Alternating Current

Smart Concept Alternating Current

  1. Ohm’s law expressed as E = IR.
    (a) may never be applied to a.c. circuits
    (b) always applied to a.c. circuits as to d.c. circuits when Z is substituted for R.
    (c) tells us that Eeff = 0.707 E0 for a.c.
    (d) None of these.
  2. The peak value of an ac supply is 440V, hen its rms value (a) 31.11 V      (b) 41.11 V             (c) 311.1V                (d) 411.1 V
  3. A Phasor is a (a) Scalar Quantity  (b) Vector Quantity (c) Tensor Quantity (d)    (d) None
  4. In the case of an Inductor (a) Voltage lags the current by 90 (b) Voltage leads the Current by 90 (c) Voltage leads the current by 60 (d) Voltage leads the current by 45
  5. A 100 ohm resistor is connected to a 220V, 50Hz ac supply. The rms value of current in the circuit is (a) 1.56 A    (b) 1.56 mA              (c) 2.2 A                (d) 2.2 mA
  6. The Mean value of emf for a complete cycle (a) 0.637 E0       (b) Zero                   (c) 0.707 E0     (d) 0.5 E0
  7. The rms value of emf for a complete cycle

0.637 E0                          (c) 0.707 E0

Zero                        (d) 0.5 E0

8. The Mean peak of emf for a complete cycle

0.637 E0                          (c) 0.707 E0

Zero                        (d) E0

9. The frequency of a.c. supplied to our houses in India is ……………

50Hz                   (c) 100Hz  

60Hz                   (d) 120 Hz

10. In an a.c. circuit, the instantaneous values of current and e.m.f. are given by: I = I0 sin(ωt +π/2) and E = E0 sin ωt. The power P consumed in the circuit is:
(a) E0 I0 √2
(b) EI √2
(c) Zero
(d) E0 I0/ 2

11. An alternating voltage is connected in series with a resistance R and an inductance L. If the potential drop across the resistance is 200 V and across the inductance is 150 V, then the applied voltage is:
(a) 350 V
(b) 500 V
(c) 250 V
(d) 300 V

12. For LCR series circuit, the current will be maximum when:
(a) XL = 0
(b) XC = O
(c) XL = XC
(d) R = XL + XC.

13. In an a.c. circuit, the current
(a) always leads the voltage,
(b) always lags behind the voltage
(c) is always in phase with the voltage
(d) may lead or lag behind or be in the same phase with the voltage.

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

15. Of the following about the capacitive reactance which is correct:
(a) capacitive reactance is inversely proportional to the frequency of a.c.
(b) capacitive reactance is measured in Farad.
(c) The reactance of the capacitor is directly proportional to its ability to store charge.
(d) None of these.

16. The Inductive reactance of an dc source

100ohm             (c) 50 ohm

Zero                   (d) 60 ohm

17. In a purely capacitive circuit if the frequency of ac source is doubled, then its capacitive reactance will be

Remains same

Halved

Doubled

Zero

18. figure shown a series LCR circuit connected to a variable frequency 230V source

The source frequency that derive the circuit in resonance is

  • 4 Hz                       (c) 6 Hz
  • 5 Hz                       (d) 8 Hz

19. The power factor of the given circuit

0.2                          (c) 0.8

0.4                          (d) 0.6

20. With increase in frequency of applied a.c. supply, the impedance of LCR series circuit:
(a) increases
(b) decreases
(c) first decreases, becomes minimum and then increases.
(d) remains constant.

21. When 100 V d.c. applied across a solenoid a current of 1.0 A flows in it. When 100 V a.c. supply is applied across the same coil, the current drops to 0.5 A. If the frequency of a.c. source is 50 Hz, the impedance and inductance of the solenoid are:
(a) 200 Ω and 0.55 H
(b) 200 Ω and 1.0 H
(c) 100 Ω and 0.93 H
(d) 100 Ω and 0.80 H

22. Assertion: An ac does not show magnetic effect

Reason: Alternating current does not vary with time.

Both the assertion & Reason are true and reason is the correct explanation of assertion.

Both the assertion & Reason are true and reason is not the correct explanation of assertion.

Assertion is true but reason is false

Both Assertion & reason are false.

23. Assertion: Average value of ac over a complete cycle is always zero.

Reason: Average value of ac is always defined over half cycl.

  • Both the assertion & Reason are true and reason is the correct explanation of assertion.
  • Both the assertion & Reason are true and reason is not the correct explanation of assertion.
  • Assertion is true but reason is false
  • Both Assertion & reason are false.

24. Assertion: The only element that dissipates energy in an ac circuit is the resistance element

Reason: There is no power losses associated with pure inductive & pure capacitive circuit.

  • Both the assertion & Reason are true and reason is the correct explanation of assertion.
  • Both the assertion & Reason are true and reason is not the correct explanation of assertion.
  • Assertion is true but reason is false
  • Both Assertion & reason are false.

25. Assertion: Resonance is exhibited by a ckt only if both L & C are present in the ckt.

Reason: Only then the voltage across L & C cancel each other, both being out of Phase.

  • Both the assertion & Reason are true and reason is the correct explanation of assertion.
  • Both the assertion & Reason are true and reason is not the correct explanation of assertion.
  • Assertion is true but reason is false
  • Both Assertion & reason are false.

Questions with answers

  1. Why core of a transformer is laminated ?
  2. In a series L–C–R circuit, voltages across inductor, capacitor, and resistor are VL, VC and VR respectively. What is the phase difference between (i) VL and VR (ii) VL and VC?
  3. Why can’t transformer be used to step up or step down dc voltage?
  4. . In an a.c. circuit, instantaneous voltage and current are V = 200 sin 300 t volt and i = 8 cos 300t ampere respectively. What is the average power dissipated in the circuit?
  5. In a circuit instantaneously voltage and current are V = 150 sin 314t volt and i = 12 cos 314t ampere respectively. Is the nature of circuit is capacitive or inductive ?
  6. Define RMS Value of Current.
  7. Can a capacitor of suitable capacitance replace an inductor in an AC circuit?
  8. . Why is it necessary for voltmeter to have a higher resistance?
  9. Can d.c. ammeter use for measurement of alternating current?
  10. A 1.5 µF capacitor is charged to 57V. The charging battery is then disconnected, and a 12 mH coil is connected in series with the capacitor so that LC Oscillations occur. What is the maximum current in the coil? Assume that the circuit has no resistance.
  11. The self inductance of the motor of an electric fan is 10H. What should be the capacitance of the capacitor to which it should be connected in order to impart maximum power at 50Hz?
  12. Show that the potential difference across the LC combination is zero at the resonating frequency in series LCR circuit
  1. Ans. To reduce loss due to eddy currents.
  2. 90 and 180
  3. In steady current no induction phenomenon will take place.
  4. 90 and 0
  5. Current is ahead the voltage by a phase difference of 90 degree. Hence circuit is a capacitive circuit.
  6. . RMS value of ac is defined as that value of direct current which produces the same heating effect in a given resistor as is produced by the given alternating current when passed for the same time
  7. Yes, because average power consumed in both is least while controlling an AC
  8. Since voltmeter is to be connected across two points in parallel, if it has low resistance, a part of current will pass through it which will decrease actual potential difference to be measured.
  9. No, it is based on the principle of torque. When ac is passing through it (of freq. 50 Hz). It will not respond to frequent change in direction due to inertia hence would show zero deflection.
  10. Easy Solution
  11. Self
  12. Apply Condition of resonance