Quick answer: An alternating current NEET PYQ analysis shows that NTA asks about one to two questions a year from this chapter, and almost all of them are short numericals from five areas: rms and peak values, reactance and phase in pure R, L and C circuits, impedance of a series LCR circuit, resonance (resonant frequency, quality factor, power factor), and transformers. A small formula set solves nearly every past question.
This guide breaks down what NTA repeats, the formulas you must know and solved PYQ-pattern numericals. Before you start, get the bigger picture from our complete NEET Physics strategy.
Want to test yourself first? Solve the Alternating Current PYQs for free, then use this guide to fix the gaps.
Table of Contents
Why Alternating Current Is Worth Mastering
The chapter has about 60 past questions, and they keep using the same six or seven formulas. Once you can find Z, φ and ω₀ quickly, most questions take under a minute. For chapter notes, use our Alternating Current class 12 notes, and to see where it sits among other chapters, check the NEET 2027 chapter weightage.
The chapter follows directly from induction, so revise our Electromagnetic Induction guide and our Magnetism PYQ analysis if self-inductance feels shaky. For speed on the numericals, our tips to solve Physics numericals faster help a lot here.
Alternating Current NEET PYQ Analysis: What NTA Repeats
When you sort the past NEET physics PYQs from this chapter, five question types cover nearly everything.
| Question type | Frequency | What it tests |
|---|---|---|
| Series LCR circuit | Most frequent | Impedance, phase angle, voltage across each element |
| Resonance | Very frequent | Resonant frequency, current at resonance, quality factor, bandwidth |
| Power and power factor | Very frequent | Average power, power factor, wattless current |
| Pure R, L and C circuits | Regular | Reactance, phase difference between current and voltage |
| Transformers and LC oscillations | Regular | Turns ratio, energy losses, energy exchange in LC circuits |
RMS and Peak Values
For v = vₘ sin ωt and i = iₘ sin ωt:
| Quantity | Value |
|---|---|
| RMS current | I = iₘ/√2 ≈ 0.707 iₘ |
| RMS voltage | V = vₘ/√2 |
| Average current over a full cycle | Zero |
| Average current over a half cycle | 2iₘ/π ≈ 0.637 iₘ |
Household supply in India is 220 V rms at 50 Hz, so the peak voltage is 220 × √2 ≈ 311 V. AC meters read rms values.
Pure Resistor, Inductor and Capacitor
| Circuit | Opposition | Phase of current | Average power |
|---|---|---|---|
| Pure resistor | R | In phase with voltage | I²R |
| Pure inductor | XL = ωL | Lags voltage by π/2 | Zero |
| Pure capacitor | XC = 1/ωC | Leads voltage by π/2 | Zero |
Because XC = 1/ωC, a capacitor blocks DC (ω = 0 gives infinite reactance) and lets high-frequency AC pass easily. An inductor does the opposite: XL rises with frequency.
The Series LCR Circuit
Impedance: Z = √[R² + (XL − XC)²]
Phase angle: tan φ = (XL − XC)/R
- If XL > XC, the circuit is inductive and the current lags the voltage.
- If XC > XL, the circuit is capacitive and the current leads the voltage.
- The voltages across L and C are always π apart, so they partly cancel. That is why the voltage across L or C alone can be larger than the source voltage.
Resonance in an LCR Circuit
Resonance happens when XL = XC. Then:
| Quantity | At resonance |
|---|---|
| Resonant angular frequency | ω₀ = 1/√(LC) |
| Resonant frequency | f₀ = 1/(2π√(LC)) |
| Impedance | Z = R (minimum) |
| Current | Maximum, I = V/R |
| Phase angle | Zero; current and voltage in phase |
| Power factor | 1 |
- Resonance needs both L and C; an RL or RC circuit cannot resonate.
- Quality factor: Q = ω₀L/R = 1/(ω₀CR). A higher Q means a sharper resonance.
- Bandwidth: 2Δω = R/L, so Q = ω₀/(2Δω).
- Resonant circuits are used in the tuning of radio and TV receivers.
Power and Power Factor
Average power: P = V I cos φ (rms values)
Power factor: cos φ = R/Z
| Case | Power factor | Power |
|---|---|---|
| Pure resistor | 1 | Maximum, VI |
| Pure inductor or capacitor | 0 | Zero; the current is a wattless current |
| LCR at resonance | 1 | Maximum |
Low power factor means large current for little useful power, so power companies add capacitors to raise the power factor in industrial circuits.
