Magnetism and Matter

A paramagnetic sample shows a net magnetisation of $6\mathrm{A}/\mathrm{m}$ when it is placed in an external magnetic field of $0.4\mathrm{T}$ at a temperature of $4\mathrm{K}$ . When the sample is placed in an external magnetic field of $0.3\mathrm{T}$ at a temperature of $24\mathrm{K}$ , then the magnetisation will be :
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Let the magnetic field on the earth be modelled by that of a point magnetic dipole at the centre of the 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

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

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 10⁸

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

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

A paramagnetic sample shows a net magnetisation of 8 Am^-1 when placed in an external magnetic field of 0.6 T at a temperature of 4 K. 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

Consider the two idealised 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 (iii) Magnetic field is constant inside the solenoid and zero outside. These idealised assumptions, however, contradict fundamental laws as below:

(a) Case I contradicts gauss’s law for electrostatic fields

(b) Case ii contradicts gauss’s law for magnetic fields

(c) Case(i) agrees with

(d) Case(ii) contradicts with _en

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

The magnetic field of the 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 the earth. At Mumbai, the 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 the earth axis passes through Greenwich
(d) Declination averaged over the earth must be always negative