## Class 12 Important Questions for Physics – Magnetism and Matter

**NCERT Exemplar Class 12 Physics** is very important resource for students preparing for XII Board Examination. Here we have provided **NCERT Exemplar Problems Solutions** along with **NCERT Exemplar Problems Class 12.**

Question from very important topics are covered by **NCERT Exemplar Class 12**. You also get idea about the type of questions and method to answer in your class 12th examination.

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Here you can get **Class 12 Important Questions Physics** based on NCERT Text book for Class XII. **Physics Class 12 Important Questions** are very helpful to score high marks in board exams. Here we have covered Important Questions on **Magnetism and Matter** for Class 12 Physics subject.

**Physics Important Questions Class 12** are given below.

### Multiple Choice Questions (MCQ I)

- A toroid of n turns, mean radius R and cross-sectional radius a carries current I. It is placed on a horizontal table taken as x-y plane. Its magnetic moment m
- (a) is non-zero and points in the z-direction by symmetry.
- (b) points along the axis of the toroid
- (c) is zero, otherwise there would be a field falling as (1/r
^{3}) at large distances outside the toroid. - (d) is pointing radially outwards.

- The magnetic field of Earth can be modelled by that of a point dipole placed at the centre of the Earth. The dipole axis makes an angle of 11.3° with the axis of Earth. At Mumbai, declination is nearly zero. Then,
- (a) the declination varies between 11.3° W to 11.3° E.
- (b) the least declination is 0°.
- (c) the plane defined by dipole axis and Earth axis passes through Greenwich.
- (d) declination averaged over Earth must be always negative.

- In a permanent magnet at room temperature
- (a) magnetic moment of each molecule is zero.
- (b) the individual molecules have non-zero magnetic moment which are all perfectly aligned.
- (c) domains are partially aligned.
- (d) domains are all perfectly aligned.

- Consider the two idealized systems: (i) a parallel plate capacitor with large plates and small separation and (ii) a long solenoid of length L >> R, radius of cross-section. In (i) E is ideally treated as a constant between plates and zero outside. In (ii) magnetic field is constant inside the solenoid and zero outside. These idealised assumptions, however, contradict fundamental laws as below:
- A paramagnetic sample shows a net magnetisation of 8 Am
^{–1}when placed in an external magnetic field of 0.6T at a temperature of 4K. When the same sample is placed in an external magnetic field of 0.2 T at a temperature of 16 K, the magnetisation will be- (a) (32/3) Am
^{–1} - (b) (2/3)Am
^{–1} - (c) 6 Am
^{–1} - (d) 2.4 Am
^{–1}

- (a) (32/3) Am

### Multiple Choice Questions (MCQ II)

- S is the surface of a lump of magnetic material.
- (a) Lines of B are necessarily continuous across S.
- (b) Some lines of B must be discontinuous across S.
- (c) Lines of H are necessarily continuous across S.
- (d) Lines of H cannot all be continuous across S.

- The primary origin(s) of magnetism lies in
- (a) atomic currents.
- (b) Pauli exclusion principle.
- (c) polar nature of molecules.
- (d) intrinsic spin of electron.

- A long solenoid has 1000 turns per metre and carries a current of 1 A. It has a soft iron core of μ
_{r}= 1000. The core is heated beyond the Curie temperature, T_{c}.- (a) The H field in the solenoid is (nearly) unchanged but the B field decreases drastically.
- (b) The H and B fields in the solenoid are nearly unchanged.
- (c) The magnetisation in the core reverses direction.
- (d) The magnetisation in the core diminishes by a factor of about 10
^{8}.

- Essential difference between electrostatic shielding by a conducting shell and magnetostatic shielding is due to
- (a) electrostatic field lines can end on charges and conductors have free charges.
- (b) lines of B can also end but conductors cannot end them.
- (c) lines of B cannot end on any material and perfect shielding is not possible.
- (d) shells of high permeability materials can be used to divert lines of B from the interior region.

