JEE Physics Magnetism And Matter MCQs Set 34

Multiple Choice Questions (MCQs) for JEE Physics: Magnetism and Matter

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Multiple Choice Questions

Question 1:
Assertion (A): Electromagnets are made of soft iron.
Reason (R): The coercivity of soft iron is small.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. An electromagnet must be easy to magnetise and demagnetise.
2. Small coercivity of soft iron makes this possible.

Teacher's Note:
1. Low coercivity.

Question 2: The work done in turning a magnet of magnetic moment M by an angle of 90 degrees from the meridian is n times the work done to turn it through 60 degrees. The value of n is
(a) 2
(b) 1
(c) 0.5
(d) 0.25
Show Answer & Explanation

Answer: (a) 2

Explanation:
1. W = MB(1 - cos θ).
2. W(90°) = MB and W(60°) = MB/2, so n = 2.

Teacher's Note:
1. W = MB(1 - cos θ).

Question 3: The relation between B, H and I in SI units is
(a) \( B=\mu_0(H+I) \)
(b) \( B=H+4\pi I \)
(c) \( H=\mu_0(B+I) \)
(d) none of these
Show Answer & Explanation

Answer: (a) \( B=\mu_0(H+I) \)

Explanation:
1. B = μ₀(H + I), where I is the magnetisation.

Teacher's Note:
1. Know the basic fact.

Question 4: The 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
Show Answer & Explanation

Answer: (a) a ferromagnetic material becomes paramagnetic

Explanation:
1. Above the Curie point the domain order is destroyed by thermal agitation and the ferromagnet behaves as a paramagnet.

Teacher's Note:
1. Know the basic fact.

Question 5: A magnet of magnetic moment 20 CGS units is freely suspended in a uniform magnetic field of intensity 0.3 CGS units. The work done in deflecting it by an angle of 30 degrees in CGS units is
(a) 6
(b) \( 3\sqrt3 \)
(c) \( 3(2-\sqrt3) \)
(d) 3
Show Answer & Explanation

Answer: (c) \( 3(2-\sqrt3) \)

Explanation:
1. W = MB(1 - cos 30°) = 20 × 0.3 × (1 - √3/2) = 3(2 - √3).

Teacher's Note:
1. W = MB(1 - cos θ).

Question 6: A steel wire of length l has a magnetic moment M. It is bent at its midpoint into an L-shape, two perpendicular arms each of length l/2. The new magnetic moment is
(a) \( M \)
(b) \( M/\sqrt2 \)
(c) \( M/2 \)
(d) \( 2M \)
Show Answer & Explanation

Answer: (b) \( M/\sqrt2 \)

Explanation:
1. M = m l.
2. After bending, the poles are separated by the diagonal √((l/2)² + (l/2)²) = l/√2.
3. The new moment is m l/√2 = M/√2.

Teacher's Note:
1. Moment = pole strength × pole separation.

Question 7: A torque of 10⁻⁵ N m is required to hold a magnet at 90 degrees with the horizontal component H of the earth's magnetic field. The torque required to hold it at 30 degrees will be
(a) \( 5\times10^{-6}\ \text{N m} \)
(b) data is insufficient
(c) \( \tfrac13\times10^{-5}\ \text{N m} \)
(d) \( 5\sqrt3\times10^{-6}\ \text{N m} \)
Show Answer & Explanation

Answer: (a) \( 5\times10^{-6}\ \text{N m} \)

Explanation:
1. τ = MH sin θ.
2. τ(30°)/τ(90°) = sin 30° = 1/2, so τ = 5 × 10⁻⁶ N m.

Teacher's Note:
1. τ is proportional to sin θ.

Question 8: The time periods of vibration of two bar magnets placed so that their moments add and so that they subtract are 4 s and 6 s respectively. The ratio of their magnetic moments M1/M2 is
(a) 6 : 4
(b) 30 : 16
(c) 2.6 : 1
(d) 1.5 : 1
Show Answer & Explanation

Answer: (c) 2.6 : 1

Explanation:
1. T is proportional to 1/√M.
2. M1 + M2 is proportional to 1/16 and M1 - M2 to 1/36.
3. M1 : M2 = (1/16 + 1/36) : (1/16 - 1/36) = 52 : 20 = 2.6 : 1.

