Content text 20. Moving Charges and Magnetism Medium.pdf
1. When a magnetic compass needle is carried nearby to a straight wire carrying current, then (I) The straight wire cause a noticeable deflection in the compass needle. (II) The alignment of the needle is tangential to an imaginary circle with straight wire as its centre and has a plane perpendicular to the wire (a) (I) is correct (b) (II) is correct (c) Both (I) and (II) are correct (d) Neither (I) nor (II) is correct 2. When one of the following is not correct about Lorentz Force? (a) In presence of electric field E(r) and magnetic field B(r) the force on a moving electric charge is F qE(r) v B(r) = + (b) The force, due to magnetic field on a negative charge is opposite to that on a positive charge. (c) The force due to magnetic field become zero of velocity and magnetic field are parallel or anti-parallel. (d) For a static charge the magnetic force is maximum. 3. A charged particle is moving on circular path with velocity v in a uniform magnetic field B, if the velocity of the charged particle is doubled and strength of magnetic field is halved, then radius becomes (a) 8 times (b) 4 times (c) 2 times (d) 16 times 4. The magnetic force F on a current carrying conductor of length l in an external magnetic field B is given by (a) l I B (b) I I B (c) I(l B) (d) I l B 2 5. A strong magnetic field is applied on a stationary electron. Then the electron (a) Moves in the direction of the field (b) Remains stationary. (c) Moves perpendicular to the direction of the field (d) Moves opposite to the direction of the field. 6. A straight wire having mass of 1.2 kg and length of 1 m carries a current of 5 A. If the wire is suspended in mid-air by a uniform horizontal magnetic field, then the magnitude of field is (a) 0.65 T (b) 1.53 T (c) 2.4 T (d) 3.2 T 7. A circular loop of radius R carrying a current I is placed in a uniform magnetic field B perpendicular to the loop. The force on the loop is (a) 2RIB (b) 2 3 2RI B (c) R IB2 (d) zero 8. A 2.5 m long straight wire having mass of 500 g is suspended in mid air by a uniform horizontal magnetic field B. If a current of 4 A is passing through the wire then the magnitude of the field is (Take g=10 m s-2 ) (a) 0.5 T (b) 0.6 T (c) 0.25 T (d) 0.8 T 9. A current of 10 A is following in a wire of length 1.5 m. A force of 15 N acts on it when it is placed in a uniform magnetic field of 2 T. The angle between the magnetic field and the direction of the current is (a) 30° (b) 45° (c) 60° (d) 90° 10. An 8 cm long wire carrying a current of 10 A is placed inside a solenoid perpendicular to its axis. If the magnetic field inside the solenoid is 0.3 T, then magnetic force on the wire is (a) 0.14 N (b) 0.24 N (c) 0.34 N (d) 0.44 N 11. The magnetic force per unit length on a wire carrying a current of 10 A and making an angle of 45° with the direction of a uniform magnetic field of 0.20 T is (a) 2 2 N m-1 (b) 2 2 N m-1 (c) 2 2 N m-1 (d) 4 2 N m-1 12. In an inertial frame of reference, the magnetic force on a moving charged particle is F .Its value in another inertial frame of reference will be (a) Remain same (b) Changed due to change in the amount of charge (c) Changed due to change in velocity of charged particle (d) Changed due to change in field direction 13. A circular coil of 20 turns and 10 cm radius is placed in a uniform magnetic field of 0.10 T normal to the plane of the coil. If the current in the coil is 5 A, cross sectional area is 10- 5 m2 and coil is made up of copper wire having free electron density about 1029 m-3 , then the average force on each electron in the coil due to magnetic field is (a) 2.5 × 10-25 N (b) 5 × 10-25 N (c) 4 × 10-25 N (d) 3 × 10-25 N 14. A charged particle with charge q enters a region of constant, uniform and mutually orthogonal fields E and B with a velocity v perpendicular to both E and B , and comes out without any change in its magnitude or direction. Then (a) 2 V B E | E = (b) 2 v E B/B = (c) 2 v B E/B = (d) 2 v E B/ E = 15. Which one of the following is correct statement about magnetic forces? (a) Magnetic forces always obey Newton’s third law
