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Class 12 Science (PCM)

Physics Part 1

Magnetism and Matter

Magnetism and Matter

Action PlanNotesQuestionsMock-Test
Study Action Plan (Sorted by Difficulty)
Magnetic Quantities, Units and Dimensions
Accuracy: 0%
Total Attempted0
Total Correct0
Total Incorrect0
Difficulty Level:
High
Ferromagnetic Materials: Hard and Soft Magnets
Accuracy: 20%
Total Attempted5
Total Correct1
Total Incorrect4
Difficulty Level:
High
Temperature Dependence of Magnetic Materials
Accuracy: 43%
Total Attempted7
Total Correct3
Total Incorrect4
Difficulty Level:
High
Bar Magnet as Equivalent Solenoid
Accuracy: 50%
Total Attempted8
Total Correct4
Total Incorrect4
Difficulty Level:
Medium
Magnetic Field of a Solenoid
Accuracy: 57%
Total Attempted7
Total Correct4
Total Incorrect3
Difficulty Level:
Medium
Magnetic Field of Bar Magnet on Axial and Equatorial Lines
Accuracy: 60%
Total Attempted5
Total Correct3
Total Incorrect2
Difficulty Level:
Medium
Magnetic Dipole and Torque
Accuracy: 69%
Total Attempted13
Total Correct9
Total Incorrect4
Difficulty Level:
Medium
Magnetisation and Magnetic Intensity
Accuracy: 70%
Total Attempted10
Total Correct7
Total Incorrect3
Difficulty Level:
Medium
Superconductors and Meissner Effect
Accuracy: 71%
Total Attempted7
Total Correct5
Total Incorrect2
Difficulty Level:
Medium
Magnetic Potential Energy
Accuracy: 75%
Total Attempted8
Total Correct6
Total Incorrect2
Difficulty Level:
Low
Classification of Magnetic Materials
Accuracy: 80%
Total Attempted10
Total Correct8
Total Incorrect2
Difficulty Level:
Low
Diamagnetism
Accuracy: 90%
Total Attempted10
Total Correct9
Total Incorrect1
Difficulty Level:
Low
Ferromagnetism and Domain Theory
Accuracy: 90%
Total Attempted10
Total Correct9
Total Incorrect1
Difficulty Level:
Low
Basic Properties of Bar Magnets
Accuracy: 93%
Total Attempted14
Total Correct13
Total Incorrect1
Difficulty Level:
Low
Electrostatic Analogy of Magnetic Dipole
Accuracy: 100%
Total Attempted1
Total Correct1
Total Incorrect0
Difficulty Level:
Low
Gauss's Law for Magnetism
Accuracy: 100%
Total Attempted13
Total Correct13
Total Incorrect0
Difficulty Level:
Low
Magnetic Field Lines of Bar Magnet and Solenoid
Accuracy: 100%
Total Attempted5
Total Correct5
Total Incorrect0
Difficulty Level:
Low
Magnetic Flux and Magnetic Field Line Validity
Accuracy: 100%
Total Attempted1
Total Correct1
Total Incorrect0
Difficulty Level:
Low
Magnetic Susceptibility and Permeability
Accuracy: 100%
Total Attempted7
Total Correct7
Total Incorrect0
Difficulty Level:
Low
Paramagnetism
Accuracy: 100%
Total Attempted4
Total Correct4
Total Incorrect0
Difficulty Level:
Low

Learning Objectives

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Learning Objectives

  • Understand the universal nature of magnetic phenomena.
  • Describe the behavior of the Earth as a magnet and the properties of bar magnets.
  • Explain the relationship between moving charges and magnetic fields.
  • Define key magnetic quantities such as magnetic moment, magnetic field, and magnetization.
  • Classify materials based on their magnetic properties: diamagnetic, paramagnetic, and ferromagnetic.
  • Apply Gauss's law for magnetism to analyze magnetic fields.
  • Calculate the magnetic field and torque on a bar magnet in an external magnetic field.
  • Discuss the significance of magnetic susceptibility and permeability in materials.
  • Analyze the behavior of magnetic domains in ferromagnetic materials.

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Revision Notes & Summary

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Chapter Five: Magnetism and Matter

5.1 Introduction

  • Magnetic phenomena are universal in nature, affecting everything from distant galaxies to tiny atoms.
  • The term 'magnet' originates from the island of Magnesia in Greece, known for its magnetic ore deposits.
  • Key points about magnetism:
    • The Earth behaves like a magnet, with its magnetic field pointing from geographic south to north.
    • A freely suspended bar magnet aligns itself in the north-south direction, with the end pointing north termed the north pole and the opposite end the south pole.

5.2 Key Concepts

  • Magnetic Field (B): Produced by moving charges or electric currents.
  • Magnetic Moment (m): A vector quantity representing the strength and direction of a magnet's magnetic field.

