DPP–5 Sub.

: Physics Magnetic Effect of Current Batch : XII + IIT

1. A proton moving with a constant velocity passes 9. A charged particle is whirled in a horizontal circle
through a region of space without any change in its on a frictionless table by attaching it to string fixed
velocity. If E and B represent the electric field and at one point. If a magnetic field is switched on in
magnetic fields respectively, this region of space the vertical direction the tension in the string
may have (a) will increase (b) will decrease
(i) E = 0, B = 0 (ii) E = 0, B  0 (c) will remain the same
(iii) E  0, B = 0 (iv) E  0, B  0 (d) may increase or decrease
then the following is true :
10. A beam consisting of protons and electrons moving
(a) (i) only (b) (iv) only
at the same speed goes through a thin region in
(c) (i), (ii) and (iv) (d) (i), (ii), (iii) and (iv)
which there is a magnetic field perpendicular to the
2. Which of the particles given below will describe the beam. The protons and the electrons
smallest circle when projected with the same (a) will go deviated
velocity perpendicular to a magnetic field (b) will be deviated by the same angle and will not
(a) electron (b) proton separate
(c) –particle (d) all will have same radius (c) will be deviated by different angles and hence
3. An electron is moving with a speed of 108 m/s separate
perpendicular to a uniform magnetic field of (d) will be deviated by the same angle but will
intensity B. Suddenly the intensity of magnetic field separate
is reduced to B/2. The radius of the path becomes 11. A uniform electric field and a uniform magnetic
(if the original value is r) field are produced pointing in the same direction.
(a) no change (b) reduces to r/2 An electron is projected with its velocity pointed in
(c) increases to 2r (d) particle stops moving the same direction
4. An electron revolves round the nucleus in a circular (a) the electron will turn to its right
orbit of radius r and with angular frequency . The (b) the electron will turn to its left
magnetic field produced at the nucleus will be (c) the electron velocity will increase in magnitude
(a) µ0e/2R (b) µ0e/4R (d) the electron velocity will decrease in magnitude
(c) µ02e/2R (d) none of the above 12. A proton beam is going from north to south and an
5. An electron is moving with velocity v around the electron beam is going from south to north.
nucleus in a circular orbit of radius r. The magnetic Neglecting the earth's magnetic field, the electron
field at the nucleus will be beam will be deflected
(a) µ0ev/4R (2) µ0ev/2R (a) towards the proton beam
(c) µ0ev/4R2 (d) none of the above (b) away from the proton beam
(c) upwards
6. An electron moves with uniform velocity v and (d) downwards
enters a region of uniform magnetic field B. If v and
B are parallel to each other, then the electron 13. When an electron moves opposite to the direction
(a) continue to move in the same direction of current carrying wire, a force F acts on it. If it
(b) moves in a direction perpendicular to B moves perpendicular to the wire at same distance,
(c) moves in a circular path then the force will be
(d) will not move (a) F (b) 2F
(c) F/2 (d) zero
7. A thin plastic ring of radius R having charge Q
14. An electron moves in circular orbit of radius r cm
coulombs is rotated about its axis with angular
with a speed v m/s in presence of magnetic field B.
frequency . The magnetic field intensity at the
The energy gained by the electron in going once
centre of the ring will be
round the circular orbit is
µ 0 Q µ 0 Q
(a) (b) mv2
4 R 2R (a) (b) 2rmv
r
µ 0 Q (c) zero (d) none of the above
(c) (d) None of the above
8 R 15. A wire of length l is bent to form a circular coil of
8. A uniform magnetic field acts right angles to the some turns. A current I is then established in the
direction of motion of electrons. As a result, the coil and it is placed in a uniform magnetic field B.
electron moves in a circular path of radius 2 cm. If The maximum torque that acts on the coil is
the speed of electrons is doubled, then the radius (a) IBl2 (b) 4IBl2
of the circular path will be Il 2 B
(a) 2.0 cm (b) 0.5 cm (c) (d) Zero
4