(b) Magnetic forces do not obey Newton’s third law (c) For very high current, magnetic forces obey Newton’s third law (d) Inside low magnetic field, magnetic forces obey Newton’s third law 16. When a positively charged particle enters a uniform magnetic field with uniform velocity, its trajectory can be (i) a straight line (ii) a circle (iii) a helix. (a) (i) only (b) (i) or (ii) (c) (i) or (iii) (d) any one of (i), and (iii) 17. An electron having momentum 22 1 2.4 10 kg m s − − enters a region of uniform magnetic field of 0.15 T. The field vector makes an angle of o 30 with the initial velocity vector of the electron. The radius of the helical path of the electron in the field shall be (a) 2 mm (b) 1 mm (c) 2 3 mm (d) 0.5 mm 18. Two particles of equal charges after being accelerated through the same potential difference enter in a uniform transverse magnetic field and describe circular paths of radii R1 and R2 .Then the ratio of their respective masses ( ) 1 M2 M / is (a) 1 R2 R / (b) ( ) 2 1 R2 R / (c) ( ) 2 R1 R / (d) ( ) 2 2 R1 R / 19. An electron of energy 1800 e V describes a circular path in magnetic field of flux density 0.4 T. The radius of path is (q 1.6 10 C,m 9.1 10 kg) 31 e −19 − = = (a) 2.58 10 m −4 (b) 3.58 10 m −4 (c) 2.58 10 m −3 (d) 3.58 10 m −3 20. Two particles have the ratio of their velocities as 3 : 2 on entering the field. If they move in different circular paths, then the ratio of the radii of their paths is (a) 2 : 3 (b) 3 : 2 (c) 9 : 4 (d) 4 : 9 21. A charged particle would continue to move with a constant velocity in a region wherein, which of the following conditions is not correct ? (a) E = 0,B 0 (b) E 0,B 0 (c) E 0,B= 0 (d) E = 0,B= 0 22. A charged particle is moving in a cyclotron, what effect on the radius of path of this charged particle will occur when the frequency of the radio frequency field is doubled? (a) It will also be doubled (b) It will be halved (c) It will be increased four times (d) It will remain unchanged. 23. If an electron enters with a velocity J ˆ v v = 0 into a cubical region in a uniform electromagnetic field and the orbit of the electron is found to spiral down inside the cube in plane parallel to the x-y plane then the configuration of electric field and magnetic field is (a) k B B ˆ J, E E ˆ = o = o (b) k B B ˆ i, E E ˆ = o = o (c) J B B ˆ i, E E ˆ = o = o (d) J B B ˆ J, E E ˆ = o = o 24. An electron is moving in a cyclotron at a speed of 7 1 3.2 10 ms − in a magnetic field of 4 5 10− T perpendicular to it. What is the frequency of this electron? (q 1.6 10 C,m 9.1 10 kg) 31 e −19 − = = (a) 1.4 10 Hz −5 (b) 1.4 10 Hz 7 (c) 1.4 10 Hz 6 (d) 1.4 10 Hz 9 25. The cyclotron frequency c is given by (a) 2 m qB (b) 2 q mB (c) qB 2m (d) qm 2B 26. Which of the following is not correct about cyclotron? (a) It is a machine to accelerate charged particles or ions to high energies. (b) Cyclotron uses both electric and magnetic fields in combination to increase the energy of charged particles. (c)The operation of the cyclotron is based on the fact that the time for one revolution of an ion is independent of its speed or radius