Class 12 Physics Chapter 20 – Alternating Current MCQs | FBISE | Grade 12 Physics - MCQs

Description: Practice Class 12 Physics Chapter 20 Alternating Current MCQs according to the FBISE HSSC-II assessment framework. Questions cover alternating current, rms and peak values, phase relationships, capacitive and inductive reactance, impedance, RLC resonance, AC power, transformers and electrical power transmission.

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1. Alternating current is a current whose magnitude and direction:

A) remain constant
B) change periodically
C) are always zero
D) change only once

2. The SI unit of alternating current is:

A) ampere
B) volt
C) ohm
D) hertz

3. The time taken by an AC quantity to complete one cycle is called its:

A) amplitude
B) frequency
C) period
D) phase

4. Frequency is the number of complete cycles occurring per:

A) hour
B) minute
C) degree
D) second

5. The relation between frequency f and period T is:

A) $f=T$
B) $f=\frac{1}{T}$
C) $f=T^2$
D) $f=2T$

6. The angular frequency of an AC quantity is:

A) $\omega=2\pi f$
B) $\omega=f/2\pi$
C) $\omega=2f$
D) $\omega=\pi/f$

7. The maximum value of an alternating current is called its:

A) rms value
B) average value
C) peak value
D) mean value

8. For a sinusoidal current, the instantaneous current may be written as:

A) $I=I_0\cos(\omega t)$ only
B) $I=I_0+\omega t$
C) $I=I_0/\omega t$
D) $I=I_0\sin(\omega t)$

9. The root-mean-square value of a sinusoidal current is:

A) $I_{\rm rms}=\frac{I_0}{\sqrt2}$
B) $I_{\rm rms}=I_0\sqrt2$
C) $I_{\rm rms}=2I_0$
D) $I_{\rm rms}=I_0/2$

10. If the peak current is 10 A, its rms value is approximately:

A) 5 A
B) 14.14 A
C) 7.07 A
D) 20 A

11. The rms value of AC is useful because it gives the equivalent:

A) frequency of DC
B) heating effect of DC
C) phase of DC
D) charge of DC

12. The average value of a pure sinusoidal AC current over one complete cycle is:

A) maximum
B) half its peak value
C) infinite
D) zero

13. In a purely resistive AC circuit, current and voltage are:

A) in phase
B) 180° out of phase
C) 90° out of phase
D) unrelated

14. For a pure resistor, Ohm’s law for AC using rms quantities is:

A) $V=IR^2$
B) $V=R/I$
C) $V_{\rm rms}=I_{\rm rms}R$
D) $V=I+R$

15. The electrical power dissipated in a pure resistor supplied by AC is:

A) $P=V_{\rm rms}/I_{\rm rms}$
B) $P=V_{\rm rms}I_{\rm rms}$
C) $P=V_{\rm rms}+I_{\rm rms}$
D) $P=I_{\rm rms}/R$

16. A capacitor in an AC circuit offers opposition to current called:

A) resistance only
B) conductance
C) inductance
D) capacitive reactance

17. Capacitive reactance is given by:

A) $X_C=\frac{1}{\omega C}$
B) $X_C=\omega C$
C) $X_C=\omega/C$
D) $X_C=C/\omega$

18. As AC frequency increases, capacitive reactance:

A) increases
B) remains constant
C) decreases
D) becomes infinite

19. In a pure capacitive AC circuit, current leads voltage by:

A) 0°
B) 90°
C) 180°
D) 45°

20. An inductor in an AC circuit offers opposition called:

A) inductive reactance
B) capacitive reactance
C) resistance only
D) electric potential

21. Inductive reactance is given by:

A) $X_L=1/\omega L$
B) $X_L=L/\omega$
C) $X_L=LC$
D) $X_L=\omega L$

22. As AC frequency increases, inductive reactance:

A) decreases
B) increases
C) remains zero
D) becomes independent of L

23. In a pure inductive AC circuit, current:

A) leads voltage by 90°
B) is in phase with voltage
C) lags voltage by 90°
D) is always zero

