CBSE Class 12 Physics VBQs Magnetism and Matter

Question. The domain formation is a necessary feature of
(a) diamagnetism
(b) Para magnetism
(c) ferromagnetism
(d) all of these
Answer. C

Question. A bar magnet is cut into two equal halves by a plane parallel to the magnetic axis. Of the following physical quantities, the one which remains unchanged is
(a) pole strength
(b) magnetic moment
(c) intensity of magnetisation
(d) None of these
Answer. C

Question. Curie temperature is the temperature above which
(a) a ferromagnetic material becomes paramagnetic
(b) a paramagnetic material becomes diamagnetic
(c) a ferromagnetic material becomes diamagnetic
(d) a paramagnetic material becomes ferromagnetic
Answer. A

Question. The susceptibility of a ferromagnetic material is χ at 27°C. At what temperature will its susceptibility be 0.5 χ ?
(a) 54°C
(b) 327°C
(c) 600°C
(d) 237°C
Answer. B

Question. Relative permittivity and permeability of a material are εr and μr, respectively. Which of the following values of these quantities are allowed for a diamagnetic material?
(a) εr=1.5, μr=0.5
(b) εr=0.5, μr=0.5
(c) εr=1.5, μr=1.5
(d) εr=0.5, μr=1.5
Answer. A

Question. If a magnetic dipole of moment M situated in the direction of a magnetic field B is rotated by 180°, then the amount of work done is
(a) MB
(b) 2MB
(c) MB/√2
(d) √MB
Answer. B

Question. The relative permeability of a medium is 0.075. What is its magnetic susceptibility?
(a) 0.925
(b) – 0.925
(c) 1.075
(d) -1.075
Answer. B

Question. Needles N1, N2 and N3 are made of a ferromagnetic, a paramagnetic and a diamagnetic substance respectively. A magnet when brought close to them will
(a) attract N1 and N2 strongly but repel N3
(b) attract N1 strongly, N2 weakly and repel N3 weakly
(c) attract N1 strongly, but repel N2 and N3 weakly
(d) attract all three of them
Answer. B

ASSERTION REASON QUESTIONS

(a) Both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
(b) Both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
(c) Assertion is correct but Reason is incorrect.
(d) Assertion and Reason are incorrect

Question. Assertion: If a compass needle be kept at magnetic north pole of the earth the compass needle may stay in any direction.
Reason: Dip needle will stay vertical at the north pole of earth
Answer. B

Question. Assertion: Diamagnetic materials can exhibit magnetism.
Reason: Diamagnetic materials have permanent magnetic dipole moment.
Answer. C

Question. Assertion: Electromagnetic are made of soft iron.
Reason: Coercivity of soft iron is small.
Answer. B

Question. Assertion: A paramagnetic sample display greater magnetisation (for the same magnetic field) when cooled.
Reason: The magnetisation does not depend on temperature.
Answer. D

Question. 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

CASE BASED QUESTIONS

1. In small fields, ferromagnetic materials typically have much larger susceptibility, and therefore larger permeability than paramagnetic materials. Ferromagnetism results because of spontaneous, self-aligning, cooperative interaction among relatively large number of iron atoms in regions called domains. As a result of molecular interactions the molecular magnetic moments in each domain are aligned parallel to one another. In other words, each domain is spontaneously magnetised to saturation even in the absence of any external magnetic field. The directions of magnetisation in different domains are random, so that the resultant magnetisation is zero and the specimen is un-magnetised.

Question. What changes will occur in specimen on placing it inside a solenoid and increasing H (magnetising intensity)?
(a) Intensity of magnetisation decreases
(b) Intensity of magnetisation increases
(c) Magnetic susceptibility of specimen increases
(d) Magnetic susceptibility of specimen decreases
Answer. B

Question. With an increase in temperature, magnetic susceptibility of a ferromagnetic material____________
(a) Decreases
(b) Increases
(c) Remains constant
(d) First increases and then decreases
Answer. A

Question. For a diamagnetic material, which of the following option is correct?
(a) Magnetic susceptibility < 0
(b) Magnetic susceptibility > 0
(c) Magnetic susceptibility = 0
(d) Magnetic susceptibility = 1
Answer. A

Question. The property possessed by a ferromagnetic substance only is
(a) Hysteresis
(b) Susceptibility
(c) Directional property
(d) Strong attraction of magnetic substance
Answer. D

2. By analogy to Gauss’s law of electrostatics, we can write Gauss’s law of magnetism as ∫B.dS= 0. We do not have an isolated magnetic pole in nature. At least none has been found to exist till date. The smallest unit of source of magnetic field is a magnetic dipole where the net magnetic pole is zero. Hence, the net magnetic pole enclosed by any closed surface is always zero. Correspondingly, the flux of the magnetic field through any close surface is zero.
Consider the two idealised systems:

