# MCQ Questions for Class 12 Physics Chapter 4 Moving Charges and Magnetism with Answers

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## Moving Charges and Magnetism Class 12 MCQs Questions with Answers

Solve this Moving Charges and Magnetism Multiple Choice Questions of Class 12 Physics Chapter 4 MCQ, so as not to miss out on any concept from being clear about what they mean!

Multiple-Choice Questions

Question 1.
Biot-Savart law indicates that the moving electrons (velocity v) produce a magnetic field B such that
(a) B ⊥ v.
(b) B || v.
(c) it obeys inverse cube law.
(d) it is along the line joining the electron and point of observation.

Question 2.
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) The electron will continue to move with uniform velocity along the axis of the solenoid.

Question 3.
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) undergoes acceleration all the time.

Question 4.
Consider a wire carrying a steady current, l placed in a uniform magnetic field B perpendicular to its length. Consider the charges inside the wire. It is known that magnetic forces do no work. This implies that
(a) motion of charges inside the conductor is unaffected by B since they do not absorb energy.
(b) some charges inside the wire move to the surface as a result of B.
(c) if the wire moves under the influence of B, no work is done by the force.
(d) if the wire moves under the influence of B, work is done by the magnetic force on the ions, assumed fixed within the wire.

Answer: (b) some charges inside the wire move to the surface as a result of B.

Question 5.
A charged particle will not move with a constant velocity in a region where in,
(o) E = 0, B ≠ 0
(b) E ≠ 0, B ≠ 0
(c) E ≠ 0, B = 0
(d) E = 0, B = 0.

Answer: (c) E ≠ 0, B = 0

Question 6.
A wire of fixed length is turned to form a coil of one turn. It is again turned to form a coil of three turns. If in both cases same amount of current is passed, then the ratio of the intensities of magnetic field produced at the centre of a coil will be
(a) 9 times of first case
(b) $$\frac { 1 }{9}$$ times of first case
(c) 3 times of first case 1
(d) $$\frac { 1 }{3}$$ times of first case

Answer: (a) 9 times of first case

Question 7.
The ratio of the magnetic field at the centre of a current-carrying circular wire and the magnetic field at the centre of a square coil made from the same length of wire will be
(a) $$\frac { π^2 }{4√2}$$
(b) $$\frac { π^2 }{8√2}$$
(c) $$\frac { π }{2√2}$$
(d) $$\frac { π }{4√2}$$

Answer: (b) $$\frac { π^2 }{8√2}$$

Question 8.
Two solenoids having lengths L and 2L and the number of loops N and 4N, both have the same current, then the ratio of the magnetic field will be
(a) 1 : 2
(b) 2 : 1
(c) 1 : 4
(d) 4 : 1

Question 9.
A battery is connected between two points A and B on the circumference of a uniform conducting ring of radius r and resistance R. One of the arcs AB of the ring subtends an angle θ at the centre. The value of the magnetic induction at the centre due to the current in the ring is
(а) Proportional to 2 (180° – θ)
(b) Inversely proportional to r
(c) Zero, only if θ =180°
(d) Zero for all values of θ

Answer: (d) Zero for all values of θ

Question 10.
An electron and a proton enter a magnetic field perpendicularly. Both have same kinetic energy. Which of the following is true
(a) Trajectory of electron is less curved
(b) Trajectory of proton is less curved
(c) Both trajectories are equally curved
(d) Both move on straight line path

Answer: (b) Trajectory of proton is less curved

Fill in the blanks

Question 1.
If the direction of the current flowing through the wire is from south (S) to north (N) and the wire is placed over the needle, the north pole of the needle is deflected towards the …………..

Question 2.
The strength of magnetic field due to a current element is …………. proportional to the current through it.

Question 3.
The magnetic field on the axial line of a conductor is …………..

Question 4.
The magnetic field due to an infinitely long straight conductor at a distance r from it is given by …………..

Answer: B = $$\frac { µ_0l }{2πr}$$

Question 5.
The magnetic field at the centre of a circular conductor is …………..

Answer: B = $$\frac { µ_0l }{2r}$$

Question 6.
The line integral of …………… around any closed path equals µ0/, where / is the total steady current passing through any surface bounded by the closed path.

Question 7.
Ampere’s law is valid only for …………. currents.

Question 8.
A solenoid is a coil of wire with a length which is …………. as compared with its diameter.

Question 9.
If a long metallic rod carries a direct current, the ……….. field exists both inside and outside the rod.

Question 10.
An electron passes undeflected when passes through a region with electric and magnetic fields. When electric field is switched off, its path will change to …………

Question 11.
A linear conductor carrying current is placed parallel to the direction of magnetic field, then it experiences ………….. force.

Question 12.
Two linear parallel conductors carrying currents in the opposite direction …………. each other.

Question 13.
When a coil carrying current is set with its plane perpendicular to the direction of magnetic field, then torque on the coil is …………

Question 14.
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.

Question 15.
The torque on a current loop in a uniform magnetic field is ……………

Answer: τ = BlA sin θ

Question 16.
For a current loop, the product IA is called the …………….. of the loop.

Answer: magnetic dipole moment or magnetic moment

Question 17.
An ideal ammeter has …………… resistance.

Question 18.
An ideal voltmeter has …………… resistance.

Question 19.
The path of charged particle moving perpendicularly with ß is …………..

Question 20.
There is no change in the ………… as a charged particle moving in a magnetic field, although magnetic force is acting on it.

Question 21.
The magnetic dipole moment of a revolving electron is …………..

Answer: M = $$\frac { ne h }{4π m}$$

Question 22.
The ratio of angular momentum (L) to magnetic moment (M) of an electron revolving in a circular orbit is …………..

τ = mB sin θ
ampere-metre
Closed loop

Question 23.
The torque of a magnetic dipole is given by …………

Answer: τ = mB sin θ

Question 24.
The S.I. unit of magnetic pole strength is ………….