# Magnetic Properties of Materials: Short Questions Answers

#### 1. What do you mean by geographical axis and geographical poles of the earth?

The axis of rotation of the earth is called the Geographical Axis and the points, where it cuts the surface of the earth are called Geographical Poles.

#### 2. What do you mean by geographic meridian?

A vertical plane that passes through the geographical axis is called the Geographical meridian.

#### 3. What do you mean by earth’s magnetic axis and the magnetic poles?

It is assumed that the earth has a big bar magnet inside it. The axis of this magnet is called the Magnetic Axis and the points, where this axis cuts the earth’s surface are called Magnetic Poles.

#### 4. What do you mean by earth’s magnetic meridian?

A vertical plane passing through the magnetic axis of the earth is called the Magnetic Meridian

#### 5. Define magnetic elements.

The elements of physical quantities, which determine the strength or intensity of the earth’s total magnetic field both the magnitude and direction, are called the Magnetic Elements. They are

(a) Magnetic declination

(b) Magnetic inclination or dip

(c) Horizontal component of earth’s magnetic field.

#### 6. What is the angle of declination and the angle of dip at a place? [HSEB 2058]

The angle between the geographical meridian and the magnetic meridian of the earth is known as declination at that place. The angle made by the direction of the earth’s total magnetic field with its horizontal component at a place is known as the angle of dip at that lace

#### 7. What are the maximum and minimum values of the angle of dip and where do they occur?

The maximum value of the angle of dip is 90° and it will be at the magnetic poles of the earth and the minimum value of it is 0° and It will be at the magnetic equator.

#### 8. How does the angle of dip vary from place to place on the earth’s surface? [HSEB 2059]

The value of the angle of the dip will be Minh-twill i.e. 0° on the magnetic equator of the earth. The value of the angle of dip goes on increasing gradually as the place is changed from the geomagnetic equator or to the magnetic poles of the earth where it will take maximum value i.e. 90°.

#### 9. Define a neutral point in the magnetic field of a bar magnet?

The point where the magnetic field intensity due to the given bar magnet is exactly equal and opposite to the horizontal component of the earth’s magnetic field intensity at that point is called the Neutral Point since they neutralize each other. At this point, if we keep a compass needle, it does not show any particular direction.

#### 10. At a neutral point, a magnetic needle is unable to show a proper direction. Why? [HSEB 2059, 061]

At a neutral point, the magnetic field intensity is exactly equal and opposite to the horizontal component of the earth’s magnetic field. Hence they cancel each other making net magnetic field intensity at that point zero. So there will be no magnetic field at that point. Hence a compass needle is unable to show a proper direction.

#### 11. What do you mean by a magnetic line of force?

A magnetic line of force is an imaginary line along which a unit north pole will move if it is free to do so in a magnetic field. It is directed from the north pole of a magnet to the south pole outside the magnet and the south pole to the north pole inside the magnet. The direction of the tangent drawn at any point on this line gives the direction of the magnetic field at that point.

#### 12. How can you show that magnetic flux is a scalar quantity?

The magnetic flux at a surface is the number of magnetic lines of force crossing normally through the surface. If a magnetic field B is applied on the surface of area A at an angle of θ with it as shown in the figure,

Magnetic flux (Φ) = A x component of B along normal to the area

= AB cosθ° = A.B
This shows that magnetic flux through an area is the dot product of A and B

Hence, the magnetic flux is a scalar quantity.

#### 13. Two magnetic lines of force of a bar magnet can not cross each other. Why?

A tangent drawn at any point on a magnetic line of force gives the direction of the magnetic field at that point. If two magnetic lines of force intersect at a point, two tangents can be drawn at the same point on two lines of force. It means there will be two directions of the magnetic field at the same point which is impossible. Hence to have a single direction of the magnetic field at a point, two lines of force can not intersect at any point.

#### 14. What are the basic differences between the magnetic lines of force and the electric lines of force? [HSEB 2060]

The differences between the magnetic lines of force and the electric lines of force are as follows:

#### 15. What are the necessary conditions to apply tangent law?

The necessary conditions to apply tangent law are as follows:

(a) both of the fields should be uniform.

(b) the two fields should be perpendicular to each other.

#### 16. What is the principle of a vibration magnetometer?

The principle used in a vibration magnetometer is that when a bar magnet is freely suspended in the earth’s magnetic field and it is disturbed from its equilibrium position, due to the torque provided by the horizontal component of the earth magnetic field and the moment of inertia of the given magnet, it executes in simple harmonic motion.

