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Physics NEET MCQ
Magnetic Effects Of Current Mcq
Quiz 7
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Q.1
When a straight conductor is carrying an electric current
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a)there are circular magnetic line of force around it
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b) there are no magnetic line of force near it
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c)there are magnetic lines of force parallel to conductor along the direction of current
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d)there are magnetic lines of force parallel to conductor opposite to the direction of current
Explanation
Answer: (a)
Q.2
The change in the kinetic energy of a charged particle moving in a uniform magnetic field will be zero when its velocity is
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a) parallel to the magnetic field
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b) perpendicular to the magnetic field
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c)at any angle with the magnetic field
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d)in all of the above cases
Explanation
Answer: (d)
Q.3
A current of 5 ampere is passed through a straight wire of length 6cms then the magnetic induction at a point 5cms from either end of the wire is
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a) 0.25 gauss
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b) 0.12 gauss
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c)0.15 gauss
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d)0.30 gauss
Explanation
sinθ=3/5 and r=4 cm=4×10-2 From formula Answer:(c)
Q.4
A conducting rod PQ of length l carrying a current I is suspended from a support as shown in figure. A uniform magnetic field B perpendicular to PQ and directed away from the reader is applied. If the mass of the bar is M, the tension in each string is
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a) Mg/2
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b) (Mg+IB/)/2
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c) (Mg -IBl)/2
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d) Mg - (IBl)/2
Explanation
Magnetic force on conductor is opposite to mg there for option "c" is correct Answer: (c)
Q.5
A beam of electrons is accelerated through a potential difference V. It is then passed normally through a uniform magnetic field where it moves in a circle of radius r. It would have moved in a circle of radius 2r if it were initially accelerated through a potential difference .....
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a) √2 V
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b) 2 V
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c)2√2 V
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d)4 V
Explanation
speed v 2 ∝V ( potential difference ) and r ∝ velocityAnswer:(d)
Q.6
The strength of the magnetic field due to a long straight conductor is
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a) same everywhere around the conductor
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b) inversely proportional to the distance from the conductor
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c) inversely proportional to the square of the distance from the conductor
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d) inversely proportional of cube of the distance from the conductor
Explanation
Answer: (b)
Q.7
A square frame of side L carries a current i and the field at the centre of the frame is B. The square frame is recast into circular coil of radius R. When same current i is passed through the circular coil, the magnetic field at its centre is B'. The ratio B/B' is
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a)8/π2
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b) 8√2 / π2
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c)16/π2
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d)(8/√2)π2
Explanation
Magnetic field at centre=4 × magnetic field due to single armWhen wire is recast in circular coil then 4L=2πR or R=2L/π Magnetic field at centre of coil B'= Answer: (d)
Q.8
Circular coil of radius 0.314 m carrying current of 2A produces a magnetic field of 2 ×10-4 T at its centre. The number of turns of the circular coil is
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a) 25
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b) 50
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c)80
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d)100
Explanation
Use formula B=(µonI)/2R Answer: (b)
Q.9
The magnetic induction at point P which is distance 4cm from along current carrying wire is 10-3 tesla. The field of induction at a distance 12cm from the same current would be
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a) 3.33 ×10-4 tesla
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b) 1.11 × 10-4 tesla
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c)3 × 10-3 tesla
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d)9 × 10-3 tesla
Explanation
B ∝ (1/r) for infinite long conductor Answer:(a)
Q.10
The magnetic field at the centre of a square loop of side L carrying current I is
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a)
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b)
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c)
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d) None of the above
Explanation
refer Q177 Answer: (c)
Q.11
