Content text 22. Electromagnetic Induction Hard.pdf
1. A circular coil of radius 5 cm has 500 turns of a wire. The approximate value of the coefficient of self induction of the coil will be (a) 25 millihenry (b) 25 × 10–3millihenry (c) 50 × 10–34millihenry (d) 50 × 10–3 henry 2. A solenoid has 2000 turns wound over a length of 0.30 metre. The area of its cross-section is 1.2 × 10–3m2 . Around its central section, a coil of 300 turns is wound. If an initial current of 2A in the solenoid is reversed in 0.25 sec, then the emf induced in the coil is (a) 6 × 10–4V (b) 4 × 10–3V (c) 6 × 10–2V (d) 48 Mv 3. The coefficient of self inductance of a solenoid is 0.18 mH. If a crode of soft iron of relative permeability 900 is inserted, then the coefficient of self inductance will become nearly (a) 5.4 Mh (b) 162 Mh (c) 0.006 mh (d) 0.0002 Mh 4. The current in an inductor is given by i = 2 + 3tamp where t is in second. The self inducedemf in it is 9 mV the energy stored in the inductor at t= 1 second is (a) 10 Mj (b) 37.5 Mj (c) 75 Mj (d) Zero 5. The number of turns in two coils A and B are 300 and 400 respectively. They are placed close to each other. Co-efficient of mutual induction between them is 24 mH. If the current passing through the coil A is 2 Amp then the flux linkage with coil B will be (a) 24 mwb (b) 12 × 10–5wb (c) 48 mwb (d) 48 × 10–5wb 6. A coil of wire of a certain radius has 600 turns and a self- inductance of 108 mH. The self-inductance of another similar coil of 500 turns will be (a) 74 Mh (b) 75 Mh (c) 76 mH (d) 77 Mh 7. A current increases uniformly from zero to one ampere in 0.01 second, in a coil of inductance 10 mH it. The induced emf will be (a) 1 V (b) 2 V (c) 3 V (d) 4 V 8. The current in a coil varies w.r.t. to time t as I = 3t2 + 2t. If the inductance of coil be 10 mH, the value of induced emf at t = 2s will be (a) 0.14 V (b) 0.12 V (c) 0.1 V (d) 0.13 V 9. What inductance would be needed to store 1 KWh of energy in a coil carrying a 200 A current (a) 100 H (b) 180 H (c) 200 H (d) 450 H 10. The self inductance of a coil is L, keeping the length and area same, the number of turns in the coil is increased to four times. The self inductance of the coil will now be (a) L 4 1 (b) L (c) 4 L (d) 16 L 11. The mutual inductance between a primary and secondary circuit is 0.5 H. The resistance of the primary and the secondary circuits are 20 ohms and 5 ohms respectively. To generate a current of 0.4 A in the secondary, current in the primary must be changed at the rate of (a) 4.0 A/s (b) 16.0 A/s (c) 1.6 A/s (d) 8.0 A/s 12. The average emf induced in a coil in which a current changes from 0 to 2 A in 0.05 s is 8 V. The self inductance of the coil is (a) 0.1 H (b) 0.2 H (c) 0.4 H (d) 0.8 H 13. A coil of Cu wire (radius-r, self inductance-L) is bent in two concentric turns each having radius is . 2 r The self inductance now (a) 2L (b) L (c) 4 L (d) L2 14. In the following circuit, the bulb will become suddenly bright if (a) Contact is made or broken (b) Contact is made (c) Contact is broken (d) Won’t becomes bright at all 15. In the figure magnetic energy stored in the coil is (a) Zero (b) Infinite (c) 25 J (d) None of these 16. An emf of 15 volt is applied in a circuit containing 5 henry inductance and 10 ohm resistance. The ratio of the currents at time t = and at t = 1 second is (a) 1 1 / 2 1 / 2 e − e (b) 1 2 2 e − e (c) 1 1 − − e (d) e– 1 17. If the rotational velocity of a dynamo armature is doubled, then the induced emf will (a) Become half (b) Become double (c) Become quadruple (d) Remain unchanged 18. In an ac dynamo, the peak value of emf is 60 volts, then the induced emf in the position, when armature makes an angle of 30o with the magnetic field perpendicular with the coil, will be 10 V 2 H 2
