Magnetism and Matter Class 12 Physics Notes | MCQs, Short & Long Questions

Magnetism is a fundamental property of materials that causes them to experience a force in the presence of a magnetic field. The chapter Magnetism and Matter in Class 12 Physics deals with concepts such as the Earth’s magnetism, magnetic properties of materials, magnetic dipole, magnetic field intensity, magnetic susceptibility, and so on. Understanding these topics is essential for board exams as well as competitive exams like JEE and NEET.

Magnetism and Matter Class 12 Physics Notes | MCQs, Short & Long Questions

Magnetism and Matter Notes

1. Magnetism: Basic Concepts

• Magnetic Pole: Every magnet has two poles – North (N) and South (S).

• Like poles repel, unlike poles attract.

• Magnetic poles always exist in pairs; a single isolated magnetic pole (monopole) does not exist.

• Magnetic axis: Imaginary line joining the magnetic poles.

• Magnetic equator: Perpendicular bisector of the line joining magnetic poles.

2. Magnetic Dipole

• A magnetic dipole consists of two equal and opposite magnetic poles separated by a small distance.

• Magnetic dipole moment (M) = Pole strength × Distance between poles

  $$M=m\times 2l$$

  where $m$ = pole strength, $2l$ = magnetic length.

• It is a vector quantity, direction from South to North pole.

3. Magnetic Field and Magnetic Lines of Force

• The space around a magnet in which the magnetic force can be detected is called the magnetic field.

• Represented by magnetic field lines:

  • Originate from North pole, end at South pole outside the magnet.

  • Inside the magnet, direction is from South to North pole.

  • Never intersect each other.

4. Bar Magnet as an Equivalent

Solenoid

• A bar magnet behaves like a solenoid with current loops.

• The magnetic field at the axial point of a bar magnet at distance $r$ from center is:

  Baxial=μ04π⋅2Mr3B_{axial} = \frac{{\mu_0}}{4\pi} \cdot \frac{2M}{r^3}Baxial​=4πμ0​​⋅r32M​

• The magnetic field at the equatorial point:

  Bequatorial=μ04π⋅Mr3B_{equatorial} = \frac{{\mu_0}}{4\pi} \cdot \frac{M}{r^3}Bequatorial​=4πμ0​​⋅r3M​

5. Earth’s Magnetism

• Earth behaves like a giant bar magnet with its magnetic South pole near the geographic North pole.

• Important terms:

  • Magnetic Declination (D): Angle between geographic meridian and magnetic meridian.

  • Inclination or Dip (I): Angle between the magnetic field vector and horizontal plane.

  • Horizontal component (Bh): $Bh=B\cos I$

  • Vertical component (Bv): $Bv=B\sin I$

6. Magnetic Properties of Materials

• Diamagnetic materials: Weakly repelled by magnetic fields (e.g., copper, bismuth).

• Paramagnetic materials: Weakly attracted by magnetic fields (e.g., aluminum).

• Ferromagnetic materials: Strongly attracted; can be permanently magnetized (e.g., iron, cobalt).

7. Magnetization and Magnetic Intensity

• Magnetization (M): Magnetic moment per unit volume.

  M=Magnetic momentVolumeM = \frac{Magnetic \, moment}{Volume}M=VolumeMagneticmoment​

• Magnetic intensity (H): External magnetic field applied.

• Relation:

  B=μ0(H+M)B = \mu_0 (H + M)B=μ0​(H+M)

8. Magnetic Susceptibility (χ)

• χ=MHχ = \frac{M}{H}χ=HM​

• Shows how easily a material can be magnetized.

9. Hysteresis Curve

• The graph between $B$ and $H$ for a ferromagnetic material.

• Retentivity: The ability to retain magnetization.

• Coercivity: The reverse magnetizing field required to bring $B=0$.

10. Permanent Magnets and Electromagnets

• Permanent magnets: High retentivity, high coercivity, e.g., Alnico.

• Electromagnets: Soft iron core with high permeability, low retentivity.

Objective Questions (MCQs) with Answers

Q1. The magnetic dipole moment is a:

a) Scalar quantity
b) Vector quantity
c) Both a and b
d) None of these

Answer: The magnetic dipole moment is a vector quantity directed from the South pole to the North pole.

Q2. Magnetic susceptibility of diamagnetic materials is:

a) Positive
b) Zero
c) Negative
d) Infinite

Answer: Diamagnetic materials have negative susceptibility because they develop a weak magnetic moment opposite to the applied field.

