Magnetism and M...

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 xy-plane. Its magnetic moment m

In a permanent magnet at room temperature

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 (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 A\(m^{–1}\) 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

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 \(\mu_r\) = 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 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