RBSE Solutions Class 12 Physics Chapter 8 Magnetism and Properties of Magnetic Substances

RBSE Class 12 Physics Chapter 8 Multiple Choice Type Questions

Question 1. If the distance between two magnetic poles of unit magnetic strength is 1 m, then force acting between them will be:
(a) 4\( \pi \) \( \times \) \( 10^{-7} \) N
(b) 4\( \pi \) N
(c) \( 10^{-7} \) N
(d) \( \frac{4 \pi}{10^{-7}} \) N
Answer: (c) \( 10^{-7} \) N
In simple words: The force between two magnetic poles is calculated using Coulomb's law for magnetism. When the distance is 1 meter and the poles have unit strength, the force comes out to be \( 10^{-7} \) Newton.

🎯 Exam Tip: Remember Coulomb's law for magnetic poles \( F = \frac{\mu_0}{4\pi} \frac{m_1 m_2}{r^2} \). For unit pole strength and 1m distance, \( m_1 m_2 / r^2 = 1 \), so \( F = \frac{\mu_0}{4\pi} \). Use the value of \( \frac{\mu_0}{4\pi} = 10^{-7} \text{ N/A}^2 \).

Question 2. Magnetic susceptibility for superconductors is :
(a) +1
(b) -1
(c) zero
(d) infinity
Answer: (b) -1
In simple words: Superconductors perfectly push out magnetic fields from inside them. This special property means their magnetic susceptibility is exactly -1.

🎯 Exam Tip: Superconductors exhibit perfect diamagnetism, known as the Meissner effect. This strong diamagnetic behavior corresponds to a magnetic susceptibility of -1.

Question 3. Magnetic susceptibility of free space is:
(a) +1
(b) -1
(c) zero
(d) infinity
Answer: (c) zero
In simple words: Free space, or a vacuum, has no material to be magnetized. So, it cannot get magnetized at all, making its magnetic susceptibility zero.

🎯 Exam Tip: Free space acts as a reference for magnetic properties. Since there are no dipoles to align or oppose an external field, its susceptibility is zero, and its relative permeability is 1.

Question 4. Magnetic susceptibility is negative and very less for :
(a) ferromagnetic substances
(b) paramagnetic substances
(c) diamagnetic substances
(d) All of the options
Answer: (c) diamagnetic substances
In simple words: Diamagnetic substances are weakly pushed away by magnets and have a small, negative magnetic susceptibility. This means they get magnetized in the opposite direction to the applied magnetic field.

🎯 Exam Tip: Remember that diamagnetic materials are repelled by magnetic fields, while paramagnetic materials are weakly attracted, and ferromagnetic materials are strongly attracted. This behavior directly correlates with their susceptibility values (negative for diamagnetic, small positive for paramagnetic, large positive for ferromagnetic).

Question 6. Unit of magnetic moment is :
(a) Wb
(b) Wb/m²
(c) A/m
(d) Am²
Answer: (d) Am²
In simple words: Magnetic moment measures a magnet's strength and orientation. Its unit is Ampere-meter squared.

🎯 Exam Tip: The magnetic moment (M) for a current loop is given by M = IA, where I is current (Amperes) and A is area (meter squared), hence Am².

Question 7. Wb x A/m is equals to :
(a) J
(b) N
(c) H
(d) W
Answer: (b) N
In simple words: When you multiply Weber (unit of magnetic flux) by Ampere per meter (unit of magnetic field strength), the resulting unit is Newton. This is related to the force on a current-carrying wire in a magnetic field.

🎯 Exam Tip: Recall the relationship between magnetic force, current, length, and magnetic field. Force is \( F = BIL \), where B is magnetic field (Tesla or Wb/m²), I is current (Amperes), L is length (meters). So N = (Wb/m²) * A * m = Wb * A/m.

Question 8. In which of the following, the magnetic field does not interplay with :
(a) magnet
(b) accelerated magnet
(c) static charge
(d) dynamic charge
Answer: (c) static charge
In simple words: A magnetic field affects moving charges (dynamic charge) and other magnets, but it does not exert any force on charges that are standing still (static charge).

🎯 Exam Tip: Remember the Lorentz force law: \( \vec{F} = q(\vec{E} + \vec{v} \times \vec{B}) \). A magnetic field \( \vec{B} \) only exerts a force on a charge \( q \) if it is moving with velocity \( \vec{v} \). For a static charge (\( \vec{v} = 0 \)), only an electric field can exert a force.

Question 10. Magnetic moment of diamagnetic substances is :
(a) infinity
(b) zero
(c) 100 Am²
(d) None of the options
Answer: (b) zero
In simple words: Diamagnetic substances have no permanent magnetic moment. Their atoms have electron orbits that cancel out, resulting in a net magnetic moment of zero in the absence of an external field.

