permeability

Relative permeability of a material is the ratio of magnetic flux density (B) in that material to the flux density (B₀ ) in air or vacuum  .

Thus ,   Relative permeability,

         μ_r = B / B₀

* The absolute (or actual) permeability μ_o of vacuum/air  is  4π x 10^-7 H/m.

* The absolute (or actual) permeability of other materials is denoted by μ .

The ratio μ / μ₀    is called relative permeability μ_r  of the material .

 

Magnetizing force or Magnetic intensity (H)

* Consider a toroid with n turns per unit length carrying a current I.

* If the absolute permeability of toroid material is μ  (=μ_r μ₀ ), then magnetic flux density B in the material is

         B =  μ.n.I

The quantity  n.I  is called the magnetizing force  or  magnetic intensity ( H ) .

    Now,     H = n . I

   So,          B =  μ.H

*  S.I  unit of  H  =   ampere –turn / meter   =   A-T / m

Now ,      μ =  B / H

* so , ratio of magnetic field and  magnetic intensity in material is constant.

 

Intensity of magnetisation (I)

When a magnetic material is placed in magnetic field , the material is magnetized.

Thus,  Magnetic moment developed per unit volume of the material is called intensity of magnetization .

Intensity of magnetization ,  I =  M / V

Where , M =  magnetic moment developed in the material

             V = volume of material

If m=  magnetic pole strength developed in material,

2l =  length of material  and

A =  cross section area of material  , then

              M =  m . 2l   and  V =  A . 2l

We have ,    I =  M / V 

So        I  =  m / a

Intensity of magnetization is also defined as, 

*  The pole strength developed per unit area of cross section of the material is called intensity of magnetization ( I ) .

* S. I unit of intensity of magnetization  =  A / m

 

Magnetic susceptibility ( X_m )

The magnetic susceptibility of a material is defined as the ratio of intensity of magnetization  developed in the material to the applied magnetizing force (H).

* It is represented by X_m (Greek alphabet Chi).

magnetic susceptibility ,    X_m =  I / H

* The magnetic susceptibility of a material indicates how easily the material can be magnetized.

* Unit of I  and H are same ( A / m ) ,  so  X_m   is unitless .

 

Relation Between  μ_r  and  X_m

Consider a current carrying toroid having core material of relative permeability μ_r ,

The Total magnetic flux density in material

        B = B₀  +  B_m

Where,  B₀ =  magnetic field due to current in coil

             B_m =  magnetic field due to magnetization of material

Now,   B₀ =  μ₀ . H   and   B_m  =  μ₀ . I

Where , I =  intensity of magnetization induced in material

So,       B =  μ₀ . H  +  μ₀ . I

Or,       B = μ₀ ( H + I  )

Now,     I  =  X_m . H

So,        B = μ₀ ( H + I  )

Or,        B =  μ₀ ( H + X_m . H )

Or,       B =  μ₀ .H ( 1 + X_m  )

          μ .H =  μ₀ .H ( 1 + X_m  )           ( B = μ .H )

          μ₀ . μ_r. H =  μ₀ .H ( 1 + X_m  )          

so,         μ_r  =   ( 1 + X_m  )          

 

CLASSIFICATION OF MAGNETIC MATERIALS

* All materials or substances are affected by external magnetic fields; some attain weak magnetic properties and some acquire strong magnetic properties.

* On the basis of their behavior in external magnetic fields, the various substances are classified into the following three categories :

1. Diamagnetic substances 2. Paramagnetic substances  3. Ferromagnetic  substances.

* A diamagnetic substance is feebly repelled by a strong magnet while a paramagnetic substance is feebly attracted by a strong magnet.  However, a ferromagnetic substance is strongly attracted by a magnet.

 

DIAMAGNETIC SUBSTANCES

* Those substances which are feebly repelled by a strong magnet are called diamagnetic substances * Diamagnetic substances are  weakly magnetized by external magnetic field in opposite direction of external field .

e.g. bismuth, copper, zinc, silver, gold, air, water, hydrogen ete.

* Diamagnetic substance loses its magnetism as soon as the external magnetic field is removed.

Properties of diamagnetic substances

* Induced magnetic field in the diamagnetic substance opposes the external field. For this reason, the resultant field  inside the substance is less than the external field .

* A diamagnetic substance is feebly repelled by a strong magnet.

* When a diamagnetic substance is placed in a magnetic field, the magnetic lines of force prefer to pass through the surrounding air rather than through the substance  .

*  When a rod of diamagnetic substance is suspended freely in a uniform magnetic field, the rod comes to rest with its longest axis at right angles to the direction of the field .It happens because a diamagnetic substance is weakly repelled by a magnet.

