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Alternating Current

NCERT Class 12 Physics Chapter 7

Alternating Current for Class 12 Physics, JEE and NEET: rms values, reactance, phasors, the series LCR circuit, resonance, power factor and transformers.

7 topics Updated 8 Nov 2026

For: Class 12, JEE and NEET

In the syllabus: JEE Class 12 syllabus, NEET Class 12 syllabus

With your textbook

NCERT Physics Part I (Class 12), Chapter 7 “Alternating Current”, reprint 2026-27.

Open the official NCERT chapter (PDF, 2.4 MB, ncert.nic.in)

Checked on 19 Oct 2026.

Learn in this order

  1. AC voltage applied to a resistor

    AC voltage applied to a resistor, current in phase, average power ½iR, I = i/√2, 311 V peak, examples.

    NCERT Section 7.2 11 min read 4 solved examples 7 practice questions

  2. Phasors: ac current and voltage as rotating vectors

    AC voltage and current as rotating vectors, phase difference, lead and lag, adding phasors.

    NCERT Section 7.3 11 min read 4 solved examples 7 practice questions

  3. AC voltage applied to an inductor

    AC voltage applied to an inductor, current lagging by π/2, XL = ωL = 2πνL, zero average power, examples.

    NCERT Section 7.4 10 min read 4 solved examples 7 practice questions

  4. AC voltage applied to a capacitor

    AC voltage applied to a capacitor, current leading by π/2, XC = 1/(ωC), why DC is blocked, examples.

    NCERT Section 7.5 10 min read 4 solved examples 7 practice questions

  5. Series LCR circuit

    Phasor-diagram derivation, impedance Z, phase angle, resonance at ω = 1/√(LC), radio tuning, examples.

    NCERT Section 7.6 15 min read 4 solved examples 7 practice questions

  6. Power in ac circuits and the power factor

    Average power VI cos φ in AC circuits and its derivation, wattless current, power at resonance, NCERT examples.

    NCERT Section 7.7 12 min read 4 solved examples 7 practice questions

  7. Transformers

    Voltage and current ratios, power factor, efficiency, leakage flux, core losses and transmission.

    NCERT Section 7.8 14 min read 4 solved examples 7 practice questions

Key formulas in this chapter

AC voltage applied to a resistor

  • v = vm sin ωt, i = im sin ωt, im = vm/R
  • average power: im^2 R/2 = I2 R
  • I = im/√2, V = vm/√2
  • P = I2 R, V = IR, P = IV

Phasors

  • v = vm sin ωt, i = im sin(ωt + φ)
  • sum of two phasors: A2 = A1^2 + A2^2 + 2A1A2 cos φ
  • phase difference = angle between phasors

AC voltage applied to an inductor

  • v = vm sin ωt, i = im sin(ωt − π/2)
  • XL = ωL = 2πνL
  • im = vm/XL
  • average power: PL = 0

AC voltage applied to a capacitor

  • v = vm sin ωt, i = im sin(ωt + π/2)
  • XC = 1/(ωC) = 1/(2πνC)
  • im = vm/XC
  • average power: PC = 0

Series LCR circuit

  • i = im sin(ωt + φ), im = vm/Z
  • Z = √(R2 + (XC − XL)2)
  • tan φ = (XC − XL)/R
  • resonance: XL = XC, ω0 = 1/√(LC), Z = R

Power in ac circuits

  • P = VI cos φ = I2 Z cos φ
  • power factor: cos φ = R/Z
  • power component: I cos φ
  • wattless component magnitude: I |sin φ|
  • at resonance: P = I2 R

Transformers

  • Vs/Vp = Ns/Np
  • Is/Ip = Np/Ns
  • ideal: Vp Ip = Vs Is
  • efficiency = output power/input power

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