(c) 4.0 cm (d) 1.0 cm
1
16. A electron whose e/m is 1.76 × 1011 c/kg enter a 21. A particle with charge q and mass m is shot with
region where there is a uniform magnetic field of kinetic energy k into the region between two plates.
induction 2 × 10–3 T with a velocity of 3 × 106 m/ If the magnetic field between plates is B, how large
sec in a direction making an angle of 45o with the must B be if the particle is just to miss collision with
field. The pitch of its helical path in the region is : the opposite plate.
(a) 1.2 cm (b) 3.8 cm
22. An electron beam is bent in a circle of radius 2 cm
(c) 5.36 cm (d) 8.4 cm
by a uniform field with B = 4.5 mT. What is the speed
17. Figure shows a wire of arbitrary shape carrying a of the electron ?
current I between points a and b. The length of the
23. The magnetic field in a certain region of space is
wire is L and the distance between points a and b is 
d. The wire lies in a plane at right angle to a uniform given by B  0.8 ˆi , a proton is shot into the field with
magnetic field B. Then the force on the wire is
× × × × ×
velocity (2iˆ  3j)
ˆ × 105 m/sec. Determine the radius
(a) ILB
L and pitch of the path that the proton follows
(b) IdB × × × × b×
24. A cyclotron is accelerating deuterons (i) what is the
(c) I(L – d) B × × × × ×
a d required frequency of the oscillating electric field of
(d) None of these × × × × × B = 1.5 T (ii) if the deutrons are required to acquire
18. A straight rod of mass m and length L is suspended 15 meV of kinetic energy and difference of potential
from the identical springs as shown in figure. The across the gape is 50 kV, how many times does the
2
spring is stretched a distance x0 due to the weight 1H undergo acceleration.
of the wire. The circuit has total resistance R. When 25. An electron is projected normally, with a velocity of
the magnetic field perpendicular to the plane of 3 × 107 m/sec in a magnetic field of strength 10 T.
paper is switched on, springs are deserved to extend Calculate the magnetic force on electron and
further by the same distance. The magnetic field compare it with the weight of the electron. (g = 10
strength is m/sec2)
2mgR mgR mgR mgR 26. A long straight wire AB carries a current of 4A, A
(a) (b) (c) (d)
LE LE 2LE E proton P travels at 4 × 106 m/sec. parallel to the
19. A charged particle of mass m and charge q in a wire, 0.2 m from it and in a direction opposite to the
uniform magnetic field B acts into the plane. The current. Calculate the force which the magnetic field
plane is frictional having coefficient of friction µ. of current exerts on the proton. Also specify the
The speed of charged particle just before entering direction of the force.
into the region is V0. The radius of curvature of the 27. A long straight conductor PQ carrying a current of
V0 60 A, is fixed horizontally. Another long conductor
path after the time is XY is kept parallel to PQ at a distance of 4 mm, in
2µg air. Conductor XY is free to move and carries a
mV0 mV0 mV0 current I. Calculate the magnitude and direction of
(a) qB (b) 2qB (c) 4qB (d) None current I for which the magnetic repulsion just
balance the weight of conductor XY (mass per unit
20. Four wires of equal length are bent in the form of length for conductor XY is 10–2 kg/m).
four loops P, Q, R and S. These are suspended in a 28. A circular coil of 100 turns, radius 10 cm carries a
uniform magnetic field and same current is passed current of 5 A. It is suspended vertically in a uniform
in each loop. horizontal magnetic field of 0.5 T. The field lines
making an angle of 60 o with the plane of coil.
Calculate the magnitude of the torque that must be
applied on it to prevent it from turning.
Q R S
P

(a) Couple on loop P will be the highest

(b) Couple on loop Q will be the highest
(c) Couple on loop R will be the highest
(d) Couple on loop S will be the highest

ANSWER KEY
1. (c) 2. (a) 3. (c) 4. (b) 5. (c) 6. (a) 7. (a) 8. (c) 9. (d) 10. (c) 11. (d) 12. (a) 13. (a) 14. (c) 15.(c)

16. (b) 17. (b) 18. (b) 19. (b) 20. (d) 21. B  2mk /qd 22. 16 × 106 m/sec 23. r = 0.0039 m, d = 0.0164 m
24.  = 11.5 × 106 Hz, N = 300 25. 4.8 × 10–11 N, 5.3 × 1018 27. 32.67 A 28. 3.927 N–m