Alternating CurrentNEET MCQs with solutions
Alternating Current covers AC circuits with R, L, C elements, impedance, resonance, power factor, transformers and LC oscillations. NEET tests impedance and phase calculations in LCR circuits, resonance condition, power factor and transformer working.
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- 12 Physics
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- 24 questions
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- 546 questions
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- NCERT topics
- 16
Practise 24 questions
Tap an option to check it. Questions from every NCERT topic in this chapter, from easy to hard.
Q1Grand Test
The peak voltage of an AC source is 311 V. What is its RMS voltage (approximately)?
Not quite — the answer is B.
RMS = V0/√2 = 311/1.414 ≈ 220 V. Option A is the peak value itself, a common confusion. Option C divides by 2 instead of √2.
Q2Important PYQ Mixed Concepts
In a series resonant LCR circuit, if the resonant current is I, the average power dissipated at resonance is:
Not quite — the answer is D.
At resonance XL = XC, net reactance is zero, and Z = R. Average power P = I²R. Reactive elements exchange energy each half-cycle but consume zero net power. Option B (zero) is the trap from confusing wattless circuits with resonant circuits.
Q3AC Through Pure Resistor
An alternating voltage v = V₀ sin(ωt) is applied across a pure resistor R. The current through the resistor is:
Not quite — the answer is A.
For a pure resistor Ohm's law applies at every instant: i = v/R = (V₀/R)sinωt. Voltage and current are in phase. Option B is wrong — cosine form would imply a 90° phase shift, which occurs only in inductors or capacitors.
Q4Alternating Current Basics
Which of the following correctly describes an alternating current?
Not quite — the answer is A.
AC varies periodically and reverses direction each half-cycle. B describes DC. C describes unidirectional pulsating current, not AC. D is physically impossible for a useful source.
Q5AC Voltage and Current Equations
An alternating voltage is represented by v = 220 sin(100πt) V. What are its peak voltage and angular frequency respectively?
Not quite — the answer is A.
Comparing with v = V₀sin(ωt): V₀ = 220 V and ω = 100π rad/s. Option B gives RMS value instead of peak. Option D halves ω by mistake, treating 100π as 2πf = 100 instead of ω = 100π.
Q6Series LCR Circuit
In a series LCR circuit, R = 20 Ω, X_L = 30 Ω and X_C = 10 Ω. The impedance of the circuit is:
Not quite — the answer is A.
Z = √[R² + (X_L − X_C)²] = √[400 + 400] = 20√2 Ω. Adding X_L and X_C instead of subtracting gives the wrong value 40 Ω. Net reactance is always |X_L − X_C|.
Q7RMS and Mean Values
The peak value of an alternating current is 10 A. What is its RMS value?
Not quite — the answer is D.
I_rms = I₀/√2 = 10/√2 = 5√2 A. Option C (10 A) confuses peak with RMS. Option A inverts: 10/√2 = 5√2 — same value, different form, used as distractor. Option B multiplies instead of dividing by √2.
Q8AC Through Pure Capacitor
For a pure capacitive AC circuit, if v = V₀ sin(ωt), the current through the capacitor is:
Not quite — the answer is B.
i = C(dv/dt) = C·d(V₀sinωt)/dt = ωCV₀cosωt. Option A is the in-phase trap. Option C inverts the reactance — it has 1/ωC in numerator, giving wrong units and wrong amplitude.
Q9AC Through Pure Inductor
An alternating voltage is applied across a pure inductor. If the current is i = I₀ sin(ωt), the voltage across the inductor is:
Not quite — the answer is B.
v_L = L(di/dt) = ωLI₀cos(ωt) = V₀cos(ωt). Voltage leads current by π/2. Option A is the in-phase trap — students assume voltage and current follow identical sinusoidal form in all AC circuits.
Q10Phasor Diagram
In a phasor diagram of a purely resistive AC circuit, the voltage and current phasors are:
Not quite — the answer is A.
For a pure resistor, voltage and current are always in phase: φ = 0. Their phasors are parallel and point in the same direction. The 90° options describe purely inductive or capacitive circuits, not resistive.
Q11Resonance in LCR Circuit
In a series LCR circuit, resonance occurs when:
Not quite — the answer is A.
At resonance X_L = X_C, so net reactance = 0 and Z reduces to its minimum value R. Option D (Z = 0) is the classic trap — resistance always remains; impedance is minimum but never zero for finite R.
Q12Power in AC Circuit
For an AC circuit, the average power consumed over one complete cycle is given by:
Not quite — the answer is A.
Average power is P = Vrms Irms cosφ. The cosφ factor determines what fraction of apparent power S = VrmsIrms is actually dissipated. Option C lacks the 1/2 factor needed to convert peak to rms values.
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Get RankUp on Google PlayQ13Power Factor
In an AC circuit, the power factor is defined as:
Not quite — the answer is B.
Power factor = cosφ where φ is the phase difference between voltage and current. Equivalently PF = real power / apparent power = P/S. Option A is the reciprocal (1/cosφ), not cosφ.
Q14Transformer
For an ideal transformer, the ratio of secondary voltage to primary voltage is equal to:
Not quite — the answer is A.
For an ideal transformer V_S/V_P = N_S/N_P — secondary voltage is directly proportional to secondary turns. Option B (N_P/N_S) is the classic inverted-ratio trap that catches students who confuse the voltage and current ratio directions.
Q15Choke Coil
A choke coil is primarily used in an AC circuit to:
Not quite — the answer is A.
