Untitled Document

Question Bank No: 1

1. A network shown below is a part of complete circuit.If at a certain instant, the corrent I is 5A and is decreasing at a rate $10^{3} A / S .$ The ( $V_{B} - V_{A}) is$
electromagnetQ23

a)		20V
b)		15V
c)		10V
d)		5V

2. The magnetic flux linked with the coil varies with time as $φ =$ 3 $t^{2} + 4 t + 9.$ The magnitude of induced emf at t= 2S is

a)		9V
b)		16V
c)		3V
d)		4V

3. In an ideal inductor L=4H and w=100 rad/S. The power developed is

a)		$V_{o} I_{0}$
b)		$V_{o} I_{0} / 2$
c)		2 $V_{o} I_{0}$
d)		Zero

4. An inductance L having a resistance R is connected to an alternating source of angular frequency w. The quality factor (Q) of the inductance is

a)		${(\frac{wL}{R})}^{2}$
b)		${(\frac{R}{wL})}^{2}$
c)		$(\frac{wL}{R})$
d)		$\frac{R}{wL}$

5. In an L-R circuit, the time constant is that time in which current grows from zero to the value

a)		0.63 $I_{0}$
b)		0.5 $I_{0}$
c)		0.37 $I_{0}$
d)		$I_{0}$

6. In a choke coil, the reactance $X_{L}$ and resistance R are such that

a)		$X_{L}$ = R
b)		$X_{L}$ > > R
c)		$X_{L}$ < < R
d)		$X_{L}$ = $\propto$

7. A st conductor of length 0.4 m is moved with a speed of 7 m/s perpendicular to magnetic field of induction 0.9wb/ $m^{2} .$ The induced emf a cross the conductor is

a)		25.2V
b)		5.04V
c)		2.52V
d)		1.26V

8. The core used in transformer is laminated in order to

a)		increase the magnetic field
b)		increasing magnetic saturation level of the core
c)		increase the residential magnetism of the core
d)		decrease the eddy current loss in the core

9. A transformer has an efficiency of 80%. It works at 4kw and 100V if secondary voltage is 240 V, The current in primary coil is

a)		0.4A
b)		4A
c)		10A
d)		40A

10. A 100 mH coil carries current of IA energy stared in magnetic field is

a)		0.05 J
b)		0.5 J
c)		1 J
d)		0.1 J

11. The average power dissipation in a pure capacitance in a.c. is

a)		1/2 ${CV}^{2}$
b)		${CV}^{2}$
c)		1/4 ) ${CV}^{2}$
d)		Zero

12. An ac. ammeter connected in series in an a.c. circuit reads 5 amp. The peak value of current is

a)		5A
b)		$\frac{5}{\sqrt{2}} A$
c)		5 $\sqrt{2} A$
d)		10 / $π A$

13. An A.C. circuit using an inductor and a capacitor in series has a max: current if L = 0.5 H and C= 8MF .The angular frequency of A.C voltage will be

a)		500
b)		5 $\times 10^{5}$
c)		4000
d)		5000

14. As the load on a d.c. shunt motor increases it speed will be

a)		Sharply increases
b)		slightly increases
c)		remains constant
d)		slightly reduce

15. In L- R Circuit, the ac source has voltage 22oV if potential difference across inductance is 176V. The potential difference across the resistance will be

a)		44 V
b)		132 V
c)		110 V
d)		220 V

16. A 5cm long solenoid having 10 $Ω$ resistance and 5mH inductance is joined to a 10V battery.At steady state, current through solenoid is

a)		5A
b)		2A
c)		1A
d)		Zero

17. A d.c motor has internal resistance 4 $Ω$ it is operated as 220v and draws 5 amp current. The back emf produced is

a)		80 V
b)		160 V
c)		200 V
d)		120 V

18. The equivalent quantity of mass in electrolyte is

a)		charge
b)		current
c)		co-efficient of self inductance
d)		potential

19. A d.c. circuit contain 10 $Ω$ of resistance in series with 10H coil. The impedence of the circuit is

a)		10 $Ω$
b)		20 $Ω$
c)		1 $Ω$
d)		0

20. Two coils X and Y are placed in a circuit such that a current changes by 2A in coil X and magnetic flux change of 0.4 wb occurs in y.The value of mutual inductance of the coil is

a)		0.8 H
b)		0.2 wb
c)		0.2 H
d)		5+1

21. An emf of 5V is produced in a coil when current changes at a steady rate from 3A to 2A in one milli Second. The Value of self inductance is

a)		5 $\times 10^{2} H$
b)		5 $\times 10^{0} H$
c)		5 $\times 10^{- 3} H$
d)		0.005mH

