# 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

Question 1.
A charge moving with a velocity v in X-direction is subjected to a field of magnetic induction in the negative X direction. As a result, the charge will:
(a) remain undefeated
(b) retard along X-axis
(c) start moving in a circular path in YZ plane.
(d) move along a helical path around X-axis.

Answer: (a) remain undefeated

Question 2.
Two thin long parallel wires separated by a distance r are carrying a current I amp. each. The magnitude of the force per unit length applied by one wire on the other is:
(a) $$\frac {µ_0I^2}{r^2}$$
(b) $$\frac {µ_0I^2}{2πr}$$
(c) $$\frac {µ_0I}{2πr}$$
(d) $$\frac {µ_0I^2}{4πr}$$

Answer: (b) $$\frac {µ_0I^2}{2πr}$$

Question 3.
A wire is placed parallel to the lines of force in a magnetic field and a current flows in the wire, then:
(a) The wire will experience a force in the direction of the magnetic field.
(b) The wire will experience a force in a direction opposite to the magnetic field.
(c) It will experience a force in a direction perpendicular to the lines of force.
(d) Tire wire will not experience a force at all.

Answer: (d) Tire wire will not experience a force at all.

Question 4.
In order to increase the sensitivity of a moving coil galvanometer,
(a) the suspension wire should be made stiff.
(b) the arc of the coil should be reduced.
(c) the magnetic field should be increased.
(d) the number of turns in the coil should be reduced.

Answer: (a) the suspension wire should be made stiff.

Question 5.
Two particles A and B having equal charges after being accelerated through the same potential difference enter a region of uniform magnetic field and describe circular path of radii R1 and R2 respectively. The ratio of the masses of A to B is:
(a) ($$\frac {R_1}{R_2}$$)1/2
(b) ($$\frac {R_2}{R_1}$$)
(c) ($$\frac {R_1}{R_2}$$)²
(d) $$\frac {R_1}{R_2}$$

Answer: (c) ($$\frac {R_1}{R_2}$$)²

Question 6.
When a charged particle moves perpendicular to a uniform magnetic field, its:
(a) Energy and momentum both change.
(b) Energy and momentum both don’t change.
(c) Energy changes but momentum remains unchanged.
(d) Momentum changes but energy remains unchanged.

Answer: (d) Momentum changes but energy remains unchanged.

Question 7.
The force on a current carrying conductor in a magnetic field is maximum, when angle between the length of the conductor and the magnetic field is:
(a) $$\frac {π}{2}$$
(b) $$\frac {π}{4}$$
(c) π
(d) zero

Answer: (a) $$\frac {π}{2}$$

Question 8.
What happens between the two streams of electrons moving parallel to each other in the same direction:
(a) attract each other
(b) cross the electric and magnetic field of each other.
(c) repel each other.
(d) none of these

Answer: (c) repel each other.

Question 9.
Magnetic field is not associated with:
(a) a charge in uniform motion.
(b) a decelerated charge.
(c) an accelerated charge.
(d) a stationary change.

Answer: (d) a stationary change.

Question 10.
A magnetic needle is kept in a non-uniform magnetic field. It experiences:
(a) a force and torque.
(b) a force but not a torque.
(c) a torque but not a force.
(d) neither a force nor a torque.

Answer: (a) a force and torque.

Question 11.
A proton and an electron enter a region in which a magnetic field is suddenly switched on. The forces experienced by them are:
(a) equal and opposite.
(b) different in magnitude but in same direction.
(c) in the ratio of 1840.
(d) same in magnitude and direction.

Answer: (a) equal and opposite.

Question 12.
Two protons move parallel to each other with equal speeds 3 × 102 m/s. The ratio of magnetic and electrical forces between them is:
(a) 10-6
(b) 10-3
(c) 10-9
(d) 1

Question 13.
A proton and an a particle enter in a uhiform magnetic field with the same velocity. The period of rotation of the α-particle will be:
(a) four times that of the proton.
(b) three times that of the proton.
(c) two times that of the proton.
(d) same as that of the proton.

Answer: (c) two times that of the proton.

