NCERT Class 12 Physics Chapter 7 Alternating Current Notes - Download Free PDF

The NCERT Notes for Class 12 Physics Chapter 7 Alternating Current provide a detailed overview of all important topics covered in the chapter. According to the latest syllabus for CBSE 2026-27, the Alternating Current Chapter contributes to 6-8 marks in the Board exam.

First, here is a quick exam-weightage snapshot for Alternating Current:

• CBSE Weightage: Theory-based 2-3 questions, including formula-based numericals and definitions contributing to 8-12 marks in the CBSE Board exams.

• JEE Main Weightage: 1-2 questions per session can be seen from the Alternating Current Chapter in the JEE Main. Questions are usually fast-calculation-based.

• JEE Advanced Weightage: In the JEE Advanced, AC questions are usually paired with numericals and graphs, contributing to around 8-12 marks (2-3 questions) in the exam.

You can find complete revision notes on alternating current class 12 NCERT chapter 7, including all formulas and numerical tips as noted for CBSE boards in the article below.

Collegedunia’s Alternating Current notes are based on NCERT, Reference books like S.L Arora, and previous years’ questions, which will help you not only in Boards but in Competitive exams as well.

Here is how much this chapter matters for Boards and JEE, and how long to study it.

• CBSE Board Weightage (2026): Usually, this chapter carries 2 marks alone, or up to 6 marks with EMI (Unit IV), with 1 easy question or simple numerical from LCR, power, or transformers.
• JEE Main Weightage (2026 trends): 2-3 questions around 6.6% weightage in Physics. More numerical and conceptual MCQs compared to boards.
• High-Yield Topics (Common): Series LCR circuit (resonance, impedance, power factor), phasor diagrams, transformer working & efficiency, RMS/average values.
• Previous Year Trend: Boards - straightforward 2-5 marks; JEE Main - resonance, power, and transformer-based questions appear consistently.

For preparation, spend 4-6 hours for CBSE Boards (theory + diagrams) and 8-12 hours for JEE (numericals + concepts + practice).

1. Basics of Alternating Current

Alternating Current (AC) is a type of electric current that continuously changes both its magnitude and direction with time. Unlike direct current (DC), which flows only in one direction, AC reverses its direction periodically, making it ideal for transmission over long distances.

Mathematical Representation (Instantaneous Value)

i = I₀ sin(ω t) or i = I₀ cos(ω t)

Where:

• I₀ = Peak current
• ω = 2π f = 2πT = Angular frequency

Average (Mean) Value (Over Half Cycle)

I_avg = 2I₀π ≈ 0.637 I₀

Result:

• I_avg = 0.637 I₀ (63.7% of peak value)

RMS (Root Mean Square) Value (Effective Value)

I_rms = I₀√2, V_rms = V₀√2

Result:

• I_rms = 0.707 I₀
• V_rms = 0.707 V₀

2. AC Circuit with Pure Resistance (R only)

When AC is applied to a resistor, current and voltage vary together without any phase difference. The circuit behaves exactly like a DC circuit.

Equations

E = E₀ sin(ω t), I = I₀ sin(ω t)

Where:

I₀ = E₀R

Key Points

• Voltage and current are in phase ( φ = 0^∘ )
• Power is continuously consumed
• Phasor diagram: Same direction

3. AC Circuit with Pure Inductance (L only)

An inductor resists a change in current due to self-induction. Hence, the current lags behind the voltage.

Current Equation (Phase Lag)

I = I₀ sin(ω t - π2)

Inductive Reactance

X_L = ω L

I₀ = E₀X_L

Key Points

• Current lags voltage by 90°
• X_L = 0 for DC
• No average power consumed

4. AC Circuit with Pure Capacitance (C only)

Current leads voltage by 90^∘ (or voltage lags current by 90^∘ ).

A capacitor allows current to change easily, so current leads voltage.

Current Equation (Phase Lead)

I = I₀ sin(ω t + π2)

Capacitive Reactance

X_C = 1ω C

I₀ = E₀X_C

Key Points

• Current leads voltage by 90°
• No power consumption
• Phasor: Current ahead

5. Series LCR Circuit (Most Important Section)

In an LCR circuit, resistance, inductance, and capacitance are connected in series, and the same current flows through all.