🎯 Exam Tip: Diamagnetism arises from the induced magnetic moments that oppose an external field, not from permanent atomic magnetic moments. This is why their net magnetic moment is zero.

Question 11. The relative permeability of ferromagnetic substance is :
(a) \( \mu_r > 1 \)
(b) \( \mu_r >> 1 \)
(c) \( \mu_r = 1 \)
(d) \( \mu_r = 0 \)
Answer: (b) \( \mu_r >> 1 \)
In simple words: Ferromagnetic substances like iron are very strongly attracted to magnets. This means they let magnetic field lines pass through them very easily, so their relative permeability is much greater than 1.

🎯 Exam Tip: Ferromagnetic materials are known for their strong magnetic properties, which are reflected in their very high relative permeability, sometimes reaching thousands.

Question 12. Vertical component of earth's magnetic field is zero at:
(a) magnetic pole
(b) geographical pole
(c) magnetic meridian
(d) None of the options
Answer: (d) None of the options
In simple words: The Earth's magnetic field has both horizontal and vertical parts. At the magnetic equator, the vertical component is zero, not at the magnetic pole, geographical pole, or magnetic meridian. Since the magnetic equator is not an option, the answer is "None of the options".

🎯 Exam Tip: The vertical component of Earth's magnetic field is zero at the magnetic equator, where a freely suspended magnet would rest horizontally (angle of dip is 0°). At the magnetic poles, the vertical component is maximum, and the horizontal component is zero (angle of dip is 90°).

Question 14. Steel is used for making permanent magnet because :
(a) energy loss is minimum
(b) density of steel is more
(c) residual magnetism of steel is more
(d) due to external magnetic field the magnetism does not destroy easily
Answer: (d) due to external magnetic field the magnetism does not destroy easily
In simple words: Steel is good for making permanent magnets because it has high retentivity, meaning it holds onto its magnetism well even after the external magnetic field is removed. It also has high coercivity, making it hard to demagnetize.

🎯 Exam Tip: Key properties for permanent magnets are high retentivity (strong residual magnetism) and high coercivity (resistance to demagnetization), which steel possesses.

Question 15. At Curie temperature, ferromagnetic substance becomes: .
(a) paramagnetic
(b) diamagnetic
(c) ferromagnetic
(d) more ferromagnetic
Answer: (c) ferromagnetic
In simple words: Above its Curie temperature, a ferromagnetic substance loses its strong magnetic properties and changes into a paramagnetic substance. This means it becomes only weakly attracted to magnets.

🎯 Exam Tip: The Curie temperature marks a critical phase transition where the thermal energy becomes sufficient to overcome the magnetic ordering (alignment of magnetic domains), causing ferromagnetic materials to become paramagnetic.

RBSE Class 12 Physics Chapter 8 Very Short Answer Type Questions

Question 1. A magnetic needle which is free to rotate in a vertical plane if placed at geomagnetic south or north pole, then in which direction it rotates?
Answer: A free magnetic needle in a vertical plane will always point vertically up or down when placed at a geomagnetic pole. It aligns itself with the direction of the Earth's magnetic field lines at the poles.
In simple words: At the Earth's magnetic poles, a compass needle that can move up and down will point straight up or straight down.

🎯 Exam Tip: At the magnetic poles, the magnetic field lines are perpendicular to the Earth's surface, causing the dip needle to align vertically. At the North Magnetic Pole, it dips downwards, and at the South Magnetic Pole, it dips upwards (if it were a conventional N-pole pointing compass).

Question 3. On going from equatorial line towards pole what will be the change in angle of dip?
Answer: As one moves from the magnetic equatorial line towards the magnetic poles, the angle of dip continuously increases. It changes from 0° at the magnetic equator to 90° at the magnetic poles.
In simple words: As you travel from the magnetic equator to the magnetic poles, the angle a compass needle dips downwards gets bigger, going from no dip to a full 90-degree dip.

🎯 Exam Tip: Remember that the angle of dip is 0° at the magnetic equator (horizontal needle) and 90° at the magnetic poles (vertical needle).

Question 4. The magnetic susceptibility of substance is – 0.085. What type of substance this is?
Answer: This substance is a diamagnetic substance. This is because diamagnetic substances have a negative and very small value for magnetic susceptibility, indicating they are weakly repelled by magnetic fields.
In simple words: Since its magnetic susceptibility is a small negative number, the substance is diamagnetic.

🎯 Exam Tip: A negative magnetic susceptibility is the defining characteristic of diamagnetic materials, while paramagnetic materials have a small positive susceptibility, and ferromagnetic materials have a large positive susceptibility.

