Chapter 5: Magnetism and Matter

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 an 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/r³ at large distances outside the toroid

d) is pointing radially outwards

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^o with the axis of the earth. At Mumbai, declination is nearly zero. Then,

a) the declination varies between 11.3^o W to 11.3^o E

b) the least declination is 0^o

c) the plane defined by the dipole axis and the earth axis passes through Greenwich

d) declination average over the earth must always be negative

In a permanent magnet at room temperature

a) magnetic moment of each molecule is zero

b) the individual molecules have a non-zero magnetic moment which is 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, the radius of the cross-section. In i) E is ideally treated as a constant between plates and zero outside. In ii) the 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

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 0.2T at a temperature of 16K, the magnetisation will be

a) 32/3 Am^-1

b) 2/3 Am^-1

c) 6 Am^-1

d) 2.4 Am^-1

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 meter and carries a current of 1 A. It has a soft iron core of μr = 1000. The core is heated beyond the Curie temperature Tc

a) the H field in the solenoid is 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 the direction

d) the magnetisation in the core diminishes by a factor of about 108

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