R, L, and C in series → same current through all.

Voltage Relations

• V_R = I₀ R (in phase)
• V_L = I₀ X_L (leads by 90°)
• V_C = I₀ X_C (lags by 90°)

Impedance

Z = √R² + (X_L - X_C)²

I₀ = E₀Z

Phase Angle

tan φ = X_L - X_CR

Resonance Condition

_r = 1√LC, f_r = 12π√LC

At Resonance

• X_L = X_C
• Z = R (minimum)
• Current = maximum
• φ = 0 , power factor = 1

Quality Factor (Q)

Q = _r LR = 1R√LC

6. Power in AC Circuit

Power in AC depends on phase difference between voltage and current.

Instantaneous Power

P = EI

Average Power

P_avg = V_rms I_rms cos φ = E₀ I₀2 cos φ

Power Factor

cos φ = RZ

Key Points

• Pure L/C → cosφ = 0 (wattless current)
• Max power when φ = 0

7. Transformers (Very High Weightage)

Construction: Soft iron laminated core + primary (P) and secondary (S) coils.

Principle: Mutual induction.

A transformer changes AC voltage using mutual induction.

Turns Ratio

E_sE_p = N_sN_p

Power Relation

E_p I_p = E_s I_s

Efficiency

η = E_s I_sE_p I_p × 100%

Key Points

• Step-up: N_s > N_p
• Step-down: N_s < N_p
• Losses: Copper, eddy current, hysteresis

Quick Formula Sheet (Memorise These!)

• X_L = ω L , X_C = 1ω C
• Z = √R² + (X_L - X_C)²
• Resonance: f_r = 12π √LC , Z_min = R
• P_avg = V_rms I_rms cos φ
• Transformer: E_sE_p = N_sN_p = I_pI_s

How Many Marks Is Alternating Current in Class 12?

Expect 2-3 marks directly, but with EMI, this unit can contribute up to 6-8 marks, making it an important scoring chapter.

Marks Breakdown (Based on Trends)

• AC alone: Usually 2-3 marks
• Combined with EMI: Can go up to 5-6 marks
• Overall Unit IV weightage: ~11-12% of the Physics paper (8/70 marks)

Question Pattern Insights

• 1 short question (1-2 marks) → theory (e.g., why AC is preferred over DC)
• OR 1 simple numerical (2-3 marks) → LCR circuit, power, or transformer
• Occasionally part of a case-based or diagram question

Alternating Current Key Formulas

Alternating Current Questions - CBSE 2026 (All Sets)

The Alternating Current (AC) chapter in CBSE 2026 carried 18-22% weightage, contributing 12-15 marks through 6-8 questions.

• Around 50-60% of questions came from LCR circuits and resonance, while short answers alone contributed around 40-45% of marks.
• The paper was 60% numerical and 40% conceptual, with case-based questions forming a high-weight segment.
• This shows that focusing on just 4-5 core topics can help students secure up to 80% of AC marks, making it a highly scoring and strategic chapter.

Source -

• CBSE Class 12 Physics Question Paper 2026 Set 1
• CBSE Class 12 Physics Question Paper 2026 Set 2
• CBSE Class 12 Physics Question Paper 2026 Set 3

Alternating Current Toppers' Strategy

CBSE focuses on theory, derivations and direct questions, while JEE Main emphasizes numericals and concept application. Focusing on 5 to 6 core topics covers 70 to 80% of AC questions. The topic-wise plan below shows where to spend time for each exam.

Topper-Level Insights for Alternating Current Preparation

• RLC + Resonance = 40-50% of AC numericals: Focus here first.
• RMS + Power formulas are directly asked: Easy marks.
• Phase concept = conceptual trap area: Avoid silly mistakes.
• Transformer = low effort, high return: Must not skip.
• Formula revision daily (5 mins): Improves speed drastically.

All the best for your boards! Revise these notes 2-3 times and solve NCERT + previous year questions

Other Resources for Class 12 Physics Chapter 7

Pair these notes with the Solutions, Handwritten Notes, Formula Sheet and official NCERT chapter linked below.

All Chapters Notes for Class 12 Physics