97 practice questions on Alternating Current , sorted Easy → Hard. Try each one first, then open its answer page for the worked explanation. Want the full theory first? Read the Alternating Current notes .
V and I Phase Relationships in AC Circuits Pure Inductor: I lags V by 90° V I Pure Capacitor: I leads V by 90° V I Memory aid "ELI the ICE man": in an inductor (L) E(V) leads I; in a capacitor (C) I leads E(V) For a pure resistor, V and I stay perfectly in phase (no lag or lead at all) In a pure inductor the current lags the voltage by 90° (energy is briefly stored in the magnetic field before current responds); in a pure capacitor the current leads the voltage by 90° (current must flow to build up charge before voltage rises) - the mnemonic "ELI the ICE man" keeps these straight.
Easy - 28 questions Q1.
Why can a transformer not operate with a steady direct current supply?
A A steady current produces no changing magnetic fluxB A steady current produces excessive output voltageC A steady current always reverses the magnetic fieldD A steady current makes the secondary resistance zeroShow answer & explanation →
Q2.
An AC generator coil completes 300 rotations per minute. What is the frequency of the generated AC?
A 2 HzB 5 HzC 10 HzD 50 HzShow answer & explanation →
Q3.
An alternating current is given by i = 5 sin(100πt + π/6) A. What is the current at t = 1/600 s?
A 2.50 AB 3.54 AC 4.33 AD 5.00 AShow answer & explanation →
Q4.
In an ideal LC circuit, the capacitor initially has maximum energy. After a time T/4, where T is the oscillation period, the energy is maximum in the:
A CapacitorB ResistorC External sourceD InductorShow answer & explanation →
Q5.
What is the average value of a sinusoidal alternating current over one complete cycle?
A 0B I<sub>0</sub>/2C 2I<sub>0</sub>/πD I<sub>0</sub>Show answer & explanation →
Q6.
In an AC circuit, the current and voltage are:
A Always in phaseB Always out of phaseC In phase for pure resistor onlyD Depends on frequency onlyShow answer & explanation →
Q7.
A transformer that increases voltage is called:
A Step-down transformerB RectifierC Step-up transformerD InductorShow answer & explanation →
Q9.
The RMS value of an AC voltage with peak value V<sub>0</sub> is:
A V<sub>0</sub>/√2B V<sub>0</sub>/2C V<sub>0</sub>√2D 2V<sub>0</sub>Show answer & explanation →
Q10.
The inductive reactance X<sub>L</sub> of an inductor L at frequency f is:
A 2πfLB 2πf/LC L/(2πf)D 1/(2πfL)Show answer & explanation →
Q11.
In a purely inductive AC circuit, the phase relationship between current and voltage is:
A Current lags voltage by 90°B Current leads voltage by 90°C Current and voltage are in phaseD Current lags voltage by 45°Show answer & explanation →
Q13.
In a purely capacitive AC circuit, the phase relationship is:
A Current leads voltage by 90°B Current lags voltage by 90°C Current and voltage are in phaseD Current leads voltage by 45°Show answer & explanation →
Q14.
The impedance Z of a series LCR circuit is:
A √(R² + (X<sub>L</sub> - X<sub>C</sub>)²)B R + X<sub>L</sub> + X<sub>C</sub>C R + X<sub>L</sub> - X<sub>C</sub>D √(R² + X<sub>L</sub>² + X<sub>C</sub>²)Show answer & explanation →
Q15.
Resonance in a series LCR circuit occurs when:
A X<sub>L</sub> = X<sub>C</sub> (inductive reactance equals capacitive reactance)B R = X<sub>L</sub>, when resistance equals inductive reactanceC R = X<sub>C</sub>, when resistance equals capacitive reactanceD Z = 0, when the total impedance vanishes entirelyShow answer & explanation →
Q18.