(a) 20 volts (b) 30 3 volts (c) 30 volts (d) 45 volts 19. An ideal transformer has 500 and 5000 turns in primary and secondary windings respectively. If the primary voltage is connected to a 6V battery then the secondary voltage is (a) 0 (b) 60V (c) 0.6 V (d) 6.0 V 20. In a step-down transformer, the transformation ratio is 0.1, current in primary is 10 mA. The current in secondary is (a) 10 Ma (b) 1 Ma (c) 1 Ma (d) 0.1 A 21. How much current is drawn by primary of a transformer connected to 220 V supply, when it power to a 110 V and 550 W refrigerator (a) 2.5 A (b) 0.4 A (c) 4 A (d) 25 A 22. A step down transformer is connected to main supply 200V to operate a 6V, 30W bulb. The current in primary is (a) 3 amp (b) 1.5 amp (c) 0.3 amp (d) 0.15 amp 23. An ideal transformer steps down 220 V to 22 V in order to operate a device with an impedance of 220 . The current in the primary is (a) 0.01 A (b) 0.1 A (c) 0.5 A (d) 1.0 A 24. The frequency for which a 5F capacitor has a reactance of 1000 1 ohm is given by (a) MHz 100 (b) Hz 1000 (c) Hz 1000 1 (d) 1000 Hz 25. Let frequency = 50 Hz, and capacitance C = 100F in an ac circuit containing a capacitor only. If the peak value of the current in the circuit is 1.57 A. The expression for the instantaneous voltage across the capacitor will be (a) E = 50 sin (100 t – 2 ) (b) E = 100 sin (50 t) (c) E = 50 sin (100 t) (d) E = 50 sin (100 t + 2 ) 26. A bulb of 60 volt and 10 watt is connected with 100 volt of ac source with an inductance coil in series. If bulb illuminates with it's full intensity then value of inductance of coil is (= 60 Hz) (a) 1.28 H (b) 2.15 H (c) 3.27 H (d) 3.89 H 27. In an ac circuit, containing an inductance and a capacitor in series, the current is found to be maximum when the value of inductance is 0.5 henry and a capacitance of 8 F . The angular frequency of the input ac voltage must be equal to (a) 500 rad/sec (b) 4 510 rad/sec (c) 4000 rad/sec (d) 5000 rad/sec 28. A resistance of 40 ohm and an inductance of 95.5 millihenry are connected in series in a 50 cycles/second ac circuit. The impedance of this combination is very nearly (a) 30 ohm (b) 40ohm (c) 50 ohm (d) 60 ohm 29. In a series circuit R = 300 , L = 0.9H , C = 2.0F and = 1000 rad / sec. The impedance of the circuit is (a) 1300 (b) 900 (c) 500 (d) 400 30. In LCR circuit, the capacitance is changed from C to 4C. For the same resonant frequency, the inductance should be changed from L to (a) 2L (b) L/2 (c) L/4 (d) 4L 31. An LCR series circuit is connected to an external e.m.f. e = 200 sin 100t . The values of the capacitance and resistance in the circuit are 2F and 100 respectively. The amplitude of the current in the circuit will be maximum when the inductance is (a) 100 Henry (b) 2 50 / Henry (c) 100 Henry (d) 2 100 Henry 32. In the circuit shown below, what will be the readings of the voltmeter and ammeter (a) 800 V, 2A (b) 300 V, 2A (c) 220 V, 2.2 A (d) 100 V, 2A 33. In the circuit shown in the figure the ac source gives a voltage V = 20 cos(2000 t) . Neglecting source resistance, the voltmeter and ammeter reading will be (a) 0V, 0.47 A (b) 1.68V, 0.47 A (c) 0V,1.4 A (d) 5.6V,1.4 A 34. In the following circuit diagram inductive reactance of inductor is 24 and capacitive reactance of capacitor is 48, then reading of ammeter will be V A 5mH 50F 6 4 220 V, 50 Hz 300 V 300 V 100 A V
(a) 5 A (b) 2.4 A (c) 2.0 A (d) 10 A 35. Shown in the figure is a circular loop of radius r and resistance R. A variable magnetic field of induction B = e-t is established inside the coil. If the key (K) is closed at t = 0, the electrical power developed is equal to R X X X X r X B K X X (a) R r 2 (b) R 10 r 3 (c) 5 r R 2 4 (d) R 10 r 4 36. An ac source of angular frequency is fed across a resistor R and a capacitor C in series. The current registered is I. If now the frequency of source is changed to /3 (but maintaining the same voltage), the current in the circuit is found to be halved. The ratio of reactance to resistance at the original frequency will be. (a) 5 3 (b) 3 5 (c) 5 3 (d) 3 5 37. Shown in the figure is a circular loop of radius r and resistance R. A variable magnetic field of induction B = e-t is established inside the coil. If the key (K) is closed at t = 0, the electrical power developed is equal to R X X X X r X B K X X (a) R r 2 (b) R 10 r 3 (c) 5 r R 2 4 (d) R 10 r 4 38. In the figure shown the magnetic field at the point P is (a) 0 2 4 3 a 2 i − (b) 0 2 4 3 a i + (c) (4 ) 3 2 i o 2 + (d) (4 ) 3 a 2 i o 2 − 39. An electron moves straight inside a charged parallel plate capacitor of uniform surface charge density . The space between the plates is filled with constant magnetic