5.3 Magnetic Properties of Materials

  • Classification of Materials:
    • Diamagnetic: Materials with negative susceptibility (X < 0).
    • Paramagnetic: Materials with small positive susceptibility (0 < X < ε).
    • Ferromagnetic: Materials with large positive susceptibility (X >> 1).

Table 5.2: Magnetic Properties of Materials

TypeSusceptibility (X)Relative Permeability (µᵣ)Magnetic Permeability (µ)
DiamagneticX < 00 ≤ µᵣ < 1µ < µ₀
Paramagnetic0 < X < εµᵣ ≈ 1µ > µ₀
FerromagneticX >> 1µᵣ >> 1µ >> µ₀

5.4 Magnetisation and Magnetic Intensity

  • Magnetisation (M): Defined as the net magnetic moment per unit volume, measured in A/m.
  • Magnetic Intensity (H): Defined as H = B₀/µ₀, where B₀ is the external magnetic field.
  • The total magnetic field B in a material is given by:
    • B = µ₀(H + M)

5.5 Important Laws and Principles

  • Gauss's Law for Magnetism: The net magnetic flux through any closed surface is zero, indicating that magnetic field lines are continuous and form closed loops.
  • Magnetic Susceptibility (X): A measure of how a material responds to an external magnetic field, influencing its magnetisation.

5.6 Applications and Examples

  • Superconductors: Exhibit perfect diamagnetism (X = -1) and perfect conductivity at low temperatures, expelling magnetic fields completely (Meissner effect).
  • Paramagnetic Materials: Weakly attracted to magnetic fields, with individual atomic dipole moments aligning in the presence of an external field.

5.7 Summary of Key Points

  • Magnetic materials are classified based on their susceptibility and permeability.
  • Understanding these properties is crucial for applications in technology and engineering.
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Practice Test – MCQs, True/False

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Multiple Choice Questions

A.

It is equal to the total magnetic charge enclosed.

B.

It is always zero.

C.

It depends on the shape of the surface.

D.

It is equal to the magnetic field strength times the area.
Correct Answer: B

Solution:

Gauss's Law for Magnetism states that the net magnetic flux through any closed surface is zero, indicating that magnetic monopoles do not exist.

Chapter Concept:

Gauss's Law for Magnetism

A.

Paramagnetic materials have a negative XX, while diamagnetic materials have a positive XX.

B.

Paramagnetic materials have a larger positive XX than diamagnetic materials.

C.

Both paramagnetic and diamagnetic materials have a negative XX.

D.

Paramagnetic materials have a positive XX, while diamagnetic materials have a negative XX.
Correct Answer: D

Solution:

Paramagnetic materials have a positive magnetic susceptibility, indicating they are weakly attracted to magnetic fields, whereas diamagnetic materials have a negative susceptibility, indicating they are repelled by magnetic fields.

Chapter Concept:

Classification of Magnetic Materials

A.

Both have isolated magnetic monopoles.

B.

Both produce similar magnetic field patterns.

C.

Cutting either results in isolated poles.

D.

Both have electric field lines that form closed loops.
Correct Answer: B

Solution:

A bar magnet can be viewed as equivalent to a current-carrying solenoid because both produce similar magnetic field patterns. Cutting a bar magnet or a solenoid results in smaller dipoles, not isolated poles.

Chapter Concept:

Bar Magnet as Equivalent Solenoid

A.

The bar magnet experiences a net force but no torque.

B.

The bar magnet experiences no net force but a torque given by τ=m×B\tau = \mathbf{m} \times \mathbf{B}.

C.

The bar magnet experiences both a net force and a torque.

D.

The bar magnet experiences neither a net force nor a torque.
Correct Answer: B

Solution:

In a uniform magnetic field, a bar magnet experiences zero net force but a torque given by τ=m×B\tau = \mathbf{m} \times \mathbf{B}, where m\mathbf{m} is the magnetic moment and B\mathbf{B} is the magnetic field.

Chapter Concept:

Magnetic Dipole and Torque

A.

The compass needle will align parallel to the field lines in both cases.

B.

The compass needle will align perpendicular to the field lines in both cases.

C.

The compass needle will not be affected by the field lines in either case.

D.

The compass needle will align parallel to the field lines for the solenoid but perpendicular for the bar magnet.
Correct Answer: A

Solution:

In both a solenoid and a bar magnet, the compass needle aligns parallel to the magnetic field lines, indicating the direction of the magnetic field.

Chapter Concept:

Magnetic Field Lines of Bar Magnet and Solenoid

A.

They become diamagnetic.

B.

They become paramagnetic.

C.

They retain their ferromagnetic properties.

D.

They become superconductors.
Correct Answer: B

Solution:

At high temperatures, ferromagnetic materials lose their ferromagnetic behavior and become paramagnetic as the domain structure breaks down.

Chapter Concept:

Temperature Dependence of Magnetic Materials

A.

Diamagnetic, μr=0\mu_r = 0

B.