In a step-up transformer, if the primary has N<sub>1</sub> turns and secondary has N<sub>2</sub> turns (N<sub>2</sub> > N<sub>1</sub>), then:
A V<sub>2</sub> > V<sub>1</sub> and I<sub>2</sub> < I<sub>1</sub>B V<sub>2</sub> < V<sub>1</sub> and I<sub>2</sub> > I<sub>1</sub>C V<sub>2</sub> = V<sub>1</sub>D V<sub>2</sub> > V<sub>1</sub> and I<sub>2</sub> > I<sub>1</sub>Show answer & explanation →
Q20.
Average power dissipated in an AC circuit is:
A V<sub>rms</sub> × I<sub>rms</sub> × cos(φ)B V<sub>peak</sub> × I<sub>peak</sub>C V<sub>rms</sub> × I<sub>rms</sub>D V<sub>peak</sub> × I<sub>peak</sub> / 2Show answer & explanation →
Q21.
Wattless current in an AC circuit is:
A The component of current 90° out of phase with voltage that does no workB The current flowing specifically at the resonant frequency in the majority of cases studiedC The total instantaneous current supplied by the AC source as widely reportedD The steady DC offset component superimposed on the AC current in standard practiceShow answer & explanation →
Q22.
The resonant frequency of an LC circuit is given by:
A f<sub>0</sub> = 1/(2π√(LC))B f<sub>0</sub> = 2π√(LC)C f<sub>0</sub> = LC/2πD f<sub>0</sub> = 2πLCShow answer & explanation →
Q23.
Energy losses in a transformer are reduced by:
A Using laminated soft iron core to minimize eddy currentsB Using thick copper windings to raise winding resistanceC Increasing the primary voltage applied to the coilD Decreasing the number of turns on the secondary windingShow answer & explanation →
Q24.
The quality factor Q of a series LCR circuit at resonance is:
A Q = ω_0 L/R = 1/(ω_0 CR)B Q = R/ω_0 L under most conditions encounteredC Q = ω_0 C/R as frequently observed in practiceD Q = RL/ω_0 in many documented casesShow answer & explanation →
Q25.
The instantaneous power in an AC circuit is:
A p(t) = v(t) × i(t)B p(t) = V<sub>rms</sub> × I<sub>rms</sub>C p(t) = V<sub>0</sub> I<sub>0</sub> / 2D p(t) = V<sub>0</sub> I<sub>0</sub>Show answer & explanation →
Q26.
In a series LCR resonant circuit, the voltage across the inductor and capacitor can be:
A Greater than the source voltageB Equal to the source voltageC Less than the source voltageD ZeroShow answer & explanation →
Q27.
For the same average power, AC circuits use RMS values because:
A RMS values of AC produce the same heating effect as equivalent DC valuesB Analog meters can mainly physically register peak instantaneous valuesC RMS values are mathematically usually smaller than peak values according to most researchersD Using RMS values removes the need for phasor calculations largely in the majority of cases studiedShow answer & explanation →
Q28.
Long-distance power transmission uses high voltage because:
A Power loss P = I²R decreases when voltage is stepped up and current is stepped downB Higher voltage transmission is inherently safer for line workers according to conventional understandingC Higher voltage transmission requires fewer step-up transformers overall in routine practiceD Transmission cables can be made thinner generally by raising the voltage overallShow answer & explanation →
Medium - 32 questions Q29.
A coil of 200 turns and area 0.01 m<sup>2</sup> rotates at 100 rad s<sup>-1</sup> in a uniform magnetic field of 0.4 T. What is the RMS value of the generated emf?
A 28.3 VB 56.6 VC 40.0 VD 80.0 VShow answer & explanation →
Q30.
A series RC circuit has RMS voltage 24 V across the resistor and 18 V across the capacitor. What is the RMS supply voltage?
Show answer & explanation →
Q31.
In a series RL circuit, the RMS voltage across the resistor is 30 V and across the inductor is 40 V. What is the power factor of the circuit?
Show answer & explanation →
Q32.
An ideal LC circuit has a capacitor of 50 µF charged to 20 V and an inductor of 0.2 H. What is the maximum current in the circuit?