A choke coil has large inductive reactance and very small resistance, limiting AC current while consuming negligible average power. This makes it far more efficient than a resistor for current control. A resistor would dissipate significant energy as heat for the same current reduction.
Q16Impedance and Phase Angle
A series LCR circuit has resistance 6 Ω and net reactance 8 Ω. The impedance of the circuit is:
Not quite — the answer is C.
Z = √(R²+X²) = √(36+64) = √100 = 10 Ω. Trap Option A (2 Ω) comes from subtracting R and X; Option D (14 Ω) from adding directly. Z always uses Pythagoras, never simple arithmetic.
Q17Alternating Current Basics
An alternating current has a frequency of 50 Hz. What is the time period of one complete cycle?
Not quite — the answer is B.
T = 1/f = 1/50 = 0.02 s. Common trap: students invert correctly but misplace the decimal, selecting 0.2 s (off by factor of 10). D confuses T with f numerically.
Q18Alternating Current Basics
A sinusoidal AC source operates at a frequency of 50 Hz. What is the angular frequency of this source?
Not quite — the answer is D.
omega = 2*pi*f = 2 * 3.14 * 50 = 314 rad/s. Option A confuses omega with f. Option C omits pi entirely. Option B uses pi*f instead of 2*pi*f.
Q19Alternating Current Basics
In a sinusoidal AC circuit, the instantaneous current becomes zero twice per cycle. Which of the following best explains this?
Not quite — the answer is B.
i = I0*sin(omega*t) = 0 when omega*t = 0, pi, 2*pi — at start and midpoint of each cycle. These are the zero crossings of the sine function. Resistance does not change; zero crossings are a property of the sinusoidal function itself.
Q20Alternating Current Basics
The frequency of an AC source is doubled while the peak current remains unchanged. Which of the following statements is correct?
Not quite — the answer is A.
T = 1/f, so doubling f halves T. Angular frequency omega = 2*pi*f also doubles, not unchanged. Number of cycles per second equals frequency, which doubles — not halves.
Q21Alternating Current Basics
A sinusoidal alternating current is expressed as i = I0*sin(omega*t). What is the instantaneous current at t = T/4?
Not quite — the answer is C.
At t = T/4, omega*t = (2*pi/T)*(T/4) = pi/2. Therefore i = I0*sin(pi/2) = I0. Option B (zero) is the value at t = T/2. Option D is the RMS value, a common confusion with instantaneous value.
Q22Alternating Current Basics
Which of the following quantities is measured in the same unit as time period?
Not quite — the answer is B.
Time period T is measured in seconds. Frequency is measured in Hz = s^-1, the reciprocal. Hertz and radian per second are also reciprocal-time units. Second is the SI unit of time period.
Q23Alternating Current Basics
An AC waveform has a time period of 25 ms. How many complete cycles will occur in 0.50 s?
Not quite — the answer is C.
f = 1/T = 1/(25*10^-3) = 40 Hz. Cycles N = f*t = 40 * 0.50 = 20. Option A arises from incorrectly using T = 0.05 s. Option D results from computing cycles = f alone without multiplying by time.
Q24Alternating Current Basics
All of the following statements about a sinusoidal alternating current are correct EXCEPT:
Not quite — the answer is A.
The instantaneous value of sinusoidal AC varies continuously with time — it is never constant. B, C, D are all true properties of sinusoidal AC. A is the exception.
ELITE question · AIR under 50 level
This chapter has 208 ELITE questions for students aiming at the very top. They are only in the app.
Unlock ELITE questions in the appKey AC Concepts
Quick revision: most questions in this chapter test these facts.
| Concept | Key Formula |
|---|---|
| RMS values | V_rms = V₀/√2; I_rms = I₀/√2; used for power calculations |
| Impedance (LCR) | Z = √(R² + (XL−XC)²); XL = ωL; XC = 1/ωC |
| Resonance | XL = XC → ω₀ = 1/√(LC); Z = R (minimum); current maximum |
| Power factor | cos φ = R/Z; P = V_rms I_rms cos φ; purely resistive: cos φ = 1 |
| Transformer | V₂/V₁ = N₂/N₁ = I₁/I₂; step-up: N₂ > N₁; ideal: P₁ = P₂ |
| Quality factor | Q = ωL/R = 1/ωCR; sharp resonance = high Q |
What the app covers in this chapter
546 questions in total, each with a detailed explanation.
| Grand Test | 97 |
| Important PYQ Mixed Concepts | 59 |
| AC Through Pure Resistor | 40 |
| Alternating Current Basics | 39 |
| AC Voltage and Current Equations | 39 |
| Series LCR Circuit | 39 |
| RMS and Mean Values | 37 |
| AC Through Pure Capacitor | 30 |
| AC Through Pure Inductor | 29 |
| Phasor Diagram | 20 |
| Resonance in LCR Circuit | 20 |
| Power in AC Circuit | 20 |
| Power Factor | 20 |
| Transformer | 20 |
| Choke Coil | 19 |
| Impedance and Phase Angle | 18 |
Questions students ask
Is Alternating Current important for NEET?
Yes — LCR circuit impedance, resonance, power factor and transformer are tested regularly. Numerical problems dominate.
Which topics should I revise first?
Master impedance formula for LCR, resonance condition and frequency, power factor and average power, transformer turns ratio, and RMS vs peak values.
How many questions from this chapter are on RankUp?
The RankUp app has 546 questions on Alternating Current, including 208 ELITE questions. Every question has a detailed explanation.
