Alternating Current Class 12 Physics MCQs 2026-27 | RMS, LCR & Resonance

Alternating Current Class 12 Physics MCQs 2026–27

Chapter 7 MCQs with answers covering RMS and peak values, phase relations, R/L/C circuits, reactance, impedance, series LCR, resonance, AC power, power factor, wattless current, AC generator and transformer.

Class 12 PhysicsChapter 7CBSE 2026–2760 MCQsAnswers + Explanations

Quick answer: This page provides 60 CBSE-focused Alternating Current MCQs with answers. The current syllabus covers peak/RMS values, reactance and impedance, LCR series circuit (phasors only), resonance, AC power, power factor, wattless current, AC generator and transformer. Revise Chapter 7 Notes before attempting the test.

How these MCQs are designed: They combine direct recall, formula application, phase reasoning, numerical reasoning, competency-style interpretation and common misconceptions. Current CBSE competency-focused material also emphasizes analysis of LCR circuits, resonance, power factor and transformer applications. CBSE competency-focused Physics material · CBSE 2026–27 curriculum.

Q1. The RMS value of a sinusoidal current with peak value I0 is

A. I0/√2

B. I0

C. 2I0/π

D. I0/2

Answer: A
For a sine wave, Irms = I0/√2.

Q2. The algebraic average of a pure sinusoidal current over one complete cycle is

A. I0/√2

B. 2I0/π

C. 0

D. I0

Answer: C
Positive and negative halves cancel over a full cycle.

Q3. For i = I0 sin(ωt), the frequency is

A. ω/2π

B. 2πω

C. 1/ω

D. ωπ

Answer: A
Since ω = 2πf, f = ω/2π.

Q4. In a pure resistive AC circuit, voltage and current are

A. 90° out of phase

B. in phase

C. 180° out of phase

D. 45° out of phase

Answer: B
For an ideal resistor, φ = 0.

Q5. In a pure inductive circuit, current

A. leads voltage by 90°

B. lags voltage by 90°

C. is in phase

D. lags by 180°

Answer: B
Inductive current lags voltage by 90°.

Q6. In a pure capacitive circuit, current

A. leads voltage by 90°

B. lags voltage by 90°

C. is in phase

D. lags by 180°

Answer: A
Capacitive current leads voltage by 90°.

Q7. Inductive reactance is

A. ω/L

B. ωL

C. 1/ωL

D. L/ω

Answer: B
XL = ωL.

Q8. If frequency doubles, ideal inductive reactance

A. halves

B. doubles

C. is unchanged

D. becomes zero

Answer: B
XL is directly proportional to f.

Q9. If frequency doubles, ideal capacitive reactance

A. doubles

B. halves

C. is unchanged

D. becomes four times

Answer: B
XC is inversely proportional to f.

Q10. The impedance of a series LCR circuit is

A. R+XL+XC

B. √[R²+(XL−XC)²]

C. XL−XC

D. R²+XL²+XC²

Answer: B
Reactive effects combine phasorially.

Q11. At series resonance

A. XL > XC

B. XL < XC

C. XL = XC

D. R = 0

Answer: C
Resonance requires cancellation of inductive and capacitive reactance.

Q12. At series resonance, impedance is

A. maximum

B. minimum and equal to R

C. zero

D. equal to XL+XC

Answer: B
Z = R at resonance.

Q13. At series resonance, power factor is

A. 0

B. 0.5

C. 1

D. infinite

Answer: C
φ = 0, so cosφ = 1.

Q14. For a fixed source voltage, current in a series LCR circuit is maximum at

A. very low frequency only

B. resonance

C. very high frequency only

D. all frequencies

Answer: B
Current is V/Z and Z is minimum at resonance.

Q15. Resonant angular frequency is

A. √LC

B. 1/√LC

C. 2π√LC

D. 1/2πLC

Answer: B
ω0 = 1/√LC.

Q16. Resonant frequency is

A. 1/(2π√LC)

B. 2π√LC

C. 1/LC

D. √LC/2π

Answer: A
f0 = 1/(2π√LC).

Q17. If XL > XC in a series LCR circuit, current

A. leads voltage

B. lags voltage

C. is in phase

D. is zero

Answer: B
The circuit is net inductive.

Q18. If XC > XL, the circuit is

A. inductive

B. capacitive

C. purely resistive

D. at resonance

Answer: B
Net reactance is capacitive.

Q19. For a series LCR circuit, power factor is

A. Z/R

B. R/Z

C. XL/XC

D. RZ

Answer: B
cosφ = R/Z.

Q20. Average power in an AC circuit is

A. VrmsIrms

B. VrmsIrms sinφ

C. VrmsIrms cosφ

D. V0I0

Answer: C
Pavg = Vrms Irms cosφ.

