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

Alternating Current (AC) is an important and high-scoring chapter in Class 12 Physics. AC usually contributes around 6–8 marks, making it one of the most scoring topics in the syllabus.

• AC Current CBSE Weightage: 8–12 marks
• AC Current JEE Main Weightage: 6–8 marks (2–3 questions)
• AC Current JEE Advanced Weightage: 8–12 marks (2–3 questions)

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.

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)

$$ i = I_0 \sin(\omega t) \quad \text{or} \quad i = I_0 \cos(\omega t) $$

Where:

• \( I_0 \) = Peak current
• \( \omega = 2\pi f = \dfrac{2\pi}{T} \) = Angular frequency

Average (Mean) Value (Over Half Cycle)

$$ I_{avg} = \frac{2I_0}{\pi} \approx 0.637 \, I_0 $$

Result:

• \( I_{avg} = 0.637 \, I_0 \)  (63.7% of peak value)

RMS (Root Mean Square) Value (Effective Value)

$$ I_{rms} = \frac{I_0}{\sqrt{2}}, \quad V_{rms} = \frac{V_0}{\sqrt{2}} $$

Result:

• \( I_{rms} = 0.707 \, I_0 \)
• \( V_{rms} = 0.707 \, V_0 \)

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

$$ E = E_0 \sin(\omega t), \quad I = I_0 \sin(\omega t) $$

Where:

$$ I_0 = \frac{E_0}{R} $$

Key Points

• Voltage and current are in phase (\( \phi = 0^\circ \))
• 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)

$$ I = I_0 \sin\left(\omega t - \frac{\pi}{2}\right) $$

Inductive Reactance

$$ X_L = \omega L $$

$$ I_0 = \frac{E_0}{X_L} $$

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 \( 90^\circ \) (or voltage lags current by \( 90^\circ \)).

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

Current Equation (Phase Lead)

$$ I = I_0 \sin\left(\omega t + \frac{\pi}{2}\right) $$

Capacitive Reactance

$$ X_C = \frac{1}{\omega C} $$

$$ I_0 = \frac{E_0}{X_C} $$

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

• \( V_R = I_0 R \) (in phase)
• \( V_L = I_0 X_L \) (leads by 90°)
• \( V_C = I_0 X_C \) (lags by 90°)

Impedance

$$ Z = \sqrt{R^2 + (X_L - X_C)^2} $$

$$ I_0 = \frac{E_0}{Z} $$

Phase Angle

$$ \tan \phi = \frac{X_L - X_C}{R} $$

Resonance Condition

$$ \omega_r = \frac{1}{\sqrt{LC}}, \quad f_r = \frac{1}{2\pi\sqrt{LC}} $$

At Resonance

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

Quality Factor (Q)

$$ Q = \frac{\omega_r L}{R} = \frac{1}{R}\sqrt{\frac{L}{C}} $$

6. Power in AC Circuit

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

Instantaneous Power

$$ P = EI $$

Average Power

$$ P_{avg} = V_{rms} I_{rms} \cos \phi = \frac{E_0 I_0}{2} \cos \phi $$

Power Factor

$$ \cos \phi = \frac{R}{Z} $$

Key Points

• Pure L/C → \( \cos\phi = 0 \) (wattless current)
• Max power when \( \phi = 0 \)

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

$$ \frac{E_s}{E_p} = \frac{N_s}{N_p} $$

Power Relation

$$ E_p I_p = E_s I_s $$

Efficiency

$$ \eta = \frac{E_s I_s}{E_p I_p} \times 100\% $$

Key Points

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

Quick Formula Sheet (Memorise These!)

• \( X_L = \omega L \),   \( X_C = \dfrac{1}{\omega C} \)
• \( Z = \sqrt{R^2 + (X_L - X_C)^2} \)
• Resonance: \( f_r = \dfrac{1}{2\pi \sqrt{LC}} \),   \( Z_{\min} = R \)
• \( P_{\text{avg}} = V_{\text{rms}} I_{\text{rms}} \cos \phi \)
• Transformer: \( \dfrac{E_s}{E_p} = \dfrac{N_s}{N_p} = \dfrac{I_p}{I_s} \)

Ques. How many marks is alternating current 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.

Overall, focusing on 5–6 core topics can cover 70–80% of AC questions, making preparation highly strategic.

• RLC circuits and resonance are the most important topics, with resonance contributing 2–3 questions in JEE.
• RMS, phase, and reactance are high-frequency areas, with RMS giving 1–2 direct CBSE questions and reactance ~15% weight in JEE.
• Phase and graphs are high-concept and error-prone, especially in JEE.
• Numericals dominate JEE, while CBSE remains more NCERT and step-based.
• Quality factor and transformer are easy scoring topics with low effort.

Topic-wise Strategy

Topper-Level Insights for Alternating Current Preparation

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