CBSE Class 12 Physics Chapter 7 Alternating Current MCQs

CBSE Class 12 Physics Chapter 7 MCQs: CBSE Class 12 Physics Chapter 7 Alternating Current is an very important topic for class 12 board exams and also for entrance tests like NEET and JEE. Students often look for practice materials like alternating current class 12 mcq and short questions to test their understanding. These chapter 7 physics class 12 mcq are designed in simple style so that every learner can revise easily and avoid common in exam. Questions like ac mcq class 12 or mcq of alternating current class 12 help in clearing basic concepts like RMS value, reactance, impedance, resonance and power factor.

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CBSE Class 12 Physics Chapter 7 MCQs

Q1. The RMS value of alternating current is given by:

a) I₀/√2

b) I₀/2

c) I₀√2

d) 2I₀

Answer: a) I₀/√2

Explanation: RMS (Root Mean Square) value is the effective value of AC current, calculated as peak current divided by √2.

Q2. In a purely resistive AC circuit, the phase difference between voltage and current is:

a) 90°

b) 45°

c) 0°

d) 180°

Answer: c) 0°

Explanation: In resistive circuits, voltage and current are in phase, meaning no phase difference exists.

Q3. The reactance of a capacitor in AC circuit is given by:

a) ωL

b) 1/ωC

c) ωC

d) 1/ωL

Answer: b) 1/ωC

Explanation: Capacitive reactance Xc = 1/ωC, where ω is angular frequency and C is capacitance.

Q4. In an AC circuit with inductor, current lags voltage by:

a) 0°

b) 45°

c) 90°

d) 180°

Answer: c) 90°

Explanation: In purely inductive circuits, current always lags behind voltage by 90°.

Q5. The power factor of a purely capacitive circuit is:

a) 1

b) 0

c) 0.5

d) √3/2

Answer: b) 0

Explanation: Power factor = cos φ. For capacitive circuit, φ = 90°, so cos 90° = 0.

Q6. In RLC series circuit, resonance occurs when:

a) XL = XC

b) XL > XC

c) XL < XC

d) R = 0

Answer: a) XL = XC

Explanation: At resonance, inductive reactance equals capacitive reactance, making impedance minimum.

Q7. The frequency of AC mains in India is:

a) 50 Hz

b) 60 Hz

c) 25 Hz

d) 100 Hz

Answer: a) 50 Hz

Explanation: Indian power supply operates at standard frequency of 50 Hz.

Q8. The impedance of RLC series circuit is:

a) R + XL + XC

b) √(R² + (XL - XC)²)

c) R² + (XL - XC)²

d) R(XL - XC)

Answer: b) √(R² + (XL - XC)²)

Explanation: Impedance Z is calculated using vector addition of resistance and net reactance.

Q9. The unit of reactance is:

a) Henry

b) Farad

c) Ohm

d) Watt

Answer: c) Ohm

Explanation: Reactance has same unit as resistance, which is Ohm (Ω).

Q10. In AC circuit, the average power consumed by pure inductor is:

a) I²XL

b) V²/XL

c) Zero

d) VIcos φ

Answer: c) Zero

Explanation: Pure inductor consumes no power as current and voltage are 90° out of phase.

Q11. The inductive reactance increases with:

a) Decreasing frequency

b) Increasing frequency

c) Constant frequency

d) Zero frequency

Answer: b) Increasing frequency

Explanation: XL = ωL = 2πfL, so reactance increases with frequency.

Q12. The quality factor (Q) of RLC circuit is:

a) R/ωL

b) ωL/R

c) ωC/R

d) R/ωC

Answer: b) ωL/R

Explanation: Quality factor Q = ωL/R = (1/R)√(L/C) at resonance.

Q13. In capacitive circuit, current leads voltage by:

a) 0°

b) 45°

c) 90°

d) 180°

Answer: c) 90°

Explanation: In purely capacitive circuits, current leads voltage by 90°.

Q14. The peak value of AC voltage is related to RMS value as:

a) V₀ = VRMS

b) V₀ = √2 VRMS

c) V₀ = VRMS/√2

d) V₀ = 2VRMS

Answer: b) V₀ = √2 VRMS

Explanation: Peak value is √2 times the RMS value in sinusoidal AC.

