CBSE Class 12 Physics Moving Charges and Magnetism MCQs Set E

Question. If a conducting wire carries a direct current through it, the magnetic field associated with the current will be __________ .
(a) both inside and outside the conductor
(b) neither inside nor outside the conductor
(c) only outside the conductor
(d) only inside the conductor

Answer: C

Question. A cyclotron’s oscillator frequency is 20 MHz. If the radius of its ‘dees’ is 40 cm, what is the kinetic energy (in MeV) of the proton beam produced by the accelerator?
(a) 7 MeV
(b) 13.25 MeV
(c) 28 MeV
(d) 3.5 MeV

Answer: B

Question. In a cyclotron, a charged particle 
(a) undergoes acceleration all the time
(b) speeds up between the dees because of the magnetic field
(c) speeds up in a dee
(d) slows down within a dee and speeds up between dees

Answer: A

Question. A compass needle is placed above a straight conducting wire. If current passes through the conducting wire from South to North. Then the deflection of the compass __________ .
(a) is towards West
(b) is towards East
(c) keeps oscillating in East-West direction
(d) no deflection

Answer: A

Question. A micro-ammeter has a resistance of 100 W and a full scale range of 50 mA. It can be used as a higher range ammeter or voltmeter provided resistance is added to it. Pick the correct range and resistance combinations.
(a) 50 V range and 10 kW resistance in series
(b) 10 V range and 200 kW resistance in series
(c) 5 mA range with 1 W resistance in parallel
(d) 10 mA range with 1 W resistance in parallel.

Answer: B. C

Question. When a charged particle moving with velocity ^→v is subjected to a magnetic field of induction ^→B, the force on it is non-zero. This implies that
(a) angle between is either zero or 180°
(b) angle between is necessarily 90°
(c) angle between can have any value other than 90°
(d) angle between can have any value other than zero and 180°

Answer: D

Question. Two charged particles traverse identical helical paths in a completely opposite sense in a uniform magnetic field B = B₀ K .
(a) They have equal z-components of momenta
(b) They must have equal charges
(c) They necessarily represent a particle, anti-particle pair
(d) The charge to mass ratio satisfy: (e/m)₁ + (e/m)₂ = 0

Answer: D

Question. Consider the following two statements about the Oersted's experiment.
Statement P: The magnetic field due to a straight current carrying conductor is in the form of circular loops around it.
Statement Q: The magnetic field due to a current carrying conductor is weak at near points from the conductor, compared to the far points.
(a) Both P and Q are true
(b) Both P and Q are false
(c) P is true, but Q is false
(d) P is false, but Q is true

Answer: C

Question. A current carrying loop is placed in a uniform magnetic field. The torque acting on it does not depend upon the
(a) shape of the loop
(b) area of the loop
(c) value of current
(d) magnetic field

Answer: C

Question. Biot-Savart law indicates that the moving electrons (velocity v) produce a magnetic field B such that
(a) B is perpendicular to v
(b) B is parallel to v
(c) it obeys inverse cube law
(d) it is along the line joining the electron and point of observation

Answer: A

Question. 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) negative
(c) positive
(d) increases with the quantum number n

Answer: A, B

Question. An electron is projected with uniform velocity along the axis of a current carrying long solenoid. Which of the following is true? 
(a) The electron will be accelerated along the axis
(b) The electron path will be circular about the axis
(c) The electron will experience a force at 45° to the axis and hence execute a helical path
(d) The electron will continue to move with uniform velocity along the axis of the solenoid

Answer: D

Question. A circular coil of 50 turns and radius 7 cm is placed in a uniform magnetic field of 4 T normal to the plane of the coil. If the current in the coil is 6 A then total torque acting on the coil is
(a) 14.78 N
(b) 0 N
(c) 7.39 N
(d) 3.69 N

Answer: B

Question. Consider the following statements about the representation of the magnetic field
Statement P: The magnetic field emerging out of the plane of the paper is denoted by a dot .
Statement Q: The magnetic field going into the plane of the paper is denoted by a cross .
(a) Both P and Q are true
(b) P is true, but Q is false
(c) P is false, but Q is true
(d) Both P and Q are false

Answer: A

Question. The sensitivity of a moving coil galvanometer increases with the decrease in:
(a) number of turns
(b) area of coil
(c) magnetic field
(d) torsional rigidity

Answer: D

Question. A current carrying circular loop of radius R is placed in the x-y plane with centre at the origin.
Half of the loop with x > 0 is now bent so that it now lies in the y-z plane.  
(a) The magnitude of magnetic moment now diminishes.
(b) The magnetic moment does not change.
(c) The magnitude of B at (0,0, z), z>>R increases.
(d) The magnitude of B at (0,0, z), z>>R is unchanged.

Answer: A

Question. In an ammeter 4% of the mains current is passing through galvanometer. If the galvanometer is shunted with a 5 Ω resistance, then resistance of galvanometer will be
(a) 116 Ω
(b) 117 Ω
(c) 118 Ω
(d) 120 Ω

Answer: D

Question. A circular current loop of magnetic moment M is in an arbitrary orientation in an external magnetic field B. The work done to rotate the loop by 30° about an axis perpendicular to its plane is
(a) MB
(b) √3(M2B)
(c) MB/2
(d) zero

Answer: D

Question. A voltmeter of range 2V and resistance 300 Ω cannot be converted to an ammeter of range:
(a) 5 mA
(b) 8 mA
(c) 1 A
(d) 10 A

Answer: A

Question. A rectangular coil of length 0.12 m and width 0.1 m having 50 turns of wire is suspended vertically in a uniform magnetic field of strength 0.2 Weber/m². The coil carries a current of 2 A. If the plane of the coil is inclined at an angle of 30° with the direction of the field, the torque required to keep the coil in stable equilibrium will be
(a) 0.24 Nm
(b) 0.12 Nm
(c) 0.15 Nm
(d) 0.20 Nm

Answer: D

Fill in the Blanks 

Question. To convert galvanometer into a voltmeter of given range, a suitable high resistance should be connected in _______________ with the galvanometer.
Answer: series

Question. When a magnetic dipole of moment M rotates freely about its axis from unstable equilibrium to stable equilibrium in a magnetic field B , the rotational kinetic energy gained by it is_______________.
Answer: 2MB

Question. An electron passes undeflected when passed through a region with electric and magnetic fields. When electric field is switched off its path will change to _______________.
Answer: circular

Question. The ratio of angular momentum (L) to magnetic moment (M) of an electron revolving in a circular orbit is _______________.
Answer: L/M = 2m/e

Question. The path of charged particle moving perpendicularly with B is _______________.
Answer: circular

Question. There is no change in the _______________ as a charged particle moving in a magnetic field, although magnetic force is acting on it.
Answer: kinetic energy

Question. Two linear parallel conductors carrying currents in the opposite direction _______________ each other.
Answer: repel

Question. When a coil carrying current is set with its plane perpendicular to the direction of magnetic field, then torque on the coil is _______________.
Answer: zero

Question. A linear conductor carrying current if placed parallel to the direction of magnetic field, then it experiences ________________ force.
Answer: zero [F = I/B sin θ and θ = 0°]

Question. Torque on a current carrying rectangular coil inside a galvanometer is maximum and constant irrespective of its orientation as it is suspended inside _______________ magnetic field.
Answer: radial