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Magnetism and Matter: questions with answers

Class 12 Physics, Chapter 5 (NCERT). 15 questions by concept, 1 with pictures. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Magnetic Gauss law and absence of monopoles

Question 1

A closed surface encloses the north end of a bar magnet but not its south end; part of the surface cuts through the magnet. What is its total outward magnetic flux?

  1. (a)It equals the flux through the outside part alone
  2. (b)Zero, because inward and outward flux balance
  3. (c)Positive, because only the north pole is enclosed
  4. (d)Negative, because the north pole absorbs field lines
Show answer

(b) Zero, because inward and outward flux balance

Gauss law for magnetism applies even when the surface passes through magnetic material. The return field inside the magnet contributes inward flux that cancels outward flux.

Question 2

A closed box lies in a nonzero spatially uniform magnetic field. Its net outward magnetic flux is zero. Which conclusion follows?

  1. (a)Flux entering the box balances flux leaving it, while B remains nonzero
  2. (b)The field must reverse direction at the centre of the box
  3. (c)The magnetic field must be zero at every point of the box surface
  4. (d)No field can cross any individual face of the box
Show answer

(a) Flux entering the box balances flux leaving it, while B remains nonzero

A uniform field can enter one side and leave another. The signed integral over the entire closed surface vanishes without requiring any local field to vanish.

Magnetic poles and field-line meaning

Question 3

A particle of positive charge moves along +x in a magnetic field directed along +y. What is the direction of its magnetic force?

  1. (a)Along +x
  2. (b)Along +y
  3. (c)Along +z
  4. (d)Zero force
Show answer

(c) Along +z

Use F=q v cross B. The vector +x cross +y points along +z; negative charge reverses that direction.

Question 4

A positive charge moves antiparallel to a nonzero uniform magnetic field. What is its magnetic force?

  1. (a)A nonzero force opposite the field
  2. (b)A nonzero force perpendicular to the field
  3. (c)Zero force
  4. (d)A nonzero force along the field
Show answer

(c) Zero force

Parallel or antiparallel velocity gives sin(theta)=0. A magnetic field line gives B direction, not a force qB along itself.

Far axial and equatorial dipole fields

Question 5

The diagram shows the moment m of a short source magnet and a far equatorial observation point P. Ignore other fields. In which direction will the north end of a small compass at P point?

mP+x+y
  1. (a)Towards +x
  2. (b)Towards -x
  3. (c)Towards +y
  4. (d)Towards -y
Show answer

(b) Towards -x

At a far equatorial point B=-(mu0/4pi)m/r³, opposite the source moment. The compass north end aligns with that local B.

Question 6

At a far equatorial point of a short bar magnet, its field points towards -y. What is the direction of the source magnetic moment?

  1. (a)Towards -x
  2. (b)Towards +y
  3. (c)Towards -y
  4. (d)Towards +x
Show answer

(b) Towards +y

The equatorial field is opposite to the source moment, so reverse the measured field direction to find m.

Torque and force on a magnetic dipole

Question 7

A magnet of moment m has measured torque magnitude mB/2 in a uniform field of magnitude B. Its moment makes an acute angle with the field. What is that angle?

  1. (a)30°
  2. (b)75°
  3. (c)60°
  4. (d)45°
Show answer

(a) 30°

The measured torque satisfies mB sin(theta)=mB/2. Thus sin(theta)=1/2 and the acute angle is 30°. The cosine error would give 60° instead.

Question 8

A permanent magnet has no translational acceleration in a uniform field, but an iron nail is attracted near a bar magnet. Which explanation accounts for the difference?

  1. (a)Both fields pull the dipole along B; the supposedly uniform-field experiment must be wrong
  2. (b)The nail is attracted because every magnetic field has zero torque
  3. (c)The bar magnet produces a field gradient, whereas a uniform field gives a dipole no net force
  4. (d)Only the nail has magnetic poles, so only the nail can feel magnetic force
Show answer

(c) The bar magnet produces a field gradient, whereas a uniform field gives a dipole no net force

The nail acquires an induced moment in the nonuniform field of the bar magnet. A gradient permits unequal opposing forces and hence net attraction. Spatial uniformity removes that net force.

Magnetic potential energy, equilibrium and work

Question 9

A magnet is exactly antiparallel to a nonzero uniform B. It can rotate freely. How should this orientation be classified?