A circular coil of 50 turns carries a current of 0.2A. The earth's magnetic field is 6×10-5 T. the net field at the centre of coil is zero. The radius of the coil is
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a)1.047 cm
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b) 10.47 cm
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c)1.047 m
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d)10.47 m
Explanation
use formula B=(µon i ) / 2R given value of B=6×10-5 TAnswer: (b)
Q.12
A circular coil of radius 4 cm having 5 turns carries a current of 2A. It is placed in uniform magnetic field of intensity 0.1 weber/mThe work done to rotate the coil from the equilibrium position through 180° is
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a) 0.1 J
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b) 0.2 J
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c)0.4 J
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d)0.8 J
Explanation
Magnetic moment of coil=NIAB=50×2×π(4×10-2)2)=0.5024Work=MB(1 - cos θ) Work=(0.5024) ( 0.1) ( 1- cos180) Work=0.1 JAnswer: (a)
Q.13
A long hollow metallic cylinder of radius R has a current i ampere. The magnetic induction B away from the axis at a distance r from the axis varies as shown in option
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a)
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b)
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c)
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d)
Explanation
No magnetic field inside hollow cylinder Answer:(a)
Q.14
The plane of a rectangular loop of wire with sides 0.05 m and 0.08 m is parallel to a uniform magnetic field of induction 1.5×10-2 tesla. A current of 10.0 amp. flows through the loop. If the side of length 0.08 m in normal and the side of length 0.05 m is parallel to the line of induction, then torque acting on it is
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a) 6000 Newton × metre
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b) Zero
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c) 1.2 × 10-2 newton × metre
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d) 6 × 10-4 newton × metre
Explanation
Equal and opposite force will be on side of length 0.08m τ=BINA sin90° τ=1.5×10-2×10×1×0.08×0.05 τ=6×10-4 N-m Answer: (d)
Q.15
A rectangular coil of cross-section A is placed in uniform magnetic field B as shown in figure. The magnetic flux through the coil is
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a)B.A
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b) B.Asinα
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c)B.Acosα
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d)none of these
Explanation
Take normal component of B to cross-sectionAnswer: (b)
Q.16
Two wires A and B carry current as shown in figure the magnetic interactions
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a) Push I2 away from I1
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b) Pull I2 closer to I1
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c)Turn I2 clockwise
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d)Turn I2 counter clockwise
Explanation
We can use right hand thumb rule and Left hand rule to determine direction of magnetic filed and Force If Wire of current I2 is not fixed at intersection then net force would be zero, but option is not given. Hence we assume wire of I2 is fixed at centre. As shown in figure equal and opposite force will rotate the wire in anti-clock wise Answer: (d)
Q.17
A solenoid has 1000 turns wound on a length of 20cm. It carries a current of 5A. The magnetic field at one end of the solenoid is
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a)0.031 T
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b) 0.0155 T
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c)0.00775 T
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d)0.062 T
Explanation
Use formula form magnetic field at the middle of solenoid B=µo n I B=4π×10-7 ×5000×5=0.031 TAT one end=0.031/2=0.0155 T Answer:(b)
Q.18
A wire carrying a current of 5A lies perpendicular to a uniform magnetic field. The force per unit length is 1.25 ×10-7 N/cm. the magnetic field is
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a) 2.5 mT
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b) 2.5 µT
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c) 2.5 mG
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d) 2.5 µG
Explanation
F=Bil given F=1.25×10-5 N/m l=1 and I=5A 1.25×10-5=B ×5×1 B=2.5 × 106 Answer: (b)
Q.19
A wire bent in the form of a regular polygon of n sides, is inscribed in a circle of radius 'a' metre . If i ampere is the current flowing through wire, then the magnetic flux density at the centre of the circle is
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a)
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b)
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c)
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d)
Explanation
Angle made by the side at centre is 2π/ n , thus θ 1=θ 2=π/ nPerpendicular distance of side from centre=acos(π/n) From formula for magnetic field due to finite conductor at centre=Answer: (b)
Q.20
Two identical coils carry equal currents have a common centre and their planes are at right angles to each other. What is the ratio of magnitudes of the resultant magnetic field and the field due to one coil alone at the centre
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a) 2 : 1
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b) 1 : 1
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c) 1 : √2
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d)√2 : 1