Question. (I) A parallel plate capacitor with large plates and small separation and (II) A long solenoid of length L>>R the radius of cross section.
In case (I) 𝐸̅ is ideally treated as a constant between the plates and zero outside. In (II) magnetic field B is constant inside the solenoid and zero outside. These idealised assumptions, however, contradict fundamental laws as
(a) Case (I) agrees with ∫𝐸.𝑑𝑙 = 0
(b) Case (II) contradicts ∫𝐵.𝑑𝑙 = I
(c) Case (II) contradicts Gauss’s law for magnetic fields
(d) Case (I) contradicts Gauss’s law for electrostatic fields.
Answer. C

Question. Net magnetic flux through any closed surface kept in a magnetic field is
(a) Zero
(b) μ0/2𝜋
(c) 2𝜋𝜇0
(d) 4 𝜇0/𝜋
Answer. A

Question. A closed surface S encloses a magnetic dipole of magnetic moment 2ml. The magnetic flux emerging from the surface
(a) μ0 m
(b) 2 μ0 m
(c) 2m/ μ0
(d) Zero
Answer. D

Question. The surface integral of a magnetic field over a surface is
(a) Proportional to mass enclosed
(b) Proportional to charge enclosed
(c) Zero
(d) Equal to its magnetic flux through that surface
Answer. C

3. Before the 19th century, scientists believed that magnetic properties were confined to a few materials like iron, cobalt and nickel. But in 1846, Curie and Faraday discovered that all the materials in the universe are magnetic to some extent. These magnetic substances are categorised in two groups. Weak magnetic materials are called diamagnetic and para magnetic materials. Strong magnetic materials are called ferromagnetic materials. According to the modern theory of magnetism, the magnetic response of any material is due to circulating electrons in the atoms. Each such electron has a magnetic moment in a direction perpendicular to the plane of circulation. In magnetic materials all these magnetic moments due to the orbit and spin motion of all the electrons in any atom vectorially add up to a resultant magnetic moment. The magnitude and direction of these resultant magnetic moment is responsible for the behaviour of the materials. For diamagnetic materials χ is small and negative and for paramagnetic materials χ is small and positive. Ferromagnetic materials have a large χ and are characterised by non–linear relation between 𝐵̅ and 𝐻̅.

Question. The universal (or inherent) property among all substance is
(a) Diamagnetism
(b) Para magnetism
(c) Ferromagnetism
(d) Both (a) and (b)
Answer. A

Question. When a bar is placed near a strong magnetic field and it is repelled, then the material of the bar is
(a) Diamagnetic
(b) Ferromagnetic
(c) Paramagnetic
(d) Anti-ferromagnetic
Answer. A

Question. Magnetic susceptibility of a diamagnetic substance
(a) Decreases with temperature
(b) Is not affected by temperature
(c) Increases with temperature
(d) First increases then decreases with temperature
Answer. B

Question. For a para magnetic material, the dependence of the magnetic susceptibility χ on the absolute temperature is given as
(a) χ ∝ T
(b) χ ∝ 1/T²
(c) χ ∝ 1/T
(d) independent
Answer. C

4. There exists a perfect diamagnet, namely, a superconductor. This is a metal at very low temperatures. In this case χ = –1, μr = 0, μ= 0. The external magnetic field is totally expelled. Interestingly, this material is also a perfect conductor. However, there exists no classical theory which ties these two properties together. A quantum-mechanical theory by Bardeen, Cooper, and Schrieffer (BCS theory) explains these effects. The BCS theory was proposed in1957 and was eventually recognised by a Nobel Prize in physics in 1970.

Question. The value of magnetic susceptibility for a superconductor is
(a) Zero
(b) Infinity
(c) +1
(d) -1
Answer. D

Question. Superconductors are
(a) Insulators
(b) Semiconductors
(c) Conductors
(d) Perfect conductors
Answer. D

Question. Resistance of a superconductor is
(a) Infinite
(b) Zero
(c) Maximum
(d) Minimum
Answer. B

Question. Which of the following is a property of superconductors?
(a) Meissner Effect
(b) Hall Effect
(c) Photoelectric effect
(d) Doppler effect
Answer. A

5. A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, cobalt, etc. and attracts or repels other magnets. A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). The resemblance of magnetic field lines for a bar magnet and a solenoid suggest that a bar magnet may be thought of as a large number of circulating currents in analogy with a solenoid. Cutting a bar magnet in half is like cutting a solenoid. We get two smaller solenoids with weaker magnetic properties. The field lines remain continuous, emerging from one face of the solenoid and entering into the other face. One can test this analogy by moving a small compass needle in the neighbourhood of a bar magnet and a current-carrying finite solenoid and noting that the deflections of the needle is similar in both cases.

