CBSE Class 12 Physics Magnetism and Matter MCQs Set A

Question: 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, 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

Answer: A

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: D

Question: Magnetic lines of force due to a bar magnet do not intersect because
(a) a point always has a single net magnetic field
(b) the lines have similar charges and so repel each other
(c) the lines always diverge from a single force
(d) the lines need magnetic lenses to be made to interest

Answer: A

Question: Choose the correct option

Assertion : The ferromagnetic substance do not obey Curie’s law.
Reason : At Curie point a ferromagnetic substance start behaving as a paramagnetic substance.
(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.

Answer: C

Question: The coercivity of a small magnet where the ferromagnet gets demagnetized is 3 × 10³ Am^–1. The current required to be passed in a solenoid of length 10 cm and number of turns 100, so that the magnet gets demagnetized when inside the solenoid, is

(a) 30 mA
(b) 60 mA
(c) 3 A
(d) 6 A

Answer: C

Question: Choose the correct option

Assertion : A paramagnetic sample display greater magnetisation (for the same magnetic field) when cooled.
Reason : The magnetisation does not depend on temperature
(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.

Answer: D

Question: The magnetic susceptibility is negative for
(a) diamagnetic material only
(b) paramagnetic material only
(c) ferromagnetic material only
(d) paramagnetic and ferromagnetic materials

Answer=A

Question: Two points A and B are situated at a distance x and 2x respectively from the nearer pole of a magnet 2 cm long. The ratio of magnetic field at A and B is
(a) 4 : 1 exactly           
(b) 4 : 1 approximately
(c) 8 : 1 approximately
(d) 1 : 1 approximately

Answer=C

Question: Choose the correct option

Assertion : Electromagnetic are made of soft iron.
Reason : Coercivity of soft iron is small.

(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.

Answer=B

Question: A magnetic needle of magnetic moment 6.7 × 10^–2 Am² and moment of inertia 7.5 × 10^–6 kg m² is performing simple harmonic oscillations in a magnetic field of 0.01 T. Time taken for 10 complete oscillations is :

(a) 6.98 s
(b) 8.76 s
(c) 6.65 s
(d) 8. 89 s

Answer=C

Question: The magnetism of magnet is due to
(a) pressure of big magnet inside the earth
(b) earth
(c) cosmic rays
(d) the spin motion of electron
Answer: D

Question: Choose the correct option

Assertion : The poles of magnet can not be separated by breaking into two pieces.
Reason : The magnetic moment will be reduced to half when a magnet is broken into two equal pieces

(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.

Answer: B

Question: A thin diamagnetic rod is placed vertically between the poles of an electromagnet. When the current in the electromagnet is switched on, then the diamagnetic rod is pushed up, out of the horizontal magnetic field. Hence the rod gains gravitational potential energy. The work required to do this comes from

(a) the current source
(b) the magnetic field
(c) the induced electric field due to the changing magnetic field
(d) the lattice structure of the material of the rod

Answer: A

Question: The given figure represents a material which is

(a) paramagnetic
(b) diamagnetic
(c) ferromagnetic
(d) none of these

Answer: B

Question: Choose the correct option

Assertion : The earth¢s magnetic field is due to iron present in its core.
Reason : At a high temperature magnet losses its magnetism

(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct

Answer: D

Question: A magnetic dipole is acted upon by two magnetic fields which are inclined to each other at an angle of 75°. One of the fields has a magnitude of 15 mT. The dipole attains stable equilibrium at an angle of 30° with this field. The magntidue of the other field (in mT) is close to

(a) 1
(b) 11
(c) 36
(d) 1060

Answer: B

Question: A dip circle is so set that its needle moves freely in the magnetic meridian. In this position, the angle of dip is 40º. Now the dip circle is rotated so that the plane in which the needle moves makes an angle of 30º with the magnetic
meridian. In this position, the needle will dip by an angle
(a) 40º         
(b) 30º
(c) more than 40º
(d) less than 40º

Answer: B

Question: A magnetic dipole is under the influence of two magnetic fields The angle between the field directions is 60° and one of the fields has a magnitude of 1.2 × 10^–2 T. If the dipole comes to stable equilibrium at an angle of 15° with this field, what is the magnitude of other field ?
(a) 4.4 × 10^–3 tesla
(b) 5.2 × 10^–3 tesla
(c) 3.4 × 10^–3 tesla
(d) 7.8 × 10^–3 tesla

Answer: A

Question: A long straight horizontal cable carries a current of 2.5 A in the direction 10° south of west to 10° north of east. The magnetic meridian of the place happens to be 10° west of the geographic meridian. The earth’s magnetic field at the location is 0.33 Gauss, and the angle of dip is zero. Locate the line of neutral points. (Ignore the thickness of the cable).
(a) 1.5 cm
(b) 2.5 cm
(c) 3.5 cm
(d) 2.0 cm

Answer: A

Question: A very small magnet is placed in the magnetic meridian with its south pole pointing north. The null point is obtained 20 cm away from the centre of the magnet. If the earth¢s magnetic field (horizontal component) at this point be 0.3 gauss, the magnetic moment of the magnet is
(a) 8.0 × 10² e.m.u.
(b) 1.2 × 10³ e.m.u.
(c) 2.4 × 10³ e.m.u.
(d) 3.6 × 10³ e.m.u.

Answer: B

Question: The time period of oscillation of a magnet in a vibration magnetometer is 1.5 sec. The time period of oscillation of another magnet similar in size, shape and mass but having 1/4 magnetic moment than that of the 1st magnet oscillating at the same place will be
(a) 0.75 sec
(b) 1.5 sec
(c) 3.0 sec
(d) 6.0 sec

Answer: C