CBSE Class 12 Physics (2026–27)
Chapter 5: Magnetism and Matter
20 Important Questions and Answers
1. What is a magnetic dipole? Define magnetic dipole moment.
Answer:
A magnetic dipole consists of two equal and opposite magnetic poles separated by a small distance. A bar magnet is a common example of a magnetic dipole. The strength of a magnetic dipole is measured by its magnetic dipole moment. It is defined as the product of the pole strength (m) and the magnetic length (2l) of the magnet.
[M = m(2l)]
The SI unit of magnetic dipole moment is ampere-meter² (A·m²). The magnetic dipole moment determines the torque experienced by a magnet in an external magnetic field. A magnet with a larger magnetic dipole moment produces a stronger magnetic effect and aligns more effectively with an external magnetic field.
2. State the properties of magnetic field lines.
Answer:
Magnetic field lines are imaginary lines used to represent magnetic fields. They emerge from the north pole and enter the south pole outside a magnet, while inside the magnet they travel from south to north, forming closed loops. Magnetic field lines never intersect each other because the magnetic field at a point has a unique direction. The closeness of field lines indicates the strength of the magnetic field; closer lines represent a stronger field. The tangent drawn at any point on a field line gives the direction of the magnetic field at that point. These properties help in understanding the nature and strength of magnetic fields.
3. Why do magnetic monopoles not exist?
Answer:
Magnetic monopoles are hypothetical particles having only a north pole or only a south pole. According to experimental observations, magnetic poles always exist in pairs. When a bar magnet is cut into two pieces, each piece behaves as a complete magnet with its own north and south poles. Further cutting continues to produce smaller dipoles rather than isolated poles. This behavior suggests that magnetic monopoles do not exist in nature. The magnetic field lines always form closed loops, which also supports the absence of isolated magnetic poles. Therefore, unlike electric charges, magnetic poles cannot be separated from each other.
4. Explain the Earth as a giant magnet.
Answer:
The Earth behaves like a giant magnet with a magnetic field surrounding it. Its magnetic south pole lies near the geographic north pole, while its magnetic north pole lies near the geographic south pole. Due to this magnetic field, a freely suspended magnetic needle aligns itself approximately along the north-south direction. The Earth’s magnetic field is believed to be produced by electric currents generated in its molten iron core. This magnetic field protects the Earth from harmful charged particles coming from space. The study of Earth’s magnetism is important for navigation, compass operation, and understanding geomagnetic phenomena.
5. What is magnetic declination?
Answer:
Magnetic declination is the angle between the geographic meridian and the magnetic meridian at a particular place. The geographic meridian passes through the geographical north and south poles, while the magnetic meridian is the direction shown by a freely suspended magnetic needle. Since the magnetic poles do not coincide with the geographic poles, these two directions are generally different. Magnetic declination varies from place to place and changes slowly with time. It is an important parameter in navigation, surveying, and map-making because accurate directional measurements require correction for magnetic declination.
6. What is the angle of dip?
Answer:
The angle of dip or magnetic inclination is the angle made by the Earth’s total magnetic field with the horizontal plane at a particular place. A magnetic needle free to rotate in a vertical plane aligns itself along the Earth’s magnetic field and makes this angle with the horizontal. At the magnetic equator, the angle of dip is 0°, while at the magnetic poles it is 90°. The value of the angle increases gradually from the equator towards the poles. The angle of dip helps determine the direction and strength of the Earth’s magnetic field at different locations.
7. Define magnetic susceptibility.
Answer:
Magnetic susceptibility is a measure of how easily a material can be magnetized when placed in an external magnetic field. It is represented by the symbol χ (chi) and is defined as the ratio of intensity of magnetization (M) to magnetizing field intensity (H).
[\chi = \frac{M}{H}]
Magnetic susceptibility is a dimensionless quantity. Materials with positive susceptibility, such as paramagnetic and ferromagnetic substances, are attracted by magnetic fields. Diamagnetic substances have negative susceptibility and are weakly repelled. The value of susceptibility helps classify materials according to their magnetic behavior and is useful in various technological and scientific applications.
8. Distinguish between diamagnetic and paramagnetic substances.
Answer:
Diamagnetic substances are weakly repelled by an external magnetic field, while paramagnetic substances are weakly attracted. Diamagnetic materials have negative magnetic susceptibility and relative permeability less than one. Examples include bismuth, copper, and silver. Paramagnetic materials have positive magnetic susceptibility and relative permeability slightly greater than one. Examples include aluminum, platinum, and oxygen. In diamagnetic substances, atomic magnetic moments cancel each other completely. In paramagnetic substances, atoms possess permanent magnetic moments that align partially with the applied magnetic field. These differences explain their distinct responses when placed in magnetic fields.
9. What are ferromagnetic substances? Give examples.
Answer:
Ferromagnetic substances are materials that are strongly attracted by magnetic fields and can retain magnetism even after the external field is removed. Their atoms possess magnetic moments that align in the same direction within small regions called domains. When an external magnetic field is applied, these domains align more completely, producing strong magnetization. Ferromagnetic substances have very high magnetic susceptibility and permeability. Common examples include iron, cobalt, nickel, and their alloys. Due to their strong magnetic properties, ferromagnetic materials are widely used in transformers, electric motors, generators, and permanent magnets.
10. What are magnetic domains?
Answer:
Magnetic domains are tiny regions inside a ferromagnetic material where the atomic magnetic moments are aligned in the same direction. In an unmagnetized ferromagnetic substance, these domains are randomly oriented, resulting in zero net magnetization. When an external magnetic field is applied, domains aligned with the field grow in size, and others shrink. This causes the material to become magnetized. The domain theory explains the strong magnetic behavior of ferromagnetic substances and their ability to retain magnetism. The concept of domains is fundamental to understanding permanent magnets and magnetic materials used in technology.
