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CHAPTER MOVING CHARGES & MAGNETISM 4 Exercise 1: NCERT Based Topic-wise MCQs 4.1 & 4.2 INTRODUCTION & MAGNETIC FORCE 1. An electron having a charge e moves with a velocity v in X-direction. An electric field acts on it in Y-direction? The force on the electron acts in NCERT Page-134 / N-109 (a) positive direction of Y-axis (b) negative direction of Y-axis (c) positive direction of Z-axis (d) negative direction of Z-axis 2. An electric charge in uniform motion produces NCERT Page-134 / N-108 (a) an electric field only (b) a magnetic field only (c) both electric and magnetic fields (d) no such field at all 3. A charge q is moving with a velocity v parallel to a magnetic field B. Force on the charge due to magnetic field is NCERT Page-135 / N-109 (a) q ∨ B (b) qB/v (c) zero (d) B v/q 4. The work done by a magnetic field, on a moving charge is NCERT Page-134 / N-109 (a) zero because F⃗ acts parallel to v⃗⃗ (b) positive because F⃗ acts perpendicular to v⃗⃗ (c) zero because F⃗ acts perpendicular to v⃗⃗ (d) negative because F⃗ acts parallel to v⃗⃗ 5. Lorentz force is NCERT Page-134 / N-109 (a) electrostatic force acting on a charged particle. (b) magnetic force acting on a moving charged particle. (c) the vector sum of electrostatic and magnetic force acting on a moving charged particle. (d) the vector sum of gravitational and magnetic force acting on a moving charged particle. 6. An electric charge +q moves with velocity v⃗⃗ = 3iˆ + 4jˆ + kˆ in an electromagnetic field given by E⃗⃗ = 3iˆ + jˆ + 2kˆ and B⃗⃗ = iˆ + jˆ − 3kˆ The y-component of the force experienced by. +q is : NCERT Page-134 / N-109 (a) 11q (b) 5q (c) 3q (d) 2q
7. A charged particle moves with velocity v⃗ = aiˆ + djˆ in a magnetic field B⃗⃗ = Aiˆ + Djˆ. The force acting on the particle has magnitude F. Then, NCERT Page-134 / N-109. (a) F = 0, if aD = dA (b) F = 0, if aD = −dA (c) F = 0, if aA = −dD (d) F ∝ (a 2 + b 2 ) 1/2 × (A 2 + D 2 ) 1/2 8. If a particle of charge 10−12 coulomb moving along the xˆ-direction with a velocity 105m/s experiences a force of 10−10 newton in yˆ-direction due to magnetic field, then the minimum magnetic field is NCERT Page-135/ N-109 (a) 6.25 × 103 Tesla in zˆ-direction (b) 10−15 Tesla in zˆ-direction (c) 6.25 × 10−3 Tesla in zˆ-direction (d) 10−3 Tesla in zˆ-direction 9. A certain region has an electric field E⃗⃗ = (2iˆ − 3jˆ)N/C and a uniform magnetic field B⃗⃗ = (5iˆ + 3jˆ + 4kˆ )T. The force experienced by a charge 1C moving with velocity (iˆ + 2ˆj)ms−1 is NCERT Page-134 / N-109 (a) (10iˆ − 7ˆj − 7kˆ ) (b) (10iˆ + 7ˆj + 7kˆ ) (c) (−10iˆ + 7jˆ + 7kˆ ) (d) (10iˆ + 7ˆj − 7kˆ ) 10. A charge particle is moving in a uniform magnetic field (2iˆ + 3jˆ)T. If it has an acceleration of (αiˆ − 4jˆ)m/s 2 , then the value of α will be : NCERT Page-134 / N-109 (a) 3 (b) 6 (c) 12 (d) 2 11. The magnetic force F⃗ on a current carrying conductor of length I in an exteranal magnetic field B⃗⃗ is given by (a) I×B⃗⃗ I⃗ NCERT Page-136 / N-110 (b) I⃗×B⃗⃗ I (c) I(I⃗ × B⃗⃗) (d) I 2 I⃗ × B⃗⃗ 12. A conducting circular loop of radius r carries a constant current i. It is placed in a uniform magnetic field B such that B is perpendicular to the plane of the loop. The magnetic force acting on the loop is NCERT Page-136 / N-110 (a) irB (b) 2πriB (c) zero (d) πriB 13. A current carrying conductor placed in a magnetic field experiences maximum force when angle between current and magnetic field is NCERT Page-136 / N-110 (a) 3π/4 (b) π/2 (c) π/4 (d) zero A current of 3A is flowing in a linear conductor having a length of 40cm. The conductor is placed in a magnetic field of strength 500 gauss and makes an angle of 30∘ with the direction of the field. It experiences a force of magnitude (a) 3 × 10−4N NCERT Page-136 / N-110 (b) 3 × 10−2N
