Chapter 5: Magnetism and Matter Application Questions | physics Class 12 CBSE

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Chapter 5: Magnetism and Matter Application Questions | physics Class 12 CBSE | ByteNotes.in

Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 1

Applied Concept

A student suspends a bar magnet freely from a thread. The magnet rotates and comes to rest. With reference to the properties of magnetic field lines and Earth's magnetism, explain what happens and why the magnet points in the north-south direction.

The Earth behaves as a large magnet with its magnetic field pointing approximately from the geographic south to the geographic north; this field exerts a torque on the freely suspended bar magnet.
The torque tau = m x B acts on the magnet, rotating it until the torque becomes zero, i.e., until m is aligned with the Earth's B field; at this point the magnet points in the north-south direction.
The north pole of the magnet points toward the geographic north because the Earth's geographic north corresponds to a magnetic south pole of Earth's equivalent internal magnet, causing attraction between unlike poles.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 2

Applied Concept

In a physics laboratory, a student places a bismuth rod between the poles of a strong electromagnet. The rod moves toward the weaker field region. Identify the magnetic property of bismuth and explain the mechanism responsible for this behaviour.

Bismuth is a diamagnetic material (mentioned in the chapter as an example); its magnetic susceptibility chi is small and negative.
The mechanism involves Lenz's law: when the field is applied, electrons with orbital moments aligned with the field slow down while those opposed speed up, inducing a net magnetic moment opposite to the applied field.
Because the induced moment opposes the field, the bismuth rod is repelled toward the weaker-field region, consistent with diamagnetic behaviour.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 3

Applied Concept

An engineer wants to design a powerful electromagnet for an MRI machine. She selects a ferromagnetic core material. Using the concepts of H, M, and mu_r from the chapter, explain why a ferromagnetic core dramatically increases the magnetic field compared to an air-core solenoid carrying the same current.

The magnetic intensity H = nI depends only on the solenoid current and turns per unit length, and is unchanged by the core material.
A ferromagnetic material has chi >> 1, so M = chi*H is very large; the additional field Bm = mu0*M is enormous compared to B0 = mu0*H of the air-core solenoid.
The total field B = mu0*(H + M) = mu0*mu_r*H; since mu_r >> 1 (exceeding 1000 for ferromagnets), B is hundreds of times larger than B0 for the same current.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 4

Applied Concept

With reference to the chapter's discussion of superconductors, explain why a magnet floats above a superconducting disk. What property of the superconductor is responsible?

A superconductor is a perfect diamagnet with chi = -1 and mu_r = 0; it completely expels any external magnetic field from its interior — this is the Meissner effect.
The magnet placed above the superconducting disk creates a magnetic field that the superconductor attempts to expel; by Lenz's law, currents are induced in the superconductor that produce a field exactly cancelling the external field inside.
By Newton's third law, the magnet is repelled upward by the force from the superconductor's induced field with equal and opposite force, causing the magnet to levitate above the disk.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 5

Applied Concept

A compass needle placed at point O is observed to align in a specific direction. Another identical magnetised needle Q is placed at various positions around O. Referring to the potential energy formula U = -m.B, identify the configuration of Q that has the lowest potential energy.

Potential energy of dipole Q in the field of P is U = -mQ.BP; lowest potential energy occurs when mQ is parallel to BP (theta = 0 degrees) giving U = -mB (minimum).
If Q is on the axis of P, the field BP is along m_P; so Q aligned parallel to BP and along the axis of P gives the lowest potential energy configuration (corresponding to PQ6 in the chapter's example).
In this configuration, Q is in stable equilibrium because any displacement increases potential energy, and the torque tends to restore Q to this orientation.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 6

Applied Concept

A geologist finds a rock sample that is strongly attracted to a magnet even after being removed from a magnetic field for a long time. Identify the type of magnetic material and name two examples from the chapter. What determines whether it will remain magnetised?

The rock sample is a hard ferromagnetic material (hard ferromagnet); it retains its magnetisation long after the external field is removed, which is the defining property of permanent magnets.
Examples from the chapter: Alnico (an alloy of iron, aluminium, nickel, cobalt and copper) and the naturally occurring lodestone.
The material remains magnetised because its domain structure does not easily revert to random orientation once aligned; the magnetisation persists until a demagnetising field is applied or the temperature exceeds the Curie temperature.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 7

Applied Concept

A solenoid with 1000 turns per metre carries a current of 2 A and has a core with relative permeability 400. Calculate the magnetic intensity H, the magnetic field B, and the magnetisation M inside the solenoid.

