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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
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
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
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
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
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
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
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
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
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
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
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
Answer
Answer: (a) 10-6
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
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
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
Answer
Answer: (b) √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
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
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
Answer
Answer: (a) 4Ω
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
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
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
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 ………………..
Answer
Answer: zero, infinite.
Question 2.
The material used for the suspension wire of a moving coil galvanometer is ………………..
Answer
Answer: phosphor bronze.
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
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.
Answer
Answer: double.
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
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
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
Answer: pendicular to both \(\vec{B}\) and \(\vec{v}\)
Question 8.
The sensitivity of a moving coil galvanometer can be increased by decreasing ………………..
Answer
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
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
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
Answer: low, parallel, high, series.
Question 12.
A charged particle enters at 30° to the direction of magnetic field. Its path will be ………………..
Answer
Answer: Helical.
Question 13.
Two parallel wires carrying current in the same direction attract each other because of ………………..
Answer
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
Answer: both inside and outside the rod.
Question 15.
The magnetic effect of electric current was discovered by ………………..
Answer
Answer: Oersted.
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 ………………..
Answer
Answer: zero.
Question 17.
A current carrying conductor produces ………………..
Answer
Answer: Both the electric and the magnetic fields.
Question 18.
If a current is passed through a vertical spring, the adjacent turns of the spring will ………………..
Answer
Answer: attract each other.
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