Q3. The angle between geographic meridian and magnetic meridian is called:

a) Dip
b) Declination
c) Retentivity
d) Coercivity

Answer: The angle between the two meridians is called Declination.

Q4. Retentivity of material refers to:

a) Ability to demagnetize
b) Ability to retain magnetization
c) Ability to conduct electricity
d) None of these

Answer: Retentivity refers to the ability to retain magnetization even after the magnetizing field is removed.

Q5. Earth’s magnetic field behaves like a:

a) Electric dipole
b) Magnetic monopole
c) Magnetic dipole
d) None of these

Answer: Earth’s magnetic field behaves like a magnetic dipole with its south pole near the geographic north.

Q6. The SI unit of magnetic dipole moment is:

a) Weber
b) Tesla
c) A·m²
d) Henry

Answer: The SI unit of magnetic dipole moment is A·m² (Ampere meter square).

Q7. The material with very high permeability is:

a) Diamagnetic
b) Paramagnetic
c) Ferromagnetic
d) None of these

Answer: Ferromagnetic materials have very high permeability because they allow magnetic field lines to pass easily.

Q8. The tangent law is used to find:

a) Electric field
b) Magnetic field
c) Gravitational field
d) None of these

Answer: The tangent law is used to find the magnetic field strength using a deflection magnetometer.

Q9. Hysteresis curve shows the relation between:

a) B and H
b) M and H
c) B and M
d) None of these

Answer: The hysteresis curve shows the relation between B (magnetic induction) and H (magnetic field intensity).

Q10. The magnetic inclination at the magnetic equator is:

a) 0°
b) 45°
c) 90°
d) 180°

Answer: At the magnetic equator, inclination or dip is 0° because the magnetic field is horizontal.

Q11. For an ideal permanent magnet, coercivity should be:

a) Zero
b) Very small
c) Very large
d) Negative

Answer: For an ideal permanent magnet, coercivity should be very large so that it is not easily demagnetized.

Q12. The relation between B, H, and M is given by:

a) B=μ0HB = \mu_0 HB=μ0​H
b) B=μ0(H+M)B = \mu_0 (H + M)B=μ0​(H+M)
c) $B=M/H$
d) $B=H-M$

Answer: The correct relation is B=μ0(H+M)B = \mu_0 (H + M)B=μ0​(H+M).

Q13. Magnetic susceptibility (χ) is defined as:

a) $\chi =H/M$
b) $\chi =M/H$
c) $\chi =M\times H$
d) $\chi =M+H$

Answer: Magnetic susceptibility is $\chi =M/H$.

Q14. Diamagnetic substances are:

a) Strongly attracted
b) Weakly attracted
c) Weakly repelled
d) Strongly repelled

Answer: Diamagnetic substances are weakly repelled by a magnetic field.

Q15. Paramagnetic susceptibility varies as:

a) $1/T$
b) $T$
c) T2T^2T2
d) 1/T21/T^21/T2

Answer: Paramagnetic susceptibility varies as $1/T$ according to Curie’s law.

Q16. At the magnetic poles of Earth, the dip is:

a) 0°
b) 30°
c) 60°
d) 90°

Answer: At the magnetic poles, the dip is 90° because the magnetic field is vertical.

Q17. The unit of magnetic field intensity H is:

a) Tesla
b) A/m
c) Weber/m²
d) Henry

Answer: The unit of magnetic field intensity H is A/m (Ampere per meter).

Q18. Magnetic field inside a long solenoid is:

a) Zero
b) Maximum at ends
c) Uniform
d) Non-uniform

Answer: Magnetic field inside a long solenoid is uniform.

Q19. Magnetic susceptibility of paramagnetic substances is:

a) Positive and large
b) Positive and small
c) Negative and small
d) Zero

Answer: Paramagnetic substances have positive and small susceptibility.

Q20. The strength of Earth’s magnetic field is of the order of:

a) 105 T10^5 \,T105T
b) 10−5 T10^{-5} \,T10−5T
c) 10−10 T10^{-10} \,T10−10T
d) 102 T10^2 \,T102T

Answer: Earth’s magnetic field is of the order of 10−5 T10^{-5} \,T10−5T (Tesla).

Long Answer Questions (LAQs)

Q1. Explain the magnetic properties of materials with examples.