Question 5. Define retentivity.
Answer: Retentivity, also known as residual magnetism, is the property of a magnetic material that describes the amount of magnetic induction (B) remaining in it after the external magnetizing field (H) has been completely removed.
In simple words: Retentivity is how much magnetism a material keeps even after the magnetizing force is taken away.

🎯 Exam Tip: Materials with high retentivity are suitable for making permanent magnets, as they retain strong magnetism after being magnetized.

Question 6. Give two examples of paramagnetic substances.
Answer:
(i) CuCl₂
(ii) Oxygen
In simple words: Two examples of substances that are weakly attracted to magnets are copper chloride and oxygen gas.

🎯 Exam Tip: Common paramagnetic examples include liquid oxygen, aluminum, sodium, platinum, and copper chloride, which have unpaired electrons causing a weak attraction to magnetic fields.

Question 7. What is magnetic meridian?
Answer: The magnetic meridian at a place is defined as the vertical plane that passes through the magnetic axis of a freely suspended small magnet. The Earth's magnetic field at that location acts along the direction of this magnetic meridian.
In simple words: The magnetic meridian is an imaginary vertical flat surface that runs through the North and South magnetic poles and shows the direction a free compass needle would point.

🎯 Exam Tip: The magnetic meridian is distinct from the geographical meridian. The angle between them is called the magnetic declination, an important element of Earth's magnetic field.

Question 8. The angles of dip at two places on the surface of the earth are respectively 0° and 90°, where are these places located?
Answer: The place where the angle of dip is 0° is the magnetic equator. The places where the angle of dip is 90° are the magnetic poles.
In simple words: An angle of dip of 0 degrees is found at the magnetic equator, and an angle of dip of 90 degrees is found at the magnetic poles.

🎯 Exam Tip: Understand that the magnetic equator is where the Earth's magnetic field is purely horizontal, and the magnetic poles are where it is purely vertical.

Question 10. Write unit of magnetic pole strength.
Answer: The unit of magnetic pole strength is Ampere-meter (A-m).
In simple words: The strength of a magnetic pole is measured in Ampere-meters.

🎯 Exam Tip: Magnetic pole strength (m) is typically defined such that magnetic moment \( M = m \times 2l \) (where 2l is the magnetic length). Since the unit of M is Am², the unit of m must be Am.

Question 11. The vertical component of earth's magnetic field at a place is \( \sqrt{3} \) times the horizontal component. What is the value of angle of dip at this place?
Answer:
Given that the vertical component \( B_V \) is \( \sqrt{3} \) times the horizontal component \( B_H \):
\( B_V = \sqrt{3} B_H \)
The angle of dip \( \theta \) is related by:
\( \tan\theta = \frac{B_V}{B_H} \)
Now, substitute the given relationship:
\( \tan\theta = \frac{\sqrt{3} B_H}{B_H} \)
\( \implies \tan\theta = \sqrt{3} \)
We know that \( \tan 60^\circ = \sqrt{3} \)
\( \implies \theta = 60^\circ \)
So, the angle of dip at this place is \( 60^\circ \).
In simple words: If the upward/downward part of Earth's magnetic field is \( \sqrt{3} \) times stronger than its horizontal part, then the magnetic dip angle there is 60 degrees.

🎯 Exam Tip: Remember the relationship \( \tan\theta = \frac{B_V}{B_H} \). This formula is key for solving problems involving the angle of dip and components of Earth's magnetic field.

Question 12. What is magnetic hysteresis?
Answer: Magnetic hysteresis is the phenomenon where the magnetic induction (B) of a ferromagnetic material lags behind the magnetizing field (H) when the material is subjected to a cycle of magnetization. This means that the value of B does not solely depend on the current value of H, but also on the previous magnetic history of the material.
In simple words: Magnetic hysteresis means that when you magnetize and demagnetize a material, its magnetic state doesn't just follow the magnetizing force; it also remembers its past magnetic states, causing a delay.

🎯 Exam Tip: The hysteresis loop is a graphical representation of this phenomenon, showing the B versus H curve for a complete cycle of magnetization and demagnetization.