field of induction → B . Neglect gravity, the time of straight line motion of the electron in the capacitor is (a) 0 B (b) 0 B (c) 0B (d) 0B 40. In the figure shown a coil of single turn is wound on a sphere of radius r and mass m. The plane of the coil is parallel to the inclined plane and lies in the equatorial plane of the sphere. If sphere is in rotational equilibrium the value of B is (current in the coil is i) (a) ir mg (b) i mgsin (c) i mgr sin (d) None 41. A particle with charge +q and mass m1, moving under the influence of a uniform electric field E ^ i and a uniform magnetic field B ^ k , follows a trajectory from P to Q as shown. The velocities at P and Q are V ^ i and - 2V ^ j . Choose the wrong statement. (a) E= 4 3 qa mv 2 (b) The rate of work done by the electric field at P is 4 3 a mv3 (c) The rate of work done by the electric field at P is 0. O 2a Q aP → E → B • → B 0 + em ⎯→• x x x x x x x x x l P (a,0) (2a,0)(3a,0) x i z y i 240 V L C ac ammeter
(d) The rate of work done by both the fields at Q is 0 42. Shown in the figure is a very long semicylindrical conducting shell of radius R and carrying a current i. An infinitely long straight current carrying conductor is lying along the axis of the semicylinder. If the current flowing through the straight wire be 0 i then the force on the semicylinder is (a) 2 0 0 R ii (b) R i i 2 0 0 (c) R i i 2 2 0 0 (d) None of these 43. A wire bent in the form of an equilateral triangle ABC of side a carries a current i. Find the magnetic field at a point having equal distance a from A, B and C (a) 2 a i 0 (b) 2 3 a i 0 (c) 3 a i 0 (d) 2 a 2 i 0 44. A solenoid has a length L = 1.23m and an inner diameter d = 3.55cm. It has five layers of windings of 850 turns each and carries a current i0 = 5.57A . What is B at its center? (a) 5mT (b) 30.8mT (c) 43.2mT (d) 24.2mT 45. A circular coil of 100 turns has an effective radius 0.05m and carries a current of 0.1ampere. How much work is required to turn it in an external magnetic field of 1.5 weber/m2 through 1800 about an axis perpendicular to the magnetic field. The plane of the coil is initially perpendicular to the magnetic field (a) 1.8J (b) 0.236J (c) 2.4J (d) 0.236J 46. An electron beam passes through a magnetic field 3 2 2 10 weber/ m − and an electric field 4 3.4 10− volts/m both acting simultaneously. If the path of electrons remains undeviated, calculate the speed of the electrons. If the electric field is removed, what will be radius of the electron path ? (a) 2.4 10 m −2 (b) 3.86 10 m −4 (c) 4.78 10 m −2 (d) 5 10 m −2 47. Two identical conducting ring A and B of radius R are in pure rolling over a horizontal conducting plane with same speed (of center of mass) v but in opposite direction. A constant magnetic field B is present pointing inside the plane of paper. Then the potential difference between the highest points of the two rings, is: A B (a) Zero (b) 2BvR (c) 4BvR (d) None of these 48. A copper disc of radius 0.1 m is rotated about its centre with 20 revolution per second in a uniform magnetic field of 0.1 T with its plane perpendicular to the field. The emf induced across the radius of the disc is - (a) 20 volt (b) 10 volt (c) 20 millivolt (d) 100 millivolt 49. A current I = 10 sin(100 t) amp. is passed in first coil, which induces a maximum e.m.f of 5 volt in second coil. The mutual inductance between the coils is- (a) 10 Mh (b) 15 mH (c) 25 mH (d) 5 mH 50. AB and CD are smooth parallel rails, separated by a distance L and inclined to the horizontal at an angle . A uniform magnetic field of magnitude B, directed vertically upwards, exists in the region. EF is a conductor of mass m, carrying a current I. For EF to be in equilibrium : A C L D B E F (a) I must flow from E to F (b) BIL = mg cos (c) BIL = mg sin (d) BIL = mg 51. An equilateral triangular loop ADC of uniform specific resistivity having some resistance is pulled with a constant velocity v out of a uniform magnetic field directed into the paper. At time t = 0 , side DC of the loop is at the edge of the magnetic field. The induced current (I) versus time (t) graph will be as: × × × × × × × × × × × × × × × × × × × × × × D C A v (a) t I (b) t I (c) t I (d) t I R 0 i i
Tiếng Việt