  1. (a)Unstable equilibrium: torque is zero and potential energy is maximum
  2. (b)Not an equilibrium: the torque magnitude is mB
  3. (c)Stable equilibrium: zero torque is sufficient for stability
  4. (d)Stable equilibrium: potential energy is minimum
Show answer

(a) Unstable equilibrium: torque is zero and potential energy is maximum

At theta=180°, sin(theta)=0 but U=-mB cos(theta)=+mB is a maximum. A slight perturbation lowers energy and moves the magnet away, so equilibrium is unstable.

Question 10

A magnet in a uniform field is initially exactly antiparallel to B and at rest. After a tiny angular displacement, what does magnetic torque tend to do?

  1. (a)Restore the magnet to antiparallel alignment because the initial torque was zero
  2. (b)Remain zero for every nearby orientation
  3. (c)Increase the displacement from antiparallel alignment
  4. (d)Keep its direction unchanged while its magnitude is always mB
Show answer

(c) Increase the displacement from antiparallel alignment

Antiparallel alignment is an energy maximum. A small deviation creates a torque that moves the magnet towards parallel alignment, farther from the unstable starting direction.

Magnetisation and magnetic intensity

Question 11

An ideal long solenoid has fixed free current and fixed turns per metre. Its air core is replaced by a uniform core of relative permeability 100. What is H_after:H_before?

  1. (a)1:1
  2. (b)99:1
  3. (c)100:1
  4. (d)1:100
Show answer

(a) 1:1

For this ideal long solenoid H=nI. Neither n nor I changes. The core changes B and M rather than multiplying H.

Susceptibility, permeability and classification

Question 12

Sample X has a small positive susceptibility and sample Y has a much larger ferromagnetic response. Which statement correctly classifies X and Y?

  1. (a)X is paramagnetic; Y is ferromagnetic
  2. (b)Both are ferromagnetic because both responses are positive
  3. (c)X is diamagnetic; Y is ferromagnetic
  4. (d)Both are diamagnetic because both can be magnetised
Show answer

(a) X is paramagnetic; Y is ferromagnetic

Sign and magnitude both matter. A small positive response is paramagnetic, whereas the much stronger ordered response belongs to ferromagnetism.

Diamagnetic response and perfect diamagnetism

Question 13

A diamagnetic sample is repelled from a north pole. The pole is replaced by a south pole with the same field-strength gradient. What happens to its induced moment and force?

  1. (a)The induced moment reverses, so the south pole attracts the sample
  2. (b)The induced moment stays fixed, so the south pole attracts the sample
  3. (c)The induced moment reverses, and repulsion persists
  4. (d)The induced moment vanishes and there is no force
Show answer

(c) The induced moment reverses, and repulsion persists

Diamagnetic magnetisation is induced opposite to the local applied field. Reversing that field reverses the moment as well, leaving the tendency to move towards weaker field unchanged.

Paramagnetic moments and thermal disorder

Question 14

A paramagnetic specimen is cooled while the applied field is held fixed. It remains in the same paramagnetic phase and below saturation. What happens to its magnetisation magnitude?

  1. (a)It reverses sign and becomes diamagnetic
  2. (b)It is forced to zero because permanent moments disappear
  3. (c)It increases as random thermal motion weakens
  4. (d)It decreases because thermal energy is needed to align moments
Show answer

(c) It increases as random thermal motion weakens

Cooling reduces thermal disorder, so the same field produces greater alignment and a larger positive magnetisation, within the stated regime.

Domains, remanence and temperature

Question 15

An idealised ferromagnetic specimen has two domains of equal volume. Each domain has the same nonzero magnetisation, but they point in opposite directions. Which statement is correct?

  1. (a)The domains must both point in the same direction for cancellation
  2. (b)Each domain must be unmagnetised because the net specimen moment is zero
  3. (c)Each domain is magnetised, but their contributions give zero net specimen moment
  4. (d)The specimen moment is twice one domain's moment, pointing in the direction of the first domain.
Show answer

(c) Each domain is magnetised, but their contributions give zero net specimen moment

Magnetic moments add vectorially. Equal and opposite domain moments cancel in the specimen total; cancellation does not erase the nonzero magnetisation inside each domain.

Common misconception in Magnetism and Matter

Open full misconception guide →

Can you separate the north pole by cutting a magnet?

Wrong

Cutting a bar magnet in half gives one north pole and one south pole.

Why it feels right. The original poles are at opposite ends.

Right

Each ordinary fragment remains a dipole with its own north and south poles.

Worked example

Before
NOne barS
After
NPiece 1S
NPiece 2S
Cutting creates new faces with opposite poles. Each piece remains a dipole.
  1. Cut across a bar magnet.
  2. Each shorter bar has an original end and a new cut end.
  3. A compass can distinguish the opposite poles on each bar.