# Physics Class 12 Chapter 5 Subjective Questions

## Magnetism and Matter

1. Define a magnet.

It is defined as a substance which can attract magnetic materials like iron etc. and rests in north-south direction when suspended freely. It has
two equal and opposite magnetic poles separated by a certain distance.

1. What is a sure test of magnetism? Name a natural magnet and give its chemical formula.

Repulsion is the sure test of magnetism. Magnetite is a natural magnet having chemical formula Fe3O4

1. Define induce magnetism.

It is defined as the magnetism attained because of another magnet without physical contact.

1. Define pole strength of a magnet. The two poles of a magnet equally strong?

It is defined as the ability of a magnet to attract small pieces of magnetic material ike iron, nickel etc. by its poles.
Yes, the two poles of a magnet are equally strong.

1. Define the following:

(a) Terrestrial magnetism.
(b) Geographic meridian.
(c) Magnetic meridian.

(a) It is defined as the study of magnetic field of earth.
(b) Geographic meridian :It is defined as the vertical plane passing through the geographic axis i.e., geographical N-S line.
(c) It is defined as the vertical plane passing through the magnetic
axis of the earth. .

1. Define geomagnetic equator.

It is defined as the great circle on earth’s surface whose plane is perpendicular to the magnetic axis.

1. What do you mean by the directive property of a magnetic dipole?

It is that properly of a freely suspended magnet due to which it always aligns itself along the N-S line.

1. What is the basic difference between magnetic lines of force and electric lines of force?

The magnetic lines of force are always closed loops. As isolated magnetic poles do not exist, they always run from N-pole to south pole
outside the magnet and S-pole to N-pole inside the magnet. On the otherhand, the electric lines of force originate from positive charge, and terminate on the negative charge and are thus discontinuous curves.

1. If a bar magnet is cut into two equal parts parallel to its length, then what happens to its dipolemoment?

Let m, 2l be the pole strength and the dipole length of the magnet. If M be the magnetic moment of the magnet, then
M = m. 2l
when it is cut into two equal pieces parallel to its length, then each
piece will be a magnet having length 2l and pole strength m/2
Thus the magnetic moment of each piece will be M/2. (2l) = 1/2 . [m = 2l] = M/2
Hence the magnetic moment will become half of that of the original magnet.

1. Define magnetic permeability.

It is defined as the degree to which the magnetic lines of force can penetrate a substance. It is denoted by g.

1. Differentiate between magnetic intensity and intensity of magnetisation.

Magnetic Intensity – It is defined as the degree to which a magnetic field line can magnetise a substance. It is denoted by H.
Intensity of Magnetisation – It is defined as the degree to which a substance is magnetised on placing it in a magnetic field. It is denoted by I.

1. What happens to the dipole moment of a bar magnet when it is cut into two equal pieces transverse to its length.

Each piece will be a magnet having pole strength m and length l if its original parameters were m and 2l. So magnetic moment of
each piece will be m. l = M/2 , where M = m. 2l. Thus the magnetic moment reduces to half value as compared to its original value.

1. Explain why ordinary piece of iron don’t behave as a magnet ?

The molecular magnets in ah ordinary piece of iron are randomly oriented and form closed chains, thus cancel the effect of each other, so it does not behave as a magnet.

1. Define the followings :

(a) Define a magnetic dipole.
(b) What is a natural fundamental dipole? Explain.

(a) It is defined as a pair of magnetic poles of equal and opposite pole strength separated by a small distance.
(b) Electron is a natural fundamental dipole. The electrons revolving in an atom behave as tiny current loops which give rise to the magnetism.

1. Define the followings :

(a) What will happen to a magnetic core placed in a solenoid carrying no current?
(b) How can you determine the polarity of a solenoid?

(a) It will not be magnetised.
(b) Polarity of a solenoid can be determined with the help of clock rule. Anticlockwise current in a face of the solenoid gives north polarity to it and clockwise current gives S-polarity.

1. Define neutral point in the magnetic field of a bar magnet.

It is defined as that point where the field due to the magnet is . completely neutralised by the horizontal component of earth’s magnetic
field i.e., Bmag = BH at the neutral point.

1. What is the direction of horizontal component of earth’s magnetic field above the surface of earth?

The direction of horizontal component of earth’s magnetic field is from geographic south to geographic north above the surface of earth.

1. How will a magnet suspended freely at equator and pole orient itself?

A magnet suspended freely at equator will orient itself parallel to the surface of earth while the magnet suspended freely at pole will stand vertically.

1. How does dip angle vary as one moves from magnetic equator to the pole?

The dip angle varies from 0° to 90° as one moves from magnetic equator to the pole.

1. In what direction would a compass needle align if taken to geographical (i) north and (ii) south pole?

A compass needle would align under the influence of vertical component because the horizontal component of earth’s magnetic field is zero at both the geographical north and south poles.

1. Define declination at a place. How does it help in navigation?

It is defined as the angle between the geographic and magnetic meridian at that place. By knowing the declination, the ship can be steered in the required direction, so as to reach the destination.

1. Define angle of dip at a place. What is its value in Britain as compared to India?

It is defined as the angle made by the direction of the earth’s total magnetic field with the horizontal direction. Britain is close to the magnetic north pole, so the angle of dip is greater in Britain than in India. Dip angle is about 70° in Britain and about 18° at a place in southern India which is ¼ th as in Britain.

1. A magnetic needle is placed on a cork floating on a still lake in the northern hemisphere. Does this needle together with the cork move towards the north of the lake?

The magnetic needle does not experience any net force and hence it along with the cork will not move towards the north of the lake and will come to rest along N-S direction. But if the needle is inclined to the magnetic meridian, a torque will act on the needle and after a few oscillations, it will set itself along the magnetic meridian.

1. The earth’s core is known to contain iron, yet geologists don’t regard this as a source of earth’s magnetism. Why?

The molten iron is above its Curie temperature, so it cannot retain magnetism. Hence it cannot remain as a magnet.

1. Define magnetic flux and magnetic flux density.

Magnetic flux—It is defined as the total number of magnetic lines of force passing normally through the unit area of a substance. It is denoted by ø It is also commonly known as magnetic field or strength of magnetic field.

1. Compare the magnetic fields of a solenoid and a bar magnet.

The magnetic field inside a solenoid is stronger than inside a bar magnet.

1. State Curie’s law in magnetism.

It states that the magnetic susceptibility of a material is inversely proportional to its absolute temperature i.e., mathematically Where C is proportionality constant known as Curie constant.
Xm ∝ 1/T
or Xm = C/T
Where C is proportionality constant known as Curie constant

1. Give the value of Curie temperature for iron, cobalt and nickel.

The Curie temperatures for iron, cobalt and nickel are 1000 K, 1375 K and about 625 K respectively.

1. Which material have permeability more than one (1) ?

Paramagnetic and Ferromagnetic materials have permeability more than one.

1. State two methods of how to demagnetise a bar magnet?

A bar magnet can be demagnetised by :
(i) hammering it repeatedly.
(ii) heating it.

1. Why a tangent galvanometer fails to work at magnetic north pole of earth?

It fails to work because horizontal component of earth’s magnetic field BH at the poles of earth is zero.

1. How does the intensity of magnetisation of a paramagnetic material vary with increasing applied magnetic field?

It directly increases with the increase in applied magnetic field.

1. What happen when a diamagnetic material is placed in a varying magnetic field?

It tends to move from the stronger to the weaker part of the magnetic field.

1. Why the electromagnets are made of soft iron?

The electromagnets are made of soft iron because coercivity of soft iron is small.

1. Why do magnetic lines of force prefer to pass through iron than air?