HSSC-II (Class-12) Solved Physics MCQs; Unit-16: Alternating current (Baluchistan Textbook Board, Quetta) With Verified Answers: 


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 the source equals the impedance of the 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 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 {ε_0μ_0}}`
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20. Electromagnetic waves do not transport




... Answer is B. Charges
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21. Which one of the following waves has the shortest wavelength?




... Answer is D. Æ”-rays wave
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22. A process in which an audio signal superimposed on a radio frequency wave is called




... Answer is C. Modulation
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