NEET · Physics

NEET Alternating Current: RMS, Reactance, LCR, Resonance and Transformer

Own the AC half of Unit 14, including sinusoidal AC, peak/RMS values, reactance, impedance, series LCR, resonance, AC power, wattless current, AC generator and transformer. Every formula must state sinusoidal steady-state/ideal assumptions where required.

Subject: Physics
Official syllabus unit: Unit 14: Electromagnetic Induction and Alternating Currents

In short

The verified 2026 Unit 14 AC scope includes peak and RMS current/voltage, reactance and impedance, series LCR circuits, resonance, AC power, wattless current, AC generator and transformer. RMS shortcuts such as I_rms = I0/sqrt(2) apply to sinusoidal waveforms, not every alternating waveform.

Subject faculty

Ashwini M.

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Physics • NEET

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Also in Physics: Kumar Gandharva Saxena

Syllabus mapping

Syllabus mapping
Unit	Topics
Focused owner of the alternating-current portion of NEET UG 2026 Physics Unit 14	AC is time-varying, not simply "current that changes", RMS with waveform condition, Reactance and impedance, Resonance, AC power, Generator and transformer

Unit
Focused owner of the alternating-current portion of NEET UG 2026 Physics Unit 14
Topics
AC is time-varying, not simply "current that changes", RMS with waveform condition, Reactance and impedance, Resonance, AC power, Generator and transformer

Before this chapter

Concepts in this chapter

1. AC is time-varying, not simply "current that changes"

Use sinusoidal voltage/current as the standard NEET model. Make phase visible on a graph before using phasor relations.

2. RMS with waveform condition

For a sine wave, RMS is peak divided by sqrt(2). Do not transfer that ratio to a non-sinusoidal waveform without computing the RMS definition.

3. Reactance and impedance

Inductive reactance increases with angular frequency; capacitive reactance decreases. In a series LCR circuit, the difference X_L-X_C determines whether current lags or leads the applied voltage.

4. Resonance

Series resonance occurs when X_L=X_C, giving the ideal resonance angular frequency 1/sqrt(LC). At resonance the reactive contributions cancel in the series impedance and current is maximum for fixed applied RMS voltage and resistance. Real circuits have losses and finite bandwidth.

5. AC power

Average power depends on the power factor cos(phi). In an ideal pure inductor or capacitor, average power over a complete cycle is zero even though instantaneous energy exchange occurs.

6. Generator and transformer

The generator block connects rotating magnetic flux to sinusoidal induced emf. The transformer relation is idealized: turns ratio controls voltage ratio, while current ratio is inverse if losses are neglected. Do not imply a transformer works on steady DC.

7. Decision aids for this chapter

Frequency-response decision strip: as frequency rises, X_L rises and X_C falls.
Series-LCR phase card: capacitive, resonant and inductive states.
RMS warning badge: peak/sqrt(2) only for a sinusoid.

Formula sheet

Sinusoidal current: i = I0 sin(omega t + phi0). Sinusoidal signal. I0 peak A.
Sinusoidal current
Use when — Sinusoidal signal. I0 peak A.
RMS sine values: I_rms = I0/sqrt(2), V_rms = V0/sqrt(2). Pure sinusoid over complete cycles.
RMS sine values
Use when — Pure sinusoid over complete cycles.
Inductive reactance: X_L = omega L. Sinusoidal steady state, ideal inductor. Unit ohm.
Inductive reactance
Use when — Sinusoidal steady state, ideal inductor. Unit ohm.
Capacitive reactance: X_C = 1/(omega C). Sinusoidal steady state, ideal capacitor. Unit ohm.
Capacitive reactance
Use when — Sinusoidal steady state, ideal capacitor. Unit ohm.
Series LCR impedance: Z = sqrt(R^2 + (X_L-X_C)^2). Sinusoidal steady state, series LCR. Unit ohm.
Series LCR impedance
Use when — Sinusoidal steady state, series LCR. Unit ohm.
Phase relation: tan(phi) = (X_L-X_C)/R. Series LCR with chosen voltage-current phase convention.
Phase relation
Use when — Series LCR with chosen voltage-current phase convention.
RMS current: I_rms = V_rms/Z. Series sinusoidal steady state.
RMS current
Use when — Series sinusoidal steady state.
Resonance: omega_0 = 1/sqrt(LC), f0 = 1/(2πsqrt(LC)). Ideal series LCR where X_L=X_C.
Resonance
Use when — Ideal series LCR where X_L=X_C.
Average power: P_avg = V_rms I_rms cos(phi). Sinusoidal steady state. Unit W.
Average power
Use when — Sinusoidal steady state. Unit W.
Generator emf: epsilon = N B A omega sin(omega t). Ideal rotating coil, uniform B, chosen phase origin.
Generator emf
Use when — Ideal rotating coil, uniform B, chosen phase origin.
Ideal transformer: V_s/V_p = N_s/N_p. Ideal transformer, sinusoidal AC, negligible losses.
Ideal transformer
Use when — Ideal transformer, sinusoidal AC, negligible losses.
Ideal transformer current: I_s/I_p = N_p/N_s. Ideal power conservation.
Ideal transformer current
Use when — Ideal power conservation.

Worked examples

L = 0.10 H and C = 100 microF.

Target concept: series-LCR resonance.

Condition: L = 0.10 H and C = 100 microF.

Method selection: use the ideal series resonance frequency because the question asks when inductive and capacitive reactances cancel.

Reasoning/calculation: LC = 1.0 x 10^-5; sqrt(LC) = 3.162 x 10^-3; f0 = 1/[2πsqrt(LC)] approx 50.3 Hz.

Units/sign check: LC has units s^2, so 1/sqrt(LC) has s^-1; frequency is positive in Hz.

PI category: Decision / Selection Error if a non-resonance model is chosen first; Execution Error for microfarad conversion.

Common mistakes and what they actually indicate

Uses peak/sqrt(2) for non-sinusoidal AC
Decision / selection error
Why it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Check waveform before RMS shortcut.
Mixes peak voltage with RMS current
Execution error
Why it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
Keep peak or RMS system consistent.
Uses series-LCR impedance for parallel circuit
Decision / selection error
Why it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Identify topology before formula.
Says current lags at all frequencies
Knowledge gap
Why it happens
Preparation Intelligence classification: Knowledge Gap.
How it is corrected
Compare X_L and X_C.
Applies transformer turns ratio to DC
Needs review
Why it happens
Preparation Intelligence classification: Needs Review.
How it is corrected
Restore changing-flux/AC condition.

FAQ