Q21. Average power consumed by an ideal pure inductor is

A. maximum

B. zero

C. VrmsIrms

D. I²R

Answer: B
The phase difference is 90°.

Q22. Wattless current is associated with

A. resistive component

B. reactive component

C. only DC

D. zero current

Answer: B
Iw = Irms sinφ.

Q23. For an ideal pure resistor, power factor is

A. 0

B. 0.5

C. 1

D. −1

Answer: C
Voltage and current are in phase.

Q24. The peak voltage of a 220 V RMS sinusoidal supply is approximately

A. 110 V

B. 220 V

C. 311 V

D. 440 V

Answer: C
V0 = √2 Vrms ≈ 311 V.

Q25. A 50 Hz AC source has time period

A. 0.5 s

B. 0.02 s

C. 50 s

D. 100 s

Answer: B
T = 1/f.

Q26. For i = 10 sin(100πt) A, the RMS current is approximately

A. 5 A

B. 7.07 A

C. 10 A

D. 14.14 A

Answer: B
10/√2 = 7.07 A.

Q27. For v = 311 sin(100πt) V, the RMS voltage is

A. 110 V

B. 220 V

C. 311 V

D. 440 V

Answer: B
311/√2 ≈ 220 V.

Q28. For v = 100 sin(200πt) V, frequency is

A. 50 Hz

B. 100 Hz

C. 200 Hz

D. 400 Hz

Answer: B
f = 200π/(2π) = 100 Hz.

Q29. If L = 0.2 H and f = 50 Hz, XL is approximately

A. 6.28 Ω

B. 31.4 Ω

C. 62.8 Ω

D. 314 Ω

Answer: C
XL = 2π(50)(0.2) ≈ 62.8 Ω.

Q30. If C increases while frequency is fixed, XC

A. increases

B. decreases

C. is unchanged

D. becomes infinite

Answer: B
XC = 1/(ωC).

Q31. At resonance, VL and VC can be

A. necessarily zero

B. equal in magnitude and opposite in phase

C. unequal and in phase

D. absent

Answer: B
Their net phasor contribution cancels.

Q32. The resonance curve of a series LCR circuit represents variation of

A. R with f

B. current with frequency

C. voltage with resistance only

D. capacitance with time

Answer: B
Current peaks at resonance.

Q33. For a net inductive series LCR circuit, phase angle φ is conventionally

A. positive

B. negative

C. always zero

D. undefined

Answer: A
tanφ = (XL−XC)/R > 0.

Q34. For a net capacitive series LCR circuit, current

A. lags voltage

B. leads voltage

C. is zero

D. is always in phase

Answer: B
XC > XL gives capacitive behaviour.

Q35. If R is doubled while XL and XC stay fixed, impedance

A. must halve

B. generally increases

C. must become zero

D. is unchanged in every case

Answer: B
Z = √[R²+(XL−XC)²].

Q36. At resonance, the reactive component of impedance is

A. R

B. zero

C. 2R

D. infinite

Answer: B
XL−XC = 0.

Q37. An AC generator converts

A. electrical to mechanical energy

B. mechanical to electrical energy

C. heat to light

D. DC to AC without motion

Answer: B
Mechanical rotation produces electrical energy.

Q38. An AC generator uses

A. split rings

B. slip rings

C. a capacitor only

D. a transformer core only

Answer: B
Slip rings provide alternating output.

Q39. Peak EMF of an N-turn rotating coil is

A. NBAω

B. NB/Aω

C. Nω/BA

D. BA/Nω

Answer: A
E0 = NBAω.

Q40. If angular speed of an ideal generator coil doubles, peak EMF

A. halves

B. doubles

C. is unchanged

D. becomes zero

Answer: B
E0 is proportional to ω.

Q41. A transformer works on the principle of

A. electrostatic shielding

B. mutual induction

C. photoelectric effect

D. Joule heating

Answer: B
Changing primary flux induces secondary EMF.

Q42. For an ideal transformer,

A. Vs/Vp = Np/Ns

B. Vs/Vp = Ns/Np

C. Vs/Vp = Ip/Is only

D. VpVs = NpNs

Answer: B
Voltage ratio equals turns ratio.

Q43. A transformer with Ns > Np is

A. step-down

B. step-up

C. an isolation-only device

D. a DC converter

Answer: B
Secondary voltage is higher ideally.

Q44. For an ideal transformer, if voltage is stepped up, secondary current

A. increases proportionally

B. decreases correspondingly

C. is unchanged

D. becomes zero

Answer: B
Power is conserved ideally.

Q45. A transformer cannot normally operate on steady DC because

A. DC has too much voltage

B. there is no continuous changing magnetic flux

C. DC has no current

D. resistance becomes zero

Answer: B
Transformer action needs changing flux.