22. If N is number of turns in a coil, the value of self inductance varries as

a)		N
b)		$N^{0}$
c)		$N^{2}$
d)		$N^{- 2}$

23. A coil having 500 square 100 ps each of side 10 cm is placed normal to a magnetic field which increases at the rate of 1 wb/ $m^{2} .$ The induced emf is

a)		0.1V
b)		5.0V
c)		0.5V
d)		1.0V

24. A gramophone disc of brass having a diameter of 30cm rotates horizontally at the rate of 100 revolutions in 3 minutes. If the vertical component of earth's magnetic field be 0.01 T., then emf induced between the centre and rim of the disc is

a)		48 ${\times 10}^{- 4}$ V
b)		12.85 ${\times 10}^{- 4}$ V
c)		24.7 ${\times 10}^{- 4}$ V
d)		36 ${\times 10}^{- 2}$ V

25. An emf of 1.5 V is applied in a circuit containing 5 henry inductance and 10 $Ω$ resistance. The ratio of current at time t= $\infty$ and t=1 sec is

a)		$\frac{e^{1 / 2}}{e^{1 / 2} - 1}$
b)		$\frac{e^{2}}{e^{2} - 1}$
c)		$e^{- 1}$
d)		$1 - e^{- 1}$

26. An inductance of 0.2 H and a resistance of 100 $Ω$ are connected in series with an A.C. supply of 220 volts 50 Hz. Phase lag of current from e.m.f. applied is

a)		$\tan^{- 1}$ (0.4 $π$ )
b)		$\tan^{- 1}$ ( $π$ )
c)		$\tan^{- 1}$ (4 $π$ )
d)		$\tan^{- 1}$ (0.2 $π$ )

27. A magnetic field of 2 $\times 10^{- 2}$ T acts at right angles to a coil of area 100 ${cm}^{2}$ with 50 turns. The average e.m.f. induced in the coil is 0.1V when it is removed from the field in time t. The value of t is

a)		0.01 sec
b)		0.1 sec
c)		0.5 sec
d)		1 sec

28. A capacitive circuit have zero resistance. When A.C voltage is applied across this circuit, then the current leads the applied A.C. voltage by an angle

a)		$30^{o}$
b)		$45^{o}$
c)		$90^{o}$
d)		$0^{o}$

29. At low frequency a condenser offers

a)		high impedance
b)		low impedance
c)		Zero impedance
d)		impedance of condenser is independent of frequency

30. The overall efficiency of a transformer is 90%. The transformer is rated for an output of 9000 watt. The primary voltage is 1000 volt. The ratio of turns in the primary to secondary coil is 5:1. The iron losses at full load are 700 watt. The primary voltage is 1000 volt. The primary coil has 1 resistance of 1 ohm. Then the voltage in secondary coil is

a)		100 volt
b)		5000 volt
c)		200 volt
d)		zero volt

31. Two coils A and B are connected in series to a supply of 240 V r.m.s and 50 cps. The resistance of A is 5 $Ω$ and the inductance of B is 0.02H. The power consumed is 3K watt and the power factor is 0.75, the resistance of coil B is

a)		5.8 $Ω$
b)		0.58 $Ω$
c)		58 $Ω$
d)		0.058 $Ω$

32. An inductor has a resistance R and inductance L. It is connected to an A.C source of e.m.f $E_{v}$ and angular frequency $ω$ , then the current $I_{v}$ in the circuit is given by

a)		$I_{v}$ = $\frac{E_{v}}{ω L}$
b)		$I_{v}$ = $\frac{E_{v}}{R}$
c)		$I_{v}$ = $\frac{E_{v}}{\sqrt{R^{2} + ω^{2} L^{2}}}$
d)		$I_{v}$ = $\sqrt{{(\frac{E_{v}}{R})}^{2} + {(\frac{E_{v}}{ω L})}^{2}}$

33. In an a.c circuit e.m.f and current are given by E= 200 sin 314t, I= Sin(314t+ $π / 3$ ), Average power consumption in the circuit will be

a)		200 W
b)		100 W
c)		50 W
d)		25 W

34. An alternating voltage is given by E= $E_{1} \sin ω t + E_{2} \cos ω t$ . Then the root mean square value of voltage is given by

a)		$\sqrt{E_{1}^{2} + E_{2}^{2}}$
b)		$\sqrt{E_{1} E_{2}}$
c)		$\sqrt{\frac{E_{1} E_{2}}{2}}$
d)		$\sqrt{\frac{E_{2}^{1} + E_{2}^{2}}{2}}$

35. If you are not to develop eddy currents in the core of the transformer then it can be obtained

a)		by increasing the number of turns in secondary coil
b)		by taking laminated core
c)		by making step - down transformer
d)		by using a weak-alternating current at high potential