Question 13. What will be the ratio in the values of magnetic field due to a bar magnet at axial and equatorial positions from mid point of bar magnet?
Answer: The magnetic field due to a short bar magnet at an axial position is given by \( B_{axial} = \frac{\mu_0}{4\pi} \frac{2M}{r^3} \).
The magnetic field due to a short bar magnet at an equatorial position is given by \( B_{equatorial} = \frac{\mu_0}{4\pi} \frac{M}{r^3} \).
Therefore, the ratio of the magnetic fields at axial to equatorial positions is:
\( \frac{B_{axial}}{B_{equatorial}} = \frac{\frac{\mu_0}{4\pi} \frac{2M}{r^3}}{\frac{\mu_0}{4\pi} \frac{M}{r^3}} \)
\( \implies \frac{B_{axial}}{B_{equatorial}} = 2 \)
Thus, the ratio \( B_{axial} : B_{equatorial} \) is 2:1.
In simple words: For a bar magnet, the magnetic field strength right along its axis is twice as strong as the field strength at the same distance in a direction perpendicular to its axis (equatorial line).

🎯 Exam Tip: Remember that these formulas are for short bar magnets, where \( r >> l \) (r is the distance from the center, l is half the magnetic length). The 2:1 ratio is a fundamental result for magnetic dipole fields.

Question 14. What will be the value of angle of dip where horizontal and vertical component of earth are equal?
Answer: If the horizontal component \( B_H \) and the vertical component \( B_V \) of the Earth's magnetic field are equal:
\( B_H = B_V \)
The angle of dip \( \theta \) is given by:
\( \tan\theta = \frac{B_V}{B_H} \)
Since \( B_V = B_H \), we can write:
\( \tan\theta = \frac{B_H}{B_H} \)
\( \implies \tan\theta = 1 \)
We know that \( \tan 45^\circ = 1 \)
\( \implies \theta = 45^\circ \)
Thus, the angle of dip will be 45° where the horizontal and vertical components of Earth's magnetic field are equal.
In simple words: If the horizontal and vertical parts of Earth's magnetic field are the same strength, the angle of magnetic dip is 45 degrees.

🎯 Exam Tip: This is a common special case. Always set up the tangent relation \( \tan\theta = B_V/B_H \) first, then substitute the given condition to find the angle.

Question 15. If a bar magnet is cut along its length in two equal parts, then what will be the change in magnetic moment of the magnet?
Answer: A bar magnet's magnetic moment (M) is defined as the product of its pole strength (m) and its magnetic length (l), so \( M = m \times l \).
When a bar magnet is cut *along its length* into two equal parts, the magnetic length (l) of each new magnet remains the same. However, the pole strength (m) of each new part becomes half of the original pole strength, i.e., \( m' = m/2 \).
Therefore, the magnetic moment of each new part \( M' \) will be:
\( M' = m' \times l \)
\( M' = \left(\frac{m}{2}\right) \times l \)
\( \implies M' = \frac{1}{2} (m \times l) \)
\( \implies M' = \frac{M}{2} \)
So, if a bar magnet is cut along its length into two equal parts, the magnetic moment of each part becomes half of the original magnet's magnetic moment.
In simple words: If you cut a bar magnet in half lengthwise, each new, thinner magnet will have half the magnetic strength of the original magnet.

🎯 Exam Tip: Understand the difference between cutting a magnet along its length versus transversely. If cut transversely, each piece retains the original pole strength but has half the length, also resulting in \( M/2 \).

RBSE Class 12 Physics Chapter 8 Long Answer Type Questions

Question 2. How can you distinguish between diamagnetic and paramagnetic rods?
Answer: Diamagnetic and paramagnetic rods can be distinguished by observing their behavior when suspended freely in a uniform magnetic field:
1. **Paramagnetic Rod:** When a paramagnetic rod is freely suspended in a uniform magnetic field, it tends to align itself parallel to the direction of the applied magnetic field. This is because paramagnetic materials are weakly attracted to magnetic fields.
2. **Diamagnetic Rod:** When a diamagnetic rod is freely suspended in a uniform magnetic field, it tends to align itself perpendicular to the direction of the applied magnetic field. This happens because diamagnetic materials are weakly repelled by magnetic fields.
In simple words: If you hang a paramagnetic rod, it will line up with a magnetic field; if you hang a diamagnetic rod, it will turn to be across (perpendicular to) the field.

🎯 Exam Tip: The key difference lies in the alignment: paramagnetic parallel (attracted), diamagnetic perpendicular (repelled). Ferromagnetic materials would align strongly parallel.

Question 3. Why do we get two neutral points for a bar magnet? Can we obtain only one neutral point? Why?
Answer:
**Why two neutral points?**
A neutral point is a location where the magnetic field produced by a magnet completely cancels out the horizontal component of the Earth's magnetic field. The resultant magnetic field at a neutral point is zero. For a bar magnet placed in the Earth's magnetic field, typically two such points exist because the magnetic field lines from the magnet spread out and can, at specific locations, be equal in magnitude and opposite in direction to the Earth's horizontal field.