Q46. Laminating a transformer core mainly reduces

A. copper loss

B. eddy-current loss

C. flux leakage

D. resistance of wire

Answer: B
Thin insulated laminations restrict eddy currents.

Q47. For an ideal transformer, input power is

A. always greater than output

B. equal to output

C. always zero

D. independent of voltage and current

Answer: B
VpIp = VsIs.

Q48. If power factor is 0.8, the magnitude of sinφ is

A. 0.2

B. 0.6

C. 0.8

D. 1.8

Answer: B
sinφ = √(1−0.8²) = 0.6.

Q49. If Vrms = 100 V, Irms = 4 A and cosφ = 0.5, average power is

A. 50 W

B. 100 W

C. 200 W

D. 400 W

Answer: C
P = 100×4×0.5 = 200 W.

Q50. A 200 V RMS source supplies 3 A at power factor 1. Average power is

A. 200 W

B. 300 W

C. 600 W

D. 0 W

Answer: C
P = 200×3 = 600 W.

Q51. A circuit has R = 6 Ω and net reactance 8 Ω. Its impedance is

A. 8 Ω

B. 10 Ω

C. 14 Ω

D. 48 Ω

Answer: B
Z = √(36+64) = 10 Ω.

Q52. For the circuit in Q51, power factor is

A. 0.4

B. 0.6

C. 0.8

D. 1

Answer: B
cosφ = R/Z = 6/10 = 0.6.

Q53. If XL = XC in a series LCR circuit, the circuit behaves as

A. purely inductive

B. purely capacitive

C. purely resistive at the terminals

D. open circuit

Answer: C
Net reactance is zero, so impedance is R.

Q54. Which quantity has the same unit as resistance?

A. Reactance

B. Frequency

C. Power factor

D. Phase angle

Answer: A
Reactance and resistance are both measured in ohms.

Q55. Which statement about RMS value is correct?

A. It is always equal to peak value

B. It represents equivalent DC heating effect for a resistor

C. It is the full-cycle algebraic average

D. It is always zero for AC

Answer: B
That is the effective-value definition.

Q56. In a pure capacitor, increasing frequency makes current amplitude for fixed voltage

A. decrease

B. increase

C. unchanged

D. zero

Answer: B
I0 = V0/XC and XC decreases with frequency.

Q57. In a pure inductor, increasing frequency for fixed voltage makes current amplitude

A. increase

B. decrease

C. unchanged

D. infinite at every frequency

Answer: B
I0 = V0/XL and XL increases.

Q58. A step-down transformer ideally has

A. Ns > Np

B. Ns < Np

C. Ns = 0

D. Np = 0

Answer: B
Lower secondary voltage requires fewer secondary turns.

Q59. Which is an appropriate CBSE 2026–27 boundary for the LCR series circuit?

A. Advanced complex-number phasors required

B. Phasors only

C. LC oscillations only

D. No phasor concept

Answer: B
The official syllabus specifies LCR series circuit (phasors only).

Q60. Which combination belongs to the current CBSE Chapter 7 scope?

A. RMS, reactance, LCR phasors, resonance, power factor, generator and transformer

B. Only DC circuits and Kirchhoff laws

C. Ray optics and interference

D. Semiconductors only

Answer: A
These are explicitly listed in the current Chapter 7 scope.

Common MCQ Traps

RMS ≠ average: full-cycle average of a sine wave is zero, but RMS is I0/√2 or V0/√2.

Lead/lag: pure L → I lags V; pure C → I leads V.

Reactance: XL rises with frequency; XC falls.

Resonance: XL = XC, Z = R, current maximum and power factor unity.

Transformer: voltage ratio follows turns ratio; ideal current ratio is inverse.

Generator: AC generator uses slip rings; transformer uses mutual induction.

How to Use These MCQs

Attempt all 60 without looking at answers. Mark every uncertain question, then revisit the relevant concept in the Chapter 7 Notes and practise the Chapter 7 Important Questions. Do not treat your score as a prediction of board marks.

Frequently Asked Questions

What are the important MCQ topics in Alternating Current Class 12?

RMS and peak values, phase relations, reactance, impedance, series LCR, resonance, AC power, power factor, wattless current, AC generator and transformer.

What is the resonance condition in a series LCR circuit?

XL = XC, giving minimum impedance Z = R and maximum current for a fixed source voltage.

What is the RMS value of a sinusoidal current?

Irms = I0/√2.

Why does a transformer require AC?

Normal transformer action requires continuously changing magnetic flux; steady DC cannot maintain that changing flux after the initial transient.

Syllabus discipline: The current CBSE 2026–27 syllabus specifies the LCR series circuit as phasors only. These MCQs avoid requiring advanced phasor mathematics. LC oscillations are not treated as Chapter 7 core content here.