12th class Physics MCQs Preparation 2022 and Onwards (Unit: Alternating Current: Set-1 (30 MCQs)) for National MDCAT, ECAT, FPSC, PPSC, JOB Exam, Physics Lecturer, SSC, HSSC (F.Sc.), BS, MS Exams NTS, ETS, etc..



1. The mean value of current over one complete cycle of A.C. is




... Answer is A. Zero
One cycle of alternating current or voltage consists of half positive cycle and half negative cycle. therefore the average (mean) value of the current or voltage over one cycle is zero.


2. Root mean square value of alternating current is equal to




... Answer is D. 70.7% of l₀
`\I_{rms}`=0.707 l₀ = 70.7% of l₀


3. When the initial phase of A.C. is `\frac {Ï€}{2}` then it will complete its one cycle at the phase of




... Answer is D. `\frac {5Ï€}{2}`
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4. Average power dissipation in an A.C. circuit across the resistor is




... Answer is C. `\V_{rms}I_{rms}`
The average power dissipation in an A.C. circuit across the resistor is equal to the product of r.m.s. value of voltage and current. < p > = `\V_{rms}I_{rms}`


5.  When a capacitor is connected across the A.C. source then




... Answer is C. I leading by V
The current is leading the voltage by 90°.


6. When the frequency of A.C. is increased then the reactance of the inductor will




... Answer is C. Increased
`\X_L` = 2 π f L (directly proportional)


7. The power dissipation in LC circuit is :




... Answer is D. Zero
The power dissipation in an inductive AC circuit over a complete cycle is zero.


8. A capacitor is an A.C. circuit is working at




... Answer is C. High frequency
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9. The unit of impedance is




... Answer is D. Ohm
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10. According to the phasor diagram, the phase difference between `\V_L` and `\V_C` is




... Answer is C. π
The phase difference between `\V_L` and `\V_C` is 180°


11. Power factor is equal to




... Answer is C. cos φ
The power factor is the ratio of true power to apparent power.


12. In a choke and a resistor series circuit, the A.C. source delivers its maximum power when




... Answer is C. R = Z
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13. The A.C. source transfers its maximum power to load when

... Answer is D. Impedance of source equals to impedance of load
`\Z_L` = `\Z_G`


14.  In an RLC-series circuit the amplitude of the current has its maximum value when




... Answer is D. `\X_L` = `\X_C`
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15. In an RCL-parallel circuit, the resonance phenomenon will be observed when the net current in the circuit is




... Answer is D. Zero
`\i_L` - `\i_C` = 0


16.. The resonance frequency is




... Answer is C. f = `\frac {1}{2Ï€ sqrt {LC}}`
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17. The propagation of electromagnetic waves was predicted by :




... Answer is B. Maxwell
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18. The phase difference between an electric and magnetic lines of forces in electromagnetic waves is




... Answer is B. 90°
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19. The speed of electromagnetic waves in free space is given by the equation




... Answer is B. c = `\frac {1}{sqrt {ε₀μ₀}}`
----------------


20. Electromagnetic waves do not transport




... Answer is B. Charges
----------------


21. Which one of the following waves has the shortest wavelength?




... Answer is D. Æ”-rays wave
--------------------------


22. A process in which an audio signal superimposed on a radio frequency wave is called




... Answer is C. Modulation
----------------------


23. The power factor of a purely resistive circuit is




... Answer is B. unity
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24. A capacitor is perfectly insulator for




... Answer is A. Direct current
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25.  The peak value of alternating current is 5√2 A. The mean square value of the current will be




... Answer is D. 5² A
= `\frac {1}{2}`i₀²


26. In the choke coil the reactance `\X_L` and resistance R are related as




... Answer is C. `\X_L` ≫ R
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27. The phase difference between the current and voltage at resonance is :




... Answer is A. 0
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28. In the AC system we generate sine waveform because




... Answer is B. It produces least disturbance in electrical circuits
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29. An alternating voltage is given by 20sin 157t. The frequency of alternating voltage is




... Answer is B. 25 Hz
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30. In LR current which one of the following statements is correct?




... Answer is D. The inductive reactance decreases with frequency.
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