Answer:
Magnetic materials are broadly classified into diamagnetic, paramagnetic, and ferromagnetic based on their behavior in a magnetic field. Diamagnetic materials like copper and bismuth develop weak magnetic moments opposite to the applied field, showing negative susceptibility. Paramagnetic materials like aluminum and platinum develop small magnetic moments in the same direction as the applied field, having small positive susceptibility. Ferromagnetic materials like iron, cobalt, and nickel show strong attraction towards the magnetic field, have large positive susceptibility, and can be permanently magnetized.

Q2. Derive the expression for the magnetic field at a point on the axial line of a bar magnet.

Answer:
Consider a bar magnet with magnetic dipole moment $M$. At a point $P$ on the axial line at distance $r$ from the center, the magnetic field due to north and south poles is calculated using Coulomb’s law for magnetic poles. The resultant field along the axial line is:

Baxial=μ04π⋅2Mr3B_{axial} = \frac{\mu_0}{4\pi} \cdot \frac{2M}{r^3}Baxial​=4πμ0​​⋅r32M​

This equation shows that the magnetic field is directly proportional to the dipole moment and inversely proportional to the cube of the distance from the center.

Q3. Describe Earth’s magnetism and define its magnetic elements.

Answer:
Earth behaves like a giant bar magnet with its magnetic south pole near the geographic north pole. Its magnetic field is described using three elements: Magnetic Declination (D), the angle between geographic and magnetic meridians; Magnetic Inclination or Dip (I), the angle between Earth’s magnetic field vector and the horizontal plane; and the Horizontal Component (Bh) of Earth’s magnetic field, given by $Bh=B\cos I$. These elements help in determining the exact direction and strength of Earth’s magnetic field at any location.

Q4. What is hysteresis? Explain the hysteresis curve with retentivity and coercivity.

Answer:
Hysteresis is the lag between changes in magnetization (B) of a material and the magnetic field strength (H) applied to it. The hysteresis curve is a graph of B vs H. Retentivity is the ability of a material to retain magnetization even after the external magnetic field is removed, while coercivity is the amount of reverse magnetic field required to reduce the magnetization to zero. Materials with high retentivity and coercivity are used for permanent magnets, while those with low values are used for electromagnets.

Q5. Distinguish between permanent magnets and electromagnets with applications.

Answer:
Permanent magnets are made from hard ferromagnetic materials like Alnico, having high retentivity and coercivity, making them suitable for devices like loudspeakers and electric meters. Electromagnets, on the other hand, are made from soft iron with high permeability and low retentivity so that they can be magnetized and demagnetized easily. They are widely used in electric motors, cranes, and magnetic relays.

Short Answer Questions (SAQs)

Q1. Define magnetic dipole moment and write its SI unit.

Answer:
Magnetic dipole moment is defined as the product of pole strength and the distance between the two poles of a magnetic dipole. It is a vector quantity directed from the south pole to the north pole. Its SI unit is A·m² (Ampere meter square).

Q2. What is Curie’s Law in magnetism?

Answer:
Curie’s Law states that the magnetic susceptibility ($\chi$) of a paramagnetic material is inversely proportional to its absolute temperature (T), i.e., $\chi \propto 1/T$. It shows that paramagnetic materials lose magnetism at higher temperatures.

Q3. What is magnetic declination?

Answer:
Magnetic declination is the angle between the geographic meridian (true north-south line) and the magnetic meridian (direction shown by a magnetic compass). It varies from place to place on Earth.

Q4. Define retentivity of a magnetic material.

Answer:
Retentivity is the ability of a material to retain its magnetization even after the external magnetizing field has been removed. Materials with high retentivity are suitable for making permanent magnets.

Q5. Write one property each of diamagnetic, paramagnetic, and ferromagnetic materials.

Answer:
Diamagnetic materials are weakly repelled by magnetic fields. Paramagnetic materials are weakly attracted by magnetic fields. Ferromagnetic materials are strongly attracted and can be permanently magnetized.

Conclusion

The chapter Magnetism and Matter in Class 12 Physics provides a strong foundation for understanding how magnetic fields interact with materials, the behavior of Earth’s magnetic field, and the principles behind magnets and electromagnets.

We studied key concepts such as magnetic dipole moment, magnetic susceptibility, hysteresis curve, magnetic properties of materials, and Earth’s magnetism in detail. We also solved 20 objective questions with answers, as well as short and long answer questions, which will help students in preparing for board exams and competitive exams like JEE, NEET, and other entrance tests.

By mastering these topics and practicing the numerical problems, students can gain clarity in both theoretical and practical aspects of magnetism, leading to better performance in physics exams.

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