* The relative permeability (μ_r) of a diamagnetic substance is always less than 1.

* The magnetic susceptibility ( X_m ) of a diamagnetic substance has a small negative value .

* The magnetic susceptibility ( X_m ) of a diamagnetic substance does not change with temperature.

* When diamagnetic substance is placed in a non-uniform magnetic field, it moves from stronger to the weaker parts of the field.

Explaination :

* Consider that a watch glass containing diamagnetic liquid is placed on two closely spaced ( small gap ) poles of a magnet .

* There is a slight depression of liquid in the middle. This is because the field is stronger at the middle than that near the poles.

* If the poles are moved apart, the magnetic field becomes weaker at the middle than that near the poles. As a result, the liquid rise in the middle.

* In either case, the diamagnetic liquid moves from stronger part of the magnetic field to the weaker part.

  

Cause of diamagnetism

* Each electron in an atom  revolves  in an orbit around the nucleus. This revolving electron is equivalent to a tiny current loop. Therefore, each revolving electron has orbital magnetic dipole moment M_o .

* An electron also spins about its own axis. This spinning motion produces an effective current loop and hence a spin magnetic moment M_s.

* The vector sum of M_o and M_s provides the net magnetic dipole moment M to the atom.

*  In a diamagnetic substance, M_o and M_s cancel each other for every atom so that the net magnetic moment of the atom is zero.

* Therefore, motion of all electrons in the atom of a diamagnetic substance can be viewed as the motion of two electrons revolving with the same speed but in the opposite directions.

* Because their magnetic moments are equal in magnitude and opposite in direction, the two magnetic moments cancel each other .

* Thus in the absence of external magnetic field, the atoms of a diamagnetic substance have no net magnetic moment. Hence the substance does not exhibit diamagnetism.

Effect of external magnetic field

* When an external uniform magnetic field  is applied then both  electrons experience  equal  magnetic force in magnitude   but in opposite direction along radius of path .

* Due to this speed of one electron is increased and speed of other electron is decreased.

* Hence,  Magnetic moment of two electrons are different in magnitude and in opposite direction.

* The vector addition of these two magnetic moments gives rise to a net dipole magnetic moment directed opposite to external magnetic field B.

* Thus when a diamagnetic material is placed in an external magnetic field, an induced magnetic moment is developed in the material which opposes the applied magnetic field.

PARAMAGNETIC SUBSTANCES

* Those substances which are  feebly attracted by a strong magnet are called paramagnetic substances.

* Paramagnetic substances are weakly magnetised in the direction of applied external magnetic field .

e.g.   aluminum, antimony, chromium, lithium, oxygen, copper chloride, sodium, tungsten etc.

* Paramagnetic substance loses its magnetism as soon as the external magnetic field is removed .

Properties of paramagnetic substances

*  A paramagnetic substance is feebly attracted by a strong magnet.

* When a paramagnetic substance is placed in a magnetic field, the magnetic lines of force prefer to pass through the substance rather than through air .

* The resultant field  inside the substance is more than the external field .

* When a rod of paramagnetic substance is suspended freely in a uniform magnetic field, the rod comes to rest with its longest axis along the direction of the external magnetic field.

* When paramagnetic substance is placed in a non-uniform magnetic field, it moves from weaker to the stronger parts of the field.

Note : Magnetic field is strong in small gap  while Magnetic field is weak in large gap

* The relative permeability (μ_r) of a paramagnetic substance is always more than 1.

* The magnetic susceptibility (X_m) of a paramagnetic substance has small positive value .

*  The magnetic susceptibility (X_m) of a paramagnetic substance varies inversely as the absolute temperature (T)    i.e  ,    X_m α  1/ T

This means that a paramagnetic substance loses magnetism with rise in temperature.

Cause of paramagnetism:

In a paramagnetic substance, the individual atom  has small net magnetic moment. In other words, the electron’s spins and orbital motions have a net circulating current that is not zero. Therefore, the atom has a net magnetic moment i.e., each acts as a magnetic dipole.

* In the absence of external magnetic field, the dipoles of the paramagnetic substance are randomly oriented. Therefore, the net magnetic moment of the substance is zero. Hence, the substance does not exhibit paramagnetism.

*  When a paramagnetic substance is placed in an external magnetic field the dipoles are partially aligned in the direction of the applied field. Therefore, the substance is feebly magnetised in the direction of the applied magnetic field. This results in a weak attractive force on the substance.

