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Physics NEET MCQ
Magnetic Effects Of Current Mcq
Quiz 8
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Q.1
Two parallel wires each of 0.5 m in length atre at distance 1 metre from each other. If current through each wire is 1 amp, then force between the two wires is
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a) 10-7 N
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b) 0.5 N
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c)107 N
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d)5 ×1014 N
Explanation
Use formula Answer:(a)
Q.2
An electron of mass m is accelerated through a potential difference of V and then it enters a magnetic field of induction B normal to the lines. the radius of circular path is
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a)
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b)
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c)
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d)
Explanation
Kinetic energy of electron eV=(1/2) mv2 v2=2eV/m Now when electron enters magnetic field it follows circular path Answer: (b)
Q.3
An electron moves in a circular arc of radius 10 m at a constant speed of 2×107 m/s with its plane of motion normal to the magnetic flux density of 10-5T. What value does, from this set of figure, give for the specific charge of the electron?
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a)2 × 102 C/kg
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b) 2 ×105 C/kg
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c)5 ×106 C/kg
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d)2 ×1011 C/kg
Explanation
use formula derived in Q200 and find value of e/mAnswer: (d)
Q.4
A battery is connected between to points A and B on the circumference of a uniform conducting ring of radius r and resistance R. One of the arc AB of the ring subtends angle θ at the centre. The value of the magnetic induction at the centre due to the current in the ring is
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a) Proportional to 2( 180° - θ)
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b) inversely proportional to r
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c)zero , only if θ=180°
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d)zero for all values of θ
Explanation
Answer: (d)
Q.5
If a proton is projected in a direction perpendicular to a uniform magnetic field with velocity v and an electron is projected along the line of force, what will happen to proton and electron?
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a) the electron will travel along a circle with constant speed and the proton will move along a straight line
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b) proton will move in a circle with constant speed and there will be no effect on the motion of electron
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c)there will not be any effect on the motion of electron and proton
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d)the electron and proton both will follow the path of a parabola
Explanation
Answer:(b)
Q.6
An electron accelerated through a potential difference of V passes through a magnetic field experiences a force F. If the accelerating potential is increased to 2V, the electron in the same magnetic field will experience force
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a) F
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b) F/2
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c) √2 F
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d) 2F
Explanation
F ∝ velocity (v) and velocity v ∝ √( potential) Thus option "c" is correct Answer: (c)
Q.7
At a distance of 10cm from long straight wire, carrying current, the magnetic field is 0.04 tesla. At the distance of 40 cm, the magnetic field will be
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a)0.01 T
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b) 0.02T
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c)0.08 T
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d)0.16 T
Explanation
Magnetic field due to infinitely long conductor ∝ (1/r)Answer: (a)
Q.8
A circular coil of radius 4 cm having 20 turns carries a current of 3A. It is placed in a magnetic field of intensity 0.5 weber/mThe magnetic dipole moment of coil is
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a) 0.15 amp × m2
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b) 0.30 amp × m2
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c)0.45 amp m2
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d)0.60 amp × m2
Explanation
use formula Magnetic moment=NIA ( A is cross-sectional area of coil )Answer: (b)
Q.9
If a particle of charge 10-12 C moving along the x-direction with velocity 105 m/s experiences a force of 10-10 N in y-direction due to magnetic field, then the minimum magnetic field is
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a) 6.26 × 103 tesla in -direction
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b) 10-15 tesla in z- direction
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c)103 tesla in z-direction
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d)10-3 tesla in z-direction
Explanation
F=qvB B=F/qv B=10-10 / (10-12 × 105) B=10-3 in magnitude Direction Negative z axis according to Fleming's left hand rule Answer:(d)
Q.10
A wire of 2m is bent in the form of a circular loop. The magnetic moment of the loop when 1A current flows through it is ......Am2.
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a) 2π
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b) π/2
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c) π/4
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d) 1/π
Explanation
USe L=2πR magnetic dipole moment=IA=I(πR2) Answer: (d)
Q.11
When current is passed through a circular wire prepared from long conducting wire, magnetic field produced at its centre is B. Now loop having two turns in prepared from the same wire and the same current is passed through it. The magnetic field at its centre will be
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a)B/4
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b) B/2
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c)4B
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d)2B
Explanation
B at centre=µonI / 2r For case I : n=1 and r ∴ B is magnetic field For case II : n=2 and r'=r/2 ∴ B'=4BAnswer: (c)
Q.12
Electric current "I" flows through a hexagonal loop of side 'a'. Magnetic field ar its centre 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 with centre=60° thus θ1=θ1=30°Thus perpendicular distance r=(a/2)(tan60) Now Magnetic field at centre due to six side=6 × Magnetic field due to one side Answer: (b)
Q.13
Two particles of mass m and charge q are attached with two ends, one with each end, of rod length 2r. The rod is rotated with angular speed ω about its mid point. The ratio of magnetic moment produced and total angular momentum of these particles to ....
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a) q/2m
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b) q/m
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c)2q/m
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d)q/πm
Explanation
Magnetic moment M=AI=πr2(2qf) Angular momentum=2mr2 ω But ω=2πf Take ratio of M and L Answer:(a)
Q.14
Two very long conducting parallel wires are separated by a distance d from each other and the same current I is passing through them in mutually opposite directions. A particle of charge q passes through a point which is at distance d/2 from both wires, with velocity v, perpendicularly to the plane formed by the wires. Resulting magnetic force acting on this particle is ...
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a)
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b)
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c)
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d) zero
Explanation
Line of action of B and v are same hence no force on charge Answer: (d)
Q.15
A stationary charged particle does not experience any net electromagnetic force, then ...