(c) 3 × 102N (d) 3 × 104N 15. A current of 10A is flowing in a wire of length 1.5m. A force of 15N acts on it when it is placed in a uniform magnetic field of 2T. The angle between the magnetic field and the direction of the current is NCERT Page-136 / N-110 (a) 30∘ (b) 45∘ (c) 60∘ (d) 90∘ 16. An 8cm long wire carrying a current of 10A is placed inside a solenoid perpendicular to its axis. If the magnetic field inside the solenoid is 0.3T, then magnetic force on the wire is NCERT Page-136 / N-110 (a) 0.14N (b) 0.24N (c) 0.34N (d) 0.44N 4.3 MOTION IN A MAGNETIC FILED 17. A particle of mass m and charge q enters a magnetic field B perpendicularly with a velocity v. The radius of the circular path described by it will be NCERT Page-138 / N-112 (a) Bq/mv (b) mq/Bv (c) mB/qv (d) mv/Bq 18. A charged particle moves through a magnetic field in a direction perpendicular to it. Then the (a) velocity remains unchanged NCERT Page-138 / N-112 (b) speed of the particle remains unchanged (c) direction of the particle remains unchanged (d) acceleration remains unchanged 19. A charged particle of mass m and charge q travels on a circular path of radius r that is perpendicular to a magnetic field B. The time taken by the particle to complete one revolution is NCERT Page-138 / N-112 (a) 2πq 2B m (b) 2πmq B (c) 2πm qB (d) 2πqB m 20. A proton and an alpha particle of the same velocity enter in a uniform magnetic field which is acting perpendicular to their direction of motion. The ratio of the circular paths described by the alpha particle and proton is: (a) 1: 4 NCERT Page-138 / N-112 (b) 4: 1 (c) 2: 1 (d) 1: 2 21. An electron with energy 0.1keV moves at right angle to the earth's magnetic field of 1 × 10−4Wbm−2 . The frequency of revolution of the electron will be (Take mass of electron = 9.0 × 10−31kg ) NCERT Page-138 / N-112 (a) 1.6 × 105Hz (b) 5.6 × 105Hz
(c) 2.8 × 106Hz (d) 1.8 × 106Hz 22. A proton and an α-particle enter a uniform magnetic field perpendicularly with the same speed. If proton takes 25μ second to make 5 revolutions, then the time period for the α-particle would be NCERT Page-138 / N-112 (a) 50μ sec (b)25μsec (c) 10μsec (d) 5μsec 23. A charged particle enters into a magnetic field with a velocity vector making an angle of 30∘ with respect to the direction of magnetic field. The path of the particle is NCERT Page-138 / N-112 (a) circular (b) helical (c) elliptical (d) straight line 24. If an electron and a proton having same momenta enter perpendicular to a magnetic field, then (a) curved path of electron and proton will be same (ignoring the sense of revolution) NCERT Page-138 / N-112 (b) they will move undeflected (c) curved path of electron is more curved than that of the proton (d) path of proton is more curved 25. If an electron describes half a revolution in a circle of radius r in a magnetic field B, the energy acquired by it is NCERT Page-138 / N-112 (a) zero (b) 1 2 mv 2 (c) 1 4 mv2 (d) πr × Bev 26. A 10eV electron is circulating in a plane at right angles to a uniform field at a magnetic induction 10−4Wb/m2 (= 1,0 gauss). The orbital radius of the electron is NCERT Page-138 / N-112 (a) . 12cm (b) 16cm (c) 11cm (d) 18cm 27. An electron (mass = 9 × 10−31kg, charge = 1.6 × 10−19C ) moving with a velocity of 106m/s enters a magnetic field. If it describes a circle of radius 0.1m, then strength of magnetic field must be NCERT Page-138 / N-112 (a) 4.5 × 10−5T (b) 1.4 × 10−5T (c) 5.5 × 10−5T (d) 2.6 × 10−5T 28. A proton and a deuterium nucleus having certain kinetic energies enter in a uniform magnetic field with same component of velocity in the direction of magnetic field. Which of the following is correct? (a) Proton has greater pitch of helical motion. NCERT Page-138/ N-112 (b) Deuterium nucleus has greater pitch of helical motion. (c) Both particles have same pitch of helical motion. (d) Which particle has greater pitch depends on the fact that which particle has greater component of velocity perpendicular to magnetic field. Element, Biot Savart's Law 4.4 MAGNETIC FILED DUE TO CURRENT ELEMENTS, BIOT-SAVART’S LAW