**Can we obtain only one neutral point?**
No, in the typical configuration of a bar magnet placed horizontally in the Earth's magnetic field, it is not possible to obtain only one neutral point. This is because the magnetic field pattern of a bar magnet, combined with the Earth's uniform horizontal magnetic field, will always result in either two neutral points (when the magnet's field lines oppose the Earth's field in two regions) or no neutral points (if the fields never cancel out). A single neutral point would require a very specific, non-symmetrical, and generally unachievable combination of magnetic fields.
In simple words: A bar magnet usually creates two neutral spots where its field perfectly cancels the Earth's field. You cannot get only one neutral point with a simple bar magnet because its field pattern and the Earth's field don't allow for just one spot where they fully cancel out.

🎯 Exam Tip: Neutral points are crucial for understanding magnetic field superposition. The number and location depend on the magnet's orientation and the strength of the Earth's magnetic field components.

Question 5. A bar magnet is placed in parallel to a magnetic field \( \overrightarrow{\mathrm{B}} \). Its magnetic moment is \( \overrightarrow{\mathrm{M}} \). How much work will be done by magnetic moment to make it perpendicular to magnetic field?
Answer: The work done (W) in rotating a magnetic dipole (bar magnet) in a uniform magnetic field is given by the change in its potential energy.
The potential energy of a magnetic dipole in a magnetic field is \( U = -MB \cos\theta \).
So, the work done to rotate the magnet from an initial angle \( \theta_1 \) to a final angle \( \theta_2 \) is:
\( W = U_2 - U_1 = (-MB \cos\theta_2) - (-MB \cos\theta_1) \)
\( W = MB (\cos\theta_1 - \cos\theta_2) \)

Given:
Initial position: The bar magnet is placed parallel to the magnetic field.
So, the initial angle \( \theta_1 = 0^\circ \).
Final position: The magnet is to be made perpendicular to the magnetic field.
So, the final angle \( \theta_2 = 90^\circ \).

Now substitute these values into the work done formula:
\( W = MB (\cos 0^\circ - \cos 90^\circ) \)
Since \( \cos 0^\circ = 1 \) and \( \cos 90^\circ = 0 \):
\( W = MB (1 - 0) \)
\( \implies W = MB \)
Therefore, the work done by the magnetic moment to make the bar magnet perpendicular to the magnetic field is \( MB \). This represents the maximum potential energy that the magnet can gain when moved from a stable equilibrium position.
In simple words: When a magnet is aligned with a magnetic field, it's in a stable state. To turn it to be at a right angle to the field, work equal to its magnetic moment times the field strength must be done.

🎯 Exam Tip: Remember that positive work is done when rotating a magnet from a more stable (lower potential energy) to a less stable (higher potential energy) configuration. The initial parallel alignment (\( \theta = 0^\circ \)) is the most stable state.

Question 6. Define angle of declination and angle of dip?
Answer:
**Angle of Declination:** The angle of declination at a specific place is the angle between the geographical meridian and the magnetic meridian. The geographical meridian is a vertical plane passing through the Earth's geographical north and south poles. The magnetic meridian, on the other hand, is a vertical plane passing through the magnetic axis of a freely suspended small magnet at that location. It essentially tells us how much a compass needle deviates from true geographical north.

**Angle of Dip:** The angle of dip, also known as magnetic inclination, is the angle that the Earth's total magnetic field vector \( \overrightarrow{\mathrm{B}} \) makes with the horizontal direction in the magnetic meridian at any given place. This angle indicates how much a magnetic needle dips downwards (or upwards) from the horizontal when freely suspended and allowed to rotate in a vertical plane.
In simple words: Magnetic declination is the angle between true North and where a compass points. Magnetic dip is the angle the compass needle makes with the ground, showing how much it points down or up.

🎯 Exam Tip: Both magnetic declination and angle of dip are essential elements of Earth's magnetic field that vary with location and are critical for navigation and geophysical studies.

Question 7. Write Curie-Weiss law and what is Curie temperature for iron?
Answer:
**Curie-Weiss Law:** This law describes the magnetic susceptibility of paramagnetic materials above their Curie point. It states that in the paramagnetic phase (i.e., at temperatures T greater than the Curie temperature \( T_C \)), the magnetic susceptibility \( X_m \) varies inversely with the difference between the absolute temperature T and the Curie temperature \( T_C \). The formula is:
\( X_m = \frac{C}{T-T_C} \quad (\text{for } T > T_C) \)
Where C is the Curie constant.

**Curie Temperature for Iron:** The Curie temperature for iron is approximately 1043 K (or 770°C). Above this temperature, iron loses its ferromagnetic properties and becomes paramagnetic.
In simple words: The Curie-Weiss law explains how a material's magnetic attraction changes with temperature above a certain point. For iron, this special temperature, called the Curie temperature, is about 1043 Kelvin, where it stops being strongly magnetic.

