Chapter-7 Alternating Current — Online MCQ Test
PHYSICS · CLASS 12 INTER II YEAR · Andhra State Board
Practice Chapter-7 Alternating Current with a free chapter-wise online MCQ test.
This chapter covers: AC circuits - RMS value - reactance - impedance - LCR circuit - resonance - transformer - AC generator.
AI-generated questions from basic to board-exam level, with instant results and explanations.
Chapter-7 Alternating Current — Important Questions & Answers
What is the RMS (root mean square) value of an alternating current with peak value I₀?
- A. I₀/2
- B. I₀/√2
- C. I₀√2
- D. 2I₀
Answer: B. I₀/√2
The RMS value of an AC current is defined as I₀/√2, where I₀ is the peak (maximum) value of the current.
The RMS value of an AC current is defined as I₀/√2, where I₀ is the peak (maximum) value of the current.
Which of the following is the SI unit of reactance?
- A. Ampere
- B. Ohm
- C. Henry
- D. Farad
Answer: B. Ohm
Reactance (inductive or capacitive) is measured in Ohms (Ω), the same unit as resistance.
Reactance (inductive or capacitive) is measured in Ohms (Ω), the same unit as resistance.
In an AC circuit, the impedance Z is related to resistance R, inductive reactance Xₗ, and capacitive reactance Xc by which expression?
- A. Z = R + Xₗ + Xc
- B. Z = √[R² + (Xₗ - Xc)²]
- C. Z = R² + (Xₗ - Xc)²
- D. Z = R + (Xₗ - Xc)
Answer: B. Z = √[R² + (Xₗ - Xc)²]
Impedance is the vector sum of resistance and net reactance: Z = √[R² + (Xₗ - Xc)²].
Impedance is the vector sum of resistance and net reactance: Z = √[R² + (Xₗ - Xc)²].
What is the inductive reactance of a pure inductor with inductance L in an AC circuit of frequency f?
- A. Xₗ = 2πfL
- B. Xₗ = L/(2πf)
- C. Xₗ = 2πfL²
- D. Xₗ = 1/(2πfL)
Answer: A. Xₗ = 2πfL
Inductive reactance is directly proportional to frequency and inductance: Xₗ = 2πfL or ωL, where ω = 2πf.
Inductive reactance is directly proportional to frequency and inductance: Xₗ = 2πfL or ωL, where ω = 2πf.
A sinusoidal AC voltage is given by V(t) = 230sin(ωt) V. What is its RMS voltage?
- A. 230 V
- B. 230/√2 V ≈ 163 V
- C. 230√2 V ≈ 325 V
- D. 115 V
Answer: B. 230/√2 V ≈ 163 V
The peak voltage is 230 V, so RMS voltage = 230/√2 ≈ 163 V. The value 230 V used in India is already the RMS value.
The peak voltage is 230 V, so RMS voltage = 230/√2 ≈ 163 V. The value 230 V used in India is already the RMS value.
In a purely inductive circuit, what is the phase difference between voltage and current?
- A. 0°
- B. 90° (voltage leads)
- C. 45°
- D. 180°
Answer: B. 90° (voltage leads)
In a pure inductor, voltage leads current by 90° because the inductor opposes changes in current.
In a pure inductor, voltage leads current by 90° because the inductor opposes changes in current.
In a series RLC circuit, if the inductance is decreased while keeping resistance and capacitance constant, the resonant frequency will:
- A. Decrease
- B. Increase
- C. Remain unchanged
- D. Become zero
Answer: B. Increase
Resonant frequency f₀ = 1/(2π√LC) is inversely proportional to √L. Decreasing L increases the resonant frequency.
Resonant frequency f₀ = 1/(2π√LC) is inversely proportional to √L. Decreasing L increases the resonant frequency.
An AC circuit contains only a capacitor (C = 20 μF) and an AC voltage source (V = 100 V, f = 100 Hz). What is the current in the circuit?
- A. 1.26 A
- B. 0.126 A
- C. 12.6 A
- D. 0.0126 A
Answer: A. 1.26 A
Xc = 1/(2πfC) = 1/(2π × 100 × 20 × 10⁻⁶) ≈ 79.6 Ω. Current I = V/Xc = 100/79.6 ≈ 1.26 A.
Xc = 1/(2πfC) = 1/(2π × 100 × 20 × 10⁻⁶) ≈ 79.6 Ω. Current I = V/Xc = 100/79.6 ≈ 1.26 A.
A resonant LCR circuit has R = 5 Ω, L = 10 mH, and the resonant frequency is 1000 Hz. What is the capacitance?
- A. 2.53 μF
- B. 25.3 μF
- C. 0.253 μF
- D. 253 μF
Answer: A. 2.53 μF
At resonance, f₀ = 1/(2π√LC), so C = 1/(4π²f₀²L) = 1/(4π² × 10⁶ × 0.01) ≈ 2.53 × 10⁻⁶ F = 2.53 μF.
At resonance, f₀ = 1/(2π√LC), so C = 1/(4π²f₀²L) = 1/(4π² × 10⁶ × 0.01) ≈ 2.53 × 10⁻⁶ F = 2.53 μF.
In an LCR series circuit, at a frequency below resonance, which type of circuit behavior is exhibited?
- A. Inductive (current lags voltage)
- B. Capacitive (current leads voltage)
- C. Purely resistive
- D. Behavior depends only on R
Answer: B. Capacitive (current leads voltage)
Below resonance, Xc > Xₗ, so the net reactance is capacitive, making the circuit capacitive in nature. Current leads voltage in a capacitive circuit.
Below resonance, Xc > Xₗ, so the net reactance is capacitive, making the circuit capacitive in nature. Current leads voltage in a capacitive circuit.