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JEE physics Assertion & Reason

Magnetism · CBSE STD 12 Science (CBSE - English Medium). 25 questions.

Questions

Assertion & Reason

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25 questions · self-marked practice — reveal the answer and mark yourself.

Question 11 Mark
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.
Answer
(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.
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Question 21 Mark
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.
Answer
(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.
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Question 31 Mark
Assertion : For making permanent magnets, steel is preferred over soft iron.
Reason $\quad$ : As retentivity of steel is smaller.
Answer
(b) Steel is preferred over soft iron for making permanent magnets, because coercivity of steel is larger.
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Question 41 Mark
Assertion : Magnetic moment of helium atom is zero.
Reason : All the electron are paired in helium atom orbitals.
Answer
(a) Helium atom has paired electrons so their electron spin are opposite to each other and hence it's net magnetic moment is zero.
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Question 51 Mark
Assertion : Gauss theorem is not applicable in magnetism.
Reason : Mono magnetic pole does not exist.
Answer
(a)
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Question 61 Mark
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.
Answer
(e) For a perfectly diamagnetic substance,$B=\mu_0(H+I)=0 \quad \therefore I=-H .$Therefore, $\chi_m=\frac{I}{H}=-1$Therefore relative permeability$\mu_r=1+\chi_m=1-1=0 . \quad \therefore \mu=\mu_0 \mu_r=\text { zero. }$i.e. for a perfectly diamagnetic material permeability is zero.
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Question 71 Mark
Assertion : The permeability of a ferromagnetic material is independent of the magnetic field.
Reason : Permeability of a material is a constant quantity.
Answer
(d) The permeability of a ferromagnetic material is not independent of magnetic field, $\vec{B}=K_m \vec{B}_0$.$B_0$ is applied field. The total magnetic field $\vec{B}$ inside a ferromagnet may be $10^3$ or $10^4$ times the applied field $B_0$. The permeability $K_m$ of a ferromagnetic material is not constant, neither the field $\vec{B}$ nor the magnetization $\vec{M}$ increases linearly with $\vec{B}$. Even at small value of $B_0$. From the hysteresis curve, magnetic permeability is greater for lower field.
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Question 81 Mark
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.
Answer
(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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Question 91 Mark
Assertion : Reduction factor $(K)$ of a tangent galvanometer helps in reducing deflection to current.
Reason $\quad$ : Reduction factor increases with increase of current.
Answer
(c) The reduction factor of tangent galvanometer is$K=\frac{B_H}{G}=B_H \times \frac{2 r}{n \mu_0}$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.
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Question 101 Mark
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.
Answer
(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.
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Question 111 Mark
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.
Answer
(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.
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Question 121 Mark
Assertion : To protect any instrument from external magnetic field, it is put inside an iron body.
Reason : Iron is a magnetic substance.
Answer
(a) Since iron is ferromagnetic in nature, therefore, lines of force due to external magnetic field prefer to pass through iron.
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Question 131 Mark
Assertion : Electromagnets are made of soft iron.
Reason : Coercivity of soft iron is small.
Answer
(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 $l$ 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.
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Question 141 Mark
Assertion : A paramagnetic sample display greater magnetisation (for the same magnetising field) when cooled.
Reason : The magnetisation does not depend on temperature.
Answer
(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.
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Question 151 Mark
Assertion : The earth's magnetic field does not affect the working of a moving coil galvanometer.
Reason : Earth's magnetic field is very weak.
Answer
(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.
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Question 161 Mark
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.
Answer
(a) A magnetic field is produced by the motion of electric charge. Since motion is relative, the magnetic field is also relative.
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Question 171 Mark
Assertion : Soft iron is used as transformer core.
Reason : Soft iron has narrow hysteresis loop.
Answer
(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.
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Question 181 Mark
Assertion : The properties of paramagnetic and ferromagnetic substance are not effected by heating.
Reason : As temperature rises, the alignment of molecular magnets gradually decreases.
Answer
(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.
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Question 191 Mark
Assertion : The ferromagnetic substance do not obey Curie's law.
Reason : At Curie point a ferromagnetic substance start behaving as a paramagnetic substance.
Answer
(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.
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Question 201 Mark
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.
Answer
(b) In tangent galvanometer the current through the coil is given by $I=\frac{2 r}{n \mu_0} \cdot B_H \tan \theta \Rightarrow \tan \theta \propto n / r$i.e. by reducing its radius or by increasing number of turns of coil we can increase the sensitivity of tangent galvanometer.
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Question 211 Mark
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.
Answer
(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.
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Question 221 Mark
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.
Answer
(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.
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Question 231 Mark
Assertion : When radius of circular loop carrying current is doubled, its magnetic moment becomes four times.
Reason $\quad$ : Magnetic moment depends on area of the loop.
Answer
(b) Magnetic dipole moment of the current loop$=\text { Ampere turns } \times \text { Area of the coil }$Initially magnetic moment $M=i \pi r$, new magnetic moment $M^{\prime}=i \pi(2 r)^2=4 i\left(\pi r^2\right)=4 M$
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Question 241 Mark
Assertion : Magnetic moment of an atom is due to both, the orbital motion and spin motion of every electron.
Reason $\quad$ : A charged particle produces a magnetic field.
Answer
(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.
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Question 251 Mark
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.
Answer
(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.
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