- Let the magnetic field on earth be modelled by that of a point magnetic dipole at the centre of earth. The angle of dip at a point on the geographical equator
- (a) is always zero.
- (b) can be zero at specific points.
- (c) can be positive or negative.
- (d) is bounded.

### Very Short Answer Type Questions

- A proton has spin and magnetic moment just like an electron. Why then its effect is neglected in magnetism of materials?
- A permanent magnet in the shape of a thin cylinder of length 10 cm has M = 10
^{6}A/m. Calculate the magnetisation current I_{M}. - Explain quantitatively the order of magnitude difference between the diamagnetic susceptibility of N2 (~5 × 10
^{-9}) (at STP) and Cu (~10^{-5}). - From molecular view point, discuss the temperature dependence of susceptibility for diamagnetism, paramagnetism and ferromagnetism.
- A ball of superconducting material is dipped in liquid nitrogen and placed near a bar magnet.

(i) In which direction will it move?

(ii) What will be the direction of it’s magnetic moment?

### Short Answer Type Questions

- Verify the Gauss’s law for magnetic field of a point dipole of dipole moment m at the origin for the surface which is a sphere of radius R.
- Three identical bar magnets are riveted together at centre in the same plane as shown in Fig. 5.1. This system is placed at rest in a slowly varying magnetic field. It is found that the system of magnets does not show any motion. The north-south poles of one magnet is shown in the Fig. 5.1. Determine the poles of the remaining two.
- Suppose we want to verify the analogy between electrostatic and magnetostatic by an explicit experiment. Consider the motion of (i) electric dipole p in an electrostatic field E and (ii) magnetic dipole m in a magnetic field B. Write down a set of conditions on E, B, p, m so that the two motions are verified to be identical. (Assume identical initial conditions.)
- A bar magnet of magnetic moment m and moment of inertia I (about centre, perpendicular to length) is cut into two equal pieces, perpendicular to length. Let T be the period of oscillations of the original magnet about an axis through the mid point, perpendicular to length, in a magnetic field B. What would be the similar period T′ for each piece?
- Use (i) the Ampere’s law for H and (ii) continuity of lines of B, to conclude that inside a bar magnet, (a) lines of H run from the N pole to S pole, while (b) lines of B must run from the S pole to N pole.

### Long Answer Type Questions

- Verify the Ampere’s law for magnetic field of a point dipole of dipole moment m = mkˆ . Take C as the closed curve running clockwise along (i) the z-axis from z = a > 0 to z = R; (ii) along the quarter circle of radius R and centre at the origin, in the first quadrant of x-z plane; (iii) along the x-axis from x = R to x = a, and (iv) along the quarter circle of radius a and centre at the origin in the first quadrant of x-z plane.
- What are the dimensions of χ, the magnetic susceptibility? Consider an H-atom. Guess an expression for χ, upto a constant by constructing a quantity of dimensions of χ, out of parameters of the atom: e, m, v, R and μ
_{0}. Here, m is the electronic mass, v is electronic velocity, R is Bohr radius. Estimate the number so obtained and compare with the value of |χ| ~10^{–5}for many solid materials. - Assume the dipole model for earth’s magnetic field B which is given

- Consider the plane S formed by the dipole axis and the axis of earth. Let P be point on the magnetic equator and in S. Let Q be the point of intersection of the geographical and magnetic equators. Obtain the declination and dip angles at P and Q.
- There are two current carrying planar coils made each from identical wires of length L. C
_{1}is circular (radius R ) and C_{2}is square (side a). They are so constructed that they have same frequency of oscillation when they are placed in the same uniform B and carry the same current. Find a in terms of R.

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## Answers to Multiple Choice Questions

(MCQ I) | (MCQ II) | ||
---|---|---|---|

Q.No. |
Answer |
Q.No. |
Answer |

1 | (c) | 1 | (a), (d) |

2 | (a) | 2 | (a), (d) |

3 | (c) | 3 | (a), (d) |

4 | (b) | 4 | (a), (c), (d) |

5 | (b) | 5 | (b), (c), (d) |

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