 * Paramagnetism is generally very weak as only a very small fraction of the dipoles are aligned in the direction of the applied magnetic field. The fraction of the dipoles that line up with the field depends upon the strength of the field and the temperature.

* Paramagnetism is quite sensitive to temperature. The lower the temperature, the stronger is the paramagnetism and vice-versa.

Ferromagnetic Substances

* Those substances which are strongly attracted by a magnet are called ferromagnetic substances.

e.g. iron, nickel, cobalt etc.

* It is  strongly magnetized in the direction of the applied external magnetic field.

* The resultant magnetic field inside the ferromagnetic substance is very large; often thousands times greater than the external field.

Properties of ferromagnetic substances

* Ferromagnetic substances show all the properties of paramagnetic substances but to a much greater degree.

* A ferromagnetic substance is strongly attracted by a magnet.

* When a ferromagnetic substance is placed in a magnetic field, the magnetic field lines tend to crowd into the substance .

* The resultant field inside a ferromagnetic substance is very large as compared to the external field * When a rod of ferromagnetic substance is suspended in a uniform magnetic field, it quickly aligns itself in the direction of the field .

* When ferromagnetic substance is placed in a non-uniform magnetic field, it moves from weaker to stronger parts of the magnetic field.

* The relative permeability (μ_r)  of a ferromagnetic substance is very large. For example, the relative permeability of soft iron is about 8000.

* The magnetic susceptibility ( X_m )  of a ferromagnetic substance is positive having a very high value.

* When a ferromagnetic substance is heated, magnetization decreases because random thermal motions tend to destroy the alignment of domains.

* When external magnetic field is removed, some ferromagnetic substances retain magnetism.

 

Cause of ferromagnetism

* Like paramagnetic substances, the atoms of ferromagnetic substances have a permanent magnetic moment.

* In a ferromagnetic substance, the atoms do not act as a dipole independently. rather they group magnetically  which are called domains.

* The region of space over which the magnetic dipole moments of the atoms are aligned in the same direction is called a domain.

 * Each domain contains a very large number of atoms whose dipole moments are parallel and pointing in one direction .Therefore, each domain has a net magnetic dipole moment.

(i) In the absence of external magnetic field, the domains of a ferromagnetic material are randomly oriented . Due to this the net magnetic moment in the material is zero. Therefore, a ferromagnetic material does not exhibit magnetism in the normal state.

(ii) When a ferromagnetic substance is placed in an external magnetic field, a net magnetic moment develops in the substance. This can occur in two ways viz.

 (a) By the displacement of boundaries of the domains i.e., the domains that already happen to be aligned with the applied field may grow in size whereas those oriented opposite to the external field reduce in size

 (b) By the rotation of the domains i.e., the domains may rotate so that their magnetic moments are more or less aligned in the direction of the applied magnetic field .

The result is that there is a net magnetic moment in the material in the direction of the applied field. Since the degree of alignment is very large even for a small external magnetic field, the magnetic field produced in the ferromagnetic material is often much greater than the external field.

Curie temperature

* Ferromagnetism decreases with the increase in temperature.

* At sufficiently high temperature, the ferromagnetic property of the substance suddenly disappears and the substance becomes paramagnetic.

The temperature at which a ferromagnetic substance becomes paramagnetic is called Curie temperature or Curie point of the substance.

The Curie temperature of iron is 770°C and that of nickel is 358°C. Therefore, if the temperature of iron becomes 770°C, it will change into a paramagnetic substance.

                                    

Cycle Of Magnetization

When a magnetic material magnetise  in both direction by one cycle of alternating current  then it  is called cycle of magnetization.

Magnetic Hysteresis

When a magnetic material is subjected to a cycle of magnetisation, then  magnetic flux density B in the material lags behind the applied magnetising force H. This phenomenon is known as hysteresis.

Hysteresis Loop

* The curve drawn between magnetizing force ( H )  and magnetic flux density ( B ) in a cycle of magnetization is called hysteresis loop.

* When the iron piece is subjected to a cycle of magnetization then resultant B-H curve traces a closed path  . This closed path  is called hysteresis loop .

Explanation :

Consider an unmagnetized iron bar AB wound with N turns .

* When current in the solenoid is zero, then   H = 0 and hence B in the iron is zero.

* As H is increased (by increasing solenoid current), the magnetic flux density (B) also increases until the point of maximum magnetic flux density (+ B) is reached. This is saturated point of magnetic material.

* Beyond saturated point, the magnetic flux density will not increase regardless of any increase in current or magnetising force.   In this stage , B-H curve follow  the path oa.

* If H is gradually reduced , it is found that magnetic flux density B does not decrease along path oa but follows the path ab.