Q15. The resonant frequency of RLC circuit is:

a) 1/2π√LC

b) 2π√LC

c) 1/√LC

d) √LC/2π

Answer: a) 1/2π√LC

Explanation: Resonant frequency f₀ = 1/2π√LC, derived from ωL = 1/ωC condition.

Q16. Transformers work on the principle of:

a) Self induction

b) Mutual induction

c) Electromagnetic induction

d) All of these

Answer: b) Mutual induction

Explanation: Transformers operate using mutual induction between primary and secondary coils.

Q17. In step-up transformer:

a) Ns > Np

b) Ns < Np

c) Ns = Np

d) Cannot determine

Answer: a) Ns > Np

Explanation: Step-up transformer has more turns in secondary (Ns) than primary (Np).

Q18. The efficiency of an ideal transformer is:

a) 50%

b) 75%

c) 90%

d) 100%

Answer: d) 100%

Explanation: Ideal transformer has no losses, hence 100% efficiency.

Q19. AC can be stepped up or down using transformer because:

a) It has varying magnitude

b) It changes direction

c) Both a and b

d) None

Answer: a) It has varying magnitude

Explanation: Changing magnetic flux due to varying AC enables transformer action.

Q20. The power loss in transformer is mainly due to:

a) Eddy currents

b) Hysteresis

c) Copper loss

d) All of these

Answer: d) All of these

Explanation: Real transformers have losses from eddy currents, hysteresis, and resistance.

Q21. In parallel RLC circuit at resonance:

a) Current is minimum

b) Current is maximum

c) Impedance is minimum

d) Power factor is zero

Answer: b) Current is maximum

Explanation: At resonance in parallel RLC, impedance is maximum, so current from source is minimum, but individual currents are maximum.

Q22. The phase relationship in AC circuit depends on:

a) Frequency only

b) Circuit elements only

c) Both frequency and circuit elements

d) Neither

Answer: c) Both frequency and circuit elements

Explanation: Phase depends on reactances which vary with frequency and component values.

Q23. Choke coil is used to:

a) Increase voltage

b) Decrease voltage

c) Control current

d) Store energy

Answer: c) Control current

Explanation: Choke coil provides high reactance to limit AC current flow.

Q24. The form factor of sinusoidal AC is:

a) 1.0

b) 1.11

c) 1.41

d) 2.0

Answer: b) 1.11

Explanation: Form factor = RMS value/Average value = π/2√2 ≈ 1.11 for sine wave.

Q25. Wattless current means:

a) Current consuming power

b) Current not consuming power

c) Zero current

d) Maximum current

Answer: b) Current not consuming power

Explanation: Wattless current occurs in reactive components where power consumption is zero.

Q26. The impedance triangle has:

a) R, XL, XC as sides

b) R, Z, (XL-XC) as sides

c) R, XL, Z as sides

d) All equal sides

Answer: b) R, Z, (XL-XC) as sides

Explanation: Impedance triangle shows R as base, (XL-XC) as height, and Z as hypotenuse.

Q27. LC oscillations have frequency:

a) f = 1/2π√LC

b) f = 2π√LC

c) f = √LC

d) f = 1/√LC

Answer: a) f = 1/2π√LC

Explanation: Natural frequency of LC circuit is f = 1/2π√LC.

Q28. The power consumed in AC circuit is given by:

a) VI

b) VIcos φ

c) VIsin φ

d) VI/cos φ

Answer: b) VIcos φ

Explanation: Real power P = VIcos φ, where φ is phase difference.

Q29. Skin effect in AC is:

a) Current flows on surface

b) Current flows uniformly

c) Current is zero

d) Current is maximum at center

Answer: a) Current flows on surface

Explanation: At high frequencies, AC tends to flow near conductor surface due to skin effect.

Q30. The value of current at t = T/4 in AC circuit is:

a) I₀

b) I₀/√2

c) I₀/2

d) Zero

Answer: b) I₀/√2

Explanation: At t = T/4, i = I₀sin(π/2) = I₀. Actually, i = I₀sin(2πt/T), so at t = T/4, sin(π/2) = 1, giving I₀. The correct answer should be a) I₀.