Teacher's Note:
1. Square the periods.

Question 9: Which magnetic materials have negative susceptibility?
(a) diamagnetic materials
(b) paramagnetic materials
(c) ferromagnetic materials
(d) all of the above
Show Answer & Explanation

Answer: (a) diamagnetic materials

Explanation:
1. Only diamagnetic materials have χ less than zero.

Teacher's Note:
1. Know the basic fact.

Question 10: A compass needle which is free to move in a horizontal plane is taken to a geomagnetic pole. It will
(a) stay in the north-south direction only
(b) stay in the east-west direction only
(c) become rigid showing no movement
(d) stay in any position
Show Answer & Explanation

Answer: (d) stay in any position

Explanation:
1. The field is vertical, so the horizontal component is zero and no torque acts on the needle in the horizontal plane.

Teacher's Note:
1. Know the basic fact.

Question 11: Two short magnets with their axes horizontal and perpendicular to the magnetic meridian are placed with their centres 40 cm east and 50 cm west of a magnetic needle. If the needle remains undeflected, the ratio of their magnetic moments M1 : M2 is
(a) 4 : 5
(b) 16 : 25
(c) 64 : 125
(d) \( 2:\sqrt5 \)
Show Answer & Explanation

Answer: (c) 64 : 125

Explanation:
1. The needle lies on the axial line of each magnet, and the two axial fields are opposite.
2. M1/40³ = M2/50³, so M1 : M2 = 64000 : 125000 = 64 : 125.

Teacher's Note:
1. Axial field is proportional to M/d³.

Question 12: A magnetic compass is not useful for navigation near the magnetic poles since
(a) R = 0
(b) V = 0
(c) H = 0
(d) θ = 0°
Show Answer & Explanation

Answer: (c) H = 0

Explanation:
1. The compass needle responds to the horizontal component H, which is zero at the poles.

Teacher's Note:
1. Know the basic fact.

Question 13: The ratio of the intensity of magnetisation to the magnetising field is called
(a) permeability
(b) magnetic intensity
(c) magnetic induction
(d) magnetic susceptibility
Show Answer & Explanation

Answer: (d) magnetic susceptibility

Explanation:
1. χ = I/H.

Teacher's Note:
1. Know the basic fact.

Question 14: Which of the following statements are incorrect? I. Diamagnetism is temperature independent. II. Paramagnetism is explained by domain theory. III. Above the Curie temperature a ferromagnetic material becomes paramagnetic. IV. The magnetic susceptibility is small and negative for a diamagnetic substance.
(a) I only
(b) II only
(c) I, II and III
(d) II and IV
Show Answer & Explanation

Answer: (b) II only

Explanation:
1. Only II is incorrect: domain theory explains ferromagnetism, not paramagnetism.

Teacher's Note:
1. Domains belong to ferromagnetism.

Question 15: Which of the following is responsible for the earth's magnetic field?
(a) convective currents in the earth's core
(b) diversive current in the earth's core
(c) rotational motion of the earth
(d) translational motion of the earth
Show Answer & Explanation

Answer: (a) convective currents in the earth's core

Explanation:
1. Convective motion of molten conducting material in the outer core is the accepted source of the earth's field.

Teacher's Note:
1. Know the basic fact.

Question 16: A substance which retains its magnetic moment for a long time is
(a) diamagnetic
(b) paramagnetic
(c) ferromagnetic
(d) non-magnetic
Show Answer & Explanation

Answer: (c) ferromagnetic

Explanation:
1. Ferromagnetic domains stay aligned, giving retentivity.

Teacher's Note:
1. Know the basic fact.

Question 17: A magnetising field of 2 × 10³ A/m produces a magnetic flux density of 8π T in an iron rod. The relative permeability of the rod will be
(a) \( 10^{2} \)
(b) 1
(c) \( 10^{4} \)
(d) \( 10^{3} \)
Show Answer & Explanation

Answer: (c) \( 10^{4} \)

Explanation:
1. μr = B/(μ₀ H) = 8π/(4π × 10⁻⁷ × 2 × 10³) = 10⁴.