* At point b, the magnetising force H is zero but magnetic flux density in the material has a finite value + B (= ob) called residual flux density.

* In other words, B lags behind H. The greater the lag, the greater is the residual magnetism (i.e. ordinate ob) retained by the iron piece. The power of retaining residual magnetism is called retentivity of the material.

* To demagnetize the iron piece (i.e. to remove the residual magnetism ob), the magnetising force H is reversed by reversing the current through the coil.

* When H is gradually increased in the reverse direction, the B-H curve follows the path bc so that when H = oc, the residual magnetism is zero.

* The value of H (= oc) required to wipe out residual magnetism is known as coercive force (H).

*  If H is further increased in the negative direction, the material again saturates (at point d) in the negative direction.

* Reducing H to zero and then increasing it in the positive direction completes the curve defa.

Thus when an iron piece is subjected to one cycle of magnetisation, the B-H curve traces a closed loop abcdefa called hysteresis loop.

* For one cycle of magnetisation, one hysteresis loop is traced.

e.g.  If a magnetic material is located within a coil through which alternating current (50 Hz) flows, 50 loops will be formed every second.

 

FACTORS AFFECTING THE SHAPE AND SIZE OF HYSTERESIS LOOP

There are three factors that affect the shape and size of hysteresis loop.

(a) The material

* The shape and size of the hysteresis loop largely depends upon the nature of the material. If the material is easily magnetized, the loop will be narrow. On the other hand, if the material does not get magnetized easily, the loop will be wide.

* Further, different materials will saturate at different values of magnetic flux density thus affecting the height of the loop.

(b) The maximum flux density

* The loop area also depends upon the maximum flux density that is established in the material.

* Loop area increases as the alternating magnetic field has progressively greater peak values.

(ii) The initial state of the specimen

The shape and size of the hysteresis loop also depends upon the initial state of the specimen.

 

HYSTERESIS LOSS

* When a magnetic material is subjected to a cycle of magnetization, an energy loss takes place due to the "molecular friction in the material. This loss is in the form of heat and is called hysteresis loss.

* Hysteresis loss is present in all those electrical machines whose iron parts are subjected to cycles of magnetization.

The obvious effect of hysteresis loss is the rise of temperature of the machine.

(i) Transformers and most electric motors operate on alternating current. In such devices, the magnetic flux in the iron changes continuously, both in value and direction. Hence hysteresis loss occurs in such machines.

(ii) Hysteresis loss also occurs when an iron part rotates in a constant magnetic field e.g. d.c. machines.

 

IMPORTANCE OF HYSTERESIS LOOP

* The shape and size of the hysteresis loop "largely depends upon the nature of the material. The choice of a magnetic material for a particular application often depends upon the shape and size of the hysteresis loop.

(i) The smaller the hysteresis loop area of a magnetic material, the less is the hysteresis loss. The hysteresis loop for silicon steel has a very small area. For this reason, silicon steel is widely used for making transformer cores and rotating machines which are subjected to rapid reversals of magnetization.

(ii) The hysteresis loop for hard steel indicates that this material has high rentivity and coercivity. Therefore, hard steel is quite suitable for making permanent magnets. But due to the large area of the loop, there is greater hysteresis loss. For this reason, hard steel is not suitable for the construction of electrical machines.

(iii) The hysteresis loop for wrought iron shows that this material has fairly good residual magnetism and coercivity. Hence, it is suitable for making cores of electromagnets.

 

APPLICATIONS OF FERROMAGNETIC MATERIALS

Ferromagnetic materials (e.g. iron, steel, nickel, cobalt etc) are widely used in a number of applications. The choice of a ferromagnetic material for a particular application depends upon its magnetic properties such as retentivity, coercivity and area of the hysteresis loop.

(i) The permanent magnets are made from hard ferromagnetic materials (steel, cobalt steel, carbon steel etc). Since these materials have high retentivity, the magnet is quite strong. Due to their high coercivity, they are unlikely to be demagnetised by stray magnetic fields.

(ii) The electromagnets or temporary magnets are made from soft ferromagnetic materials (e.g. soft iron). Since these materials have low coercivity, they can be easily demagnetised. Due to high saturation flux density, they make strong magnets.

(iii) The transformer cores are made from soft ferromagnetic materials. When a transformer is in use, its core is taken through many cycles of magnetisation. Energy is dissipated in the core in the form of heat during each cycle. The energy dissipated is known as hysteresis loss and is proportional to the area of hysteresis loop. Since the soft ferromagnetic materials have narrow hysteresis loop (i.e. smaller hysteresis loop area), they are used for making transformer cores.