11. Define intensity of magnetization.
Answer:
Intensity of magnetization, represented by M, is defined as the magnetic dipole moment developed per unit volume of a magnetic material. It indicates the degree to which a material becomes magnetized under the influence of an external magnetic field.
[M = \frac{\text{Magnetic Dipole Moment}}{\text{Volume}}]
The SI unit of intensity of magnetization is ampere per meter (A/m). A larger value of magnetization indicates stronger alignment of magnetic dipoles within the material. Intensity of magnetization is directly related to magnetic susceptibility and plays an important role in studying the magnetic properties of different substances.
12. What is relative permeability?
Answer:
Relative permeability is the ratio of the permeability of a material to the permeability of free space. It is represented by μr.
[\mu_r = \frac{\mu}{\mu_0}]
It indicates how easily a magnetic field can pass through a material compared to vacuum. Relative permeability is a dimensionless quantity. For diamagnetic materials, it is slightly less than one, while for paramagnetic materials it is slightly greater than one. Ferromagnetic substances have very high relative permeability values. The concept is important in designing magnetic circuits, transformers, inductors, and other electromagnetic devices where efficient magnetic flux transmission is required.
13. Why does a compass needle point north-south?
Answer:
A compass needle is a small bar magnet that is free to rotate in the horizontal plane. The Earth behaves like a giant magnet and produces a magnetic field around it. When the compass is placed at any location, the needle experiences a torque due to the Earth’s magnetic field. As a result, it aligns itself along the magnetic meridian, approximately in the north-south direction. The north-seeking end of the compass points towards the Earth’s magnetic south pole located near the geographic north pole. This principle makes the compass an important instrument for navigation and direction finding.
14. State Coulomb’s law for magnetic poles.
Answer:
Coulomb’s law for magnetic poles states that the force between two magnetic poles is directly proportional to the product of their pole strengths and inversely proportional to the square of the distance between them.
[F = \frac{\mu_0}{4\pi}\frac{m_1m_2}{r^2}]
where m₁ and m₂ are the pole strengths and r is the distance between them. Like poles repel each other, while unlike poles attract each other. This law is analogous to Coulomb’s law in electrostatics. It helps explain interactions between magnets and forms the basis for understanding magnetic forces and magnetic field calculations.
15. Explain magnetic meridian.
Answer:
The magnetic meridian at a place is the vertical plane passing through the Earth’s magnetic north and south poles. A freely suspended magnetic needle aligns itself in this plane. The direction of the magnetic meridian may differ from that of the geographic meridian due to magnetic declination. The magnetic meridian is important in navigation and magnetic measurements because it provides the natural direction of the Earth’s magnetic field at a location. Instruments such as compasses operate based on the alignment of their needles along the magnetic meridian, helping determine directions accurately.
16. What is the relation between magnetic susceptibility and relative permeability?
Answer:
Magnetic susceptibility (χ) and relative permeability (μr) are related by the expression:
[\mu_r = 1 + \chi]
This relation is valid for most magnetic materials. If susceptibility is positive, relative permeability becomes greater than one, indicating attraction toward magnetic fields. If susceptibility is negative, relative permeability becomes less than one, indicating repulsion. For ferromagnetic substances, susceptibility is very large, resulting in very high relative permeability. This relationship helps connect the microscopic magnetization properties of materials with their macroscopic magnetic behavior and is widely used in the study of magnetic materials.
17. Why are magnetic field lines closed curves?
Answer:
Magnetic field lines form closed curves because magnetic poles always exist in pairs. Outside a magnet, field lines emerge from the north pole and enter the south pole. Inside the magnet, they travel from the south pole back to the north pole, completing a continuous loop. Since isolated magnetic monopoles do not exist, magnetic field lines can never start or end independently. This closed-loop nature distinguishes magnetic field lines from electric field lines, which originate from positive charges and terminate at negative charges. The concept helps explain the continuity of magnetic fields in nature.
18. What is magnetic permeability?
Answer:
Magnetic permeability is the property of a material that determines how easily magnetic field lines can pass through it. It is represented by μ and is defined as:
[\mu = \frac{B}{H}]
where B is magnetic flux density and H is magnetic field intensity. The SI unit of permeability is henry per meter (H/m). Materials with high permeability allow magnetic fields to pass through them more easily. Ferromagnetic substances have very high permeability compared to air or vacuum. Magnetic permeability is an important factor in designing electrical machines, transformers, electromagnets, and magnetic shielding devices.
19. What is the significance of the magnetic equator?
Answer:
The magnetic equator is an imaginary line on the Earth’s surface where the angle of dip is zero. At this location, the Earth’s magnetic field is entirely horizontal. A dip needle placed at the magnetic equator remains horizontal because there is no vertical component of the magnetic field. The magnetic equator does not coincide exactly with the geographic equator and may shift slightly over time. It is significant in geomagnetism because it divides the Earth into northern and southern magnetic hemispheres and helps scientists study variations in the Earth’s magnetic field.
20. Explain the difference between magnetic and geographic poles.
Answer:
Geographic poles are the points where the Earth’s axis of rotation intersects its surface. They determine the true north and south directions. Magnetic poles are the locations where the Earth’s magnetic field is strongest and where magnetic field lines are nearly vertical. The magnetic poles do not coincide exactly with the geographic poles and change their positions slowly over time. This difference causes magnetic declination. Geographic poles are used in maps and navigation systems, whereas magnetic poles determine the direction indicated by a compass. Understanding both is essential for accurate navigation and surveying.