Question 14.
The magnetic field at the centre of a square loop of side a carrying a current I is:
(a) $$\frac {µ_0}{4π}$$ √2 $$\frac {1}{a}$$
$$\frac {µ_0}{4π}$$ $$\frac {4\sqrt{21}}{4π}$$
(c) $$\frac {µ_0}{4π}$$ 8√2 $$\frac {1}{a}$$
(d) none

Answer: (c) $$\frac {µ_0}{4π}$$ 8√2 $$\frac {1}{a}$$

Question 15.
A circular of radius R carries a current I. The magnetic field at its centre is B. At what distance from the centre on the axis of the coil, the magnetic field will be $$\frac {B}{8}$$?
(a) √2R
(b) √3R
(c) 2R
(d) 3R

Question 16.
A voltmeter has a resistance RQ and range V. What resistance should be. connected in series with it to increase its range to nV?
(a) nR0
(b) (n + 1)R0
(c) (n – 1)R0
(d) $$\frac {R_0}{n}$$

Answer: (c) (n – 1)R0

Question 17.
An ammeter has a resistance R0 and range 1. What resistance should be connected in parallel with it to increase its range tonl?
(a) $$\frac {R_0}{(n-1)}$$
(b) $$\frac {R_0}{n}$$
(c) $$\frac {R_0}{(n+1)}$$
(d) None of these

Answer: (a) $$\frac {R_0}{(n-1)}$$

Question 18.
The resistivity of a galvanometer is 60 division/amp. When a shunt is used, its sensitivity becomes 10 divisions/amp. If galvanometer is of resistance 20Ω, the value of shunt used is:
(a) 4Ω
(b) 5Ω
(c) 20Ω
(d) $$\frac {20}{7}$$Ω

Question 19.
A galvanometer has resistance G Ω. It is shunted by a resistance of SΩs. How much resistance should be added in series so that the manrcurrent in the circuit remains unchanged?
(d) $$\frac {S}{S+G}$$
(d) $$\frac {G}{G+S}$$
(c) $$\frac {G^2}{G+S}$$
(d) $$\frac {GS}{G+S}$$

Answer: (c) $$\frac {G^2}{G+S}$$

Question 20.
A galvanometer may be converted into an ammeter or a volmeter in which of the following cases, the resistances of the device so obtained will be the largest?
(a) Ammeter of range 1 A
(b) Ammeter of range 10 A
(c) Voltmeter of range 1 V
(d) Voltmeter of range 10 V

Answer: (d) Voltmeter of range 10 V

Question 21.
Which device will have the least resistance?
(a) Ammeter of range 1A
(b) Ammeter of range 10 A
(c) Voltmeter of range 1 V
(d) Voltmeter of range 10 V

Answer: (b) Ammeter of range 10 A

Fill in the Blanks

Question 1.
The resistance of an ideal ammeter is and the resistance of an ideal voltmeter is ………………..

Question 2.
The material used for the suspension wire of a moving coil galvanometer is ………………..

Question 3.
If µ0 and ∈0 be the magnetic permeability and electric permittivity of free space, p and ∈ be the corresponding quantities for medium, then the refractive index of the medium in terms of these parameters is ………………..

Answer: $$\sqrt{\frac {µ∈}{µ_0∈_0}}$$

Question 4.
When the radius of a circular current carrying coil is doubled and the current in it is halved, then the magnetic dipole-moment of the coil becomes ……………….. the original value.

Question 5.
When a particle of charge q and mass m moving with velocity v, enters a magnetic field B at right angles to the direction of the field, the particle takes a circular path of radius ……………….. and time period of its oscillation is ………………..

Answer: $$\frac {mv}{qB}$$, $$\frac {2πm}{Bq}$$

Question 6.
Newton/Amp-meter and Weber/metre2 are the units of same physical quantity i.e ………………..

Answer: magnetic field of induction or magnetic field.

Question 7.
A positive particle enters the magnetic field $$\vec{B}$$ per pendicularly with a velocity $$\vec{v}$$ The Lorentz force will act on it in a direction ………………..

Answer: pendicular to both $$\vec{B}$$ and $$\vec{v}$$

Question 8.
The sensitivity of a moving coil galvanometer can be increased by decreasing ………………..

Answer: The couple per unit twist (k) of the suspension wire.

Question 9.
An electron enters a magnetic field whose direction is perpendicular to the velocity of the electron then the speed of the electron will ………………..

Answer: remain the same.

Question 10.
A current carrying loop is placed in a uniform magnetic field. The torque acting on it does not depend upon ………………..

Answer: the shape of the loop.

Question 11.
To convert a galvanometer into an ammeter a ……………….. resistance is connected in ……………….. and to convert the galvanometer into a voltmeter a …………… resistance is connected in ………………..

Answer: low, parallel, high, series.

Question 12.
A charged particle enters at 30° to the direction of magnetic field. Its path will be ………………..

Question 13.
Two parallel wires carrying current in the same direction attract each other because of ………………..

Answer: magnetic force between them.

Question 14.
If a long copper rod carries a direct current, the magnetic field associated with the current will be ………………..

Answer: both inside and outside the rod.

Question 15.
The magnetic effect of electric current was discovered by ………………..

Question 16.
A current I flows along the length of an infinitely long, straight, thin wailed pipe, then the magnetic field at any point inside the pipe is ………………..

Question 17.
A current carrying conductor produces ………………..