#### 17. What is the principle used in deflection magnetometer?

A deflection magnetometer works on the principle of tangent law i.e. if a magnet is freely suspended in two mutually perpendicular uniforms magnetic fields B and BH, then the magnet comes to rest at an angle θ with the direction of BH such that B = BH tanθ.

#### 18. What is the difference between a deflection magnetometer and a vibration magnetometer?

The principle used in the deflection magnetometer is the principle of tangent law whereas the principle used in the vibration magnetometer is the principle of simple harmonic motion. To work deflection magnetometer, we need two mutually perpendicular uniform magnetic fields whereas, in vibration magnetometer, we need only one uniform magnetic field. In the deflection magnetometer, we measure the deflection of the suspended magnet, whereas in the oscillation magnetometer we observe the time period of oscillation of the suspended magnet.

#### 19. What is the time period of the suspended magnet in a vibration magnetometer?

The time period of the suspended magnet in a vibration magnetometer is given by t = 2π√(I/MH)

where I is the moment of inertia of the bar magnet, M be the magnetic moment of the magnet and H be the horizontal component of the earth’s magnetic field.

#### 20. On what factors does the time period of a magnet in a vibration magnetometer depend?

The time period of a magnet suspended in a vibration magnetometer depends upon

(i) the moment of inertia (I) of the magnet

(ii) the magnetic moment (M) of the magnet

(iii) the horizontal component (H) of the earth’s magnetic field.

#### 21. What happens to the time period of oscillation of the freely suspended magnet in a vibration magnetometer if its dimension is made larger?

We know that the time period of oscillation of a magnet in a vibration magnetometer is directly proportional to the square root of the moment of inertia of the magnet. The moment of inertia of a bar magnet is given by

I = (l^2+b^2)/12 x mass of the magnet.

where l, b be the length and breadth of the magnet respectively. This shows that when we increase the dimension of the magnet, the moment of inertia will be increased which increases the time period of oscillation.

#### 22. Can a vibration magnetometer be used at the magnetic poles? Give a reason? [HSEB 2056]

To work the vibration magnetometer, the torque needed is provided by the horizontal component of the earth’s magnetic field. But at the geomagnetic poles of the earth, the magnetic field becomes vertical having no horizontal component. Hence it can not be used at the geomagnetic poles.

#### 23. Can a detection magnetometer be used at the geomagnetic poles to compare the magnetic moments?

To use a deflection magnetometer, it should be kept in two mutually perpendicular uniform magnetic fields. One of them is the horizontal component of the earth’s magnetic field. At the geomagnetic poles, the horizontal component of the earth’s magnetic field is zero. Hence a deflection magnetometer can not be used at the magnetic poles of the earth to compare the magnetic moments.

#### 24. What is the reason for the magnetic properties of materials?

Spin and orbital motion of electrons in an atom give the magnetic property to an atom. The magnetic property of the material is due to the atomic magnets.

#### 25. How many types of magnetic substances do you know?

There are three types of magnetic substances. They are

(a) Diamagnetic substance

(b) Paramagnetic substance

(c) Ferromagnetic substance

#### 26. What is a diamagnetic substance? Give examples.

A substance, which when placed in a magnetic field gets feebly magnetized in a direction opposite to that of the magnetizing field, is called a Diamagnetic Substance. e.g. zinc, bismuth, etc.

#### 27. Why is diamagnetism, independent of temperature?

The atoms of a diamagnetic do not have an intrinsic magnetic dipole moment. Hence they are not affected by the thermal motion of atoms i.e. diamagnetism is independent of temperature.

#### 28. What is paramagnetism? Give examples.

A substance, which when placed in a magnetic field gets feebly magnetized in the direction of the magnetizing field, is called a Paramagnetic Substance. e.g. Aluminium, manganese, etc.

#### 29. How does the magnetic induction of a paramagnetic material vary with temperature?

When the temperature is increased, the magnetic induction of a paramagnetic material decreases.

#### 30. Why do some paramagnetic substances show greater magnetization on cooling?

The temperature has a negative effect on paramagnetic substances. Hence on cooling, magnetic induction of a paramagnetic substance increases on cooling and shows greater magnetization.

#### 31. Why does a paramagnetic sample display greater magnetization, when it is cooled?

When it is cooled, the tendency of the thermal agitation to disrupt the alignment of magnetic dipoles decreases. Hence it displays greater magnetization when it is cooled.

