Untitled Document

Question Bank No: 3

1. A lamp is connected in series with a capacitor and an ac source. What happens if the capacity of the capacitor is reduced ?

a)		the lamp shines more brightly
b)		the lamp shines less brightly
c)		there is no change in the brightness of the lamp
d)		brightness may increases or decreases depending on the frequency of ac

2. A coil of wire when connected across an ac of 200v, 50Hz. The current through it is 2A. The power consumed is 200W. The power factor is

a)		0.5
b)		2
c)		1
d)		zero

3. In a step up transformer the turns ratio is 1 : 2. A Laclanche cell of emf 1.5V is connected to primary. The voltage across the secondary is

a)		3V
b)		0.75V
c)		zero
d)		1.5V

4. A transformer is a device which corverts

a)		low voltage low current into high voltage high current
b)		high voltage low current in to low voltage high current
c)		high voltage high current into low voltage low current
d)		high voltage ac into high voltage dc

5. The Q factor in a resonant circuit is

a)		$\frac{R}{L ω}$
b)		$\frac{R}{C ω}$
c)		C $ω R$
d)		$\frac{1}{C ω R}$

6. A choke is used as a resistor in

a)		an ac circuit
b)		dc circuit
c)		half wave rectifier
d)		none

7. The resonant frequency of an LC circuit is given by

a)		$\frac{1}{2 π \sqrt{LC}}$
b)		$\frac{1}{\sqrt{2 π LC}}$
c)		$\frac{\sqrt{LC}}{2 π}$
d)		$\frac{\sqrt{C / L}}{2 π}$

8. An ac source of variable frequency f is connected to a series LCR circuit. Which one of the graphs shown below represents the variation of current i with frequency

a)
b)
c)
d)

9. The power factor in an LCR circuit at resonance is

a)		zero
b)		1/2
c)		0.8
d)		1

10. One kilo watt power is supplied to a 200 turn primary of a transformer at 0.5A. The secondary gives 200v. The number of turns of the secondary is

a)		10
b)		20
c)		100
d)		200

11. The voltage and current flowing through an ac circuit are given by V = 2cos100 $π$ t and I = 4sin 100 $π$ t. The power dissipated in the circuit is

a)		1W
b)		2 W
c)		zero
d)		4W

12. An inductance coil has a resistance 100 $π Ω$ . When an ac signal of frequency 1000Hz is fed to the coil, the applied voltage leads the current by $45^{\circ}$ . The inductance of the coil is

a)		5H
b)		50mH
c)		40mH
d)		30mH

13. If the inductance and capacitance are both doubled in LCR circuit, the resonance frequency of the circuit will

a)		decreases to one -half the original value
b)		decreases to one - fourth the original value
c)		increases to twice the original value
d)		decreases to one -eight the original value

14. The power factor of a circuit is unity. Then the impedance of the circuit is

a)		inductive
b)		capacitive
c)		resistive and capacitive
d)		resistive

15. In an ac circiut with voltage v and current I, the power developed is

a)		VI
b)		$\frac{VI}{2}$
c)		$\frac{VI}{\sqrt{2}}$
d)		depends on phase between V and I

16. Alternating voltage

a)		is independent of time
b)		varies inversely with time
c)		varies directly with time
d)		varies insoideelly

17. One complete set of negative and positive values of alternating quantities is called

a)		time period
b)		amplitude
c)		frequency
d)		cycle

18. AC measuring instrument measures

a)		peak value
b)		rms value
c)		any value
d)		average value

19. The instantaneous value of an ac is given by I = $5 \sin (wt + φ)$ . The rms value of current is

a)		5A
b)		$\sqrt{2} \times 5 A$
c)		$\frac{5}{\sqrt{2}} A$
d)		2.5 A

20. The reactance of a condenser Xc is an ac circuit varies with frequency $f$ of a source voltage which one of the following graphs represents this variation?

a)
b)
c)
d)

21. Inductive reactance $L ω$ of a coil is expressed as

a)		ampere
b)		ohm
c)		volt
d)		weber

22. The frequency of an AC main in India is

a)		110Hz
b)		50Hz
c)		60Hz
d)		120Hz

23. The average value of alternating current over a complete cycle is

a)		zero
b)		I rms
c)		$\frac{i}{\sqrt{2}}$
d)		$\frac{i}{2}$

24. In an a.c, the rms value of current I rms is related to peak value $I_{0}$ by the relation

a)		I rms = $\sqrt{2} I_{0}$
b)		I rms = $π I_{0}$
c)		I rms = $\frac{I_{0}}{π}$
d)		I rms = $\frac{I_{0}}{\sqrt{2}}$

