Moving Charges and Magnetism Class 12 Physics MCQs Pdf
13. If a charged particle moves through a magnetic field perpendicular to it
(a) both momentum and energy of particle change.
(b) momentum as well as energy are constant.
(c) energy is constant but momentum changes.
(d) momentum is constant but energy changes.
Answer/Explanation
Answer: c
Explaination:
(c) Since the direction of velocity of a particle varies so momentum changes but
direction of magnetic force is always perpendicular to direction of charged
particle. So no work is done, i.e. energy remains the same.
14. A current carrying closed loop of an irregular shape lying in more than one plane
when placed in uniform magnetic field, the force acting on it
(a) will be more in the plane where its larger position is covered.
(b) is zero.
(c) is infinite.
(d) may or may not be zero.
Answer/Explanation
Answer: b
Explaination:
(b) A current carrying closed loop of any shape when placed in a uniform
magnetic field does not experience any force.
15. The maximum current that can be measured by a galvanometer of resistance 40 Ω is
10 mA. It is converted into voltmeter that can read upto 50 V. The resistance to be
connected in the series with the galvanometer is
(a) 2010 Ω
(b) 4050 Ω
(c) 5040 Ω
(d) 4960 Ω
Answer/Explanation
Answer: d
Explaination:
16. A current loop placed in a non-uniform magnetic field experiences
(a) a force of repulsion.
(b) a force of attraction.
(c) a torque but not force.
(d) a force and a torque.
Answer/Explanation
Answer: d
Explaination:
(d) In non-uniform magnetic field, current loop experiences an unequal opposite
force which forms torque.
17. What is the net force on the rectangular coil?
(a) 25 × 10-7 N towards wire.
(b) 25 × 10-7 N away from wire.
(c) 35 × 10-7 N towards wire.
(d) 35 × 10-7 N away from wire.
Answer/Explanation
Answer: a
Explaination:
18. If the beams of electrons and protons move parallel to each other in the same
direction, then they
(a) attract each other.
(b) repel each other.
(c) no relation.
(d) neither attract nor repel.
Answer/Explanation
Answer: b
Explaination:
(b) As current carried by electrons and protons are in opposite direction.
19. A conducting circular loop of radius r carries a constant current i. It is placed
in a uniform magnetic field B, such that B is perpendicular to the plane of the
loop. The magnetic force acting on the loop is
(a) irB.
(b) 2πriB
(c) zero
(d) πriB
Answer/Explanation
Answer: c
Explaination:
(c) Net force on a current carrying closed loop is always zero, if it is placed
in an uniform magnetic field.
20. The gyro-magnetic ratio of an electron in an H-atom, according to Bohr model,
is
(a) independent of which orbit it is in.
(b) neutral
(c) positive
(d) increases with the quantum number n.
Answer/Explanation
Answer: a
Explaination:
(a) The gyro-magnetic ratio is given by
\(\mu_{l}=\frac{-e}{2 m} L, \mathrm{L}\) – Angular momentum of electron
21. An electron is projected along the axis of a circular conductor carrying the same
current. Electron will experience
(а) a force along the axis.
(б) a force perpendicular to the axis.
(c) a force at an angle of 4° with axis.
(d) no force experienced.
Answer/Explanation
Answer: d
Explaination:
(d) Since electron is moving parallel to direction of magnetic field of the
conductor
Force (F) = qvB sin 0 = 0
22. Three long, straight parallel wires, carrying current are arranged as shown in
the figure. The force experienced by a 25 cm length of wire C is
(a) 10-3 N
(b) 2.5 × 10-3 N
(c) zero
(d) 1.5 × 3 N
Answer/Explanation
Answer: c
Explaination:
(c) Force of repulsion by wire D and G on wire C is equal and opposite.
23. In a circular coil of radius r, the magnetic field at the centre is proportional
to
(a) r²
(b) r
(c) \(\frac{1}{r}\)
(d) \(\frac{1}{r²}\)
Answer/Explanation
Answer: c
Explaination:
24. A positive charge enters in a magnetic field and travels parallel to but opposite
the field. If experiences
(a) an upward force.
(b) a downward force.
(c) an accelerated force.
(d) no force.
Answer/Explanation
Answer: c
Explaination: (c) Force of repulsion by wire D and G on wire C is equal and
opposite.