🎯 Exam Tip: The Curie-Weiss law is a modification of Curie's law, specifically applying to paramagnetic materials where interactions between magnetic moments become significant near the Curie temperature.

Question 9. What are the behaviour of diamagnetic, paramagnetic and ferromagnetic substances in non-uniform magnetic field?
Answer: The behavior of diamagnetic, paramagnetic, and ferromagnetic substances in a non-uniform magnetic field depends on their magnetic susceptibility:
1. **Diamagnetic Substances:** These substances are weakly repelled by magnetic fields. In a non-uniform magnetic field, a diamagnetic substance will tend to move from regions of stronger magnetic field to regions of weaker magnetic field. This is because they acquire a magnetic moment opposite to the applied field.
2. **Paramagnetic Substances:** These substances are weakly attracted to magnetic fields. In a non-uniform magnetic field, a paramagnetic substance will tend to move from regions of weaker magnetic field to regions of stronger magnetic field, aligning its induced magnetic moment with the field.
3. **Ferromagnetic Substances:** These substances are strongly attracted to magnetic fields. In a non-uniform magnetic field, a ferromagnetic substance will move very strongly from regions of weaker magnetic field to regions of stronger magnetic field. They acquire a very large magnetic moment parallel to the field.
In simple words: In a magnetic field that isn't even everywhere, diamagnetic materials are pushed towards the weak spots, while paramagnetic and ferromagnetic materials are pulled towards the strong spots, with ferromagnetic materials being pulled much more strongly.

🎯 Exam Tip: Remember that "non-uniform field" is key. The gradient of the field causes the net force. Diamagnetic materials move against the gradient, while paramagnetic and ferromagnetic materials move along the gradient.

Question 10. What is Gauss's law in magnetism? What does it represent?
Answer:
**Gauss's Law in Magnetism:** This law states that the net magnetic flux through any closed surface is always zero. Mathematically, it is expressed as:
\[ \oint \vec{B} \cdot d\vec{S} = 0 \]
Where \( \vec{B} \) is the magnetic field and \( d\vec{S} \) is an infinitesimal area vector on the closed surface.

**What it Represents:** This law fundamentally represents that isolated magnetic poles (monopoles) do not exist. Since the net magnetic flux through any closed surface is zero, it implies that magnetic field lines always form continuous closed loops; they never start or end at any point within the closed surface. This means that if magnetic field lines enter a closed surface, an equal number of lines must exit it, reflecting the dipole nature of all magnetic sources.
In simple words: Gauss's Law for magnetism says that if you draw any closed shape, the total number of magnetic field lines going into it will always be the same as the total number coming out. This tells us there are no separate North or South magnetic poles; magnets always have both.

🎯 Exam Tip: Contrast this with Gauss's law in electrostatics, \( \oint \vec{E} \cdot d\vec{S} = Q_{enc}/\epsilon_0 \), which accounts for isolated electric charges (monopoles). The zero on the right side of the magnetic version emphasizes the absence of magnetic monopoles.

Question 11. Magnetic lines of force form closed curve. Why?
Answer: Magnetic lines of force form continuous closed curves because magnetic poles always exist in pairs (North and South). Outside a magnet, the field lines emerge from the North pole and enter the South pole. Inside the magnet, these lines continue from the South pole to the North pole, completing the loop. This continuous path demonstrates that magnetic monopoles do not exist, and field lines have no beginning or end.
In simple words: Magnetic field lines always make complete loops, starting from a magnet's North pole, going to its South pole, and then continuing through the magnet back to the North pole. This is because there are no single magnetic poles, only pairs.

🎯 Exam Tip: The closed-loop nature of magnetic field lines is a direct consequence of Gauss's law for magnetism, which states that the net magnetic flux through any closed surface is zero.

Question 12. Compare the magnetic fields of a bar magnet and current carrying solenoid.
Answer:
**Similarities between a Bar Magnet and a Solenoid:**
1. Both align themselves in the North-South direction when suspended freely, acting like a compass.
2. Both attract magnetic substances like iron.
3. Both have two poles, a North pole and a South pole.
4. Like poles repel each other, and unlike poles attract each other in both cases.
5. Both show the phenomenon of magnetic induction, meaning they can magnetize other magnetic materials.

**Dissimilarities between a Bar Magnet and a Solenoid:**

🎯 Exam Tip: Focus on controllability and permanence as key distinguishing factors. A solenoid's magnetism is transient and adjustable, whereas a permanent bar magnet's field is constant.

