Tiêu đề 6.ELECTROMAGNETIC INDUCTION - Questions.pdf ✅

6.ELECTROMAGNETIC INDUCTION (1.)An inductor (L = 100 mH), a resistor (R = 100Ω) and a battery (E = 100V) are initially connected in series as shown in the figure. After a long time the battery is disconnected after short circuiting the points A and B. The current in the circuit 1 ms after the short circuit is (a.) e A (b.) 0.1 A (c.) 1 A (d.) 1/e A (2.)Whenever a magnet is moved either towards or away from a conducting coil, an emf is induced, the magnitude of which is independent of (a.) The strength of the magnetic field (b.) The speed with which the magnet is moved (c.) The number of turns is the coil (d.) The resistance of the coil (3.)A physicist works in a laboratory where the magnetic field is 2T. She wears a necklace enclosing area 0.01m2 in such a way that the plane of the necklace is normal to the field and is having a resistance R = 0.01 Ω. Because of power failure, the field decays to 1T in time 10−3 seconds. Then what is the total heat produced in her necklace? (T = tesla) (a.) 10 J (b.) 20 J (c.) 30 J (d.) 40 J (4.)A coil of area 80 square cm and 50 turns is rotating with 2000 revolutions per minute about an axis perpendicular to a magnetic filed of 0.05 tesla. The maximum value of the e.m.f. developed in it is (a.) 200 π volt (b.) 10π 3 volt (c.) 4π 3 volt (d.) 2 3 volt (5.)When a magnet is pushed in and out of a circular coil C connected to a very senstitive galvanometer G as shown in the adjoining diagram with a frequency v, then (a.) Constant deflection is observed in the galvanometer (b.) Visible small oscillations will be observed in the galvanometer if v is about 50 Hz (c.) Oscillations in the deflection will be observed clearly if v = 1 or 2 Hz (d.) No variation in the deflection will be seen if v = 1 or 2 Hz (6.)In the circuit shown below, the key K is closed at t = 0. The current through the battery is (a.) V(R1+R2 ) R1R2 at t = 0 and V R2 at t = ∞ (b.) V(R1+R2 ) √R1 2R2 2 at t = 0 and V R2 at t = ∞ (c.) V R2 at t = 0 and V(R1+R2 ) R1R2 at t = ∞ (d.) V R2 at t = 0 and V(R1+R2 ) √R1 2R2 2 at t = ∞ (7.)Induced potential in a coil is developed by change of magnetic flux from 1 wb to 0.1 wb in 0.1 second is (a.) 1/9 volt (b.) 0.09 volt (c.) 1 volt (d.) 9 volt (8.)A square coil of 10−2 m2 area is placed perpendicular to a uniform magnetic field of intensity 103 Wb/m2 . The magnetic flux through the coil is (a.) 10 weber (b.) 10−5weber (c.) 105weber (d.) 100 weber (9.)An AC generator of 220 V having internal resistance r = 10 Ω and external resistance R = 100 Ω. What is the power developed in the external circuit (a.) 484 W G N S
(b.) 400 W (c.) 441 W (d.) 369 W (10.)The current carrying wire and the rod AB are in the same plane. The rod moves parallel to the wire with a velocity v. Which one of the following statements is true about induced emf in the rod (a.) End A will be at lower potential with respect to B (b.) A and B will be at the same potential (c.) There will be no induced e.m.f. in the rod (d.) Potential at A will be higher than that at B (11.)Two conducting circular loops of radii R1 and R2 are placed in the same plane with their centres coinciding. If R1 ≫ R2, the mutual inductance M between them will be directly proportional to (a.) R1/R2 (b.) R2/R1 (c.) R1 2 /R2 (d.) R2 2 /R1 (12.)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 × 10−3millihenry (d.) 50 × 10−3millihenry (13.)A conducting circular loop is placed in a uniform magnetic field of induction B tesla with its plane normal to the field. Now, the radius of the loop starts shrinking at the rate ( dr dt). Then, the induced emf at the instant when the radius is r,is (a.) πrB ( dr dt) (b.) 2πrB ( dr dt) (c.) πr 2 ( dB dt) (d.) ( πr 2 2 )B ( dr dt) (14.)Which of the following figure correctly depicts the Lenz’s law. The arrows show the movement of the labelled pole of a bar magnet into a closed circular loop and the arrows on the circle show the direction of the induced current (a.) (b.) (c.) (d.) (15.)The variation of induced emf(ε) with time (t ) in a coil if a short bar magnet is moved along its axis with a constant velocity is best represented as (a.) (b.) (c.) (d.) (16.)A copper disc of radius 0.1 m is rotated about its centre with 20 rev − s −1 in a uniform magnetic field of 0.1 T with its plane perpendicular to the A i B v
field. The emf induced across the radius of the disc is (a.) π 20 V (b.) π 10 V (c.) 20π mV (d.) 10π mV (17.)What is increased in step-down transformer (a.) Voltage (b.) Current (c.) Power (d.) Current density (18.)When a low flying aircraft passes over head, we sometimes notice a slight shaking of the picture on our TV screen. This is due to (a.) Diffraction of the signal received from the antenna. (b.) Interference of the direct signal received by the antenna with the weak signal reflected by the passing aircraft. (c.) Change of magnetic flux occuring due to the passage of aircraft (d.) Vibration created by the passage of aircraft (19.)A long horizontal metallic rod with length along the east-west direction is falling under gravity. The potential difference between its two ends will (a.) Be zero (b.) Be constant (c.) Increase with time (d.) Decrease with time (20.)According to Faraday’s law of electromagnetic induction (a.) The direction of induced current is such that it opposes the cause producing it (b.) The magnitude