A 0.316 AB 0.224 AC 0.158 AD 0.500 AShow answer & explanation →
Q33.
A coil in an AC generator produces an RMS emf of 70.7 V and is connected to a 50 Ω purely resistive load. What is the RMS current?
A 0.707 AB 1.41 AC 1.00 AD 3.54 AShow answer & explanation →
Q34.
A transformer has 200 primary turns and 2000 secondary turns. Input voltage 220 V, efficiency 100%. Input current if output current is 2 A:
A 0.2 AB 2 AC 20 AD 200 AShow answer & explanation →
Q35.
In an LCR circuit at resonance, impedance:
A Is maximum during normal conditionsB Equals R (minimum)C Equals L/C as generally observedD Is zero in typical laboratory settingsShow answer & explanation →
Q36.
Phase angle in a purely inductive circuit:
A 0 degreesB 45 degreesC 90 degrees (current lags)D 90 degrees (current leads)Show answer & explanation →
Q37.
RMS voltage is related to peak voltage V<sub>0</sub> by:
A Vrms = V<sub>0</sub>B Vrms = V<sub>0</sub>/sqrt(2)C Vrms = V<sub>0</sub> x sqrt(2)D Vrms = V<sub>0</sub>/2Show answer & explanation →
Q39.
In a step-down transformer, the secondary coil has:
A More turns than primaryB Same turns as primaryC Fewer turns than primaryD More resistanceShow answer & explanation →
Q41.
A 100 Ω resistor, 10 mH inductor, and 100 μF capacitor are connected in series to 200 V, 50 Hz AC. Find X<sub>L</sub> and X<sub>C</sub>.
A X<sub>L</sub> = 3.14 Ω, X<sub>C</sub> = 31.8 ΩB X<sub>L</sub> = 31.4 Ω, X<sub>C</sub> = 31.8 ΩC X<sub>L</sub> = 314 Ω, X<sub>C</sub> = 3.18 ΩD X<sub>L</sub> = 3.14 Ω, X<sub>C</sub> = 318 ΩShow answer & explanation →
Q42.
A series LCR circuit has R = 20 Ω, L = 1.5 H, C = 35 μF. At resonance, what is the impedance?
A 20 ΩB 0 ΩC InfiniteD 55 ΩShow answer & explanation →
Q44.
A transformer has 200 primary turns and 1000 secondary turns. Primary voltage is 220 V. Secondary voltage is:
A 1100 VB 44 VC 220 VD 2200 VShow answer & explanation →
Q46.
At resonance in a series LCR circuit (R=10Ω, L=0.5H, V=100V), what is the quality factor if f<sub>0</sub>=50Hz?
Show answer & explanation →
Q47.
In an AC circuit, V = 100 sin(100t) and I = 10 sin(100t - π/3). The average power is:
A 250 WB 500 WC 1000 WD 750 WShow answer & explanation →
Q48.
A choke coil (inductor) is preferred over a resistor for reducing AC current in a circuit because:
A It dissipates very little power while limiting currentB A choke coil is generally cheaper to manufacture than a resistorC It has a much higher ohmic resistance than an equivalent resistorD It amplifies the AC current instead of limiting itShow answer & explanation →
Q49.
If the frequency of AC is doubled, how does X<sub>L</sub> change?
A DoublesB HalvesC Stays sameD QuadruplesShow answer & explanation →
Q50.
An AC source (V<sub>0</sub> = 100 V) is connected to a 5 Ω resistor. Peak current and RMS current are:
A 20 A peak, 14.14 A RMSB 100 A peak, 70.7 A RMSC 5 A peak, 3.54 A RMSD 10 A peak, 7.07 A RMSShow answer & explanation →
Q51.
In an LCR circuit, if L = 40 mH and C = 250 μF, at what frequency is X<sub>L</sub> = X<sub>C</sub>?
A 50.3 HzB 100 HzC 159 HzD 25 HzShow answer & explanation →
Q52.