36. A generator produces a voltage that is given by V=240 sin 120t Volt where t is in seconds. The frequency and r.m.s voltage are

a)		60 hz and 240 volt
b)		19 hz and 120 volt
c)		19 hz and 170 volt
d)		745 hz and 170 volt

37. The equation of an alternating current is = 50 $\sqrt{2}$ sin 400 $π$ t amp, then the frequency and the root mean square value of the current are respectively

a)		200 hertz, 50 amp
b)		400 hertz, 50 $\sqrt{2}$ amp
c)		200 hertz, 50 $\sqrt{2}$ amp
d)		50 hertz, 200 amp

38. An alternating e.m.f of E= 100 sin (100 $π t$ ) is connected to a choke of negligible resistance and current oscillations of amplitude 1 A are to be produced. The inductance of the choke should be

a)		100 H
b)		$\frac{1}{π}$ H
c)		1 H
d)		$π$ H

39. In an ideal parallel LC Circuit, the capacitor is charged by connecting it to a d.c source, which is then diconnected from the source. The current in the circuit is

a)		Oscillates Intantly
b)		become zero instantly
c)		decays monotonically
d)		grows monotonically

40. The power factor of a.c circuit having L and R connected in series to an a.c source of angular frequency $ω$ is given by

a)		$\frac{\sqrt{R^{2} ω^{2} L^{2}}}{R}$
b)		$\frac{R}{\sqrt{R^{2} ω^{2} L^{2}}}$
c)		$\frac{ω L}{R}$
d)		$\frac{R}{ω L}$

41. A straight conductor of length 0.4 is moved with a speed of 7 m/s perpendicular to a magnetic field of induction 0.9 weber per $m^{2}$ . The induced emf across the conducter is

a)		25.2 V
b)		1.26 V
c)		5.04 V
d)		2.52 V

42. Current in a coil is increases from 1 A to 11 A in a 4 mili sec. If the inductance of the coil in 40 mH then emf induced in the coil during this process is

a)		40 volt
b)		0.4 Volt
c)		100 V
d)		440 V

43. In LCR circuit R =100 $Ω$ , L= 8 Henry and C= 0.5 $μ$ F are connected in this series, the resonance frequency is

a)		6000 radian per sec.
b)		500 radian per sec.
c)		600 $π$ radian per sec.
d)		250 $π$ radian per sec.

44. The number of turns in the primary and the secondary of a transformer are $N_{p}$ and $N_{s}$ and the resistance of secondary circuit is $R_{s}$ , then equivalent resistance of the load from the point of view of primary circuit is

a)		$R_{s}$ ${(\frac{N_{P}}{N_{s}})}^{2}$
b)		$R_{s}$ $(\frac{N_{P}}{N_{S}})$
c)		$R_{s}$ ${(\frac{N_{s}}{N_{p}})}^{2}$
d)		$R_{s}$ $(\frac{N_{P}}{N_{S}})$

45. In the LCR Series circuit the voltmeter across R and ammeter readings are

a)		V=100V, I=1 Amp
b)		V =100 V I=5 A
c)		V=100 V, I= 2 A
d)		V=100 I=3A

46. When a current in a coil changes from 2 amp to 6 amp in 0.05 sec. an emf of 8 volt is induced in the coil. the coefficient of self induction of the coil is

a)		0.1 H
b)		0.4 H
c)		0.2 H
d)		2 H

47. An Electric motor runs on d.c. source of emf 200V and draws a current of 10A. If the efficiency is 40%. Then the resistance o the armature is

a)		5 $Ω$
b)		12 $Ω$
c)		120 $Ω$
d)		160 $Ω$

48. A long solenoid of cross section A, length l and turns $N_{1}$ has a wound about its centre a small coil of turns $N_{2}$ . Then the mutual inductance of the two coil is

a)		$\frac{μ_{0} N_{2}^{1} N_{2}}{l}$
b)		$\frac{μ_{0} N_{1}^{2} N_{2} A}{l}$
c)		$\frac{μ_{0} N_{1} N_{2} A}{l}$
d)		$μ_{0} A N_{1} N_{2} l$

49. An ideal coil of 10H is connected in series with a resistor of 5 $Ω$ and a source of emf of 5 volt. Current flowing through the circuit after 2 sec. The circuit is switched on is

a)		e
b)		1 $- e^{- 1}$
c)		$e^{- 1}$
d)		1 $- e$

50. A short circuited coil is placed in a time varying magnetic filed. Electrical power is dissipated due to the current induced in the coil. If the number of turns were to be quadrupled and the wire radius halved, the electrical power dissipated would be

a)		quadrupled
b)		halved
c)		doubled
d)		the same