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B. Current whose magnitude and direction change periodically with time
C. Current that remains constant with time
D. Current produced only by batteries
Answer: B. Current whose magnitude and direction change periodically with time
Explanation: AC is defined as electric current whose magnitude changes continuously and direction reverses periodically, typically following a sinusoidal pattern.
2. The RMS value of an alternating current I = I0 sin(ωt) is:
A. I0
B. I0/√2
C. I0/2
D. 2I0
Answer: B. I0/√2
Explanation: RMS (root mean square) value of a sinusoidal AC is Irms = I0/√2 ≈ 0.707 I0, representing the equivalent steady DC that produces the same heating effect.
3. The average value of alternating current over one complete cycle is:
A. I0
B. I0/√2
C. Zero
D. 2I0/π
Answer: C. Zero
Explanation: Over a full cycle, the positive and negative half cycles cancel out exactly, so the average value of a sinusoidal AC over one complete cycle is zero.
4. Capacitive reactance (Xc) of a capacitor of capacitance C in an AC circuit of angular frequency ω is:
A. Xc = ωC
B. Xc = 1/(ωC)
C. Xc = ω/C
D. Xc = ωC²
Answer: B. Xc = 1/(ωC)
Explanation: Capacitive reactance is given by Xc = 1/(ωC) = 1/(2πfC); it decreases as frequency increases, since a capacitor charges/discharges faster at higher frequency.
5. Inductive reactance (XL) of an inductor of inductance L in an AC circuit of angular frequency ω is:
A. XL = ωL
B. XL = 1/(ωL)
C. XL = ω/L
D. XL = L/ω
Answer: A. XL = ωL
Explanation: Inductive reactance is XL = ωL = 2πfL; it increases with frequency because a changing current induces a larger back-EMF at higher rates of change.
6. In a purely resistive AC circuit, the phase difference between voltage and current is:
A. 90°
B. 180°
C. Zero
D. 45°
Answer: C. Zero
Explanation: In a pure resistor, voltage and current are always in phase (phase difference = 0°) since V = IR holds instantaneously at every point of the cycle.
7. In a purely inductive AC circuit, the current:
A. Leads the voltage by 90°
B. Lags the voltage by 90°
C. Is in phase with voltage
D. Lags the voltage by 180°
Answer: B. Lags the voltage by 90°
Explanation: In a pure inductor, the induced back-EMF opposes change in current, causing the current to lag behind the applied voltage by a phase angle of 90° (π/2).
8. In a purely capacitive AC circuit, the current:
A. Leads the voltage by 90°
B. Lags the voltage by 90°
C. Is in phase with voltage
D. Leads the voltage by 180°
Answer: A. Leads the voltage by 90°
Explanation: In a pure capacitor, current must flow first to build up charge before voltage develops across it, so current leads the applied voltage by 90°.
9. The impedance (Z) of a series RLC circuit is given by:
A. Z = R + XL + XC
B. Z = √(R² + (XL − XC)²)
C. Z = R × XL × XC
D. Z = R² + XL² + XC²
Answer: B. Z = √(R² + (XL − XC)²)
Explanation: Since XL and XC act in opposite phase to R, impedance is found by combining them as a right-angle vector sum: Z = √(R² + (XL − XC)²).
10. Resonance occurs in a series LCR circuit when:
A. XL = 0
B. XC = 0
C. XL = XC
D. R = 0
Answer: C. XL = XC
Explanation: At resonance, inductive and capacitive reactances become equal (XL = XC) and cancel each other, leaving impedance purely resistive and minimum.
11. The resonant frequency (f0) of a series LCR circuit is:
A. f0 = 1/(2π√(LC))
B. f0 = 2π√(LC)
C. f0 = 1/(2πLC)
D. f0 = √(LC)/2π
Answer: A. f0 = 1/(2π√(LC))
Explanation: Setting XL = XC gives ωL = 1/(ωC), so ω0 = 1/√(LC), and hence resonant frequency f0 = ω0/2π = 1/(2π√(LC)).
