# Physics MCQs for Class 12 Chapter 7 Alternating Current with Answers

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#### MCQs based on Alternating Current:

Q.1. In general in an alternating current circuit
(a) the average value of current is zero
(b) the average value of square of the current is zero
(c) average power dissipation is zero
(d) the phase difference between voltage and current is zero

(a)

Q.2. The frequency of A.C. mains in India is
(a) 30 c/s
(b) 50 c/s
(c) 60 c/s
(d) 120 c/s

(b) In India the frequency of A.C. current is 50 Hz.

Q.3. A.C. power is transmitted from a power house at a high voltage as
(a) the rate of transmission is faster at high voltages
(b) it is more economical due to less power loss
(c) power cannot be transmitted at low voltages
(d) a precaution against theft of transmission lines

(b)

Q.4. The electric mains supply in our homes and offices is a voltage that varies like a sine function with time such a voltage is called … A… and the current driven by it in a circuit is called the … B… Here, A and B refer to
(a) DC voltage, AC current
(b) AC voltage, DC current
(c) AC voltage, DC voltage
(d) AC voltage, AC current

(d) The electric mains supply in our homes and offices is a voltage that varies like a sine function with time. Such a voltage is called alternating voltage and the current driven by it in a circuit is called the alternating current.

Q.5. Alternating currents can be produced by a
(a) dynamo
(b) choke coil
(c) transformer
(d) electric motor

(a)

Q.6. The peak value of the a.c. current flowing throw a resistor is given by
(a) I0 = e0/R
(b) I = e/R
(c) I0= e0
(d) I0 = R/e0

(a)

Q.7. The alternating current can be measured with the help of
(a) hot wire ammeter
(b) hot wire voltmeter
(c) moving magnet galvanometer
(d) suspended coil type galvanometer

(a)

Q.8. Alternating current can not be measured by D.C. ammeter, because
(a) A. C. is virtual
(b) A. C. changes its direction
(c) A. C. can not pass through D.C. ammeter
(d) average value of A. C for complete cycle is zero

(d)

Q.9. The heat produced in a given resistance in a given time by the sinusoidal current I0sinωt will be the same as that of a steady current of magnitude nearly
(a) 0.71 I0
(b) 1.412 I0
(c) I0
(d) √I0

(a)

Q.10. An A.C. source is connected to a resistive circuit. Which of the following is true?
(b) Current lags behind voltage in phase
(c) Current and voltage are in same phase
(d) Any of the above may be true depending upon the
value of resistance.

(c) When resistance is connected to A.C source, then current & voltage are in same phase.

Q.11. In which of the following circuits the maximum power dissipation is observed?
(a) Pure capacitive circuit (b) Pure inductive circuit
(c) Pure resistive circuit (d) None of these

(c)

Q.12. With increase in frequency of an A.C. supply, the inductive reactance
(a) decreases
(b) increases directly with frequency
(c) increases as square of frequency
(d) decreases inversely with frequency

(b)

Q.13. If the frequency of an A.C. is made 4 times of its initial value, the inductive reactance will
(a) be 4 times
(b) be 2 times
(c) be half
(d) remain the same

(a)

Q.14. A capacitor acts as an infinite resistance for
(a) DC
(b) AC
(c) DC as well as AC
(d) neither AC nor DC

(a)

Q.15. The capacitive reactance in an A.C. circuit is
(a) effective resistance due to capacity
(b) effective wattage
(c) effective voltage
(d) None of these

(a)

Q.16. Of the following about capacitive reactance which is correct?
(a) The reactance of the capacitor is directly proportional to its ability to store charge
(b) Capacitive reactance is inversely proportional to the frequency of the current
(c) Capacitive reactance is measured in farad
(d) The reactance of a capacitor in an A.C. circuit is similar to the resistance of a capacitor in a D.C. circuit

(b)

Q.17. Phase difference between voltage and current in a capacitor in an ac circuit is
(a) π
(b) π/2
(c) 0
(d) π/3

(b)

Q.18. A capacitor has capacitance C and reactance X, if capacitance and frequency become double, then reactance will be
(a) 4X
(b) X/2
(c) X/4
(d) 2X

(c)

Q.19. When an ac voltage of 220 V is applied to the capacitor C, then
(a) the maximum voltage between plates is 220 V.
(b) the current is in phase with the applied voltage.
(c) the charge on the plate is not in phase with the applied voltage.
(d) power delivered to the capacitor per cycle is zero.