24. The SI unit of inductance is:

A) henry
B) farad
C) tesla
D) weber

25. The SI unit of capacitance is:

A) henry
B) ohm
C) tesla
D) farad

26. The opposition offered by a circuit to AC is generally called:

A) conductance
B) impedance
C) power
D) charge

27. The SI unit of impedance is:

A) ampere
B) farad
C) ohm
D) henry

28. In a series RLC circuit, impedance is:

A) $Z=\sqrt{R^2+(X_L-X_C)^2}$
B) $Z=R+X_L+X_C$
C) $Z=R^2+X_L^2-X_C^2$
D) $Z=X_L/X_C$

29. At resonance in a series RLC circuit:

A) $X_L>X_C$
B) $X_C>X_L$
C) $X_L+X_C=0$
D) $X_L=X_C$

30. The resonant angular frequency of an ideal series RLC circuit is:

A) $\omega_0=LC$
B) $\omega_0=\frac{1}{\sqrt{LC}}$
C) $\omega_0=\sqrt{LC}$
D) $\omega_0=L/C$

31. At resonance, the impedance of a series RLC circuit is:

A) maximum
B) infinite
C) minimum
D) zero for every R

32. At resonance in a series RLC circuit, current is:

A) maximum
B) minimum
C) zero
D) independent of voltage

33. Power factor of a purely resistive AC circuit is:

A) 0
B) 0.5
C) infinite
D) 1

34. The average power in an AC circuit can be written as:

A) $P=V_{\rm rms}I_{\rm rms}\sin\phi$
B) $P=V_{\rm rms}I_{\rm rms}\cos\phi$
C) $P=V_{\rm rms}/I_{\rm rms}$
D) $P=I_{\rm rms}^2/X_L$

35. In the power formula, φ represents the phase difference between:

A) two resistors
B) two currents
C) voltage and current
D) two voltages

36. In a purely inductive circuit, the average power consumed is ideally:

A) zero
B) maximum
C) equal to VI
D) infinite

37. In a purely capacitive circuit, the average power consumed is ideally:

A) maximum
B) equal to VI
C) one-half VI
D) zero

38. A transformer works with:

A) steady DC only
B) alternating current
C) static charge only
D) mechanical waves

39. A transformer transfers electrical energy between two coils through:

A) direct electrical contact
B) chemical reaction
C) electromagnetic induction
D) electrostatic induction only

40. A step-up transformer has a secondary voltage that is:

A) greater than the primary voltage
B) less than the primary voltage
C) always zero
D) equal to the resistance

41. For an ideal transformer, the voltage ratio is related to turns ratio by:

A) $\frac{V_s}{V_p}=\frac{N_p}{N_s}$
B) $V_sV_p=N_sN_p$
C) $\frac{V_s}{V_p}=N_pN_s$
D) $\frac{V_s}{V_p}=\frac{N_s}{N_p}$

42. For an ideal transformer, input power is approximately:

A) greater than output power
B) equal to output power
C) zero
D) independent of output

43. A step-down transformer is commonly used to:

A) increase voltage for transmission
B) increase frequency
C) reduce voltage for suitable loads
D) convert AC directly into DC

44. High-voltage transmission of electrical power helps reduce:

A) current and resistive power loss
B) frequency
C) transformer turns
D) electrical energy

45. The power loss in a transmission line of resistance R is proportional to:

A) $I$
B) $1/I^2$
C) $I/R$
D) $I^2R$

46. If the current in a transmission line is doubled, its resistive power loss becomes:

A) double
B) four times
C) half
D) unchanged

47. A generator converts mechanical energy into electrical energy by:

A) electrolysis
B) electrostatic charging
C) electromagnetic induction
D) nuclear fission

48. The frequency of AC generated by an alternator depends on the rotational speed and the number of:

A) magnetic poles
B) resistors
C) capacitors
D) batteries

49. A sinusoidal AC waveform has a positive half-cycle followed by a:

A) second positive half-cycle
B) constant value
C) zero-frequency interval
D) negative half-cycle

50. The phase difference between voltage and current in a pure resistor is:

A) 90°
B) 0°
C) 180°
D) 270°

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Syllabus & SLO Alignment

FBISE syllabus: National Curriculum of Pakistan 2022–23, FBISE Assessment Framework for Physics Grade XII.

Content area: Domain E – Electricity and Magnetism

Relevant SLOs: P-12-E-14 to P-12-E-26

Alignment: Alternating current; rms/peak values and phase; rectification and smoothing; mutual/self-inductance; chokes; capacitive/inductive reactance and impedance.

Assessment note: The FBISE framework distinguishes summative SLOs used in the final examination from formative SLOs used in teaching-learning. MCQs on this page should be treated as practice aligned to the listed content/SLOs, not as a reproduction of the official FBISE paper.

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