The bandwidth of a resonant circuit with Q = 100 and f<sub>0</sub> = 1 MHz is:
A 10 kHzB 100 kHzC 1 kHzD 100 HzShow answer & explanation →
Q53.
A transformer with efficiency 80% has primary power input 1000 W. Output power is:
A 800 WB 200 WC 1000 WD 1250 WShow answer & explanation →
Q54.
The phase angle φ in a series LCR circuit with R=10Ω, X<sub>L</sub>=20Ω, X<sub>C</sub>=10Ω is:
A 45° (lagging)B 45° (leading)C 0°D 90°Show answer & explanation →
Q55.
A pure inductor of 0.5 H is connected to 220 V, 50 Hz AC. What is the RMS current?
A 1.4 AB 2.8 AC 0.7 AD 44 AShow answer & explanation →
Q56.
A capacitor of 10 μF is connected to 100 V, 100 Hz AC. The RMS current is:
A 0.628 AB 6.28 AC 0.0628 AD 62.8 AShow answer & explanation →
Q57.
An AC generator converts:
A Mechanical energy to electrical energy using electromagnetic inductionB Electrical energy back into mechanical energy, like a motorC Direct current into alternating current, like an inverter circuitD Thermal energy directly into electrical energy, like a thermocoupleShow answer & explanation →
Q58.
The form factor of a sinusoidal AC wave is:
A V<sub>rms</sub>/V<sub>avg</sub> = π/(2√2) ≈ 1.11B V<sub>0</sub>/V<sub>rms</sub> = √2 as frequently observed in practiceC V<sub>avg</sub>/V<sub>rms</sub> in many documented casesD V<sub>rms</sub>/V<sub>0</sub> according to conventional understandingShow answer & explanation →
Q59.
In a series RLC circuit, the current at resonance is:
A V/R (maximum possible current)B V/Z (with Z minimum at resonance)C 0D V/(X<sub>L</sub> + X<sub>C</sub>)Show answer & explanation →
Q60.
Power factor cos(φ) = R/Z. For an LCR circuit above resonance frequency:
A Power factor is lagging (capacitive behavior)B Power factor is leadingC Power factor = 1D Power factor = 0Show answer & explanation →
Hard - 37 questions Q61.
The angular speed of an AC generator is doubled while its number of turns, coil area and magnetic field remain unchanged. At the same time, the resistance of its purely resistive load is reduced to half. How does the RMS current change?
A It becomes twice the original valueB It becomes three times the original valueC It becomes four times the original valueD It becomes eight times the original valueShow answer & explanation →
Q62.
An ideal LC circuit starts with maximum charge Q<sub>0</sub> on its capacitor. At time T/6, where T is the oscillation period, what fraction of the total energy is stored in the inductor?
Show answer & explanation →
Q63.
In a series RLC circuit, the RMS voltages across the resistor, inductor and capacitor are 60 V, 80 V and 30 V respectively. What are the RMS supply voltage and the magnitude of the phase angle between supply voltage and current?
A 78.1 V and 39.8°B 110 V and 36.9°C 170 V and 53.1°D 78.1 V and 50.2°Show answer & explanation →
Q64.
An AC generator has E<sub>0</sub> = NBAω. If the number of turns is increased by 50%, the coil area is doubled, and the angular speed is reduced to half while the magnetic field remains unchanged, how does the peak emf change?
A It becomes 1.25 timesB It becomes 1.50 timesC It becomes 2.00 timesD It remains unchangedShow answer & explanation →
Q65.
A power station transmits a fixed power through a line having fixed resistance. If the transmission voltage is increased from V to 3V while the transmitted power remains constant, how does the power loss in the line change?
A It becomes one-thirdB It becomes three timesC It becomes one-ninthD It remains unchangedShow answer & explanation →
Q67.
An LCR series circuit has R=10, L=0.1 H, C=100 microF, f=50 Hz. Impedance:
A 10 ohmB 17.6 ohmC 31.4 ohmD 41.5 ohmShow answer & explanation →
Q69.