The arrangement of iron filings surrounding a bar magnet. The pattern mimics magnetic field lines. The pattern suggests that the bar magnet is a magnetic dipole.

Question. What happens if a bar magnet is cut into two pieces: (i) transverse to its length, (ii) along its length?
Answer. In either case, one gets two magnets, each with a north and south pole.

Question. A magnetised needle in a uniform magnetic field experiences a torque but no net force. An iron nail near a bar magnet, however, experiences a force of attraction in addition to a torque. Why?
Answer. No force if the field is uniform. The iron nail experiences a non-uniform field due to the bar magnet. There is induced magnetic moment in the nail, therefore, it experiences both force and torque. The net force is attractive because the induced south pole (say) in the nail is closer to the north pole of magnet than induced north pole.

Question. Must every magnetic configuration have a north pole and a south pole?
Answer. Not necessarily. True only if the source of the field has a net non-zero magnetic moment.

Question. Two identical looking iron bars A and B are given, one of which is definitely known to be magnetised. (We do not know which one.) How would one ascertain whether or not both are magnetised? If only one is magnetised, how does one ascertain which one? [Use nothing else but the bars A and B.]
Answer. Try to bring different ends of the bars closer. A repulsive force in some situation establishes that both are magnetised. If it is always attractive, then one of them is not magnetised. In a bar magnet the intensity of the magnetic field is the strongest at the two ends (poles) and weakest at the central region. This fact may be used to determine whether A or B is the magnet. In this case, to see which one of the two bars is a magnet, pick up one, (say, A) and lower one of its ends; first on one of the ends of the other (say, B), and then on the middle of B. If you notice that in the middle of B, A experiences no force, then B is magnetised. If you do not notice any change from the end to the middle of B, then A is magnetised.

1 MARK QUESTIONS

Question. How does the pole strength and magnetic moment of each part of a bar magnet change if it is cut into two equal pieces transverse to its length.
Answer. i) pole strength remains same.
ii)magnetic moment of each part is halved.

Question. Can two magnetic lines of force intersect? Justify your answer.
Answer. No

Question. Magnetic lines of force always forms closed loops. Comment.
Answer. Magnetic poles always exist in pairs.

Question. How are magnetic field lines different from electrostatic field lines.
Answer. Electric field lines do not form closed loops, but magnetic field lines form closed loops.

Question. Why is a current loop considered a magnetic dipole?
Answer. A current loop possesses a magnetic dipole moment and it experiences a torque in a magnetic field.

Question. Why is diamagnetism independent of temperature?
Answer. The induced dipole moment in a diamagnetic sample is always opposite to the magnetising field no matter what the internal motion of the atom is.

Question. What happens when a diamagnetic substance is placed in a varying magnetic field?
Answer. Tend to move from stronger to weaker parts of the field.

Question. How can paramagnetic and diamagnetic material rods be distinguished in a magnetic field?
Answer. When suspended freely in a magnetic field the paramagnetic rod sets itself parallel to the field and the diamagnetic rod sets itself perpendicular to the field.

Question. An iron bar is heated to 1000°C and then cooled in a magnetic field free space-will it retain magnetism?
Answer. Iron loses its magnetism when heated to 1000°C and it does regain its magnetism.

Question. A certain region is to be shielded from magnetic fields. Suggest a method.
Answer. Surround the region by soft iron rings. Magnetic field lines will be drawn into the rings, and the enclosed space will be free of magnetic field.

2 MARK QUESTIONS

Question. A coil of N turns and radius R carries a current I. It is unwound and rewound to make another coil of radius R/2, current remaining the same. Calculate the ratio of the magnetic moments of the new coil and the original coil.
Answer. If L is the length of the wire, then
L = N×(2πR) = N′ 2πR/2
Let the number of turns in new coil be N′=2N
Original magnetic moment, M=NIA= NIπR²
New magnetic moment,
M′ = N′IA′ = 2NI × (πR/2)² = NI × πR²/2
M′/M = 1/2= 1:2

Question. Define ‘intensity of magnetization’ of a magnetic material. How does it vary with temperature for a paramagnetic material?
Answer. The intensity of magnetization of a paramagnetic material varies inversely with its temperature

Question. Two identical bars, one of paramagnetic material and other of diamagnetic material are kept in a uniform external magnetic field parallel to it. Draw diagrammatically the modifications in the magnetic field pattern in each case.
Answer. i) A para magnetic material develops feeble magnetization in the direction of the magnetizing field. Hence the lines of force become denser in the paramagnetic material.
ii)Lines of force become less dense in the diamagnetic material.

