Class 12 Magnetism and Matter Mock Test (CBSE)

Topics in Magnetism and Matter

• The bar magnet
• Magnetic field lines
• Bar magnet as an equivalent solenoid
• Dipole in a uniform magnetic field
• Magnetism and Gauss's law
• Magnetisation and magnetic intensity
• Magnetic properties of materials

Magnetism and Matter practice questions with answers

1. Q1Multiple choice·1 mark
   Which of the following classes of magnetic materials has a small and negative magnetic susceptibility?

   1. (a)Ferrimagnetic
   2. (b)Diamagnetic
   3. (c)Paramagnetic
   4. (d)Ferromagnetic
   Show answer and step marks

   1. Correct option: (b) Diamagnetic+1

   Answer: (b)
2. Q2Multiple choice·1 mark
   The magnetic dipole moment of a bar magnet having pole strength $m$ and magnetic length $2l$ is equal to:

   1. (a)$m l$
   2. (b)$m l^2$
   3. (c)$\frac{m}{2l}$
   4. (d)$2 m l$
   Show answer and step marks

   1. Correct option: (d) $2 m l$+1

   Answer: (d)
3. Q3Short answer·2 marks
   State any two properties of magnetic field lines. Explain why two magnetic field lines can never intersect each other.
   Show answer and step marks

   1. First property, any one of: magnetic field lines form continuous closed loops; the tangent at any point gives the direction of $\vec{B}$ there; the more lines crossing a unit area, the stronger the field; outside a magnet they run from N to S and inside from S to N; or any other valid property.+0.5
   2. Second property, a different one from the same list.+0.5
   3. If two field lines intersected, there would be two tangents, i.e. two directions of the magnetic field, at the same point, which is impossible since the field has a single direction at each point.+1

   Answer: Any two of: field lines form closed loops; the tangent gives the direction of $\vec{B}$; closer lines mean a stronger field; outside they run N to S, inside S to N. They cannot intersect because the field would then have two directions at one point.
4. Q4Short answer·2 marks
   State any two properties of magnetic field lines. Explain why two magnetic field lines never intersect each other.
   Show answer and step marks

   1. First property, any one of: magnetic field lines form continuous closed loops; the tangent at any point gives the direction of $\vec{B}$ there; the more lines crossing a unit area, the stronger the field; outside a magnet they run from N to S and inside from S to N; or any other valid property.+0.5
   2. Second property, a different one from the same list.+0.5
   3. Explanation: if two magnetic field lines intersected, there would be two tangents at the point of intersection, which implies two directions of the magnetic field at the same point, which is physically impossible.+1

   Answer: Any two of: field lines form closed loops; the tangent gives the direction of $\vec{B}$; closer lines mean a stronger field; outside they run N to S, inside S to N. They never intersect because the field would then have two directions at one point.
5. Q5Short answer·3 marks
   Explain briefly how a finite current-carrying solenoid behaves like a bar magnet. Using this analogy, state the expression for the magnetic field along the axis of a solenoid of length $2l$ and radius $a$ carrying current $I$ with $n$ turns per unit length at a distant axial point ($r \gg l$ and $r \gg a$), and relate its magnetic moment to that of an equivalent bar magnet.
   Show answer and step marks

   1. The magnetic field lines of a finite current-carrying solenoid are similar to those of a bar magnet, with its two ends acting like North and South magnetic poles.+1
   2. At a distant axial point ($r \gg l$, $r \gg a$): $B = \frac{\mu_0 n I (2l) a^2}{2 r^3}$.+1
   3. Magnetic moment $M = NIA = n(2l) I \pi a^2$, so $B = \frac{\mu_0}{4\pi}\frac{2M}{r^3}$, the same as the axial field of a bar magnet of moment $M$.+1

   Answer: A finite solenoid's field lines resemble a bar magnet's, its ends acting as N and S poles. For $r \gg l, a$: $B = \frac{\mu_0 n I (2l) a^2}{2r^3} = \frac{\mu_0}{4\pi}\frac{2M}{r^3}$, where $M = NIA = n(2l)I\pi a^2$ is the magnetic moment of the equivalent bar magnet.

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