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

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

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Question 1

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State the properties of magnetic field lines of a bar magnet.

Magnetic field lines form continuous closed loops, unlike electric field lines which begin and end on charges.
The tangent to the field line at any point gives the direction of the net magnetic field B at that point.
Field lines never intersect each other; if they did, the direction of B at the point of intersection would be ambiguous.

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Question 2

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Why are magnetic monopoles said not to exist? What happens when a bar magnet is broken into two halves?

Magnetic monopoles (isolated north or south poles) are not known to exist in nature, unlike isolated electric charges.
When a bar magnet is broken into two halves (in any direction), each piece becomes a complete magnet with its own north and south pole.
This is analogous to cutting a solenoid, where each piece retains continuous field lines emerging from one face and entering the other.

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Question 3

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Write the expression for torque on a magnetic dipole placed in a uniform magnetic field and explain the significance of each term.

The torque is given by the vector relation tau = m x B, where m is the magnetic moment vector and B is the external magnetic field.
In magnitude, tau = mB sin(theta), where theta is the angle between the magnetic moment m and the field B.
This torque is restoring in nature; it tends to align the dipole along the direction of the external field, reaching zero when theta = 0 or 180 degrees.

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Question 4

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State Gauss's law for magnetism and explain how it differs from Gauss's law for electrostatics.

Gauss's law for magnetism states that the net magnetic flux through any closed surface is zero: phi_B = sum B.Delta_S = 0.
In electrostatics, the flux through a closed surface equals q/epsilon0, where q is the enclosed charge; the non-zero value reflects the existence of isolated charges.
The difference arises because magnetic monopoles do not exist; there are no sources or sinks of B, making B-field lines always continuous closed loops.

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Question 5

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Define magnetisation M of a material. Write its SI unit and dimensional formula.

Magnetisation M is defined as the net magnetic moment per unit volume of a material: M = m_net / V.
It is a vector quantity with SI unit A m^-1 (ampere per metre).
Its dimensional formula is [L^-1 A], representing magnetic moment divided by volume.

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Question 6

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Define magnetic susceptibility. How does it determine whether a material is diamagnetic or paramagnetic?

Magnetic susceptibility chi is a dimensionless quantity defined by the relation M = chi*H, measuring how a material responds to an external magnetic field.
For diamagnetic materials, chi is small and negative (-1 <= chi < 0), indicating M is opposite to H.
For paramagnetic materials, chi is small and positive (0 < chi < epsilon), indicating M is in the same direction as H.

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Question 7

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Write the relation between relative magnetic permeability mu_r, magnetic susceptibility chi, and magnetic permeability mu.

The relative magnetic permeability is related to susceptibility by mu_r = 1 + chi, and is dimensionless.
The magnetic permeability of the material is mu = mu0 * mu_r, where mu0 is the permeability of free space.
Combining both: mu = mu0 (1 + chi); the three quantities chi, mu_r, and mu are interrelated and only one is independent.

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Question 8

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Write the expression for magnetic potential energy of a dipole in a uniform field. At which orientations is the energy minimum and maximum?

The magnetic potential energy is U_m = -m.B = -mB cos(theta), where theta is the angle between m and B; zero is fixed at theta = 90 degrees.
Potential energy is minimum (U = -mB) when theta = 0 degrees, i.e., the dipole is parallel to the field — this is the most stable equilibrium position.
Potential energy is maximum (U = +mB) when theta = 180 degrees, i.e., the dipole is antiparallel to the field — this is the most unstable equilibrium position.

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Question 9

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How does a bar magnet act as an equivalent solenoid? What is the relation between their magnetic moments?

A bar magnet can be thought of as a large number of circulating currents, in analogy with a solenoid, since both produce similar magnetic field patterns.
At large distances, the axial field of a finite solenoid is B = (mu0/4pi)(2m/r^3), which is identical to the axial field formula of a bar magnet.
The magnetic moment of a bar magnet equals the magnetic moment of an equivalent solenoid that produces the same magnetic field.

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Question 10

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What is the magnetic intensity H? Write the relation connecting B, H, and M.

Magnetic intensity H is a vector field defined as H = B/mu0 - M, where B is the total magnetic field and M is the magnetisation of the material; it is measured in A/m.
H represents the contribution of external factors (such as current in a solenoid) to the total magnetic field, independent of the material.
The total magnetic field in a material is B = mu0 (H + M), showing B depends on both external (H) and material (M) contributions.

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Question 11

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Explain why magnetic field lines inside a toroid form closed loops without violating any physical principle.

Magnetic field lines inside a toroid are completely confined within its windings and form closed loops circling the current-carrying conductors.
This is physically correct because each closed loop of a magnetic field line encloses a region through which a current passes.
Magnetostatic field lines can form closed loops only when they enclose a current-carrying region; empty-space loops are not permitted.

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Question 12

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State the condition for stable and unstable equilibrium of a magnetic dipole in a uniform magnetic field.

A magnetic dipole is in stable equilibrium when its magnetic moment m is parallel to the external field B (theta = 0 degrees); potential energy is minimum (-mB) and net torque is zero.
Unstable equilibrium occurs when m is antiparallel to B (theta = 180 degrees); potential energy is maximum (+mB) and any small disturbance will cause the dipole to rotate.
In both cases the net force on the dipole in a uniform field is zero; only the torque determines stability.

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Question 13

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What is the Meissner effect? What are the values of chi and mu_r for a superconductor?

The Meissner effect is the phenomenon of perfect diamagnetism in superconductors, where the external magnetic field is completely expelled from the interior of the material.
For a superconductor, chi = -1 and mu_r = 0, meaning the material has zero magnetic permeability and no field penetrates it.
Superconductors are metals cooled to very low temperatures that simultaneously exhibit perfect electrical conductivity and perfect diamagnetism.

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Chapter 5: Magnetism and Matter | Short Questions

State the properties of magnetic field lines of a bar magnet.

Why are magnetic monopoles said not to exist? What happens when a bar magnet is broken into two halves?

Write the expression for torque on a magnetic dipole placed in a uniform magnetic field and explain the significance of each term.

State Gauss's law for magnetism and explain how it differs from Gauss's law for electrostatics.

Define magnetisation M of a material. Write its SI unit and dimensional formula.

Define magnetic susceptibility. How does it determine whether a material is diamagnetic or paramagnetic?

Write the relation between relative magnetic permeability mu_r, magnetic susceptibility chi, and magnetic permeability mu.

Write the expression for magnetic potential energy of a dipole in a uniform field. At which orientations is the energy minimum and maximum?

How does a bar magnet act as an equivalent solenoid? What is the relation between their magnetic moments?

What is the magnetic intensity H? Write the relation connecting B, H, and M.

Explain why magnetic field lines inside a toroid form closed loops without violating any physical principle.

State the condition for stable and unstable equilibrium of a magnetic dipole in a uniform magnetic field.

What is the Meissner effect? What are the values of chi and mu_r for a superconductor?

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