Question 13. Explain the possible cause of earth's magnetism.
Answer: The Earth's magnetic field is primarily caused by the dynamic movement of molten iron and nickel in its outer core. This phenomenon is known as the geodynamo theory. The Earth's rotation (Coriolis effect) causes these electrically conducting liquids to flow in swirling patterns, creating electric currents. These circulating electric currents, in turn, generate a vast magnetic field that extends far into space, protecting our planet from solar winds.
In simple words: The Earth acts like a giant magnet because the liquid iron inside its core constantly moves and swirls, creating electric currents that generate the magnetic field.

🎯 Exam Tip: The key concept is the "geodynamo effect," involving the convection of molten metallic fluids in the Earth's outer core and the Coriolis effect from Earth's rotation. Avoid referring to a large permanent magnet inside the Earth.

Question 14. What are the uses of hysteresis curve?
Answer: The hysteresis curve, or B-H loop, is a powerful tool used to understand and select magnetic materials for various applications based on their magnetic properties. Here are its main uses:
1. **Material Selection for Permanent Magnets:** For permanent magnets, materials with a large area under the B-H loop, which means high retentivity (strong residual magnetism) and high coercivity (resistance to demagnetization), are preferred. Steel and Alnico are examples of such materials.
2. **Material Selection for Electromagnets and Transformer Cores:** For electromagnets and transformer cores, materials with a narrow hysteresis loop are desired. This indicates low retentivity and low coercivity, allowing them to be easily magnetized and demagnetized, and minimizing energy loss during cyclic magnetization. Soft iron is an excellent example of such a material.
3. **Calculation of Energy Loss:** The area enclosed by the hysteresis loop represents the energy dissipated as heat per unit volume during each cycle of magnetization. This information is critical in designing devices like transformers and motors to minimize energy waste.
4. **Characterization of Magnetic Materials:** By analyzing the shape and size of the hysteresis loop, one can determine fundamental magnetic properties like retentivity, coercivity, and permeability, which are crucial for classifying and understanding different magnetic materials.
In simple words: Hysteresis curves help engineers choose the right materials for magnets, like strong steel for permanent magnets or easily-magnetized soft iron for electromagnets. They also show how much energy is lost as heat in magnetic devices.

🎯 Exam Tip: Always link specific characteristics of the hysteresis loop (e.g., wide loop, narrow loop, large area) to their practical implications in choosing materials for permanent magnets, electromagnets, or minimizing energy loss in devices.

RBSE Class 12 Physics Chapter 8 Long Answer Type Questions

Question 1. Name and define three elements required to specify the earth's magnetic field at a given place. Draw a labelled diagram to define these elements.
Answer: The Earth's magnetic field at any location can be fully described by three key elements:
(i) **Angle of Declination:** This is the angle between the geographical meridian (a vertical plane through true north and south poles) and the magnetic meridian (a vertical plane through the magnetic axis of a freely suspended magnet) at a specific place.
(ii) **Angle of Dip:** This is the angle that the Earth's total magnetic field vector \( \vec{B} \) makes with the horizontal direction within the magnetic meridian at a particular place. This angle changes depending on the location on Earth's surface. At the magnetic equator, the dip angle is \( 0^\circ \) (horizontal), and at the magnetic poles, it is \( 90^\circ \) (vertical). Everywhere else, it falls between \( 0^\circ \) and \( 90^\circ \).
(iii) **Horizontal Component of Earth's Magnetic Field:** This is the part of the Earth's total magnetic field \( \vec{B} \) that acts horizontally in the magnetic meridian. If \( \theta \) is the angle of dip, then the horizontal component \( B_H \) is given by \( B_H = B \cos\theta \). The vertical component \( B_V = B \sin\theta \).
In simple words: The Earth's magnetic field is defined by three things: how much it's tilted from true north (declination), how much it dips into the ground (dip angle), and how strong its horizontal part is. These help us understand its direction and strength at any spot.

🎯 Exam Tip: Remember to clearly define each element and how it's measured. A well-labelled diagram helps in scoring full marks.