12. The power factor of an AC circuit is defined as:
A. cos φ = R/Z
B. cos φ = Z/R
C. cos φ = XL/Z
D. cos φ = XC/R
Answer: A. cos φ = R/Z
Explanation: Power factor is the cosine of the phase angle between voltage and current, cos φ = R/Z, and determines what fraction of apparent power does real work.
13. At resonance in a series LCR circuit, the power factor is:
A. Zero
B. 0.5
C. One
D. Undefined
Answer: C. One
Explanation: At resonance Z = R (minimum, purely resistive), so cos φ = R/Z = 1, meaning voltage and current are in phase and power dissipation is maximum.
14. Average power dissipated in a pure inductor or pure capacitor over a complete cycle is:
A. Maximum
B. Zero
C. Equal to Vrms Irms
D. Negative
Answer: B. Zero
Explanation: Since current and voltage differ in phase by 90° in a pure L or C, the average of the power term (proportional to cos 90° = 0) over a cycle is zero — hence called wattless current.
15. A transformer works on the principle of:
A. Self-induction only
B. Mutual induction
C. Electrostatic induction
D. Thermoelectric effect
Answer: B. Mutual induction
Explanation: A transformer transfers electrical energy between two coils linked by a common changing magnetic flux, which is the phenomenon of mutual induction.
16. In a step-up transformer:
A. Number of turns in secondary is less than primary
B. Number of turns in secondary is greater than primary
C. Both coils have equal turns
D. It converts AC to DC
Answer: B. Number of turns in secondary is greater than primary
Explanation: A step-up transformer increases voltage, which requires the secondary coil to have a greater number of turns than the primary coil (Ns > Np).
17. Which of the following is NOT a source of energy loss in a real transformer?
A. Copper loss (I²R heating)
B. Iron loss due to hysteresis
C. Eddy current loss in core
D. Loss due to mutual induction
Answer: D. Loss due to mutual induction
Explanation: Mutual induction is the very principle by which a transformer operates and transfers energy — it is not a loss mechanism, unlike resistive, hysteresis, and eddy current losses.
18. A choke coil is used in AC circuits (instead of a resistor) to reduce current because:
A. It consumes large average power
B. It reduces current with negligible power loss due to its low resistance and high inductive reactance
C. It converts AC to DC
D. It has zero reactance
Answer: B. It reduces current with negligible power loss due to its low resistance and high inductive reactance
Explanation: A choke coil has high inductive reactance but very low ohmic resistance, so it limits AC current effectively while dissipating very little power, unlike a resistor.
19. The quality factor (Q) of a series LCR resonant circuit is given by:
A. Q = R/(ω0L)
B. Q = ω0L/R
C. Q = ω0RL
D. Q = 1/(ω0RL)
Answer: B. Q = ω0L/R
Explanation: Quality factor, which measures the sharpness of resonance, is defined as Q = ω0L/R = (1/R)√(L/C); a higher Q means a sharper, narrower resonance peak.
20. An AC voltage V = V0 sin(ωt) is applied to a series LCR circuit with XL > XC. The circuit behaves as:
A. Purely resistive
B. Predominantly inductive
C. Predominantly capacitive
D. Purely capacitive
Answer: B. Predominantly inductive
Explanation: When XL > XC, the net reactance is inductive in nature, so the current lags behind the voltage and the circuit behaves as a predominantly inductive circuit.
This page contains a chapter-wise objective practice set. Try the questions without checking the answer first, then use the explanation to understand the point you missed. The current page contains 20 questions, so you can practise in smaller sets when you are revising.
Quick revision tip
After the practice set, write down the question numbers you missed and revise only those concepts once more. This makes MCQ practice more useful than repeatedly attempting the same questions without reviewing mistakes.
