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magnetism -2
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20 May 2007 21:50:38 IST
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20 May 2007 21:50:38 IST
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magnetism -2
Physics , Electromagnetism

 

MAGNETISM, ELECTROMAGNETIC INDUCTION & ALTERNATING CURRENT
1.       When the current in a circuit falls from 5 A to 0 A in 0.1 s, the emf induced is 1 V. Then the self inductance of the circuit is:  a. 50 mH  b. 10 mH  c. 20 mH  d. 30 mH  e. 100 mH
  1. A coil of area 200 cm² is placed at an angle 60º with a magnetic field, and the flux through it was found to be 3 m Wb. The magnetic induction of the field is:  a. 20/?3 T  b. ?3/10 T  c. 20?3 T  d. ?3/20 T
3.       When the rate of change of current in a coil is unity, the induced emf in it is:  a. total flux through the coil  b. coefficient of self-induction of the coil  c. number of turns in the coil  d. current in the coil  e. none of these
  1. The time required for the current in an L-R circuit with D. C to reach maximum value depends on:
a. only R  b. LR  c. L/R  d. only L  e. L²/R
  1. For a step-up transformer, the turns-ratio is:  a. =1  b. <1  c. >1  d. ?1  e. ?1
  2. The armature current is maximum in a motor, when it has:  a. attained maximum speed  b. minimum speed  c. just started motion  d. just when switched off  e. current is the same irrespective of speed   
  3. Instantaneous magnetic flux through a coil, which is rotated in a magnetic field, is given by  ? = 3t² + 4t ? 5 m Wb. Induced emf in it at t = 2 s is: a. 16 mV  b. 16 V  c. 11 mV 
d. 11 V  e. 15 mV
  1. Two ideal inductors each of self-inductance L are joined in series and well separated from each other. The effective self-inductance of the arrangement is:  a. L  b. 2L  c. L/2  d. L/4  e. 4L
  2. A straight conductor of length 50 cm is moved with a speed 10 m/s, in a direction making an angle 30º with the direction of a magnetic field of induction 1 T. Induced e. m. f in it is: 
a. 0  b. 2.5 V  c. 5 V d.4.33 V
  1. A generator produces an emf 440 V and has an internal resistance 400 ohm. If it is connected to a load of 4000 ohm, the output voltage is:  a. 440 V  b. 400 V  c. 200 V  d. 220 V  e. none of these
  2. A coil of 200 turns and area 100 cm² is placed with its plane making an angle 30º with a uniform field of magnetic induction 2 mT. If it is turned through 180º in 0.1 s, the induced emf in it is:  a. 40 mV  b. 80 mV  c. 40?3 mV  d. 80?3 mV  e. none of these
  3. Induced emf in a coil of 50 turns rotating at 50 Hz in a magnetic field is E1 and that in another coil of 100 turns and same area as the first, rotating at 150 Hz in the same field is E2 . Then, E1 /E2 is:  a. 6  b. 1/6  c. 3  d. 1/3  e. none of these
  4. Two coils of self-inductance 4 mH and 2 mH, when placed one above the other is found to have a mutual inductance ?2 mH. Then, the coefficient of coupling between the coils is:  a. 1  b. 2  c. ½  d. 0.25
  5. Two identical, coaxial circular coils carry a current of ?i? each circulating in the same direction. If the loops approach each other:  a. current in each decreases  b. current in each increases  c. current remains the same in both the coils  d. current in one increases while that in the other decreases
  6. A current from A to B increases in magnitude. The direction of induced
e. m. f. in the loop is:  a. clockwise b. anti clockwise  c. there is no induced
e. m. f.  d. the induced e. m. f. is a. c.
  1. R. M. S. value of an a. c. is 200 V. Then, the peak value is:              A                           B 
a. 220 V  b. 282.8 V  c. 300 V  d. 254 V  e. 0
17.   An a. c. circuit has resistance 4 ohm and impedence 8 ohm. Power factor is:  a. ½  b. 2  c. ¼  d. 4
  1. An a. c of frequency 50 Hz has peak value 100 V. Starting from zero, the e. m. f. after 1/600 a is:
a. 50 V  b. 50?3 V  c. 100/?3 V  d. 100?2 V  e. 100/?2 V
  1. An inductance of 0.1 H is connected to an a. c. 314 V/50 Hz. Then, current is:  a. 0  b. 1 A             c. 10A               d. 5A                    e. none of these
  2. A 240 V a. c. is connected to a resistance of 12 ohm, an inductance of 4 mH and a capacitance of 8 ?F, in series. Current in the circuit at resonance is:  a. 10 A  b. 12 A 
c. 15 A  d. 20 A  e. 0
  1. A step down transformer, with its primary connected to 240 V, is used to operate a 12 V/36 W lamp. If the transformer is ideal, current in the primary is:  a. 6 A  b. 60 A  c. 1.5 A d. 0.15 A  e. 0.6 A
  2. A step-up transformer has turns ratio 1:4. A cell of e. m. f. of 2 V is connected to its primary. Then, the voltage in the secondary is:  a. 8 V  b. 1/8 V  c. 4 V  d. ¼ V  e. 0
  3. When the primary of a transformer is connected to 120 V at a current of 1.8 mA, current in the secondary is found to be 150 mA. If the efficiency of the transformer is 90%, what is the voltage in the secondary?  a. 1.29V  b. 0.81 V  c. 9.6 V  d. 1.49 V  e. none of these
24.   Current in an a. c. circuit is i = 2Sin314t and voltage is v = 4 Cos314t. Power consumed by the circuit is:  a. 4 W  b. 8 W  c. 2 W  d. 0  e. none of these.
  1. A transformer steps down the voltage from 220 V to 11 V. Current in the primary and secondary are respectively 5 A and 90 A. Efficiency of the transformer is:  a. 100% 
R
b. 90%  c. 80%  d. 40%  e. 0%
  1. L
    An electric circuit with a D.C source of 2 V is show in the diagram.                          
The inductance is ideal. When the key is closed, current through the
20 ohm resistance is:  a. .04 A  b. 0.1 A  c. 0  d. 0.067 A  e. none of these                      
  1. A bar magnet and a circular coil are held as in the diagram.
If the magnet is now moved towards the coil, the
current induced, as seen from the left hand side, in the coil is:            key       2 V  
 