Magnetic intensity H = nI = 1000 x 2 = 2 x 10^3 A/m; note that H depends only on the solenoid geometry and current, not on the core material.
Magnetic field B = mu_r * mu0 * H = 400 x 4pi x 10^-7 x 2 x 10^3 = 1.0 T.
Magnetisation M = (mu_r - 1) * H = (400 - 1) x 2000 = 399 x 2000 approximately 8 x 10^5 A/m; M is much larger than H, reflecting the strong response of the core material.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 8

Applied Concept

With reference to Gauss's law for magnetism, a student draws a diagram where magnetic field lines emerge from a single point in space with no current present. Point out the error and state the correct principle.

The diagram is incorrect because magnetic field lines can never emanate from a single point in space; there are no sources or sinks of B (no magnetic monopoles exist).
Gauss's law for magnetism states that the net magnetic flux through any closed surface is zero; pictorially, as many field lines must enter a closed surface as leave it, with no net flux.
The field lines drawn (emanating from a point) would actually represent the electric field of a long positively charged wire, not a magnetic field; correct magnetic field lines must form closed loops.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 9

Applied Concept

A short bar magnet of magnetic moment 0.48 J/T is placed with its axis along a uniform external field of 0.25 T. Explain qualitatively what torque the magnet experiences and what would happen to its potential energy if it were rotated to be perpendicular to the field.

When the magnet's axis is along the field (theta = 0), torque tau = mB sin(0) = 0 and potential energy U = -mB is at its minimum; this is the most stable equilibrium.
If rotated to theta = 90 degrees (perpendicular to field), the torque becomes maximum tau = mB = 0.48 x 0.25 = 0.12 N.m, tending to restore it back toward the field direction.
The potential energy increases from U = -mB to U = 0 (since cos 90 = 0), meaning work must be done by an external torque to achieve this rotation against the restoring torque.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 10

Applied Concept

An oxygen cylinder at STP and an iron nail are both brought near a strong bar magnet. Describe and compare the behaviour of each object, identifying the type of magnetism involved in each case.

Oxygen at STP is paramagnetic (listed as an example in the chapter); it is weakly attracted toward the bar magnet and moves from the region of weak field to strong field, with chi small and positive.
The iron nail is ferromagnetic; it develops a strong induced magnetisation when brought near the magnet, experiences a large attractive force, and the induced south pole is closer to the magnet's north pole, producing a net attraction.
The iron nail is much more strongly attracted than oxygen because chi >> 1 for ferromagnets versus chi << 1 for paramagnets; the nail also experiences a torque due to the non-uniform field of the magnet.

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Chapter 5: Magnetism and Matter | Application Questions

A student suspends a bar magnet freely from a thread. The magnet rotates and comes to rest. With reference to the properties of magnetic field lines and Earth's magnetism, explain what happens and why the magnet points in the north-south direction.

In a physics laboratory, a student places a bismuth rod between the poles of a strong electromagnet. The rod moves toward the weaker field region. Identify the magnetic property of bismuth and explain the mechanism responsible for this behaviour.

With reference to the chapter's discussion of superconductors, explain why a magnet floats above a superconducting disk. What property of the superconductor is responsible?

A compass needle placed at point O is observed to align in a specific direction. Another identical magnetised needle Q is placed at various positions around O. Referring to the potential energy formula U = -m.B, identify the configuration of Q that has the lowest potential energy.

A geologist finds a rock sample that is strongly attracted to a magnet even after being removed from a magnetic field for a long time. Identify the type of magnetic material and name two examples from the chapter. What determines whether it will remain magnetised?

A solenoid with 1000 turns per metre carries a current of 2 A and has a core with relative permeability 400. Calculate the magnetic intensity H, the magnetic field B, and the magnetisation M inside the solenoid.

With reference to Gauss's law for magnetism, a student draws a diagram where magnetic field lines emerge from a single point in space with no current present. Point out the error and state the correct principle.

A short bar magnet of magnetic moment 0.48 J/T is placed with its axis along a uniform external field of 0.25 T. Explain qualitatively what torque the magnet experiences and what would happen to its potential energy if it were rotated to be perpendicular to the field.

An oxygen cylinder at STP and an iron nail are both brought near a strong bar magnet. Describe and compare the behaviour of each object, identifying the type of magnetism involved in each case.

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