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a)magnetic field may be zero or non-zero
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b) magnetic field must be zero
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c)electric field may be zero or non zero
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d)electric field must be zero
Explanation
Non zero force is possible if E=0 and B may or may bot be zero as magnetic field do not exert any force on stationary charge Answer: (a,d)
Q.16
Electric current I flows through the wall of conducting pipe of infinite length, in a direction parallel to its length, then...
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a) magnetic field is non-zero and equal at all points in inner region of the pipe
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b) magnetic field is zero at all points in the inner region of pipe
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c)electric field is zero only on the axis of the pipe
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d)magnetic field is different at different points in the inner region of the pipe
Explanation
On taking any Amperean loop in pipe . We find no current is enclosedAnswer: (b)
Q.17
No magnetic force acts if ... is stationary
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a) an electric dipole
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b) magnetic dipole
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c)a coil carrying current
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d)a straight conductor carrying current
Explanation
Answer:(a)
Q.18
A very long solenoid of length L has n layers. There are N turns in each layer. Diameter of the solenoid is D and current I passes through it, then magnetic field;d at the centre of solenoid is
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a) directly proportional to D
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b) inversely proportional to D
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c) independent of D
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d) directly proportional to L
Explanation
Answer: (c)
Q.19
There are 200 turns per centimeter of avery long solenoid. If 2.5 A current passes through it, then the magnetic field at its centre on the axis is ... T
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a)3.14 × 10-2
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b) 6.28 × 10-2
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c)9.42 × 10-2
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d)12.56 × 10-2
Explanation
Answer: (b)
Q.20
If a proton is moving with velocity 3 ×105 m/s at an angle of 30° in a magnetic field 0.3 T, then the radius of curvature of its trajectory will be .. [ for proton e/m=108 C/kg]
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a) 0.5 cm
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b) 0.02 cm
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c)1.25 cm
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d)2.0 cm
Explanation
component of velocity perpendicular to magnetic field is v'=vsin30Now USe formula r=mv'/BqAnswer: (a)
Q.21
A moving charge gains energy due to ....
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a) electric field
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b) magnetic field
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c)both these fields
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d)none of these fields
Explanation
Answer:(a)
Q.22
In a region E and B represent electric and magnetic field respectively. If an electron moves with constant velocity (v) through that region, then the region may not have ....
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a)E=0, B=0
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b) E=0 , B ≠ 0
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c)E ≠ 0, B=0
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d)E ≠ 0 , B ≠ 0
Explanation
In question direction of magnetic filed and electric fields are not given E and B may be perpendicular or any other orientation Given electrons moves with constant velocity resultant force is zero. Option (a) may not have E=0 and B=0 is true but in question it is not stated E and B are perpendicular. Option "a" is not correct Option b: If region may not have E=0 it implies E ≠ 0 and region may not have B ≠ 0, implies B = 0 then velocity can not remain constant. Option "b" not correct Option d: If region may not have E ≠ 0 and B ≠ 0 implies E=0 and B = 0, Stated in question either both or one of the E and B exist , then optio Option "d" not correct Option c: correct Region may not have E &nq 0 it implies E =0. Then V remains unchanged And direction of B is in the direction of electron B ≠ 0 , V remains constant In option "d" it is given region may not have B=0 , it implies B≠0 Answer: (c)
Q.23
Two beams of protons moving parallel and in the same direction, will
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a) Attract each other
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b) exerts no force on each other
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c)repel each other
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d)can't say anything
Explanation
They will constitute the current in the same directionAnswer: (a)
Q.24
Unit of permeability is ........
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a) (T - A)/m
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b) (T-m)/A
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c)(m-A)/T
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d)none of them
Explanation
Use formula for magnetic filed due to long current carrying conductor Answer:(b)
Q.25
Two wires carrying current are parallel to each other, the force existing between them is F, IF the current flowing through them is doubled and distance between them is made four times, then force acting between them will be
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a) 2F
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b) F
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c) F/2
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d) F/4
Explanation
Answer: (b)
Q.26
Assume the mass of proton to be nearly 2000 times the mass of an electron. A proton and an electron having equal momentum enter into a magnetic field at right angle to the direction of field. The ratio of the radii of their tracks will be .....
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a)2000 : 1
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b) 1 : 20000
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c)1 : 1
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d)none of these
Explanation
r=mv/qB r ∝ p( momentum)Answer: (c)
Q.27
Certain length of wire carries a steady current. It is bent to form a circular loop. The magnetic field induced at the centre of the loop is B. If the same length is now bent to give a double loop of smaller radius then the magnetic field at the centre caused by the same current is ....
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a) B/2
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b) B
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c)2B
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d)4B
Explanation
Answer: (d)
Q.28
An electron moves with velocity v in x-direction. If the magnetic field acts in y-direction then the force on the electron acts in .....
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a) x-direction
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b) y-direction
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c)z-direction
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d)-z-direction
Explanation
F=-e(vi × Bi) Answer:(d)
Q.29
The magnetic field produced inside a current carrying long solenoid does not depend on....
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a) number of turns per unit length
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b) current flowing through it
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c) radius of solenoid
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d) all of them
Explanation
Answer: (c)
Q.30
In a hydrogen atom, if an electron makes 6.6×1015 rev/s around the nucleus in an orbit of radius 2.1 Å then the magnetic moment will be ....Am2
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a)1.46 × 10-15
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b) 1.46 × 10-10
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c)1.46 × 10-22
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d)1.46 × 10-26
Explanation
Answer: (c)
0 h : 0 m : 1 s
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