CBSE Class 12 Physics Magnetic Effect of Current and Magnetism Assignment Set B

Question. Magnetism in substances is caused by
(a) orbital motion of electrons only
(b) spin motion of electrons only
(c) due to spin and orbital motions of electrons both
(d) hidden magnets

Answer: C

Question. 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 ( ^→m = mφ).
(c) is zero, otherwise there would be a field falling as 1/r³ at large distances outside the toroid.
(d) is pointing radially outwards.

Answer: C

Question. A magnetic needle is kept in a uniform magnetic field. It experiences
(a) a force and a torque
(b) a force but not a torque
(c) a torque but not a force
(d) neither a torque nor a force

Answer: C

Question. Above Curie’s temperature ferromagnetic substances becomes
(a) paramagnetic
(b) diamagnetic
(c) superconductor
(d) no change

Answer: A

Question. A magnetic needle is kept in a non-uniform magnetic field. It experiences
(a) a force and a torque
(b) a force but not a torque
(c) a torque but not a force
(d) neither a force nor a torque

Answer: A

Question. 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 , then
(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.

Answer: A, D

Question. Electro-magnets are made of soft iron because soft iron has
(a) small susceptibility and small retentivity
(b) large susceptibility and small retentivity
(c) large permeability and large retentivity
(d) small permeability and large retentivity.

Answer: B

Question. 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

Answer: B

Question. If the horizontal and vertical components of earth’s magnetic field are equal at a certain place, the angle of dip is
(a) 90°
(b) 60°
(c) 45°
(d) 0°

Answer: C

Question. In the hysteresis cycle, the value of H needed to make the intensity of magnetisation zero is called
(a) retentivity
(b) coercive force
(c) Lorentz force
(d) none of the above

Answer: B

Question. 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.

Answer: C

Question. The meniscus of a liquid contained in one of the limbs of a narrow U-tube is placed between the pole-pieces of an electromagnet with the meniscus in a line with the field. When the electromagnet is switched on, the liquid is seen to rise in the limb. This indicates that the liquid is
(a) ferromagnetic
(b) paramagnetic
(c) diamagnetic
(d) non-magnetic

Answer: B

Question. If a magnetic substance is kept in a magnetic field, then which of the following substances is thrown out?
(a) Paramagnetic
(b) Ferromagnetic
(c) Diamegnetic
(d) Antiferromagnetic

Answer: C

Question. 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.

Answer: A

Question. In a plane perpendicular to the magnetic meridian, the dip needle will be
(a) vertical
(b) horizontal
(c) inclined equal to the angle of dip at that place
(d) pointing in any direction

Answer: A

Question. Susceptibility is positive for
(a) paramagnetic substances
(b) ferromagnetic substances
(c) non-magnetic substances
(d) diamagnetic substances

Answer: A, B

Question. A permanent magnet attracts
(a) all substances
(b) only ferromagnetic substances
(c) some substances and repels others
(d) ferromagnetic substances and repels all others

Answer: B

Question. Points A and B are situated perpendicular to the axis of a 2 cm long bar magnet at large distances x and 3x from its centre on opposite sides. The ratio of the magnetic fields at A and B will be approximately equal to
(a) 1: 9
(b) 2: 9
(c) 27: 1
(d) 9: 1

Answer: C

Fill in the Blanks

Question. The unit of magnetic dipole moment is _______________.
Answer: Am²

Question. SI unit of magnetic pole strength is _______________.
Answer: ampere-meter

Question. The angle between the magnetic moment of a bar magnet and its magnetic field at an equatorial point is _______________.
Answer: 180°

Question. For paramagnetic materials magnetic susceptibility is related with temperature as inversely proportional to _______________.
Answer: T

Question. Diamagnetic substances when placed in a magnetic field, are magnetised in the direction _______________ to the magnetic field.
Answer: opposite

Question. Inside the body of a magnet the direction of magnetic field lines is from _______________.
Answer: South pole to North pole

Question. There is no effect of temperature on _______________ type of materials.
Answer: diamagnetic

Question. Paramagnetic materials when placed in a magnetic field are magnetised in the direction _______________ to the magnetic field.
Answer: parallel

Question. The ability of a material to retain magnetism after removal of magnetizing field is called as _______________.
Answer: retentivity

Question. Ferromagnetism can be explained on the basis of formation of _______________ within the materials.
Answer: domain