Paramagnetic, μr=1\mu_r = 1

C.

Ferromagnetic, μr>1\mu_r > 1

D.

Superconductor, μr=1\mu_r = 1
Correct Answer: A

Solution:

A material with X=−1X = -1 is a perfect diamagnet, such as a superconductor, where μr=0\mu_r = 0. This means the material completely expels the magnetic field.

Chapter Concept:

Magnetisation and Magnetic Intensity

A.

Ferromagnetic materials have randomly oriented domains that never align.

B.

Ferromagnetic materials have domains that align in the direction of an external magnetic field, leading to strong magnetization.

C.

Ferromagnetic materials do not have any domains.

D.

Ferromagnetic materials lose their magnetization completely at any temperature.
Correct Answer: B

Solution:

Ferromagnetic materials have domains that align in the direction of an external magnetic field, resulting in strong magnetization. This alignment is affected by temperature, but not all magnetization is lost unless the temperature is very high.

Chapter Concept:

Ferromagnetism and Domain Theory

A.

Magnetic field lines can start and end on charges.

B.

Magnetic field lines form closed loops, unlike electrostatic field lines.

C.

Magnetic field lines can intersect each other.

D.

Magnetic field lines are not affected by materials.
Correct Answer: B

Solution:

Magnetic field lines form continuous closed loops, whereas electrostatic field lines begin on positive charges and end on negative charges or at infinity.

Chapter Concept:

Gauss's Law for Magnetism

A.

M=χHM = \chi H

B.

B=μ0(1+χ)HB = \mu_0 (1 + \chi) H

C.

μ=μ0μr\mu = \mu_0 \mu_r

D.

μr=1+χ\mu_r = 1 + \chi
Correct Answer: D

Solution:

The relative permeability μr\mu_r is defined as 1+χ1 + \chi, where χ\chi is the magnetic susceptibility.

Chapter Concept:

Magnetic Susceptibility and Permeability

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True or False

Correct Answer: True

Solution:

A solenoid behaves like a bar magnet and at large distances, its axial field resembles that of a bar magnet. This allows it to be treated as an equivalent magnetic dipole with a magnetic moment.

Chapter Concept :

Magnetic Field of a Solenoid

Correct Answer: True

Solution:

The given formulas for the axial and equatorial magnetic fields of a short bar magnet at a large distance rr are correct as per the standard equations for magnetic fields.

Chapter Concept :

Magnetic Field of Bar Magnet on Axial and Equatorial Lines

Correct Answer: True

Solution:

The magnetic field lines of a current-carrying solenoid resemble those of a bar magnet, and it can be considered an equivalent magnetic dipole due to its magnetic moment.

Chapter Concept :

Magnetic Field of a Solenoid

Correct Answer: True

Solution:

Magnetic field lines of both a bar magnet and a finite solenoid form continuous closed loops, which is a key characteristic of their magnetic fields. This similarity suggests that a bar magnet can be thought of as a large number of circulating currents, analogous to a solenoid.

Chapter Concept :

Magnetic Field Lines of Bar Magnet and Solenoid

Correct Answer: True

Solution:

Ferromagnetic materials consist of domains that can align with an external magnetic field, resulting in strong magnetisation. The alignment and size of these domains can be affected by temperature, as high temperatures can cause a ferromagnet to become a paramagnet.

Chapter Concept :

Ferromagnetism and Domain Theory

Correct Answer: True

Solution:

Magnetic susceptibility χ\chi is indeed dimensionless and measures the response of a material to an external magnetic field. It is related to the relative permeability μr\mu_r by the formula μr=1+χ\mu_r = 1 + \chi, indicating how the material's permeability compares to the permeability of free space.

Chapter Concept :

Magnetic Susceptibility and Permeability

Correct Answer: True

Solution:

Gauss's law for magnetism asserts that the net magnetic flux through any closed surface is zero, indicating that magnetic monopoles do not exist and magnetic field lines form closed loops.

Chapter Concept :

Gauss's Law for Magnetism

Correct Answer: True

Solution:

The relationship between magnetic susceptibility χ\chi and relative permeability μr\mu_r for linear materials is given by μr=1+χ\mu_r = 1 + \chi. This equation describes how the relative permeability is derived from the susceptibility.

Chapter Concept :

Magnetic Susceptibility and Permeability

Correct Answer: True

Solution:

The formula Um=−m⋅BU_m = -m \cdot B correctly represents the potential energy of a magnetic dipole in a magnetic field, indicating the energy depends on the orientation of the magnetic moment mm relative to the magnetic field BB.

Chapter Concept :

Magnetic Potential Energy

Correct Answer: True

Solution:

The formula B=μ0(H+M)B = \mu_0(H + M) correctly represents the total magnetic field inside a material, where HH is the contribution from the external magnetising field and MM is the magnetisation of the material.

Chapter Concept :

Magnetisation and Magnetic Intensity

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