 
                                                            S                     N
                                   
 
a. clockwise  b. anticlockwise  c. an a. c.  d. 0
28.   A conductor of length 2 m, held in the east-west direction is falling from rest, from a height 50 m. Voltage induced in it in the first second of its fall is (horizontal earth?s magnetic induction = 0.0004 T):
a. 4 m V  b. 40 m V  c. 0.4 m V  d. 4 V  e. 0.4 V
  1. An aircraft is flying horizontally with a speed 360 km/h. Its wingspan is 20 m, earth?s total magnetic induction is 0.0008 T and the dip at the place is 30º. Induced e. m. f. across the wing is:  a. 0.8 V      b. 8 V           c. 0.8 m V     d. 8 m V     e. none of these
  2. The magnitude of induced e. m. f. in a coil, when a magnet is inserted into it does not depend on: 
a. the number of turns in the coil  b. resistance of the coil  c. magnetic moment of the magnet  d. speed of approach of the magnet  e. area of the coil
  1. The number of turns in a coil is doubled without changing the length of the coil. Then its self-inductance:  a. becomes four times the initial value  b. becomes twice the initial value  c. remains unchanged  d. becomes half the earlier value  e. becomes one-fourth the initial value.
  2. Charge induced in a circuit of resistance 10 ohm, when the flux through it of 10 m Wb is reduced to zero in 2 s is:  a. 1 m C  b. 0.5 m C  c. 1 C  d. 0.5 C  e. 0
  3. An electric lamp with rating 120 V/60 W is to be operated with a source of 240 V/50 Hz. The inductance to be connected is:  a. 1.32 H  b. 1.32 m H  c. 0.132 H  d. 0.132 m H
  4. Current in an inductance falls by 50% from the maximum value in 0.693 s. Time constant of the coil is (ln 2 = 0.693):  a. 0.693 s  b. 1 s  c. 1.44 s  d. 0.1 s  e. 6.93 s
  5. A 50 m H coil carries a current 2 A. Energy store in it is:  a. 1 J  b. 2 J  c. 0.1 J  d. 0.5 J 
e. 0.05 J
  1. A series L ? C ? R circuit has L = 0.1 m H, C = 1/4?² ? F and R = 100 ohm. The current in the circuit is maximum when the frequency of a. c. connected to it is:  a. 50 k Hz 
b. 5 k Hz  c. 100 k Hz  d. 10 kHz
  1. R
    L
    In the above problem, if the voltage connected is 240 V, current at resonance is:  a. 2.4 A  b. 24 A  c. 240 A  d. 0.24 A  e. cannot be calculated.                
 
  1. In the circuit shown in the diagram, R = 50 ohm
XL = 120 ohm and XC = 120 ohm. Then the voltmeter                   
 
reading is:  a. 0  b. 100 V  c. 200 V  d. 82.75 V                     V                        200 V/50 Hz
  1. In the above problem the power consumed is:                                    
a. 0  b. 800 W  c. 4 W  d. 1.67 W  e. none of the above 
  1. In the problem no. 38, phase difference between voltage and current is: 
a. 0       b. ?/6      c. ?/3                        d. ?/4               e. ?/2
  1. Phase difference between voltage and current in a circuit with 1 m H and 1 m F in series with 220V/50 Hz a. c. is:  a. 90º  b. 0º  c. 45º  d. 30º  e. 180º
  2. In the above problem, current in the circuit is:  a. 220 A  b. 550 A  c. 2.2 A  d. 5.5 A 
e. none of these
  1. Two identical coils, one placed over the other has mutual inductance 4 m H. Assuming the coupling to be 100%, value of each inductance is:  a. 4 m H  b. 8 m H  c. 2 m H  d. 1 m H  e. 16 m H
  2. Two inductances while in parallel has effective value 6 m H and while in parallel, 4/3 m H. The value of the individual inductance is:  a. 4, 2 m H  b. 3, 3 m H  c. 5, 1 m H  d. none of the above
  3. A coil of inductance 5 m H and negligible resistance is connected to an a. c voltage V = 10 Sin 100 t. The peak current in the circuit is:  a. 2 A b. 5 A  c. 10 A  d. 20 A  e. 0
  4. Frequency of ac mains in India is:   a. 30 Hz   b. 60 Hz   c. 100 Hz    d. 120 Hz    e. 50 Hz
  5. An ac generator produces an output voltage E = 170 Sin 377t volt. Frequency of the ac is:
a. 50 Hz   b. 110 Hz   c. 60 Hz   d. 230 Hz   e. 75 Hz
  1. If the instantaneous current in a circuit is I = 2 Sin (?t + ?) ampere, the r. m. s. value of current is:
a. 2 A    b. ?2 A   c. 2?2 A    d. 0    e. 1 A
  1. A direct current of 5 A is superimposed on an alternating current of I = 10 Sin ?t, flowing through a wire. The resulting current is:   a. 7.5 A   b. 5?3 A    c. 5?5 A   d. 15 A   
e. 10?2 A
  1. The reactance of a coil with 220 V/50 Hz ac is 100 ?. The self-inductance of the coil is:
a. 3.2 H   b. 0.22 H   c. 0.32 H    d. 2.2 H    e. 0.22 m H
 
 

 
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