Teacher's Note:
1. μr = B/(μ₀H).

Question 18: A magnetic needle is kept in a non-uniform magnetic field. It experiences
(a) neither a force nor a torque
(b) a torque but not a force
(c) a force but not a torque
(d) a force and a torque
Show Answer & Explanation

Answer: (d) a force and a torque

Explanation:
1. The poles feel unequal forces in a non-uniform field, so there is a net force as well as a torque.

Teacher's Note:
1. Know the basic fact.

Question 19: At the magnetic north pole of the earth, the horizontal component of the earth's magnetic field and the angle of dip are respectively
(a) zero, maximum
(b) maximum, minimum
(c) maximum, maximum
(d) minimum, minimum
Show Answer & Explanation

Answer: (a) zero, maximum

Explanation:
1. At the pole the field is vertical, so B_H = 0 and the dip is 90° (maximum).

Teacher's Note:
1. Know the basic fact.

Question 20: A ferromagnetic material is heated above its Curie temperature. Which of the following statements are incorrect? I. Ferromagnetic domains are perfectly arranged. II. Ferromagnetic domains are not influenced. III. The ferromagnetic material changes into a diamagnetic material.
(a) I and II
(b) I, II and III
(c) II and III
(d) I and III only
Show Answer & Explanation

Answer: (b) I, II and III

Explanation:
1. Above the Curie temperature the domains are destroyed and the material becomes paramagnetic.
2. So I, II and III are all incorrect.

Teacher's Note:
1. It becomes paramagnetic.

Question 21: Two bar magnets of the same mass, length and breadth but with magnetic moments M and 2M are joined pole for pole and suspended by a string. The time period of the assembly in a magnetic field of strength H is 3 s. If the polarity of one of the magnets is now reversed and the combination is again made to oscillate in the same field, the time of oscillation is
(a) \( \sqrt3\ \text{s} \)
(b) \( 3\sqrt3\ \text{s} \)
(c) \( 3\ \text{s} \)
(d) \( 6\ \text{s} \)
Show Answer & Explanation

Answer: (b) \( 3\sqrt3\ \text{s} \)

Explanation:
1. Initially the net moment is 3M and later 2M - M = M.
2. T is proportional to 1/√M, so T' = 3 × √(3M/M) = 3√3 s.

Teacher's Note:
1. T = 2π√(I/MB).

Question 22: Which of the following statements are incorrect? I. A paramagnetic material tends to move from a strong field to a weak field. II. A magnetic material is in the paramagnetic phase below its Curie temperature. III. The resultant magnetic moment in an atom of a diamagnetic substance is zero.
(a) I only
(b) II only
(c) I and II
(d) I, II and III
Show Answer & Explanation

Answer: (c) I and II

Explanation:
1. A paramagnet moves from weak to strong field, so I is false.
2. A ferromagnet is paramagnetic above its Curie temperature, so II is false.
3. III is true.

Teacher's Note:
1. Check the direction of motion.

Question 23: The magnetic dipole moment is a vector quantity directed from
(a) south pole to north pole
(b) north pole to south pole
(c) east to west
(d) west to east
Show Answer & Explanation

Answer: (a) south pole to north pole

Explanation:
1. By convention the moment points from the south pole to the north pole inside the magnet.

Teacher's Note:
1. Know the basic fact.

Question 24: A current I flows in a coil of area A and N turns. The magnetic moment of the coil is equal to
(a) NIA
(b) NI/A
(c) \( NI/\sqrt A \)
(d) \( N^2AI \)
Show Answer & Explanation

Answer: (a) NIA

Explanation:
1. M = N I A.

Teacher's Note:
1. Know the basic fact.

Question 25:
Assertion (A): The earth's magnetic field is due to the iron present in its core.
Reason (R): At a high temperature a magnet loses its magnetism.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (d) A is false, but R is true.

Explanation:
1. The core is too hot for iron to be ferromagnetic, so the assertion is false.
2. The reason is true: above the Curie point magnetism is lost.

Teacher's Note:
1. Core is above the Curie temperature.

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