MAGNETISM AND MATTER
Read the assertion and reason carefully to mark the correct option out of the options given below :
(a) If both assertion and reason are true and the reason is the correct explanation of the
assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the
assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.
(e) If assertion is false but reason is true.
1. Assertion : We cannot think of magnetic field configuration with three poles.
Reason : A bar magnet does exert a torque on itself due to its own field.
2. Assertion : The poles of magnet cannot be separated by breaking into two pieces.
Reason : The magnetic moment will be reduced to half when a magnet is broken into
two equal pieces.
3. Assertion : Basic difference between an electric line and magnetic line of force is that
former is discontinuous and the latter is continuous or endless.
Reason : No electric lines of forces exist inside a charged body but magnetic lines do
exist inside a magnet.
4. Assertion : Magnetic moment of an atom is due to both, the orbital motion and spin
motion of every electron.
Reason : A charged particle produces a magnetic field.
5. Assertion : When radius of circular loop carrying current is doubled, its magnetic
moment becomes four times.
Reason : Magnetic moment depends on area of the loop.
6. Assertion : The earth’s magnetic field is due to iron present in its core.
Reason : At a high temperature magnet losses its magnetic property or magnetism.
7. Assertion : A compass needle when placed on the magnetic north pole of the earth
rotates in vertical direction.
Reason : The earth has only horizontal component of its magnetic field at the north
poles.
8. Assertion : The tangent galvanometer can be made more sensitive by increasing the
number of turns of its coil.
Reason : Current through galvanometer is proportional to the number of turns of coil.
9. Assertion : The ferromagnetic substance do not obey Curie’s law.
Reason : At Curie point a ferromagnetic substance start behaving as a paramagnetic
substance.
10. Assertion : The properties of paramagnetic and ferromagnetic substance are not
effected by heating.
Reason : As temperature rises, the alignment of molecular magnets gradually
decreases.
11. Assertion : Soft iron is used as transformer core.
Reason : Soft iron has narrow hysteresis loop.
12. Assertion : Magnetism is relativistic.
Reason : When we move along with the charge so that there is no motion relative to
us, we find no magnetic field associated with the charge.
13. Assertion : The earth’s magnetic field does not affect the working of a moving coil
galvanometer.
Reason : Earth’s magnetic field is very weak.
14. Assertion : A paramagnetic sample display greater magnetisation (for the same
magnetising field) when cooled.
Reason : The magnetisation does not depend on temperature.
15. Assertion : Electromagnets are made of soft iron.
Reason : Coercivity of soft iron is small.
16. Assertion : To protect any instrument from external magnetic field, it is put inside an
iron body.
Reason : Iron is a magnetic substance.
17. Assertion : When a magnet is brought near iron nails, only translatory force act on it.
Reason : The field due to a magnet is generally uniform.
18. Assertion : When a magnetic dipole is placed in a non uniform magnetic field, only a
torque acts on the dipole.
Reason : Force would also acts on dipole if magnetic field were uniform.
19. Assertion : Reduction factor (K) of a tangent galvanometer helps in reducing deflection
to current.
Reason : Reduction factor increases with increase of current.
20. Assertion : The susceptibility of diamagnetic materials does not depend upon
temperature.
Reason : Every atom of a diamagnetic material is not a complete magnet in itself.
21. Assertion : The permeability of a ferromagnetic material is independent of the magnetic
field.
Reason : Permeability of a material is a constant quantity.
22. Assertion : For a perfectly diamagnetic substance permeability is always one.
Reason : The ability of a material of permit the passage of magnetic lines of force
through it is called magnetic permeability.
23. Assertion : Gauss theorem is not applicable in magnetism.
Reason : Mono magnetic pole does not exist.
24. Assertion : Magnetic moment of helium atom is zero.
Reason : All the electron are paired in helium atom orbitals.
25. Assertion : For making permanent magnets, steel is preferred over soft iron.
Reason : As retentivity of steel is smaller.
Explanation / Solutions
1. (d) It is quite clear that magnetic poles always exists in pairs. Since, one can imagine
magnetic field configuration with three poles. When north poles or south poles of two
magnets are glued together. They provide a three pole field configuration. It is also known
that a bar magnet does not exert a torque on itself due to own its field.
2. (b) As we know every atom of a magnet acts as a dipole, So poles cannot be separated.
When magnet is broken into two equal pieces, magnetic moment of each part will be half of
the original magnet.
3. (a) In case of the electric field of an electric dipole, the electric lines of force originate
from positive charge and end at negative charge. Since isolated magnetic lines are closed
continuous loops extending through out the body of the magnet.
