Important Questions and Answers: Magnetic Materials

MAGNETIC MATERIALS

1. On the basic of spin how the materials are classified as para, ferro, antiferro and ferrimagnetic.

(i) Paramagnetic materials have few unpaired electron spins of equal magnitudes.

(ii) Ferro magnetic materials have many unpaired electron spins with equal magnitudes.

(iii) Anti ferro magnetic materials have equal magnitude of spins but in antiparallel manner.

(iv)Ferrimagnetic materials have spins in antiparallel manner but with unequal magnitudes.

2. What is Bohr magneton?

The orbital magnetic moment and the spin magnetic moment of an electron in an atom can be expressed in terms of atomic unit of magnetic moment called Bohr magneton.

3 What is ferromagnetism?

Certain materials like iron (Fe), Cobalt (Co), Nickel (Ni) and certain alloys exhibit Spontaneous magnetization ie., they have a small amount of magnetization (atomic moments are aligned) even in the absence of an external magnetic field.This phenomenon is known as ferromagnetism.

4. What are ferromagnetic materials?

The materials which exhibit ferromagnetism are called as ferromagnetic materials.

5. What are the properties of ferromagnetic materials?

(i)   All the dipoles are aligned parallel to each other due to the magnetic interaction between any two dipoles.

(ii) They have permanent dipole moment. They attract the magnetic field strongly.

(iii)  They exhibit magnetisation even in the absence of magnetic field. This property of ferromagnetic materials is called as spontaneous magnetization.

6. What is domain theory of ferromagnetism?

According to domain theory, a virgin specimen of ferromagnetic materials consists of a number of regions or domains which are spontaneously magnetized due to parallel alignment of all magnetic dipoles. The direction of spontaneous magnetisation varies from domain to domain.

7. Mention the energies involved in origin of domains in ferromagnetic material.

(i) Magnetostatic energy

(ii) Crystalline energy

(iii) Domain wall energy

(iv)Magnetostriction energy

8. What is antiferromagnetism?

In anti-ferromagnetism, electron spin of neighbouring atoms are aligned antiparallel. Anti-ferromagnetic susceptibility is small and positive and it depends greatly on temperature.

9. What are ferrites and mention its types.

Ferrites are modified structure of iron with no carbon and in which the adjacent magnetic moments are of unequal magnitudes aligned in antiparallel direction. Its general formula is given by X²⁺ Fe2³⁺ O4^2-.

Types: normally there are two types of structure. 1. Regular spinel 2. Inverse spinel.

10.            State the applications of ferrites.

(i) They are used in transformer cores for high frequencies upto microwaves.

(ii)   They are used in ratio receivers to increase the sensitivity and selectivity of the receiver.

(iii)  Ferrites are used in digital computers and data processing circuits as magnetic storage elements.

(iv) They are used as an isolator, gyrator and circulator which are used in microwave devices.

11.            What is hysteresis in magnetic materials?

The lagging of magnetic induction (B) behind the applied field strength (H) is called hysteresis.

12. What is meant by hysteresis loss?

When the specimen is taken through a cycle of magnetization, there is a loss of energy in the form of heat. This is known as hysteresis loss 13. What are soft-magnetic materials?

Materials which are easy to magnetize and demagnetize are called soft magnetic materials.

14.            State the properties of soft magnetic materials.

(i) They have high permeability

(ii) They have low coercive force.

(iii) They have low hysteresis loss.

15.            Mention few soft magnetic materials and their applications. Soft magnetic materials:

(i) Pure or ingot iron

(ii) Cast iron (carbon above 2.5%)

(iii) Carbon steel

Applications:

(i) Cast iron used in the structure of electrical machinery and frame work of d.c.machine

(ii) Carbon steel has high mechanical strength used in making motor of turbo alternators.

16. What ate hard magnetic materials?

Materials which retain their magnetism and are difficult to demagnetize are called hard magnetic materials.

17. State the properties of hard magnetic materials.

They possess high value of B-H product

They have high retentivity

They have high coercivity

They have low permeability.

18. What are ferromagnetic materials?

Materials which exhibit ferrimagnetism are called ferromagnetic materials. They are also

known as ferrites.

19.            Mention the properties of ferromagnetic materials.

(i)  These are the ferromagnetic materials in which equal number of opposite spins with

different magnitudes such that the orientation of neighbouring spins is in anti parallel manner.

(ii) Susceptibility is positive and very large for these materials.

20. Differentiate soft and hard magnetic materials.

S.No Soft magnetic materials

1. Magnetic materials which can be easily magnetized and demagnetized

2. They have high permeability

3. Magnetic energy stored is not high

4. Low hysteresis losses due to small hysteresis loop area

Hard magnetic materials

Magnetic materials which cannot be easily magnetized and demagnetized

They have low permeability

Magnetic energy stored is high

High hysteresis losses due to small hysteresis loop area

21. Why ferrites are advantageous for use as transformer cores?

Ferrites are used as transformer cores for frequencies up to microwaves. This is because the eddy current problem which prevents the penetration of magnetic flux into the materials is much less severe in ferrites than in iron.

22. What is the origin of magnetic moment in magnetic materials?

The magnetic moment originates from the orbital motion and spinning motion of electrons.

23. What is diamagnetism?

When a material is placed in a magnetic field, the material becomes magnetized. The direction of the induced dipole moment is opposite to the externally applied magnetic field.

Due to this effect, the material gets very weakly repelled in the magnetic field. This phenomenon is known as diamagnetism.

24. What are diamagnetic materials?

The materials which exhibit diamagnetism are called diamagnetic material.

25.        What are the properties of diamagnetic materials?

(i) Diamagnetic materials repel the magnetic lines of force.

(ii)   There is no permanent dipole moment. Therefore, the magnetic effects are very

small.

The magnetic susceptibility is negative and is independent of temperature and applied magnetic field strength.

26. What is paramagnetism?

In certain materials, net magnetic moment is zero though each atom or molecule possesses a permanent magnetic moment in the absence of an external magnetic field.

But when an external magnetic field is applied the magnetic dipoles tend to align themselves in the direction of the magnetic field and the material becomes magnetized. This effect is known as paramagnetism.

27. What are paramagnetic materials?

The magnetic materials which exhibit paramagnetism are called paramagnetic material.

28.            What are properties of paramagnetic materials?

(i) Paramagnetic materials attract the magnetic lines of force.

(ii) They possess permanent dipole moment.

(iii) The susceptibility is positive