Q.1 What is the phase difference between voltage and current in a pure resistor connected to an AC source?
0°
90°
180°
45°
Explanation - In a pure resistor, current and voltage are in the same phase, meaning their peaks and zeros occur at the same time.
Correct answer is: 0°
Q.2 In a pure inductor connected to an AC source, the current lags the voltage by:
0°
90°
180°
45°
Explanation - In a pure inductor, voltage leads current by 90°, or equivalently, current lags voltage by 90°.
Correct answer is: 90°
Q.3 In a pure capacitor connected to an AC source, the current leads the voltage by:
0°
90°
180°
45°
Explanation - In a capacitor, the charging current leads the voltage across the capacitor by 90°.
Correct answer is: 90°
Q.4 The effective value (RMS value) of an AC current of peak value I₀ is:
I₀
I₀/√2
I₀√2
2I₀
Explanation - RMS current is defined as I₀/√2 for sinusoidal alternating current.
Correct answer is: I₀/√2
Q.5 The power factor of a purely inductive or purely capacitive circuit is:
1
0
0.5
–1
Explanation - For purely inductive or capacitive circuits, the phase difference between voltage and current is 90°, hence power factor = cos90° = 0.
Correct answer is: 0
Q.6 In a series RLC circuit at resonance, the impedance is equal to:
R
X_L
X_C
R + X_L
Explanation - At resonance, inductive and capacitive reactances cancel each other, so impedance equals resistance R.
Correct answer is: R
Q.7 The condition for resonance in an RLC circuit is:
X_L = X_C
R = X_L
R = X_C
X_L > X_C
Explanation - Resonance occurs when inductive reactance equals capacitive reactance, i.e., X_L = X_C.
Correct answer is: X_L = X_C
Q.8 The unit of power factor is:
Watt
No unit
Joule
Ampere
Explanation - Power factor is the cosine of the phase angle between voltage and current. Being a ratio, it has no unit.
Correct answer is: No unit
Q.9 The average power consumed in a purely inductive circuit is:
Zero
Maximum
Depends on frequency
Infinite
Explanation - In a purely inductive circuit, voltage and current are 90° out of phase, so average power is zero.
Correct answer is: Zero
Q.10 If the frequency of applied AC voltage is doubled, the capacitive reactance becomes:
Doubled
Halved
Unchanged
Quadrupled
Explanation - Capacitive reactance X_C = 1/(ωC). When frequency doubles, reactance becomes half.
Correct answer is: Halved
Q.11 Which quantity is measured in ohms in an AC circuit?
Resistance only
Reactance only
Impedance
Power
Explanation - Impedance is the effective opposition to AC and is measured in ohms, like resistance.
Correct answer is: Impedance
Q.12 The phase angle between current and voltage in a purely resistive circuit is:
0°
90°
180°
45°
Explanation - In a pure resistor, current and voltage are always in phase.
Correct answer is: 0°
Q.13 The quality factor (Q-factor) of an RLC circuit is defined as:
Energy stored / Energy dissipated per cycle
Power / Voltage
Resistance / Reactance
Voltage / Current
Explanation - Q-factor gives the sharpness of resonance and is defined as the ratio of energy stored to energy dissipated per cycle.
Correct answer is: Energy stored / Energy dissipated per cycle
Q.14 The instantaneous power in an AC circuit can be negative when:
Voltage and current are in phase
Circuit is purely resistive
Circuit contains reactance
Power factor is unity
Explanation - In reactive components, current and voltage are out of phase, leading to periods of negative instantaneous power.
Correct answer is: Circuit contains reactance
Q.15 At resonance in an RLC circuit, the current is:
Maximum
Minimum
Zero
Constant
Explanation - At resonance, impedance is minimum (equal to R), hence current is maximum.
Correct answer is: Maximum
Q.16 If the resistance in an AC circuit increases, the power factor will:
Increase
Decrease
Remain same
Become zero
Explanation - Higher resistance reduces phase difference, increasing power factor towards unity.
Correct answer is: Increase
Q.17 The SI unit of inductive reactance is:
Henry
Ohm
Tesla
Weber
Explanation - Inductive reactance is given by X_L = ωL and has the unit of ohm.
Correct answer is: Ohm
Q.18 If current leads the voltage in a circuit, the circuit must contain:
Resistor only
Inductor
Capacitor
Resistor and Inductor
Explanation - In a capacitor, current leads the voltage by 90°.
Correct answer is: Capacitor
Q.19 The impedance of a series RLC circuit at resonance is:
√(R² + (X_L - X_C)²)
R
X_L
X_C
Explanation - At resonance, X_L = X_C, so impedance reduces to R.
Correct answer is: R
Q.20 In an AC circuit, the average value of current over one complete cycle is:
I₀
I₀/√2
0
I₀/2
Explanation - Since positive and negative half cycles cancel, the average current over one cycle is zero.
Correct answer is: 0
Q.21 The product of RMS voltage and RMS current gives:
Apparent power
True power
Reactive power
Power factor
Explanation - Apparent power = V_rms × I_rms, measured in VA.
Correct answer is: Apparent power
Q.22 The true power consumed in an AC circuit is given by:
VI
VI cosφ
VI sinφ
VI tanφ
Explanation - True power is P = VI cosφ, where cosφ is the power factor.
Correct answer is: VI cosφ
Q.23 The frequency of AC supply in India is:
50 Hz
60 Hz
100 Hz
25 Hz
Explanation - Standard frequency of mains AC supply in India is 50 Hz.
Correct answer is: 50 Hz
Q.24 The capacitive reactance of a capacitor of 100 µF at 50 Hz is approximately:
31.8 Ω
63.6 Ω
159 Ω
318 Ω
Explanation - X_C = 1/(2πfC) = 1/(2 × 3.14 × 50 × 100×10⁻⁶) ≈ 31.8 Ω.
Correct answer is: 31.8 Ω
Q.25 In a transformer working on AC, the core is laminated to reduce:
Resistance
Eddy current loss
Hysteresis loss
Inductance
Explanation - Laminating the core increases resistance to eddy currents, reducing power loss.
Correct answer is: Eddy current loss