Average power in an AC circuit with phase difference phi between V and I:
A Vrms x IrmsB Vrms x Irms x sin(phi)C Vrms x Irms x cos(phi)D V<sub>0</sub> x I<sub>0</sub>/2Show answer & explanation →
Q70.
An LCR series circuit has L = 2 H, C = 200 nF, R = 100 Ω. At resonance, find Q factor and bandwidth.
A Q = 100, BW = 8 HzB Q = 447, BW = 50 HzC Q = 200, BW = 25 HzD Q = 316, BW = 25 HzShow answer & explanation →
Q71.
In an LC circuit with no resistance, energy oscillates between the inductor and capacitor. The frequency of oscillation is:
A f = 1/(2π√(LC))B f = 2π√(LC)C f = 1/√(LC)D f = RC/LShow answer & explanation →
Q72.
A 200 V, 50 Hz source drives a series RLC circuit with R=20 Ω, L=100 mH, C=10 μF. The total impedance is:
A √(20² + (31.4-318.3)²) Ω ≈ 287 ΩB 20 ΩC 100 ΩD 287 ΩShow answer & explanation →
Q73.
In a series LCR circuit at resonance, the voltage across L is V<sub>L</sub> and across C is V<sub>C</sub>. These voltages are:
A Equal in magnitude and opposite in phase (cancel)B Equal in magnitude and same phase (add) according to standard textbooksC Zero in general practice as frequently describedD Both equal to source voltage in most textbook accountsShow answer & explanation →
Q74.
A step-down transformer (20:1 ratio) is connected to 2200 V AC. The secondary is connected to a 44 W bulb. What is the secondary current and primary current?
A I<sub>2</sub> = 0.4 A, I<sub>1</sub> = 0.02 AB I<sub>2</sub> = 2 A, I<sub>1</sub> = 0.1 AC I<sub>2</sub> = 0.04 A, I<sub>1</sub> = 2 AD I<sub>2</sub> = 44 A, I<sub>1</sub> = 2.2 AShow answer & explanation →
Q75.
An AC circuit has R = 40 Ω and XL = 30 Ω. The apparent power is 500 VA. The real (true) power dissipated is:
A 400 WB 300 WC 500 WD 250 WShow answer & explanation →
Q76.
In an LCR circuit, at half-power frequencies (f<sub>1</sub> and f<sub>2</sub>), the current is I<sub>max</sub>/√2. The bandwidth is:
A f<sub>2</sub> - f<sub>1</sub> = R/(2πL) = R/ω_0/QB f<sub>2</sub> - f<sub>1</sub> = f<sub>0</sub> during normal conditionsC f<sub>2</sub> - f<sub>1</sub> = 1/(RC) as generally observedD f<sub>2</sub> - f<sub>1</sub> = 1/Q in typical laboratory settingsShow answer & explanation →
Q77.
A transmission line has resistance 100 Ω per conductor. Power is transmitted at 1000 V RMS, 100 kW. Power loss in transmission is:
A 1000 W (1 kW)B 100 W under usual circumstancesC 10 kW according to most researchersD 5 kW in the majority of cases studiedShow answer & explanation →
Q78.
For maximum power transfer from AC source to load, the load impedance should satisfy:
A Z<sub>load</sub> = Z<sub>source</sub>* (complex conjugate)B Z<sub>load</sub> = Z<sub>source</sub>, matching magnitude and phase without conjugating itC Z<sub>load</sub> = 0, shorting the load terminals together largelyD Z<sub>load</sub> = ∞, leaving the load terminals open with little current flowShow answer & explanation →
Q79.
An inductor coil has resistance R and inductance L. Its power factor at frequency f is:
A R/√(R² + (2πfL)²)B R/L as widely reportedC L/R in standard practiceD 2πfL/R under most conditions encounteredShow answer & explanation →
Q80.
In an LC circuit, if the charge on capacitor at t=0 is Q<sub>0</sub>, the maximum current is:
A Q<sub>0</sub>/√(LC)B Q<sub>0</sub> √(LC)C Q<sub>0</sub>/LCD Q<sub>0</sub> LCShow answer & explanation →
Q81.