of its orbit. (d)The charged particles and ions in cyclotron can move on any arbitrary path. 27. A proton is accelerating on a cyclotron having oscillating frequency of 11MHz in external magnetic field of 1 T. If the radius of its dees is 55 cm, then its kinetic energy (in Me V) is (m 1.67 10 kg,e 1.6 10 C) 27 19 P − − = = (a) 13.36 (b) 12.52 (c) 14.89 (d) 14.19 28. If a electron is moving in a magnetic field of 4 5.4 10− Ton a circular path of radius 32 cm having a frequency of 2.5MHz, then its speed will be (a) 6 1 8.56 10 ms − (b) 6 1 5.024 10 m s − (c) 4 1 8.56 10 m s − (d) 4 1 5.024 10 m s − 29. If a charged particle is moving in a cyclotron, then (a) It speeds up within a dee (b) Slows down within a dee and speeds up between dees (c) Speeds up between the dees because of external magnetic field (d) Undergoes acceleration all times
30. A proton an -particle enter in a uniform magnetic field perpendiculary with same speed. The ratio of time periods of both particle T Tp will be (a) 1 : 2 (b) 1 : 3 (c) 2 :1 (d) 3 : 1 31. A proton, a deutron and an -particle with same kinetic energy enter perpendiculary in a uniform magnetic field, then the ratio of radii of their circular paths is (a) 1:1: 2 (b) 2 : 1 : 1 (c) 1: 2 : 1 (d) 1 : 2 : 2 32. The energy of emergent protons in Me V from a cyclotron having radius of its dees 1.8 m and applied magnetic field 0.7 T is (mass of proton = 1.67 10 kg −27 ) (a) 50 (b) 60 (c) 70 (d) 76 33. Current flows through uniform, square frames as shown in the figure. In which case is the magnetic field at the centre of the frame not zero? (a) (b) (c) (d) 34. Five long wire A, B, C, D and E each carrying current I are arranged to form edges of a pentagonal prism as shown in figure. Each wire carries current out of the plane of paper, the magnetic induction at a point on the axis O is (Axis O is a distance R from each wire) is (a) Equal to zero (b) Less than zero (c) More than zero (d) Infinite 35. An element of i ˆ 0.05 m is placed at the origin as shown in figure which carries a large current of 10 A. The magnetic field at a distance of 1 m in perpendicular direction is (a) 4.5 × 10-8 T (b) 5.5 × 10-8 T (c) 5.0 × 10-8 T (d) 7.5 × 10-8 T 36. A 4 A current carrying loop consist of three identical quarter circles of radius 5 cm lying in the positive quadrants of the x- y, y-z and z-x planes with their centres at the origin joined together, value of at the origin is (a) (ˆ i ˆ j k ˆ)T 10 0 + − (b) ( ˆ i ˆ j k ˆ)T 10 0 − + + (c) ( ˆ i ˆ j k ˆ)T 5 0 − + + (d) 10o (ˆ i + ˆ j+ k ˆ)T 37. The magnitude of the magnetic field at the centre of the tightly wound 150 turn coil of radius 12 cm carrying a current of 2 A is (a) 18 G (b) 19.7 G (c) 15.7 G (d) 17.7 G 38. A tightly wound 90 turn coil of radius 15 cm has a magnetic field 4 × 10-4 Tat its centre. The current flowing through it is (a) 1.06 A (b) 2.44 A (c) 3.44 A (d) 4.44 A 39. A circular loop of radius 3 cm is having a current of 12.5 A. The magnitude of magnetic field at a distance of 4 cm on its axis is (a) 5.65 × 10-5 T (b) 5.27 × 10-5 T (c) 6.54 × 10-5 T (d) 9.20 × 10-5 T 40. A circular coil of wire consisting of 100 turns each of radius 9 cm carries a current of 0.4 A. The magnitude of magnetic field at the centre of the coil is (a) 2.4 × 10-4 T (b) 3.5 × 10-4 T (c) 2.79 × 10-4 T (d) 3 × 10-4 T 41. A current I is flowing through a loop. The direction of the current and the shape of the loop are as shown in the figure. The magnetic field at the centre of the loop is R I 0 times (Here, MA = R, MB = 2R,< DMA = 90°) (a) 16 5 , out of the plane of the paper. (b) 16 5 , into the plane of the paper.