LC Oscillations
When a charged capacitor is connected to an inductor, charge oscillates at ω₀ = 1/√(LC). Energy moves back and forth between the capacitor’s electric field (q²/2C) and the inductor’s magnetic field (Li²/2), and the total stays constant if there is no resistance. The system is like a mass on a spring: L acts like mass, and 1/C acts like the spring constant.
Transformers
For an ideal transformer:
Vₛ/Vₚ = Nₛ/Nₚ = Iₚ/Iₛ
| Type | Turns | Voltage | Current |
|---|---|---|---|
| Step-up | Nₛ > Nₚ | Increases | Decreases |
| Step-down | Nₛ < Nₚ | Decreases | Increases |
Energy losses and how they are reduced:
| Loss | Reduced by |
|---|---|
| Flux leakage | Winding primary and secondary over each other |
| Resistance of windings (copper loss) | Using thick copper wire |
| Eddy currents | A laminated iron core |
| Hysteresis | A soft iron core with low hysteresis loss |
A transformer works only on AC, because it needs a changing magnetic flux.
Worked PYQ-Pattern Questions
Q1. The peak value of a 220 V rms AC supply is about: (a) 156 V (b) 220 V (c) 311 V (d) 440 V
Answer: (c) 311 V, because vₘ = 220 × √2.
Q2. In a series LCR circuit, R = 30 Ω, XL = 80 Ω and XC = 40 Ω. Find the impedance and power factor.
Z = √(30² + (80 − 40)²) = √(900 + 1600) = 50 Ω. Power factor = R/Z = 30/50.
Answer: Z = 50 Ω, power factor = 0.6.
Q3. Find the resonant frequency of an LCR circuit with L = 0.1 H and C = 10 µF.
ω₀ = 1/√(0.1 × 10 × 10⁻⁶) = 1/√(10⁻⁶) = 1000 rad/s, so f₀ = 1000/(2π).
Answer: About 159 Hz.
Q4. The current through a pure inductor is called wattless because:
Answer: The phase difference between current and voltage is π/2, so the power factor cos φ is zero and the average power is zero.
Q5. A transformer has 100 primary and 500 secondary turns. If the primary voltage is 220 V, what is the secondary voltage?
Vₛ = 220 × (500/100).
Answer: 1100 V, a step-up transformer. The secondary current is one-fifth of the primary current.
Q6. Why is the core of a transformer laminated?
Answer: To reduce eddy current losses.
Common Traps in This Chapter
- Saying current leads voltage in an inductor (it lags)
- Forgetting that a capacitor’s reactance falls as frequency rises
- Saying an RC circuit can show resonance
- Using peak values in P = VI cos φ (use rms values)
- Calling the power factor of a pure inductor 1 (it is 0)
- Saying a transformer works on DC
Practise Every Alternating Current PYQ
This alternating current NEET PYQ analysis shows you where the marks are. Solving real past questions makes the formulas automatic. Every Alternating Current previous year question is available chapter-wise, free:
👉 Solve all Alternating Current NEET PYQs
Keep the formula tables beside you for the first round, then solve the second round without them.
Conclusion
Alternating Current rewards practice more than reading. Use this alternating current NEET PYQ analysis to master rms values, reactance and phase, LCR impedance, resonance, power factor and transformers. Then solve the past numericals until each one takes under a minute.
❓ FAQ
Q: How many questions come from Alternating Current in NEET? A: Usually one to two questions a year, mostly numericals on LCR circuits, resonance and power factor.
Q: What is the most important topic in Alternating Current for NEET? A: The series LCR circuit, especially impedance and resonance, followed by power factor and transformers.
Q: What happens at resonance in a series LCR circuit? A: XL equals XC, the impedance is minimum (equal to R), the current is maximum, and the power factor is 1.
Q: What is wattless current? A: Current in a purely inductive or purely capacitive circuit, where the phase difference is π/2, so no average power is used.
Q: Why does a capacitor block DC? A: Its reactance is 1/ωC. For DC, ω = 0, so the reactance is infinite and no steady current flows.
Q: Where can I practise Alternating Current NEET PYQs for free? A: You can solve every Alternating Current previous year question chapter-wise for free using the practice links in this article.