(d)

Q.20. In LCR circuit if resistance increases quality factor
(a) increases finitely
(b) decreases finitely
(c) remains constant
(d) None of these

(b)

Q.21. An inductor, a resistor and a capacitor are joined in series with an AC source. As the frequency of the source is slightly increased from a very low value, the reactance of the
(a) inductor increases
(b) resistor increases
(c) capacitor increases
(d) circuit increases

(a)

Q.22. With increase in frequency of an A.C. supply, the impedance of an L-C-R series circuit
(a) remains constant
(b) increases
(c) decreases
(d) decreases at first, becomes minimum and then increases.

(d)

Q.23. If an LCR series circuit is connected to an ac source, then at resonance the voltage across
(a) R is zero
(b) R equals the applied voltage
(c) C is zero
(d) L equals the applied voltage

(b)

Q.24. In an L.C.R. series a.c. circuit, the current
(a) is always in phase with the voltage
(b) always lags the generator voltage
(c) always leads the generator voltage
(d) None of these

(d)

Q.25. An LCR series circuit, connected to a source E, is at resonance. Then the voltage across
(a) R is zero
(b) R equals applied voltage
(c) C is zero
(d) L equals applied voltage

(b)

Q.26. At resonance frequency the impedance in series LCR circuit is
(a) maximum
(b) minimum
(c) zero
(d) infinity

(b)

Q.27. At resonant frequency the current amplitude in series LCR circuit is
(a) maximum
(b) minimum
(c) zero
(d) infinity

(a)

Q.28. In tuning, we vary the capacitance of a capacitor in the tunning circuit such that the resonant frequency of the circuit becomes nearly equal to the frequency of the radio signal received. When this happens, the …A… with the frequency of the signal of the particular radio station in the circuit is maximum. Here A refers to
(a) resonant frequency
(b) impedance
(c) amplitude of the current
(d) reactance

(a) When this happens the amplitude of the current with the frequency of the signal of the particular radio station in the circuit is maximum.

Q.29. The power factor in a circuit connected to an A.C. The value of power factor is
(a) unity when the circuit contains an ideal inductance only
(b) unity when the circuit contains an ideal resistance only
(c) zero when the circuit contains an ideal resistance only
(d) unity when the circuit contains an ideal capacitance only

(b)

Q.30. Current in a circuit is wattless if
(a) inductance in the circuit is zero
(b) resistance in the circuit is zero
(c) current is alternating
(d) resistance and inductance both are zero

(b)

Q.31. Power factor is one for
(a) pure inductor
(b) pure capacitor
(c) pure resistor
(d) either an inductor or a capacitor.

(c)

Q.32. Power factor of the A. C. circuit varies between
(a) 0 to 0.5
(b) 0.5 to 1
(c) 0 to 1
(d) 1 to 2

(c)

Q.33. The graph between inductive reactance and frequency is
(a) parabola
(b) straight line
(c) hyperbola
(d) an arc of a circle

(b)

Q.34. For minimum dissipation of energy in the circuit the power factor should be
(a) large
(b) small
(c) moderate
(d) can not say

(a)

Q.35. The opposition offered by ohmic and non ohmic components is
(a) inductive reactance
(b) capacitive reactance
(c) impedance
(d) all of these

(c)

Q.36. The transformer voltage induced in the secondary coil of a transformer is mainly due to
(a) a varying electric field
(b) a varying magnetic field
(c) the vibrations of the primary coil
(d) the iron core of the transformer

(b)

