# Alternating Current Notes, Class 12 Physics | SwaVid

Alternating Current, NCERT Class 12 Physics: notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers.

Canonical: https://www.swavid.com/cbse/class-12/physics/chapter/alternating-current

Source: https://www.swavid.com/cbse/class-12/physics/chapter/alternating-current

# Alternating Current

## Formulas and definitions

## Key ideas

## Common mistakes

## Quick checks

## Keep going

## Questions parents ask

## Class 12 Physics on SwaVid

### AC voltage applied to a resistor

### AC voltage applied to an inductor

### AC voltage applied to a capacitor

### Series LCR circuit and impedance

### Resonance in series LCR circuit

### Power in AC circuits and power factor

### Transformers

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SwaVid › CBSE › Class 12 › Physics › Alternating Current

Notes written by SwaVid from the NCERT textbook.

Class 12 Physics, Chapter 7. Key ideas, formulas and common mistakes.

CBSE prints 17 marks for Units III-IV together, not for this chapter alone.

Short answer. This chapter deals with alternating voltages and currents that vary sinusoidally with time in resistive, inductive, and capacitive circuits. It explains phasor diagrams, impedance, resonance in series LCR circuits, power factor, and the working of transformers.

Source: NCERT Physics Part I (Reprint 2026-27) , Chapter 7, Alternating Current

An alternating voltage v = vₘ sin(ωt) drives a current i = iₘ sin(ωt) through a pure resistor. The voltage and current are in phase, and the average power dissipated over a complete cycle is (vₘ iₘ) / 2 = V I, where V and I are rms values.

In a purely inductive circuit, the induced emf opposes the current change. The current lags the voltage by π/2 radians or 90°. The inductive reactance is X L = ωL, and the average power consumed over a full cycle is zero.

In a purely capacitive circuit, alternating current leads the applied voltage by π/2 radians or 90°. The capacitive reactance is X C = 1 / (ωC), which decreases as the frequency increases. The average power over a complete cycle is zero.

When a resistor, inductor, and capacitor are in series across an AC source, the total opposition to current is the impedance Z = √[R² + (X L - X C )²]. The phase angle between voltage and current satisfies tan ϕ = (X L - X C ) / R.

Resonance occurs when X L = X C at the resonant angular frequency ω₀ = 1 / √(LC). At resonance, the impedance is minimum and equal to R, producing a maximum current amplitude that is entirely in phase with the source voltage.

The average power dissipated in an AC circuit is P = V I cos ϕ, where cos ϕ = R / Z is the power factor. Power dissipation is maximum when the circuit is purely resistive (cos ϕ = 1) and zero when it is purely reactive (cos ϕ = 0).

A transformer changes alternating voltage levels through mutual induction between two coils wound on a soft iron core. For an ideal transformer, V s / V p = N s / N p = I p / I s . Energy losses occur due to flux leakage, winding resistance, eddy currents, and hysteresis.

Try each question first. Then tap it to see the answer.

I = V / R = 200 / 50 = 4 A

iₘ = √2 × I = 1.414 × 4 = 5.656 A

rms current = 4 A, peak current = 5.656 A

X L = 2πfL = 2 × 3.142 × 50 × 0.1 = 31.42 Ω

I = V / X L = 220 / 31.42 = 7.00 A

X L = 31.42 Ω, I = 7.00 A

ω₀ = 1 / √(LC) = 1 / √(2.0 × 32 × 10⁻⁶) = 1 / √(64 × 10⁻⁶)

ω₀ = 1 / (8 × 10⁻³) = 125 rad s⁻¹

At resonance, X L = X C , so Z = R = 10 Ω

ω₀ = 125 rad s⁻¹, Z = 10 Ω

Electromagnetic Waves .

Electromagnetic Induction .

Current Electricity .

The 14 chapters of Class 12 Physics .

CBSE does not give marks to one chapter. It prints 17 marks for Units III-IV together (Magnetic Effects of Current and Magnetism; Electromagnetic Induction and Alternating Currents). Alternating Current is part of that group.

No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed.

Electromagnetic Waves.

SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works.

- Ch 7 NCERT book
- 17 marks, Units III-IV together
- 3 quick checks
- Adding AC voltages algebraically across series components . Voltages across R, L, and C have different phase angles. Find the net voltage using phasor addition, V = √[V R ² + (V L - V C )²], not simple scalar addition.
- Using peak voltage instead of rms voltage in standard power calculations . Standard meters and problem statements specify rms values unless peak is stated. The formula P = V I cos ϕ strictly uses rms values V and I.
- Confusing whether current leads or lags in capacitive and inductive circuits . Remember that current leads voltage by π/2 in a pure capacitor, and current lags voltage by π/2 in a pure inductor.
- Assuming a transformer works on direct current . A transformer relies on a continuously changing magnetic flux to induce an emf. Constant direct current gives zero rate of change of flux, so no secondary voltage is induced.

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