4. (c) In an atom, electrons revolve around the nucleus and as such the circular orbits of
electrons may be considered as the small current loops. In addition to orbital motion, an
electron has got spin motion also. So the total magnetic moment of electron is the vector sum
of its magnetic moments due to orbital and spin motion. Charge particles at rest do not
produce electric field.
5. (b) Magnetic dipole moment of the current loop = Ampere turns Area of the coil
Initially magnetic moment M = ir
2, new magnetic moment
2 M'
i(2r) 4 ( ) 4 .
2
i r M
So magnetic moment becomes four times when radius is doubled.
6. (e) The temperature inside the earth is so high that it is impossible for iron core to
behave as magnet and act as a source of magnetic field. The magnetic field of earth is
considered to be due to circulating electric current in the iron (In molten state) and other
conducting materials inside the earth.
7. (d) The earth has only vertical component of its magnetic field at the magnetic poles.
Since compass needle is only free to rotate in horizontal plane. At north pole the vertical
component of earth’s field will exert torque on the magnetic needle so as to align it along its
direction. As the compass needle can not rotate in vertical plane, it will rest horizontally,
when placed on the magnetic pole of the earth.
8. (b) In tangent galvanometer the current through the coil is given by
. tan
2
0
BH
n
r
I
tan n /r
i.e. by reducing its radius or by increasing number of turns of coil we can increase the
sensitivity of tangent galvanometer.
9. (b) The susceptibility of ferromagnetic substance decreases with the rise of temperature
in a complicated manner. After Curies point the susceptibility of ferromagnetic substance
varies inversely with its absolute temperature. Ferromagnetic substance obey’s Curies law
only above its Curie point.
10. (e) The properties of substance is due to alignment of molecules in it. When these
substance are heated, molecules acquire some kinetic energy. Some of molecules may get
back to the closed chain arrangement (produce zero resultant). So they lose their magnetic
property or magnetism. Therefor the properties of both ferromagnetic and paramagnetic are
effected by heating.
11. (a) The core of a transformer undergoes cycles of magnetisation again and again. During
each cycle of magnetisation, energy numerically equal to the area of the hysteresis loop is
spent per unit volume of the core. Therefore, for high efficiency of transformer, the energy
loss will be lesser if the hystersis loop is of lesser area, i.e. narrow. That’s why the soft iron is
used as core, which has narrow hysteresis loop (or area of
B H
curve is very small). Also soft
iron (ferromegnetic substance) has high permeability, high retentivity, low coercivity and low
hystersis loss.
12. (a) A magnetic field is produced by the motion of electric charge. Since motion is relative,
the magnetic field is also relative.
13. (a) In a moving coil galvanometer, the coil is suspended in a very strong uniform
magnetic field created by two magnetic pole pieces. The earth’s magnetic field is quite weak
as compared to that field, therefore, it does not effect the working of magnetic field.
14. (c) A paramagnetic sample display greater magnetisation when cooled, this is because at
lower temperature, the tendency to disrupt the alignment of dipoles (due to magnetising field)
decreases on account of reduced random thermal motion.
15. (a) Electromagnets are magnets, which can be turned on and off by switching the
current on and off. As the material in electromagnets is subjected to cyclic change
(magnification and demagentisation), the hysteresis loss of the material must be small. The
material should attain high value of I and B with low value of magnetising field intensity H.
As soft iron has small coercivity, so it is a best choice for this purpose.
16. (a) Since iron is ferromagnetic in nature, therefore, lines of force due to external
magnetic field prefer to pass through iron.
17. (d) In general, the field due to a magnet is non-uniform. Therefore, it exerts both, a net
force and a torque on the nails which will translate and also rotate the nails before striking to
north pole of magnet with their induced south poles and vice-versa.
18. (d) In a non-uniform magnetic field, both a torque and a net force acts on the dipole. If
magnetic field were uniform, net force on dipole would be zero.
19. (c) The reduction factor of tangent galvanometer is
0
2
n
r
B
G
B
K H
H
Thus reduction factor of a tangent galvanometer depends upon the geometry of its coil. It
increases with increase of radius and decreases with increase in number of turn of the coil of
the galvanometer.
20. (c) Diamagnetism is non-cooperative behaviour of orbiting electrons when exposed to an
applied magnetic field. Diamagnetic substance are composed of atom which have no net magnetic moment (i.e., all the orbital shells are filled and there are no unpaired electrons).
When exposed to a field, a negative magnetization is produced and thus the susceptibility is negative.
Behaviour of diamagnetic material is that the susceptibility is temperature independent.
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21. (d)
22. (e)
Hints :- for a perfectly diamagnetic material permeability is zero.
23. (a)
24. (a) Helium atom has paired electrons so their electron spin are opposite to each other and hence it’s net magnetic moment is zero.
25. (b) Steel is preferred oversoft iron for making permanent magnets, because coercivity of steel is larger.