#### 32. What difference can you see substances in a magnetic field?

When paramagnetic and diamagnetic materials are kept in a magnetic field, paramagnetic material moves from the weaker part of the field to the stronger part whereas diamagnetic material moves from the stronger part to the weak part of the field.

#### 33. What are the ferromagnetic domains? [HSEB 2057]

A ferromagnetic substance consists of many microscopic regions called Magnetic Domains. Each domain consists of atomic magnetic moments which orient randomly in the absence of an applied magnetic field and a ferromagnetic substance is observed as a non-magnetic substance when a magnetic field is applied, their magnetic moments are aligned parallel to each other, giving a resultant magnetic moment. Saturation of magnetization takes place when all magnetic moments align in the direction of the field. When the field is removed, there is a tendency for the domain to stay they have been aligned. This results from the remanence field and the hysteresis effect.

#### 34. What is remanent magnetism or residual magnetism?

The value of the intensity of magnetization of the magnetic material, even when the magnetizing field is reduced to zero, is called its Retentivity or Remanent Magnetism or Residual Magnetism. 34. in paramagnetic and diamagnetic

#### 35. What is coercivity?

The value of the reverse magnetizing field, which has to be applied to the magnetic material so as to reduce the residual magnetism to zero, is called its Coercivity.

#### 36. What is the retentivity and coercivity of a ferromagnetic material? [HSEB 2072]

Retentivity of a ferromagnetic material is a measure of the magnetism remaining in the specimen when the magnetizing field is reduced to zero. It measures the strength of the magnetization of the material. The coercivity of a ferromagnetic material is the reverse magnetic field needed to reduce the magnetism to zero in the material.

#### 37. What is hysteresis?

Hysteresis means coming late. During the process of applying a magnetizing field by varying its magnitude and direction continuously, the intensity of magnetization does not become zero on making the magnetizing field zero but does a little late. This lagging of intensity of magnetization behind the magnetizing field in a ferromagnetic material taken through a complete cycle of magnetization is called the Hysteresis.

#### 38. Soft iron is used to make electromagnets. Why? OR Why do we prefer soft iron for the core of the transformer?

Soft iron has a small area of hysteresis loop with greater retentivity and smaller coercivity. Due to this, the loss of energy per unit volume is small when it is taken over a complete cycle of magnetization. Also, a high magnetic field is produced due to high retentivity which is required for an electromagnet or the core of a transformer.

#### 39. Why is it possible to make permanent magnets out of steel? OR Why do we prefer to use the alloy alnico for making permanent magnets?

Steel or alnico has a greater value of coercivity and a fairly large value of retentivity, it is used to make permanent magnets. Since it has a greater value of coercivity, it is difficult to destroy the magnetism induced in it i.e. very high reverse magnetizing field is required to reduce the residual magnetism to zero. Hence it is used to make permanent magnets.

#### 40. Why does a magnet not retain its magnetism when it is melted? [HSEB 2064]

When a magnet is melted, the atomic magnets do not retain in a direction, but orient in random directions. It makes the net magnetism zero and the material does not retain magnetism.

#### 41. A ferromagnetic substance becomes paramagnetic above Curie temperature. Why? [HSEB 2063, 065]

A ferromagnetic substance obeys curie law. When the temperature is increased, the alignment of atomic magnetic moments in it is disturbed and above Curie temperature, the alignment of these atomic magnetic moments becomes completely random and the substance becomes paramagnetic.

#### 42. When a ferromagnetic material is placed in a magnetic field, the magnetic field inside it is greater than outside of it. Why?

A ferromagnetic material has a higher value of susceptibility and permeability than that of air. Hence it has a greater field inside the material than outside when it is kept in a magnetic field.

#### 43. What is Curie’s point?

The temperature for a ferromagnetic substance above which, it behaves as a paramagnetic substance is known as Curie point.

#### 44. Steal is used in making permanent magnets whereas soft iron is preferred for making the core of the transformer. Why? [HSEB 2066, 069]

Steal has a greater value of coercivity. So it is difficult to destroy the magnetism induced in it. Hence it is used to make permanent magnets. But soft iron has greater retentivity and small coercivity and due to this, the loss of energy per unit volume will be small when it is taken over the complete cycle of magnetization. Also, a high magnetic field is produced due to high retentivity which is required for an electromagnet or the core of a transformer.