When capacitor and inductor are in parallel (tank circuit), the impedance at resonance is:
A Very high (theoretically infinite for ideal components)B Zero, as occurs instead in a series LCR circuit at resonanceC R (only resistance matters), ignoring the reactive elements entirelyD Z = X<sub>L</sub> = X<sub>C</sub>, equating the impedance to the equal reactance valuesShow answer & explanation →
Q82.
An AC source drives three loads: R = 10 Ω, X<sub>L</sub> = 10 Ω, X<sub>C</sub> = 10 Ω, all in series. The power factor is:
A 1 (pure resistive at resonance)B 0, as if the circuit carried no real power whatsoeverC 0.707, the value corresponding to a 45 degree phase angleD 0.5, the value corresponding to a 60 degree phase angleShow answer & explanation →
Q83.
An LCR circuit has L = 10 mH, C = 10 μF, R = 10 Ω. If V = 100 V at resonance, what is V<sub>L</sub> (voltage across inductor)?
A Q × 100 V = 10 × 100 = 1000 VB 100 V, equal to the source voltage with little quality-factor amplificationC 0 V, as if the inductor carried no voltage drop at resonance whatsoeverD 50 V, half of the applied source voltage with little resonance factor includedShow answer & explanation →
Q84.
The average power dissipated in a full cycle by an ideal inductor or capacitor is:
A Zero (energy just oscillates)B Maximum at resonanceC Proportional to reactanceD Equal to apparent powerShow answer & explanation →
Q85.
In an AC circuit with V = V<sub>0</sub> cos(ωt) and I = I<sub>0</sub> cos(ωt + φ), the average power is:
A (V<sub>0</sub> I<sub>0</sub>/2) cos(φ)B V<sub>0</sub> I<sub>0</sub> cos(φ)C (V<sub>0</sub> I<sub>0</sub>/2) sin(φ)D 0Show answer & explanation →
Q86.
The skin depth δ in a conductor (where current density falls to 1/e) decreases with:
A Increasing frequencyB Decreasing frequencyC Increasing resistivityD No relation to frequencyShow answer & explanation →
Q87.
A 1 kVA transformer operates at 0.85 power factor. The active (real) power it can deliver is:
A 850 WB 1000 WC 1175 WD 500 WShow answer & explanation →
Q88.
For a series RLC circuit, the condition for the current to lead the voltage (capacitive behavior) is:
A X<sub>C</sub> > X<sub>L</sub> (f < f<sub>0</sub>)B X<sub>L</sub> > X<sub>C</sub> (f > f<sub>0</sub>)C X<sub>L</sub> = X<sub>C</sub>D R > X<sub>L</sub>Show answer & explanation →
Q89.
In an AC circuit, the reactive power Q<sub>r</sub> = V<sub>rms</sub> I<sub>rms</sub> sin(φ) is measured in:
A VAR (volt-ampere reactive)B Watts as frequently observed in practiceC VA (volt-ampere) in many documented casesD Joules according to conventional understandingShow answer & explanation →
Q91.
A series LC circuit has L = 1 H and C = 1 μF. Its resonant frequency is about:
A 100 HzB 159 HzC 318 HzD 1000 HzShow answer & explanation →
Q92.
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 the voltageD lags by 45°Show answer & explanation →
Q95.
A step-up transformer has 100 turns in the primary and 1000 in the secondary. If the primary voltage is 100 V, the secondary voltage (ideal transformer) is:
A 10 VB 100 VC 110 VD 1000 VShow answer & explanation →
Q96.
In a series RLC circuit at resonance:
A the current is maximumB the current is zeroC the impedance is maximumD the source voltage is zeroShow answer & explanation →
Q97.
In an AC circuit, V<sub>rms</sub> = 200 V, I<sub>rms</sub> = 5 A and the power factor is 0.8. The average power consumed is:
A 500 WB 640 WC 800 WD 1000 WShow answer & explanation →