Question 2. What do you mean by magnetic hysteresis hysteresis curve? Draw hysteresis curve and define terms related to it.
Answer: **Magnetic Hysteresis Curve:** Hysteresis refers to the lagging of magnetic induction (B) behind the magnetizing field (H) when a ferromagnetic material is repeatedly magnetized and demagnetized. This means that the material's magnetic state depends not only on the current magnetizing field but also on its past magnetic history.
**Terms Related to Hysteresis Curve:**
1. **Retentivity (or Remanence):** This is the value of magnetic induction (B) that remains in the material even after the magnetizing field (H) has been reduced to zero. It shows how much magnetism a material "retains" after the external field is removed.
2. **Coercivity:** This is the reverse magnetizing field (H) required to reduce the residual magnetic induction (retentivity) to zero. It indicates how much effort is needed to demagnetize the material.
A hysteresis curve (B-H loop) visually represents this phenomenon, showing how the magnetic field inside a material changes as the external magnetizing field is increased, decreased, and reversed.
In simple words: Magnetic hysteresis means that a material's magnetism doesn't just switch off when the external magnetic field is removed; it holds onto some magnetism. The hysteresis curve shows this "memory" effect. Retentivity is the leftover magnetism, and coercivity is the reverse push needed to remove that leftover magnetism.

🎯 Exam Tip: When defining terms, use clear, concise language. For the hysteresis curve, focus on correctly identifying retentivity and coercivity on the diagram.

Question 3. Explain paramagnetism and specify its characteristics. Write five differences between diamagnetic and paramagnetic substance.
Answer: **Paramagnetism Explanation:**
Paramagnetic substances are materials that become weakly magnetized in the same direction as an applied external magnetic field. They are attracted to magnets and move from weaker to stronger magnetic field regions. This happens because their atoms or molecules have a permanent magnetic moment due to unpaired electrons. In the absence of an external magnetic field, these tiny magnetic moments are randomly oriented due to thermal motion, so there is no overall magnetism. However, when an external magnetic field is applied, these moments slightly align with the field, causing a weak net magnetization.
**Characteristics of Paramagnetic Substances:**
1. They have a small, positive magnetic susceptibility (\( \chi_m > 0 \)).
2. They are weakly attracted by strong magnets.
3. They move from weaker to stronger parts of a magnetic field.
4. Their magnetic susceptibility decreases as temperature increases (following Curie's Law, \( \chi_m \propto \frac{1}{T} \)).
5. When placed in a non-uniform magnetic field, a paramagnetic liquid accumulates where the field is stronger.

**Examples:** Aluminum (Al), Chromium (Cr), Manganese (Mn), Platinum (Pt), Oxygen (O₂), Copper Chloride (\( \text{CuCl}_2 \)), Sodium (Na), Lithium (Li), Magnesium (Mg), Tungsten.

**Diamagnetism Explanation:**
Diamagnetic substances are materials that develop a weak magnetization in the opposite direction to an applied external magnetic field. They are feebly repelled by magnets and tend to move from stronger to weaker parts of a magnetic field. This behavior arises because the electrons orbiting the nucleus generate orbital angular momentum, creating tiny magnetic moments. In diamagnetic substances, the resultant magnetic moment in each atom is initially zero. When an external magnetic field is applied, it induces a change in the electron's orbital motion (due to Lenz's law), creating a magnetic moment that opposes the applied field. This induced moment is why they are repelled.
**Characteristics of Diamagnetic Substances:**
1. They have a small, negative magnetic susceptibility (\( \chi_m < 0 \)).
2. They are weakly repelled by strong magnets.
3. They move from stronger to weaker parts of a magnetic field.
4. Their magnetic susceptibility is largely independent of temperature.
5. When placed in a non-uniform magnetic field, a diamagnetic liquid moves away from stronger field regions.

**Examples:** Copper (Cu), Zinc (Zn), Bismuth (Bi), Mercury (Hg), Hydrogen (H₂), Nitrogen (N₂), Gold (Au), Silver (Ag), Lead (Pb), Water, Diamond, Silicon, Quartz, Alcohol, Marble, Glass, Helium, Argon.

**Five Differences between Diamagnetic and Paramagnetic Substances:**
1. **Direction of Magnetization:** Diamagnetic substances magnetize opposite to the field; paramagnetic substances magnetize in the same direction.
2. **Interaction with Magnets:** Diamagnetic substances are weakly repelled; paramagnetic substances are weakly attracted.
3. **Magnetic Susceptibility (\( \chi_m \)):** Diamagnetic substances have negative \( \chi_m \); paramagnetic substances have positive \( \chi_m \).
4. **Temperature Dependence:** Diamagnetic \( \chi_m \) is nearly independent of temperature; paramagnetic \( \chi_m \) decreases with increasing temperature (Curie's Law).
5. **Electron Configuration:** Diamagnetic atoms have no unpaired electrons (zero net magnetic moment); paramagnetic atoms have unpaired electrons (permanent net magnetic moment).
In simple words: Paramagnetic materials are slightly pulled towards magnets and have their tiny internal magnets align with the external field. Diamagnetic materials are slightly pushed away because their internal magnets align opposite to the field. Paramagnetic strength depends on temperature, while diamagnetic strength usually doesn't.