Explanation
Equal magnetic field at centre due to coils are equal in magnitude but perpendicular to each otherAnswer: (d)
Q.21
A wire bent in the form of sector of radius r subtending an angle θo at centre, as shown in figure is carrying a current i. the magnetic field at O is
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a)
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b)
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c)
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d)zero
Explanation
Magnetic field due to coil of n turns at the centre is given by Now 2π=1 turn ∴ if angle is θ then number of turns=θ/2π=θ/360 On substituting value of n as θ/360 we get option "c" as correct Answer:(c)
Q.22
A uniform wire of mass 10-3 kg/cm is held at rest in horizontal position without any support by placing it in a uniform, perpendicular magnetic field. The wire is carrying a current of 10A. The magnetic field is
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a) 0.1 T
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b) 1.0 T
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c) 10-2 T
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d) 10-3 T
Explanation
MAgnetic force on coil=gravitational force Bil=mg Answer: (a)
Q.23
Two thick wire and two thin wires all of the same material and same length form a square in three different ways P, Q, and R as shown in figure. With the current connections as shown, the magnetic field at the centre of the square is zero in case of :
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a) P only
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b) P and Q only
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c)Q and R only
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d)P and R only
Explanation
Answer: (d)
Q.24
Current I flows through a long conducting wire bent at right angles as shown in figure. The magnetic field at a point P on the right bisector or ∠ XOY at distance r from O is
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a)
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b)
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c)
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d)
Explanation
Field at P due to OX Field at P due to OY Direction of magnetic field due to both the wire is perpendicular to plane of paper thus magnitude gets added. NEt magnetic field at P is Answer: (d)
Q.25
A circular coil of radius 20cm having 50 turns carries a current of 2A. The magnetic moment of the coil is
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a) 0.4 π A-m2
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b) 4π A-m2
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c)40 π A-m2
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d)400 π A-m2
Explanation
Answer:(b)
Q.26
Magnetic field due to a current carrying torodal solenoid is independent of
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a)its number of turns
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b) current
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c)radius
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d)none of the above
Explanation
Answer: (c)
Q.27
What is the magnetic field in the empty space enclosed by the toroidal solenoid of radius R?
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a) infinity
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b) µoI /(4πR)
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c)µoπI /(4πR)
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d)zero
Explanation
Answer: (d)
Q.28
A wire bent in the form of right angled triangle ABC as shown in figure carries a current 1 A. It is placed in the region of uniform magnetic field B=0.2T as shown in figure. IF AC=1m, the net force on the wire is
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a) 1.73 N
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b) 3.46 N
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c) 2.732 N
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d) Zero
Explanation
Force on BC=0 as current is antiparallel to Magnetic field Force on AB=Bil direction going inside the plane of paper AB==AC cos60=1 ×(√3)/2=(√3)/2 Force on AB=Bi [(√3)/2]direction going inside the plane of paper Force on AC component of Ac normal to magnetic field is AC sin θ=1×sin60=(√3)/2 Force On Ac=Bi(√3)/2 direction coming out of paper Thus resultant force is zero Answer: (d)
Q.29
If a long copper rod carries a direct current the magnetic field associated with current will be
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a)inside the rod only
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b) outside the rod only
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c)both inside and outside the rod
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d)neither inside nor outside the rod
Explanation
Answer: (c)
Q.30
As shown in figure rectangular coil of cross-section area A placed in uniform magnetic field B directed along Z-axis. The torque on the coil is
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a) ABI along +Ve Y-axis
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b) ABI along -ve Y-axis
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c)ABI along +VE X-axis
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d)zero
Explanation
From the formula for torque acting on the coil kept in magnetic field τ=NAIBsinθ, Here θ is the angle between Area vector of plane of coil and Magnetic field in given case it is ZeroThus torque is zeroAnswer: (d)
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