25. The conductivity of a super conductor is

a)		infinite
b)		very large
c)		very small
d)		zero

26. When a current flows in a conductor, the order of magnitude of drift velocity of electrons through it is

a)		$10^{- 7} cm / s$
b)		$10^{- 2} cm / s$
c)		$10^{4} cm / s$
d)		$10^{10} cm / s$

27. Two cells of 1.25V and 0.75V are connected in parallel. The effectice voltage will be

a)		0.75 V
b)		1.25 V
c)		2 V
d)		0.5 V

28. The appropriate material to be used in the construction of resistance boxes is

a)		copper
b)		iron
c)		maganin
d)		aluminium

29. Three resistors of 4 $Ω$ , 6 $Ω$ and 12 $Ω$ are connected in parallel and the combination is connected in series with 4V battery with internal resistance of 2 $Ω$ . The battery current is

a)		0.5 A
b)		1A
c)		2 A
d)		10 A

30. Given three resistors, how many different combinations of these three resistors can be made?

a)		3
b)		4
c)		5
d)		6

31. A weight W can be just supported on a rough inclined plane by a force F either acting along the plane or horizontally. If $θ$ is the angle of friction, then F/W is

a)		tan $θ$
b)		sec $θ$
c)		sin $θ$
d)		cos $θ$

32. A plumb bob is hanging from the ceiling of a car. If the car moves with the acceleration 'a' the angle made by the string with the vertical is

a)		$\sin^{-} 1 (\frac{a}{g})$
b)		$\sin^{-} 1 (\frac{g}{a})$
c)		$\tan^{-} 1 (\frac{a}{g})$
d)		$\tan^{-} 1 (\frac{g}{a})$

33. The distance x covered in time t by a body having initial velocity u and having constant acceleration a is given by $x = ut + \frac{1}{2} {at}^{2} .$ This result follows from

a)		Newton's First Law
b)		Newton's Second Law
c)		Newton's Third Law
d)		None of the above

34. The mechanical advantage of a system of pulley s is four. The force needed to lift a mass of 100 kg will be

a)		20kg. Wt
b)		25kg. Wt
c)		5kg. Wt
d)		15kg. Wt

35. Two blocks A (20kg) lying on a frictionless table are connected by a light string. The system is pulled horizontally with an acceleration of $2 m / s^{2}$ by a force F on B. The tension in the string is

a)		10N
b)		40N
c)		100N
d)		120N

36. Two coils of self inductance $L_{1}$ and $L_{2}$ are placed close together so that effective flux in one coil is completely linked with the other. If m is the mutual inductance between them, then

a)		$M = L_{1} L_{2}$
b)		$\frac{L_{1} L_{2}}{2}$
c)		$M = (L_{1} + L_{2})^{2}$
d)		$M = \sqrt{L_{1} L_{2}}$

37. A induction may store energy in

a)		Its electric field
b)		Its coils
c)		Its magnetic field
d)		Both electric and magnetic fields

38. Inductive reactance of a coil expressed as

a)		Ampere
b)		ohm
c)		volt
d)		weber

39. The power factor in a circuit is unity. Then the impedance of the circuit is

a)		inductive
b)		capertive
c)		partially inductive and partially conductive
d)		resistive

40. If the conductance and capacitance are both doubled in LCR circuit, the resonant frequency of the circuit will

a)		decrease to one half the original value
b)		decrease to one-forth the original value
c)		increase to twice the original value
d)		decrease to twice the original value

41. In a purely inductive circuit the current

a)		is in phase with the voltage
b)		is out of phase with the voltage
c)		eads the voltage by $90^{0}$
d)		lags behind the voltage by $90^{0}$

42. The rms value of current ( $I_{rms}$ ) is

a)		$b / \sqrt{2}$
b)		$\sqrt{2 I_{o}}$
c)		$ω$
d)		$2 I_{0}$

43. Power consumed in an AC circuit become zero if

a)		inductance and resistance are both high
b)		inductance and resistance are both low
c)		inductance very high and resistance negligible
d)		inductance low and resistance high

44. A transformer works on

a)		DC only
b)		AC only
c)		both AC and DC
d)		high voltage only

45. In AC circuits choke is preferred to resistors because

a)		choke coil is cheap
b)		voltage increases
c)		energy is not wasted
d)		current increases

46. In a step-down transformer, the number of turns in

a)		Primary are less
b)		Primary are more
c)		Primary and secondary equal
d)		secondary are infinite

47. The Q-factor of a resonant circuit is equal to

a)		$\frac{1}{CWR}$
b)		$\frac{1}{WL}$
c)		CWR
d)		$\frac{f}{CW}$