of induced e.m.f. produced in a coil is directly proportional to the rate of change of magnate flux (c.) The direction of induced e.m.f. is such that it opposes the cause producing it (d.) None of the above (21.)Which of the following is not an application of eddy currents (a.) Induction furnace (b.) Galvanometer damping (c.) Speedometer of automobiles (d.) X-ray crystallography (22.)A coil of Cu wire (radius-r, self inductance-L) is bent in two concentric turns each having radius r 2 . The self inductance now (a.) 2L (b.) L (c.) 4L (d.) L/2 (23.)A 10 metre wire kept in east-west direction is falling with velocity 5m/sec perpendicular to the field 0.3 × 10−4Wb/m2 . The induced e.m.f. across the terminal will be (a.) 0.15 V (b.) 1.5 mV (c.) 1.5 V (d.) 15.0 V (24.)The coil of dynamo is rotating in a magnetic field. The developed induced e.m.f. changes and the number of magnetic lines of force also changes. Which of the following conditions is correct (a.) Lines of force minimum but induced e.m.f. is zero (b.) Lines of force maximum but induced e.m.f. is zero (c.) Lines of force maximum but induced e.m.f. is not zero (d.) Lines of force maximum but induced e.m.f. is also maximum (25.)According to Lenz’s law of electromagnetic induction (a.) The induced emf is not in the direction opposing the change in magnetic flux. (b.) The relative motion between the coil and magnet produces change in magnetic flux (c.) Only the magnet should be moved towards coil (d.) Only the coil should be moved towards magnet (26.)The formula for induced e.m.f. in a coil due to change in magnetic flux through the coil is (here A = area of the coil, B = magnetic field) (a.) e = −A. dB dt (b.) e = −B. dA dt (c.) e = − d dt (A. B) (d.) e = − d dt (A × B) (27.)Eddy currents are produced when (a.) A metal is kept in varying magnetic field (b.) A metal is kept in the steady magnetic field (c.) A circular coil is placed in a magnetic field (d.) Through a circular coil, current is passed (28.)If a coil made of conducting wires is rotated between poles pieces of the permanent magnet. The motion will generate a current and this device is called (a.) An electric motor (b.) An electric generator (c.) An electromagnet (d.) All of the above (29.)A conducting rod of length l is moving in a transverse magnetic field of strength B with velocity v. The resistance of the rod is R. the current in the rod is
(a.) Blv R (b.) Blv (c.) Zero (d.) B 2v 2l 2 R (30.)A simple pendulum with bob of mass m and conducting wire of length L swings under gravity through an angle 2 θ. The earth’s magnetic field component in the direction perpendicular to swing is B. Maximum potential difference induced across the pendulum is (a.) 2 BL sin ( θ 2 ) (gL) 1/2 (b.) BL sin( θ 2 ) (gL) (c.) BL sin ( θ 2 ) (gL) 3/2 (d.) BL sin ( θ 2 ) (gL) 2 (31.)The self inductance of a solenoid of length L, area of cross-section A and having N turns is (a.) μ0N2A L (b.) μ0NA L (c.) μ0N 2LA (d.) μ0NAL (32.)Plane figures made of thin wires of resistance R + 50 milli ohm/metre are located in a uniform magnetic field perpendicular into the plane of the figures and which decrease at the rate dB/dt = 0.1 m T/s. The current in the inner and outer boundary are inner radius a = 10 cm and outer radius b = 20 cm) (a.) 10−4A (Clockwise), 2 × 10−4A (Clockwise) (b.) 10−4A (Anticlockwise), 2 × 10−4A (Clockwise) (c.) 2 × 10−4A (Clockwise), 10−4A (Anticlockwise) (d.) 2 × 10−4A (Anticlockwise), 10−4A (Anticlockwise) (33.)In a dc motor, induced e.m.f. will be maximum (a.) When motor takes maximum speed (b.) When motor starts rotating (c.) When speed of motor increases (d.) When motor is switched off (34.)If a coil of metal wire is kept stationary in a non-uniform magnetic field, then (a.) An e.m.f. is induced in the coil (b.) A current is induced in the coil (c.) Neither e.m.f. nor current is induced (d.) Both e.m.f. and current is induced (35.)The magnitude of magnetic induction for a current carrying toroid of uniform cross-section is (a.) Uniform over the whole cross-section (b.) Maximum on the outer edge (c.) Maximum on the inner edge (d.) Maximum at the center of cross-section (36.)An air core solenoid has 1000 turns and is one metre long. Its cross-sectional area is 10 cm2 . Its self inductance is (a.) 0.1256 mH (b.) 12.56 mH (c.) 1.256 mH (d.) 125.6 mH (37.)The horizontal component of the earth’s magnetic field at a place is 3 × 10−4 T and the dip is tan−1 ( 4 3 ). A metal rod of length 0.25 m placed in the north-south position and is moved at a constant speed of 10 cm/s towards the east. The emf induced in the rod will be (a.) Zero (b.) 1 μV (c.) 5 μV (d.) 10 μV (38.)In a uniform magnetic field of inductionB, a wire in the form of semicircle of radius r rotates about the diameter of the circle with angular frequency ω. If the total resistance of the circuit is R, the mean power generated per period of rotation is (a.) Bπr 2ω 2R (b.) (Bπr 2ω) 2 5 Rt (c.) (Bπrω) 2 2 R (d.) (Bπrω2) 2 8 R (39.)A current passing through a coil of self inductance of 2mH changes at the rate of 20mAs−1 . The emf induced in the coil is (a.) 10 μV (b.) 40 μV (c.) 10 mV (d.) 40 mV (40.)A magnet is made to oscillate with a particular frequency, passing through a coil as shown in figure. The time variation of the magnitude of e.m.f. generated across the coil during one cycle is                                  b D a C