Q.37. A transformer is employed to
(a) convert A.C. into D.C.
(b) convert D.C. into A.C.
(c) obtain a suitable A.C. voltage
(d) obtain a suitable D.C. voltage

(c)

Q.38. Transformers are used
(a) in DC circuit only
(b) in AC circuits only
(c) in both DC and AC circuits
(d) neither in DC nor in AC circuits

(b)

Q.39. The loss of energy in the form of heat in the iron core of a transformer is
(a) iron loss
(b) copper loss
(c) mechanical loss
(d) None of these

(a)

Q.40. A transformer is based on the principle of
(a) mutual induction
(b) self induction
(c) Ampere’s law
(d) X-ray crystallography

(a)

Q.41. Quantity that remains unchanged in a transformer is
(a) voltage
(b) current
(c) frequency
(d) None of these

(c) A transformer does not change the frequency of ac.

Q.42. Eddy currents in the core of transformer can’t be developed by
(a) increasing the number of turns in secondary coil
(b) taking laminated transformer
(c) making step down transformer
(d) using a weak a.c. at high potential

(b)

Q.43. The core of transformer is laminated to reduce
(a) flux leakage
(b) hysteresis
(c) copper loss
(d) eddy current

(d) The core of a transformer is laminated to reduce eddy current.

Q.44. The transformation ratio in the step-up transformer is
(a) one
(b) greater than one
(c) less than one
(d) the ratio greater or less than one depends on the other factor

(b)

Q.45. Which of the following statements is/are correct ?
I. In LCR series ac circuit, as the frequency of the source increases, the impedence of the circuit first decreases and then increases.
II. If the net reactance of an LCR series ac circuit is same as its resistance, then the current lags behind the voltage by 45°.
III. Below resonance, voltage leads the current while above it, current leads the voltage.
(a) I only (b) II only
(c) I and III (d) I and II

(d)

Q.46. The parallel combination of inductor and capacitor is called as
(a) rectifier circuit
(b) tank circuit
(c) acceptor circuit
(d) filter circuit

(b)

Q.47. Consider the following statements and then select the correct statements.
I. Most of the electrical device we use require AC voltage.
II. Most of the electrical energy sold by power companies is transmitted and distributed as alternating current.
III. AC voltage can be easily and efficiently converted from one to the other by means of transformers.
(a) I is correct, II and III are incorrect
(b) I III are correct, II is incorrect
(c) I II are correct, III is incorrect
(d) I, II and III are correct

(d) Most of the electrical devices we use require AC voltage. This is mainly because most of the electrical energy sold by power companies is transmitted and distributed as alternating current. The main reason for preferring use of AC voltage over DC voltage is that Ac voltage can be easily and efficiently converted from one voltage to the other by means of transformers.

Q.48. The correct variation of capacitive reactance of a capacitor with frequency is represented by

(d)

Q.49. The correct formula to determine the Q factor of series resonance circuit is

(d)

Q.50. An AC voltage source of variable angular frequency ω and fixed amplitude V0 is connected in series with a capacitance C and an electric bulb of resistance R (inductance zero). When ω is increased then,
(a) The bulb glows dimmer
(b) The bulb glows brighter
(c) Total impedance of the circuit is unchanged
(d) Total impedance of the circuit increases

(b)

Q.51. The dimensions of Inductance/Resistance is same as the dimension of
(a) time
(b) capacitance
(c) capacitive reactance
(d) inductive reactance

(a)

Q.52. A bulb connected in series with a solenoid is lit by an AC source. If a soft iron core is introduced in the solenoid then,
(a) the bulb will glow brighter
(b) the bulb will glow dimmer
(c) there will be no effect on the light produced by the bulb
(d) bulb may glow more brighter or dimmer

(b)The bulb will glow dimmer because when a soft iron core is introduced in the solenoid then, its inductive reactance (XL = ωL) will increase, as a result of which the current through the bulb decreases.

You may also check the MCQs on other chapters from the links provided below:

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