(c) 16 7 , out of the plane of the paper. (d) 16 7 , into the plane of the paper. 42. Two identical current carrying coaxial loops, carry current I in opposite sense. A simple amperian loop passes through both of them once. Calling the loop as C, then which statement is correct? (a) = c 0 Bd l 2 I (b) The value of B.d l is independent of sense of C (c) There may be a point on C where B and dl are parallel. (d) None of these 43. The correct plot of the magnitude of magnetic field B vs distance r from centre of the wire is, if the radius of wire is R (a) (b) (c) (d) 44. Ampere’s circuital law is given by (a) = 0 enc H.dl I (b) = 0 enc B.dl I (c) B.dl = J0 (d) H.dl = J0 45. If a long straight wire carries a current of 40 A, then the magnitude of the field B at a point 15 cm away from the wire is (a) 5.34 × 10-5 T (b) 8.34 × 10-5 T (c) 9.6 × 10-5 T (d) 10.2 × 10-5 T 46. A long straight wire in the horizontal plane carries a current of 75 A in north to south direction, magnitude and direction of field B at a point 3 m east of the wire is (a) 4 × 10-6 T, vertical up (b) 5 × 10-6 T, vertical down (c) 5 × 10-6 T, vertical up (d) 4 × 10-6 T, vertical down 47. A solenoid of length 50 cm, having 100 turns carries a current of 2.5 A. The magnetic field at one end of the solenoid is (a) 3.14 × 10-4 T (b) 6.28 × 10-4 T (c) 1.57 × 10-4 T (d) 9.42 × 10-4 T 48. A solenoid of length 0.6 m has a radius of 2 cm and is made up of 600 turns. If it carries a current of 4 A, then the magnitude of the magnetic field inside the solenoid is (a) 6.024 × 10-3 T (b) 8.24 × 10-3 T (c) 5.024 × 10-3 T (d) 7.024 × 10-3 T 49. Which of the following statement is correct? (a) The magnetic field in the open space inside the toroid is constant. (b)The magnetic field in the open space exterior to the toroid is constant. (c)The magnetic field inside the core of toroid is constant (d)The magnetic field inside the core of toroid is zero 50. The inner and outer radius of a toroid core are 28 cm and 29 cm respectively and around the core 3700 turns of a wire are wounded. If the current in the wire is 10 A, then the magnetic field inside the core of the toroid is (a) 2.60 × 10-2 T (b) 2.60 × 10-2 T (c) 4.52 × 10-2 T (d) 4.52 × 10-2 T 51. A 90 cm long soleniod has six layers of windings of 450 turns each. If the diameter of solenoid is 2.2 cm and current carried is 6 A, then the magnitude of field inside the solenoid, near its centre is (a) 50G (b) 60G (c) 72G (d) 80G 52. A long straight wire carrying current of 30 A rests on a table. Another wire AB of length 1 m, mass 3 g carries the same current but in the opposite direction, the wire AB is free to side up and down. The height upto which AB will rise is (a) 0.6 cm (b) 0.7 cm (c) 0.4 cm (d) 0.5 cm 53. The nature of parallel and anti-parallel currents are (a) Parallel currents repel and antiparallel currents attact. (b) Parallel currents attract and antiparallel currents repel. (c) Both currents attract. (d) Both currents repel. 54. Two parallel wires 2 m apart carry currents of 2 A and 5 A respectively in same direction, the force per unit length acting between these two wires is (a) 2 × 10-6 N m-1 (b) 3 × 10-6 N m-1 (c) 1 × 10-6 N m-1 (d) 4 × 10-6 N m-1 55. A circular coil of radius 10 cm having 100 turns carries a current of 3.2 A. The magnetic field at the center of the coil is (a) 2.01 × 10-3 T (b) 5.64 × 10-3 T (c) 2.64 × 10-4 T (d) 5.64 × 10-4 T 56. The magnetic moment of a current I carrying circular coil of radius r and number of turns N varies as
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