Question. State any four properties of magnetic field lines.
Answer. a) magnetic field lines are closed curves.
b) never cross each other.
c) tangent to the field line at any point gives the direction of magnetic field in that region.
d) relative closeness of the field line gives the strength of the field.

Question. The susceptibility of a magnetic material is -0.085. Identify the magnetic type of the substance. A specimen of this material is kept in a uniform magnetic field. Draw the modified field pattern.
Answer. As the susceptibility has a small negative value, the given material is diamagnetic in nature.

Question. A bar magnet is held perpendicular to a uniform field, Find the angle by which the magnet is to be rotated so that the torque acting on it becomes half.
Answer. τ₁=mB sin 90=mB
Τ₂=mB sinθ=mB/2
Sinθ=1/2    θ=30º

Question. A magnet of magnetic moment 2J/T is aligned in the direction of magnetic field of 0.1T, What is the net work done to bring the magnet normal to the magnetic field?
Answer. W=MB(cosθ₁-cosθ₂)
=2x0.1x(cos0-cos90)
=0.2J

Question. A short bar magnet placed with its axis at 30º with a uniform external magnetic field of 0.50T experiences a torque of magnitude equal to 9.0x10-2J. What is the magnetic moment of the magnet?
Answer. m=τ/Bsinθ
=9.0x10^-2 /0.5xsin30
=0.36J/T

Question. Three identical specimens of magnetic materials Nickel, Antimony, Aluminium are kept in a uniform magnetic field. Draw the modification in the field lines in each case. Justify your answer.
Answer. Nickel -ferromagnetic
Antimony-diamagnetic
Aluminium-paramagnetic

Question. The relative magnetic permeability of a magnetic material is 800. Identify the nature of the magnetic material and state its two properties.
Answer. As relative magnetic permeability is large, it is ferromagnetic.
1)they are strongly attracted by magnets.
2)susceptibility is positive and large.

3 MARK QUESTIONS

Question. Two magnets of magnetic moments M and M√3 are joined to form a cross. The combination is suspended in a uniform magnetic field B. The magnetic moment M now makes an angle θ with the field direction. Find the value of angle θ.
Answer. In the position of equilibrium
MBsinθ= √3 MBsin(90-θ) =√3 MBcosθ

Question. The magnetic moment of a current loop is 2.1x10^-25Am². Find the magnetic field on the axis of the loop at a distance of 1.0Aº from the loop.
Answer. The current loop is a magnetic dipole.
So Baxial=μ0 2m/4πr³ =4.2X10^-2 T

Question. An iron ring of relative permeability μr has windings of insulated copper wire of n turns per metre. When the current in the windings is I, find the expression for the magnetic field in the ring.
Answer. The magnetic field inside the ring has constant magnitude and tangential direction at every point.
Total number of turns in the windings=2πrn
I enclosed =2πrnI
From Amperes circuital law, we have
BX2πr=μ0 μrx2πrnI
B=μ0μr nI

Question. What is the basic difference between the atom or molecule of a diamagnetic and paramagnetic material?
Answer. In a diamagnetic substance, the electrons occur in pairs and their magnetic moments cancel out. The net magnetic moment of the atom is zero.
In a paramagnetic substance, the atoms have a net dipole moment due to the presence of some unpaired electron or due to the non-cancellation of the spins of two electrons.

Question. From molecular point of view, discuss the temperature dependence of susceptibility for diamagnetism, paramagnetism and ferromagnetism.
Answer. Diamagnetism is due to orbital motion of electrons developing magnetic moments opposite to applied field and hence is not much affected by temperature.
Para magnetism and ferromagnetism is due to alignments of atomic magnetic moments in the direction of the applied field. As temperature increases, this alignment is disturbed and hence susceptibilities of both decrease as temperature increases.

Question. A ball of superconducting material is dipped in liquid nitrogen and placed near a bar magnet. (i) In which direction will it move? (ii) What will be the direction of its magnetic moment?
Answer. As the ball is dipped in liquid nitrogen, its temperature becomes less than the critical temperature and it becomes a super conductor and it becomes diamagnetic. The ball moves away
from the bar magnet.

Question. A planar loop of irregular shape encloses an area of 7.5x10^-4 m² and carries a current of 12A. The sense of flow of current appears to be clockwise to an observer. What is the magnitude and direction of the magnetic moment vector associated with the current loop?
Answer. Magnetic moment associated with the loop is m=IA
=12X7.5X10^-4 =9.0X10^-3J/T
The direction of magnetic moment is along the normal to the plane of the loop away from the observer.

Question. A current of 3A flows through a plane circular coil of radius 4cm and having 20 number of turns. The coil has been placed in a uniform magnetic field of 0.5T. Find 1) dipole moment of the coil 2) potential energy of the dipole.
Answer. 
1). m=NIA=0.3 Am²
2) U= -mB=-0.15J